Transcript for:
Chemical Foundations of the Human Body

introduction chapter objectives after studying this chapter you will be able to describe the fundamental composition of matter identify the three subatomic particles identify the four most abundant elements in the body explain the relationship between an atom's number of electrons and its relative stability distinguish between ionic bonds covalent bonds and hydrogen bonds explain how energy is invested stored and released via chemical reactions particularly those reactions that are critical to life explain the importance of the inorganic compounds that contribute to life such as water salts acids and 41 bases compare and contrast the four important classes of organic carbon-based compounds proteins carbohydrates lipids and nucleic acids according to their composition and functional importance to human life the smallest most fundamental material components of the human body are basic chemical elements in fact chemicals called nucleotide bases are the foundation of the genetic code with the instructions on how to build and maintain the human body from conception through old age there are about three billion of these base pairs in human dna human chemistry includes organic molecules carbon-based and biochemicals those produced by the body human chemistry also includes elements in fact life cannot exist without many of the elements that are part of the earth all of the elements that contribute to chemical reactions to the transformation of energy and to electrical activity and muscle contraction elements that include phosphorus carbon sodium and calcium to name a few originated in stars these elements in turn can form both the inorganic and organic chemical compounds important to life including for example water glucose and proteins this chapter begins by examining elements and how the structures of atoms the basic units of matter determine the characteristics of elements by the number of protons neutrons and electrons in the atoms the chapter then builds the framework of life from there 2.1 elements and atoms the building blocks of matter by the end of this section you will be able to discuss the relationships between matter mass elements compounds atoms and subatomic particles distinguish between atomic number and mass number identify the key distinction between isotopes of the same element explain how electrons occupy electron shells and their contribution to an atom adam's relative stability the substance of the universe from a grain of sand to a star is called matter scientists define matter as anything that occupies space and has mass an object's mass and its weight are related concepts but not quite the same an object's mass is the amount of matter contained in the object and the object's mass is the same whether that object is on earth or in the zero gravity environment of outer space an object's weight on the other hand is its mass as affected by the pull of gravity where gravity strongly pulls on an object's mass its weight is greater than it is where gravity is less strong an object of a certain mass weighs less on the moon for example than it does on earth because the gravity of the moon is less than that of earth in other words weight is variable and is influenced by gravity a piece of cheese that weighs a pound on earth weighs only a few ounces on the moon elements and compounds all matter in the natural world as composed of one or more of the 92 fundamental substances called elements an element is a pure substance that is distinguished from all other matter by the fact that it cannot be created or broken down by ordinary chemical means while your body can assemble many of the chemical compounds needed for life from their constituent elements it cannot make elements they must come from the environment a familiar example of an element that you must take in is calcium ca plus plus calcium is essential to the human body it is absorbed and used for a number of processes including strengthening bones when you consume dairy products your digestive system breaks down the food into components small enough to cross into the bloodstream among these is calcium which because it is an element cannot be broken down further the elemental calcium in cheese therefore is the same as the calcium that forms your bones some other elements you might be familiar with are oxygen sodium and iron the elements in the human body are shown in figure 2.2 beginning with the most abundant oxygen o carbon c hydrogen h and nitrogen n each element's name can be replaced by a one or two letter symbol you will become familiar with some of these during this course all the elements in your body are derived from the foods you eat and the air you breathe 42. figure 2.2 elements of the human body the main elements that compose the human body are shown from most abundant to least abundant in nature elements rarely occur alone instead they combine to form compounds a compound is a substance composed of two or more elements joined by chemical bonds for example the compound glucose is an important body fuel it is always composed of the same three elements carbon hydrogen and oxygen moreover the elements that make up any given compound always occur in the same relative amounts in glucose there are always six carbon and six oxygen units for every 12 hydrogen units but what exactly are these units of elements atoms and subatomic particles an atom is the smallest quantity of an element that retains the unique properties of that element in other words an atom of hydrogen is a unit of hydrogen the smallest amount of hydrogen that can exist as you might guess atoms are almost unfathomably small the period at the end of this sentence is millions of atoms wide atomic structure and energy atoms are made up of even smaller subatomic particles three types of which are important the proton neutron and electron the number of positively charged protons and non-charged neutral neutrons gives mass to the atom and the number of each in the nucleus of the atom determine the element the number of negatively charged electrons that spin around the nucleus at close to the speed of light equals the number of protons an electron has about 1 2 000 the mass of a proton or neutron figure 2.3 shows two models that can help you imagine the structure of an atom in this case helium he in the planetary model helium's two electrons are shown circling the nucleus in a fixed orbit depicted as a ring although this model is helpful in visualizing atomic structure in reality electrons do not travel in fixed orbits but whiz around the nucleus erratically in a so-called electron cloud dot 43 figure 2.32 models of atomic structure uh in the planetary model the electrons of helium are shown in fixed orbits depicted as rings at a precise distance from the nucleus somewhat like planets orbiting the sun b in the electron cloud model the electrons of carbon are shown in the variety of locations they would have at different distances from the nucleus over time and atoms protons and electrons carry electrical charges protons with their positive charge are designated p plus electrons which have a negative charge are designated e and atoms neutrons have no charge they are electrically neutral just as a magnet sticks to a steel refrigerator because their opposite charges attract the positively charged protons attract the negatively charged electrons this mutual attraction gives the atom some structural stability the attraction by the positively charged nucleus helps keep electrons from straying far the number of protons and electrons within a neutral atom are equal thus the atom's overall charge is balanced atomic number and mass number in atom of carbon is unique to carbon but a proton of carbon has not one proton is the same as another whether it is found in an atom of carbon sodium na or iron pha the same is true for neutrons and electrons so what gives an element its distinctive properties what makes carbon so different from sodium or iron the answer is the unique quantity of protons each contains carbon by definition as an element whose atoms contain six protons no other element has exactly six protons in its atoms moreover all atoms of carbon whether found in your liver or in a lump of coal contain six protons thus the atomic number which is the number of protons in the nucleus of the atom identifies the element because an atom usually has the same number of electrons as protons the atomic number identifies the usual number of electrons as well in their most common form many elements also contain the same number of neutrons as protons the most common form of carbon for example has six neutrons as well as six protons for a total of 12 subatomic particles in its nucleus an element's mass number is the sum of the number of protons and neutrons in its nucleus so the most common form of carbon's mass number is 12 electrons have so little mass that they do not appreciably contribute to the mass of an atom carbon is a relatively light element uranium u in contrast has a mass number of 238 and is referred to as a heavy metal its atomic number is 92 it has 92 protons but it contains 146 neutrons it has the most mass of all the naturally occurring elements 44 the periodic table of the elements shown in figure 2.4 is a chart identifying the 92 elements found in nature as well as several larger unstable elements discovered experimentally the elements are arranged in order of their atomic number with hydrogen and helium at the top of the table and the more massive elements below the periodic table is a useful device because for each element it identifies the chemical symbol the atomic number and the mass number while organizing elements according to their propensity to react with other elements the number of protons and electrons in an element are equal the number of protons and neutrons may be equal for some elements but are not equal for all figure 2.4 the periodic table of the elements credit r adra goset a musgrove c w clarke w c martin visit this website http colon slash slash open stacks college dot org slash l slash table closing parenthesis to view the periodic table in the periodic table of the elements elements in a single column have the same number of electrons that can participate in a chemical reaction these electrons are known as valence electrons for example the elements in the first column all have a single valence electron an electron that can be donated in a chemical reaction with another atom what is the meaning of a mass number shown in parentheses 45 isotopes although each element has a unique number of protons it can exist as different isotopes an isotope is one of the different forms of an element distinguished from one another by different numbers of neutrons the standard isotope of carbon has 12c commonly called carbon-12 12c has six protons and six neutrons for a mass number of twelve all of the isotopes of carbon have the same number of protons therefore 13c has 7 neutrons and 14c has 8 neutrons the different isotopes of an element can also be indicated with the mass number hyphenated for example c minus 12 instead of 12c hydrogen has three common isotopes shown in figure 2.5 figure 2.5 isotopes of hydrogen protium designated has one proton and no neutrons it is by far the most abundant isotope of hydrogen in nature deuterium designated 2h has one proton and one neutron tritium designated 3h has 2 neutrons an isotope that contains more than the usual number of neutrons is referred to as a heavy isotope an example is 14c heavy isotopes tend to be unstable and unstable isotopes are radioactive a radioactive isotope is an isotope whose nucleus readily decays giving off subatomic particles and electromagnetic energy different radioactive isotopes also called radioisotopes differ in their half-life the time it takes for half of any size sample of an isotope to decay for example the half-life of tritium a radioisotope of hydrogen is about 12 years indicating it takes 12 years for half of the tritium nuclei in a sample to decay excessive exposure to radioactive isotopes can damage human cells and even cause cancer and birth defects but when exposure is controlled some radioactive isotopes can be useful in medicine for more information see the career connections 46 interventional radiologists the controlled use of radioisotopes has advanced medical diagnosis and treatment of disease interventional radiologists are physicians who treat disease by using minimally invasive techniques involving radiation many conditions that could once only be treated with a lengthy and traumatic operation can now be treated non-surgically reducing the cost pain length of hospital stay and recovery time for patients for example in the past the only options for a patient with one or more tumors in the liver were surgery and chemotherapy the administration of drugs to treat cancer some liver tumors however are difficult to access surgically and others could require the surgeon to remove too much of the liver moreover chemotherapy is highly toxic to the liver and certain tumors do not respond well to it anyway in some such cases an interventional radiologist can treat the tumors by disrupting their blood supply which they need if they are to continue to grow in this procedure called radio embolization the radiologist accesses the liver with a fine needle threaded through one of the patient's blood vessels the radiologist then inserts tiny radioactive seeds into the blood vessels that supply the tumors in the days and weeks following the procedure the radiation emitted from the seeds destroys the vessels and directly kills the tumor cells in the vicinity of the treatment radioisotopes emit subatomic particles that can be detected and tracked by imaging technologies one of the most advanced uses of radioisotopes in medicine is the positron emission tomography pet scanner which detects the activity in the body of a very small injection of radioactive glucose the simple sugar that cells use for energy the pet camera reveals to the medical team which of the patient's tissues are taking up the most glucose thus the most metabolically active tissues show up as bright hot spots on the images figure 2.6 pet can reveal some cancerous masses because cancer cells consume glucose at a high rate to fuel their rapid reproduction figure 2.6 pet scan pet highlights areas in the body where there is relatively high glucose use which is characteristic of cancerous tissue this pet scan shows sites of the spread of a large primary tumor to other sites dot 47 the behavior of electrons in the human body atoms do not exist as independent entities rather they are constantly reacting with other atoms to form and to break down more complex substances to fully understand anatomy and physiology you must grasp how atoms participate in such reactions the key is understanding the behavior of electrons although electrons do not follow rigid orbits a set distance away from the atom's nucleus they do tend to stay within certain regions of space called electron shells an electron shell is a layer of electrons that encircle the nucleus at a distinct energy level the atoms of the elements found in the human body have from one to five electron shells and all electron shells hold eight electrons except the first shell which can only hold two this configuration of electron shells is the same for all atoms the precise number of shells depends on the number of electrons in the atom hydrogen and helium have just one and two electrons respectively if you take a look at the periodic table of the elements you will notice that hydrogen and helium are placed alone on either sides of the top row they are the only elements that have just one electron shell figure 2.7 a second shell is necessary to hold the electrons in all elements larger than hydrogen and helium lithium li whose atomic number is three has three electrons two of these fill the first electron shell and the third spills over into a second shell the second electron shell can accommodate as many as eight electrons carbon with its six electrons entirely fills its first shell and half fills its second with 10 electrons neon nay entirely fills its two electron shells again a look at the periodic table reveals that all of the elements in the second row from lithium to neon have just two electron shells atoms with more than 10 electrons require more than two shells these elements occupy the third and subsequent rows of the periodic table figure 2.7 electron shells electrons orbit the atomic nucleus at distinct levels of energy called electron shells a with one electron hydrogen only half fills its electron shell helium also has a single shell but its two electrons completely fill it b the electrons of carbon completely fill its first electron shell but only half fills its second c neon an element that does not occur in the body has 10 electrons filling both of its electron shells the factor that most strongly governs the tendency of an atom to participate in chemical reactions as the number of electrons in its valence shell a valence shell is an atom's outermost electron shell if the valence shell is full the atom is stable meaning its electrons are unlikely to be pulled away from the nucleus by the electrical charge of other atoms if the valence shell is not full the atom is reactive meaning it will tend to react with other atoms in ways that make the valence shell full consider hydrogen with its one electron only half filling its valence shell this single electron is likely to be drawn into 48 relationships with the atoms of other elements so that hydrogen single valence shell can be stabilized all atoms except hydrogen and helium with their single electron shells are most stable when there are exactly eight electrons in their valence shell this principle is referred to as the octet rule and it states that an atom will give up gain or share electrons with another atom so that it ends up with eight electrons in its own valence shell for example oxygen with six electrons in its valence shell is likely to react with other atoms in a way that results in the addition of two electrons to oxygen's valence shell bringing the number to eight when two hydrogen atoms each share their single electron with oxygen covalent bonds are formed resulting in a molecule of water h2o in nature atoms of one element tend to join with atoms of other elements in characteristic ways for example carbon commonly fills its valence shell by linking up with four atoms of hydrogen in so doing the two elements form the simplest of organic molecules methane which also is one of the most abundant and stable carbon containing compounds on earth as stated above another example is water oxygen needs two electrons to fill its valence shell it commonly interacts with two atoms of hydrogen forming h2o incidentally the name hydrogen reflects its contribution to water hydro equals water gen equals maker thus hydrogen as the water maker 2.2 chemical bonds by the end of this section you will be able to explain the relationship between molecules and compounds distinguish between ions cations and anions identify the key difference between ionic and covalent bonds distinguish between nonpolar and polar covalent bonds explain how water molecules linked via hydrogen bonds atoms separated by a great distance cannot rather they must come close enough for the electrons in their valence shells to interact but do atoms ever actually touch one another most physicists would say no because the negatively charged electrons in their valence shells repel one another no force within the human body or anywhere in the natural world is strong enough to overcome this electrical repulsion so when you read about atoms linking together or colliding bear in mind that the atoms are not merging in a physical sense instead atoms link by forming a chemical bond a bond is a weak or strong electrical attraction that holds atoms in the same vicinity the new grouping is typically more stable less likely to react again than its component atoms were when they were separate a more or less stable grouping of two or more atoms held together by chemical bonds is called a molecule the bonded atoms may be of the same element as in the case of h2 which is called molecular hydrogen or hydrogen gas when a molecule is made up of two or more atoms of different elements it is called a chemical compound thus a unit of water or h2o is a compound as is a single molecule of the gas methane or ch4 three types of chemical bonds are important in human physiology because they hold together substances that are used by the body for critical aspects of homeostasis signaling and energy production to name just a few important processes these are ionic bonds covalent bonds and hydrogen bonds ions and ionic bonds recall that an atom typically has the same number of positively charged protons and negatively charged electrons as long as this situation remains the atom is electrically neutral but when an atom participates in a chemical reaction that results in the donation or acceptance of one or more electrons the atom will then become positively or negatively charged this happens frequently for most atoms in order to have a full valence shell as described previously this can happen either by gaining electrons to fill a shell that is more than half full or by giving away electrons to empty a shell than is less than half full thereby leaving the next smaller electron shell as the new full valence shell an atom that has an electrical charge whether positive or negative is an ion dot 49 visit this website http colon slash open stacks college dot org slash l slash elect energy closing parenthesis to learn about electrical energy and the attraction repulsion of charges what happens to the charged electroscope when a conductor is moved between its plastic sheets and why potassium k for instance is an important element in all body cells its atomic number is 19. it has just one electron in its valence shell this characteristic makes potassium highly likely to participate in chemical reactions in which it donates one electron it is easier for potassium to donate one electron than to gain seven electrons the loss will cause the positive charge of potassium's protons to be more influential than the negative charge of potassium's electrons in other words the resulting potassium ion will be slightly positive a potassium ion is written k plus indicating that it has lost a single electron a positively charged ion is known as a cation now consider fluorine f a component of bones and teeth its atomic number is nine and it has seven electrons in its valence shell thus it is highly likely to bond with other atoms in such a way that fluorine accepts one electron it is easier for fluorine to gain one electron than to donate seven electrons when it does its electrons will outnumber its protons by one and it will have an overall negative charge the ionized form of fluorine is called fluoride and is written as f a negatively charged ion is known as an anion atoms that have more than one electron to donate or accept will end up with stronger positive or negative charges a cation that has donated two electrons has a net charge of plus two using magnesium mg as an example this can be written mg plus plus or mg two plus an anion that has accepted two electrons has a net charge of minus two the ionic form of selenium say for example is typically written say two the opposite charges of cations and anions exert a moderately strong mutual attraction that keeps the atoms in close proximity forming an ionic bond an ionic bond is an ongoing close association between ions of opposite charge the table salt you sprinkle on your food owes its existence to ionic bonding as shown in figure 2.8 sodium commonly donates an electron to chlorine becoming the cation na plus when chlorine accepts the electron it becomes the chloride anion cl with their opposing charges these two ions strongly attract each other 50. figure 2.8 ionic bonding a sodium readily donates the solitary electron in its valence shell to chlorine which needs only one electron to have a full valence shell b the opposite electrical charges of the resulting sodium cation and chloride anion result in the formation of a bond of attraction called an ionic bond c the attraction of many sodium and chloride ions results in the formation of large groupings called crystals water is an essential component of life because it is able to break the ionic bonds in salts to free the ions in fact in biological fluids most individual atoms exist as ions these dissolved ions produce electrical charges within the body the behavior of these ions produces the tracings of heart and brain function observed as waves on an electrocardiogram ekg or ecg or an electroencephalogram egg the electrical activity that derives from the interactions of the charged ions is why they are also called electrolytes covalent bonds unlike ionic bonds formed by the attraction between a cation's positive charge and an anion's negative charge molecules formed by a covalent bond share electrons in a mutually stabilizing relationship like next door neighbors whose kids hang out first at one home and then at the other the atoms do not lose or gain electrons permanently instead the electrons move 51 back and forth between the elements because of the close sharing of pairs of electrons one electron from each of two atoms covalent bonds are stronger than ionic bonds nonpolar covalent bonds figure 2.9 shows several common types of covalent bonds notice that the two covalently bonded atoms typically share just one or two electron pairs though larger sharings are possible the important concept to take from this is that in covalent bonds electrons in the outermost valence shell are shared to fill the valence shells of both atoms ultimately stabilizing both of the atoms involved in a single covalent bond a single electron is shared between two atoms while in a double covalent bond two pairs of electrons are shared between two atoms there even are triple covalent bonds where three atoms are shared figure 2.9 covalent bonding you can see that the covalent bonds shown in figure 2.9 are balanced the sharing of the negative electrons is relatively equal as is the electrical pull of the positive protons in the nucleus of the atoms involved this is why covalently bonded molecules that are electrically balanced in this way are described as nonpolar that is no region of the molecule is either more positive or more negative than any other polar covalent bonds groups of legislators with completely opposite views on a particular issue are often described as polarized by news writers in chemistry a polar molecule is a molecule that contains regions that have opposite electrical charges polar molecules occur when atoms share electrons unequally in polar covalent bonds the most familiar example of a polar molecule is water figure 2.10 the molecule has three parts one atom of oxygen the nucleus of which contains eight protons and two hydrogen atoms whose nuclei each contain only one proton because every proton exerts an identical positive charge a nucleus that contains eight protons exerts a charge eight times greater than a nucleus that contains one proton this means that the negatively charged electrons present in the water molecule are more strongly attracted to the oxygen nucleus than to the hydrogen nuclei each hydrogen atom's single negative electron therefore migrates toward the oxygen atom making the oxygen end of their bond slightly more negative than the hydrogen end of their bond 52 figure 2.10 polar covalent bonds in a water molecule what is true for the bonds is true for the water molecule as a whole that is the oxygen region has a slightly negative charge and the regions of the hydrogen atoms have a slightly positive charge these charges are often referred to as partial charges because the strength of the charge is less than one full electron as would occur in an ionic bond as shown in figure 2.10 regions of weak polarity are indicated with the greek letter delta delta and a plus plus or minus sine even though a single water molecule is unimaginably tiny it has mass and the opposing electrical charges on the molecule pull that mass in such a way that it creates a shape somewhat like a triangular tint see figure 2.10 b this dipole with the positive charges at one end formed by the hydrogen atoms at the bottom of the tent and the negative charge at the opposite end the oxygen atom at the top of the tint makes the charged regions highly likely to interact with charged regions of other polar molecules for human physiology the resulting bond is one of the most important formed by water the hydrogen bond hydrogen bonds a hydrogen bond is formed when a weakly positive hydrogen atom already bonded to one electronegative atom for example the oxygen in the water molecule is attracted to another electronegative atom from another molecule in other words hydrogen bonds always include hydrogen that is already part of a polar molecule the most common example of hydrogen bonding in the natural world occurs between molecules of water it happens before your eyes whenever two raindrops merge into a larger bead or a creek spills into a river hydrogen bonding occurs because the weakly negative oxygen atom in one water molecule is attracted to the weakly positive hydrogen atoms of two other water molecules figure 2.11 dot 53 figure 2.11 hydrogen bonds between water molecules notice that the bonds occur between the weakly positive charge on the hydrogen atoms and the weakly negative charge on the oxygen atoms hydrogen bonds are relatively weak and therefore are indicated with a dotted rather than a solid line water molecules also strongly attract other types of charged molecules as well as ions this explains why table salt for example actually is a molecule called a salt in chemistry which consists of equal numbers of positively charged sodium not plus and negatively charged chloride cl dissolves so readily in water in this case forming dipole ion bonds between the water and the electrically charged ions electrolytes water molecules also repel molecules with nonpolar covalent bonds like fats lipids and oils you can demonstrate this with a simple kitchen experiment pour a teaspoon of vegetable oil a compound formed by nonpolar covalent bonds into a glass of water instead of instantly dissolving in the water the oil forms a distinct bead because the polar water molecules repel the nonpolar oil 2.3 chemical reactions by the end of this section you will be able to distinguish between kinetic and potential energy and between exergonic and endergonic chemical reactions identify four forms of energy important in human functioning describe the three basic types of chemical reactions identify several factors influencing the rate of chemical reactions one characteristic of a living organism as metabolism which is the sum total of all of the chemical reactions that go on to maintain that organism's health and life the bonding processes you have learned thus far are anabolic chemical reactions that is they form larger molecules from smaller molecules or atoms but recall that metabolism can proceed in another direction in catabolic chemical reactions bonds between components of larger molecules break releasing smaller molecules or atoms both types of reaction involve exchanges not only of matter but of energy the role of energy in chemical reactions chemical reactions require a sufficient amount of energy to cause the matter to collide with enough precision and force that old chemical bonds can be broken and new ones formed in general kinetic energy is the form of energy powering any type of matter in motion imagine you are building a brick wall the energy it takes to lift and place one brick atop another as kinetic energy the energy matter possesses because of its motion once the wall is in place it stores potential energy potential energy is the energy of position or the energy matter possesses because of the positioning or structure of its components if the brick wall collapses the stored potential energy is released as kinetic energy as the bricks fall in the human body potential energy is stored in the bonds between atoms and molecules chemical energy is the form of potential energy in which energy is stored in chemical bonds when those bonds are formed chemical energy is invested and when they break chemical energy is released notice that chemical energy like all energy is neither created nor destroyed rather it is converted from one form to another when you eat an energy bar before heading out the door for a hike the honey nuts and other foods the bar contains are broken down and rearranged by your body into molecules that your muscle cells convert to kinetic energy chemical reactions that release more energy than they absorb are characterized as exergonic the catabolism of the foods in your energy bar as an example some of the chemical energy stored in the bar is absorbed into molecules your body uses 54 for fuel but some of it is released for example as heat in contrast chemical reactions that absorb more energy than they release are endergonic these reactions require energy input and the resulting molecule stores not only the chemical energy in the original components but also the energy that fueled the reaction because energy is neither created nor destroyed where does the energy needed for endergonic reactions come from in many cases it comes from exergonic reactions forms of energy important in human functioning you have already learned that chemical energy is absorbed stored and released by chemical bonds in addition to chemical energy mechanical radiant and electrical energy are important in human functioning mechanical energy which is stored in physical systems such as machines engines or the human body directly powers the movement of matter when you lift a brick into place on a wall your muscles provide the mechanical energy that moves the brick radiant energy is energy emitted and transmitted as waves rather than matter these waves vary in length from long radio waves and microwaves to short gamma waves emitted from decaying atomic nuclei the full spectrum of radiant energy is referred to as the electromagnetic spectrum the body uses the ultraviolet energy of sunlight to convert a compound in skin cells to vitamin d which is essential to human functioning the human eye evolved to see the wavelengths that comprise the colors of the rainbow from red to violet so that range in the spectrum is called visible light electrical energy supplied by electrolytes in cells and body fluids contributes to the voltage changes that help transmit impulses in nerve and muscle cells characteristics of chemical reactions all chemical reactions begin with a reactant the general term for the one or more substances that enter into the reaction sodium and chloride ions for example are the reactants in the production of table salt the one or more substances produced by a chemical reaction are called the product in chemical reactions the components of the reactants the elements involved and the number of atoms of each are all present in the products similarly there is nothing present in the products that are not present in the reactants this is because chemical reactions are governed by the law of conservation of mass which states that matter cannot be created or destroyed in a chemical reaction just as you can express mathematical calculations in equations such as two plus seven equals nine you can use chemical equations to show how reactants become products as in math chemical equations proceed from left to right but instead of an equal sign they employ an arrow or arrows indicating the direction in which the chemical reaction proceeds for example the chemical reaction in which one atom of nitrogen and three atoms of hydrogen produce ammonia would be written as n plus three h right pointing arrow in h3 correspondingly the breakdown of ammonia into its components would be written as nh3 right pointing arrow in plus 3h notice that in the first example a nitrogen n atom and three hydrogen h atoms bond to form a compound this anabolic reaction requires energy which is then stored within the compound's bonds such reactions are referred to as synthesis reactions a synthesis reaction as a chemical reaction that results in the synthesis joining of components that were formerly separate figure 2.120 again nitrogen and hydrogen are reactants in a synthesis reaction that yields ammonia as the product the general equation for a synthesis reaction as a plus b right pointing arrow a b dot 55 figure 2.12 the three fundamental chemical reactions the atoms and molecules involved in the three fundamental chemical reactions can be imagined as words in the second example ammonia is catabolized into its smaller components and the potential energy that had been stored in its bonds is released such reactions are referred to as decomposition reactions a decomposition reaction is a chemical reaction that breaks down or decomposes something larger into its constituent parts see figure 2.12 b the general equation for a decomposition reaction as a b right pointing arrow a plus b an exchange reaction as a chemical reaction in which both synthesis and decomposition occur chemical bonds are both formed and broken and chemical energy is absorbed stored and released see figure 2.12 c the simplest form of an exchange reaction might be a plus bc right pointing arrow a b plus c notice that to produce these products b and c had to break apart in a decomposition reaction whereas a and b had to bond in a synthesis reaction a more complex exchange reaction might be a b plus c d right pointing arrow ac plus b d another example might be a b plus c d right pointing arrow ad plus bc in theory any chemical reaction can proceed in either direction under the right conditions reactants may synthesize into a product that is later decomposed reversibility is also a quality of exchange reactions for instance a plus bc right pointing arrow a b plus c could then reverse to a b plus c right pointing arrow a plus b c this reversibility of a chemical reaction as indicated with a double arrow a plus b c a b plus c still in the human body many chemical reactions do proceed in a predictable direction either one way or the other you can think of this more predictable path as the path of least resistance because typically the alternate direction requires more energy factors influencing the rate of chemical reactions if you pour vinegar into baking soda the reaction is instantaneous the concoction will bubble and fizz but many chemical reactions take time a variety of factors influence the rate of chemical reactions this section however will consider only the most important in human functioning properties of the reactants if chemical reactions are to occur quickly the atoms in the reactants have to have easy access to one another thus the greater the surface area of the reactants the more readily they will interact when you pop a cube of cheese into your mouth you chew it before you swallow it among other things chewing increases the surface area of the food so that digestive chemicals can more easily get at it as a general rule gases tend to react faster than liquids or solids again because it takes energy to separate particles of a substance and gases by definition already have space between their particles similarly the larger the molecule the greater the number of total bonds so reactions involving smaller molecules with fewer total bonds would be expected to proceed faster in addition recall that some elements are more reactive than others reactions that involve highly reactive elements like hydrogen proceed more quickly than reactions that involve less reactive elements reactions involving stable elements like helium are not likely to happen at all temperature nearly all chemical reactions occur at a faster rate at higher temperatures recall that kinetic energy is the energy of matter in motion the kinetic energy of subatomic particles increases in response to increases in thermal energy the higher the temperature the faster the particles move and the more likely they are to come in contact and react 56. concentration and pressure if just a few people are dancing at a club they are unlikely to step on each other's toes but as more and more people get up to dance especially if the music is fast collisions are likely to occur it is the same with chemical reactions the more particles present within a given space the more likely those particles are to bump into one another this means that chemists can speed up chemical reactions not only by increasing the concentration of particles the number of particles in the space but also by decreasing the volume of the space which would correspondingly increase the pressure if there were 100 dancers in that club and the manager abruptly moved the party to a room half the size the concentration of the dancers would double in the new space and the likelihood of collisions would increase accordingly enzymes and other catalysts for two chemicals in nature to react with each other they first have to come into contact and this occurs through random collisions because heat helps increase the kinetic energy of atoms ions and molecules it promotes their collision but in the body extremely high heat such as a very high fever can damage body cells and be life threatening on the other hand normal body temperature is not high enough to promote the chemical reactions that sustain life that is where catalysts come in in chemistry a catalyst is a substance that increases the rate of a chemical reaction without itself undergoing any change you can think of a catalyst as a chemical change agent they help increase the rate and force at which atoms ions and molecules collide thereby increasing the probability that their valence shell electrons will interact the most important catalysts in the human body are enzymes an enzyme is a catalyst composed of protein or ribonucleic acid rna both of which will be discussed later in this chapter like all catalysts enzymes work by lowering the level of energy that needs to be invested in a chemical reaction a chemical reaction's activation energy is the threshold level of energy needed to break the bonds in the reactants once those bonds are broken new arrangements can form without an enzyme to act as a catalyst a much larger investment of energy is needed to ignite a chemical reaction figure 2.13 figure 2.13 enzymes enzymes decrease the activation energy required for a given chemical reaction to occur a without an enzyme the energy input needed for a reaction to begin as high b with the help of an enzyme less energy is needed for a reaction to begin enzymes are critical to the body's healthy functioning they assist for example with the breakdown of food and its conversion to energy in fact most of the chemical reactions in the body are facilitated by enzymes dot 572.4 inorganic compounds essential to human functioning by the end of this section you will be able to compare and contrast inorganic and organic compounds identify the properties of water that make it essential to life explain the role of salts in body functioning distinguish between acids and bases and explain their role in ph discuss the role of buffers in helping the body maintain ph homeostasis the concepts you have learned so far in this chapter govern all forms of matter and would work as a foundation for geology as well as biology this section of the chapter narrows the focus to the chemistry of human life that is the compounds important for the body's structure and function in general these compounds are either inorganic or organic an inorganic compound as a substance that does not contain both carbon and hydrogen a great many inorganic compounds do contain hydrogen atoms such as water h2o and the hydrochloric acid hcl produced by your stomach in contrast only a handful of inorganic compounds contain carbon atoms carbon dioxide colorado 2 is one of the few examples an organic compound then is a substance that contains both carbon and hydrogen organic compounds are synthesized via covalent bonds within living organisms including the human body recall that carbon and hydrogen are the second and third most abundant elements in your body you will soon discover how these two elements combine in the foods you eat in the compounds that make up your body structure and in the chemicals that fuel your functioning the following section examines the three groups of inorganic compounds essential to life water salts acids and bases organic compounds are covered later in the chapter water as much as seventy percent of an adult's body weight as water this water is contained both within the cells and between the cells that make up tissues and organs its several roles make water indispensable to human functioning water as a lubricant and cushioned water as a major component of many of the body's lubricating fluids just as oil lubricates the hinge on a door water in synovial fluid lubricates the actions of body joints and water in pleural fluid helps the lungs expand and recoil with breathing watery fluids help keep food flowing through the digestive tract and ensure that the movement of adjacent abdominal organs is friction free water also protects cells and organs from physical trauma cushioning the brain within the skull for example and protecting the delicate nerve tissue of the eyes water cushions a developing fetus in the mother's womb as well water as a heat sink a heat sink is a substance or object that absorbs and dissipates heat but does not experience a corresponding increase in temperature in the body water absorbs the heat generated by chemical reactions without greatly increasing in temperature moreover when the environmental temperature soars the water stored in the body helps keep the body cool this cooling effect happens as warm blood from the body's core flows to the blood vessels just under the skin and is transferred to the environment at the same time sweat glands release warm water in sweat as the water evaporates into the air it carries away heat and then the cooler blood from the periphery circulates back to the body core water as a component of liquid mixtures a mixture as a combination of two or more substances each of which maintains its own chemical identity in other words the constituent substances are not chemically bonded into a new larger chemical compound the concept is easy to imagine if you think of powdery substances such as flour and sugar when you stir them together in a bowl they obviously do not bond to form a new compound the room air you breathe is a gaseous mixture containing three discrete elements nitrogen oxygen and argon and one compound carbon dioxide there are three types of liquid mixtures all of which contain water as a key component these are solutions colloids and suspensions 58 for cells in the body to survive they must be kept moist in a water-based liquid called a solution in chemistry a liquid solution consists of a solvent that dissolves a substance called a solute an important characteristic of solutions is that they are homogeneous that is the solute molecules are distributed evenly throughout the solution if you were to stir a teaspoon of sugar into a glass of water the sugar would dissolve into sugar molecules separated by water molecules the ratio of sugar to water in the left side of the glass would be the same as the ratio of sugar to water in the right side of the glass if you were to add more sugar the ratio of sugar to water would change but the distribution provided you had stirred well would still be even water is considered the universal solvent and it is believed that life cannot exist without water because of this water is certainly the most abundant solvent in the body essentially all of the body's chemical reactions occur among compounds dissolved in water because water molecules are polar with regions of positive and negative electrical charge water readily dissolves ionic compounds and polar covalent compounds such compounds are referred to as hydrophilic or water loving as mentioned above sugar dissolves well in water this is because sugar molecules contain regions of hydrogen oxygen polar bonds making it hydrophilic nonpolar molecules which do not readily dissolve in water are called hydrophobic or water fearing concentrations of solutes various mixtures of solutes and water are described in chemistry the concentration of a given solute is the number of particles of that solute in a given space oxygen makes up about 21 of atmospheric air in the bloodstream of humans glucose concentration is usually measured in milligram mg per deciliter dl and in a healthy adult averages about 100 milligrams per deciliter another method of measuring the concentration of a solute is by its molarily t which is moles m of the molecules per liter l the mole of an element is its atomic weight while a mole of a compound is the sum of the atomic weights of its components called the molecular weight an often used example is calculating a mole of glucose with the chemical formula c6h12o6 using the periodic table the atomic weight of carbon c is 12.011 grams g and there are six carbons in glucose for a total atomic weight of 72.066 grams doing the same calculations for hydrogen h and oxygen oh the molecular weight equals 180.156 grams the gram molecular weight of glucose when water is added to make one liter of solution you have one mole one m of glucose this is particularly useful in chemistry because of the relationship of moles to avogadro's number a mole of any solution has the same number of particles in it six point zero two times ten twenty three many substances in the bloodstream and other tissue of the body are measured in thousandths of a mole or millimoles m a colloid is a mixture that is somewhat like a heavy solution the solute particles consist of tiny clumps of molecules large enough to make the liquid mixture opaque because the particles are large enough to scatter light familiar examples of colloids are milk and cream in the thyroid glands the thyroid hormone is stored as a thick protein mixture also called a colloid a suspension is a liquid mixture in which a heavier substance is suspended temporarily in a liquid but over time settles out this separation of particles from a suspension is called sedimentation an example of sedimentation occurs in the blood test that establishes sedimentation rate or sed rate the test measures how quickly red blood cells in a test tube settle out of the watery portion of blood known as plasma over a set period of time rapid sedimentation of blood cells does not normally happen in the healthy body but aspects of certain diseases can cause blood cells to clump together and these heavy clumps of blood cells settle to the bottom of the test tube more quickly than do normal blood cells the role of water in chemical reactions two types of chemical reactions involve the creation or the consumption of water dehydration synthesis and hydrolysis in dehydration synthesis one reactant gives up an atom of hydrogen and another reactant gives up a hydroxyl group o in the synthesis of a new product in the formation of their covalent bond a molecule of water is released as a byproduct figure 2.14 this is also sometimes referred to as a condensation reaction in hydrolysis a molecule of water disrupts a compound breaking its bonds the water as itself split into h and o one portion of the severed compound then bonds with the hydrogen atom and the other portion bonds with the hydroxyl group these reactions are reversible and play an important role in the chemistry of organic compounds which will be discussed shortly dot 59 figure 2.14 dehydration synthesis and hydrolysis monomers the basic units for building larger molecules form polymers two or more chemically bonded monomers a in dehydration synthesis two monomers are covalently bonded in a reaction in which one gives up a hydroxyl group and the other a hydrogen atom a molecule of water is released as a byproduct during dehydration reactions b in hydrolysis the covalent bond between two monomers is split by the addition of a hydrogen atom to one and a hydroxyl group to the other which requires the contribution of one molecule of water salts recall that salts are formed when ions form ionic bonds in these reactions one atom gives up one or more electrons and thus becomes positively charged whereas the other accepts one or more electrons and becomes negatively charged you can now define a salt as a substance that when dissolved in water dissociates into ions other than h plus or o this fact is important in distinguishing salts from acids and bases discussed next a typical salt sodium chloride dissociates completely in water figure 2.15 the positive and negative regions on the water molecule the hydrogen and oxygen ends respectively attract the negative chloride and positive sodium ions pulling them away from each other again whereas nonpolar and polar covalently bonded compounds break apart into molecules in solution salts dissociate into ions these ions are electrolytes they are capable of conducting an electrical current in solution this property is critical to the function of ions in transmitting nerve impulses and prompting muscle contraction 60 figure 2.15 dissociation of sodium chloride in water notice that the crystals of sodium chloride dissociate not into molecules of sodium chloride but into not plus cations and cln ions each completely surrounded by water molecules many other salts are important in the body for example bile salts produced by the liver help break apart dietary fats and calcium phosphate salts form the mineral portion of teeth and bones acids and bases acids and bases like salts dissociate in water into electrolytes acids and bases can very much change the properties of the solutions in which they are dissolved acids and acid is a substance that releases hydrogen ions h plus in solution figure 2.160 because an atom of hydrogen has just one proton and one electron a positively charged hydrogen ion is simply a proton this solitary proton is highly likely to participate in chemical reactions strong acids are compounds that release all of their h plus in solution that is they ionize completely hydrochloric acid hcl which is released from cells in the lining of the stomach is a strong acid because it releases all of its h-plus in the stomach's watery environment this strong acid aids in digestion and kills ingested microbes weak acids do not ionize completely that is some of their hydrogen ions remain bonded within a compound in solution an example of a weak acid is vinegar or acetic acid it is called acetate after it gives up a proton dot 61 figure 2.16 acids and bases a in aqueous solution an acid dissociates into hydrogen ions h plus and anions nearly every molecule of a strong acid dissociates producing a high concentration of h plus b in aqueous solution a base dissociates into hydroxyl ions o and cations nearly every molecule of a strong base dissociates producing a high concentration of o bases a base as a substance that releases hydroxyl ions o in solution or one that accepts h plus already present in solution see figure 2.16 b the hydroxyl ions also known as hydroxide ions or other basic substances combined with h plus present to form a water molecule thereby removing h plus and reducing the solution's acidity strong bases release most or all of their hydroxyl ions weak bases release only some hydroxyl ions or absorb only a few h plus food mixed with hydrochloric acid from the stomach would burn the small intestine the next portion of the digestive tract after the stomach if it were not for the release of bicarbonate hco3 a weak base that attracts h plus bicarbonate except some of the h plus protons thereby reducing the acidity of the solution the concept of ph the relative acidity or alkalinity of a solution can be indicated by its ph a solutions ph is the negative base 10 logarithm of the hydrogen ion h plus concentration of the solution as an example a ph 4 solution has an h plus concentration that is 10 times greater than that of a ph 5 solution that is a solution with a ph of 4 is 10 times more acidic than a solution with a ph of 5. the concept of ph will begin to make more sense when you study the ph scale like that shown in figure 2.17 the scale consists of a series of increments ranging from 0 to 14. a solution with a ph of 7 is considered neutral neither acidic nor basic pure water has a ph of 7. the lower the number below 7 the more acidic the solution or the greater the concentration of h plus the concentration of hydrogen ions at each ph value is 10 times different than the next ph for instance a ph value of 4 corresponds to a proton concentration of 10 to 4 m or 0.0001 m while a ph value of 5 corresponds to a proton concentration of 10 to 5 m or 0.0001 m the higher the number above 7 the more basic alkaline the solution or the lower the concentration of h plus human urine for example is 10 times more acidic than pure water and hcl is 10 million times more acidic than water 62 figure 2.17 the ph scale buffers the ph of human blood normally ranges from 7.35 to 7.45 although it is typically identified as ph 7.4 at this slightly basic ph blood can reduce the acidity resulting from the carbon dioxide colorado 2 constantly being released into the bloodstream by the trillions of cells in the body homeostatic mechanisms along with exhaling co2 while breathing normally keep the ph of blood within this narrow range this is critical because fluctuations either too acidic or too alkaline can lead to life-threatening disorders all cells of the body depend on homeostatic regulation of acid-base balance at a ph of approximately 7.4 the body therefore has several mechanisms for this regulation involving breathing the excretion of chemicals in urine and the internal release of chemicals collectively called buffers into body fluids a buffer is a solution of a weak acid and its conjugate base a buffer can neutralize small amounts of acids or bases in body fluids for example if there is even a slight decrease below 7.35 in the ph of a bodily fluid the buffer in the fluid in this case acting as a weak base will bind the 63 excess hydrogen ions in contrast if ph rises above 7.45 the buffer will act as a weak acid and contribute hydrogen ions acids and bases excessive acidity of the blood and other body fluids is known as acidosis common causes of acidosis are situations and disorders that reduce the effectiveness of breathing especially the person's ability to exhale fully which causes a buildup of co2 and h plus in the bloodstream acidosis can also be caused by metabolic problems that reduce the level or function of buffers that act as bases or that promote the production of acids for instance with severe diarrhea too much bicarbonate can be lost from the body allowing acids to build up in body fluids in people with poorly managed diabetes ineffective regulation of blood sugar acids called ketones are produced as a form of body fuel these can build up in the blood causing a serious condition called diabetic ketoacidosis kidney failure liver failure heart failure cancer and other disorders also can prompt metabolic acidosis in contrast alkalosis is a condition in which the blood and other body fluids are too alkaline basic as with acidosis respiratory disorders are a major cause however in respiratory alkalosis carbon dioxide levels fall too low lung disease aspirin overdose shock and ordinary anxiety can cause respiratory alkalosis which reduces the normal concentration of h plus metabolic alkalosis often results from prolonged severe vomiting which causes a loss of hydrogen and chloride ions as components of hcl medications also can prompt alkalosis these include diuretics that cause the body to lose potassium ions as well as antacids when taken in excessive amounts for instance by someone with persistent heartburn or an ulcer 2.5 organic compounds essential to human functioning by the end of this section you will be able to identify four types of organic molecules essential to human functioning explain the chemistry behind carbon's affinity for covalently bonding in organic compounds provide examples of three types of carbohydrates and identify the primary functions of carbohydrates in the body discuss four types of lipids important in human functioning describe the structure of proteins and discuss their importance to human functioning identify the building blocks of nucleic acids and the roles of dna rna and atp in human functioning organic compounds typically consist of groups of carbon atoms covalently bonded to hydrogen usually oxygen and often other elements as well created by living things they are found throughout the world in soils and seas commercial products and every cell of the human body the four types most important to human structure and function are carbohydrates lipids proteins and nucleotides before exploring these compounds you need to first understand the chemistry of carbon the chemistry of carbon what makes organic compounds ubiquitous as the chemistry of their carbon core recall that carbon atoms have four electrons in their valence shell and that the octet rule dictates that atoms tend to react in such a way as to complete their valence shell with eight electrons carbon atoms do not complete their valence shells by donating or accepting four electrons instead they readily share electrons via covalent bonds commonly carbon atoms share with other carbon atoms often forming a long carbon chain referred to as a carbon skeleton when they do share however they do not share all their electrons exclusively with each other rather carbon atoms 1064 to share electrons with a variety of other elements one of which is always hydrogen carbon and hydrogen groupings are called hydrocarbons if you study the figures of organic compounds in the remainder of this chapter you will see several with chains of hydrocarbons in one region of the compound many combinations are possible to fill carbons for vacancies carbon may share electrons with oxygen or nitrogen or other atoms in a particular region of an organic compound moreover the atoms to which carbon atoms bond may also be part of a functional group a functional group is a group of atoms linked by strong covalent bonds and tending to function in chemical reactions as a single unit you can think of functional groups as tightly knit clicks whose members are unlikely to be parted five functional groups are important in human physiology these are the hydroxyl carboxyl amino methyl and phosphate groups table 2.1 functional groups important in human physiology functional group structural formula importance hydroxyl o h hydroxyl groups are polar they are components of all four types of organic compounds discussed in this chapter they are involved in dehydration synthesis and hydrolysis reactions carboxyl o c all carboxyl groups are found within fatty acids amino acids and many other acids amino n h2 amino groups are found within amino acids the building blocks of proteins methyl c h3 methyl groups are found within amino acids phosphate p o4 ii phosphate groups are found within phospholipids and nucleotides table 2.1 carbon's affinity for covalent bonding means that many distinct and relatively stable organic molecules nevertheless readily form larger more complex molecules any large molecule is referred to as macromolecule macro equals large and the organic compounds in this section all fit this description however some macromolecules are made up of several copies of single units called monomer mono equals one myrrh equals part like beads in a long necklace these monomers linked by covalent bonds to form long polymers poly equals many there are many examples of monomers and polymers among the organic compounds monomers form polymers by engaging in dehydration synthesis see figure 2.14 as was noted earlier this reaction results in the release of a molecule of water each monomer contributes one gives up a hydrogen atom and the other gives up a hydroxyl group polymers are split into monomers by hydrolysis lysis equals rupture the bonds between their monomers are broken via the donation of a molecule of water which contributes a hydrogen atom to one monomer in a hydroxyl group to the other carbohydrates the term carbohydrate means hydrated carbon recall that the root hydro indicates water a carbohydrate is a molecule composed of carbon hydrogen and oxygen in most carbohydrates hydrogen and oxygen are found in the same two to one relative proportions they have in water in fact the chemical formula for a generic molecule of carbohydrate as ch2on carbohydrates are referred to as saccharides a word meaning sugars three forms are important in the body monosaccharides are the monomers of carbohydrates disaccharides d equals two are made up of two monomers polysaccharides are the polymers and can consist of hundreds to thousands of monomers monosaccharides a monosaccharide is a monomer of carbohydrates five monosaccharides are important in the body three of these are the hexose sugars so called because they each contain six atoms of carbon these are glucose fructose and galactose shown in figure 2.18 a the remaining monosaccharides are the two pinto sugars each of which contains five atoms of carbon they are ribose and deoxyribose shown in figure 2.18 b 65 figure 2.185 important monosaccharides disaccharides a disaccharide as a pair of monosaccharides disaccharides are formed via dehydration synthesis and the bond linking them is referred to as a glycosidic bond glyco equals sugar three disaccharides shown in figure 2.19 are important to humans these are sucrose commonly referred to as table sugar lactose or milk sugar and maltose or malt sugar as you can tell from their common names you consume these in your diet however your body cannot use them directly instead in the digestive tract they are split into their component monosaccharides via hydrolysis 66 figure 2.193 important disaccharides all three important disaccharides form by dehydration synthesis 67 watch this video http colon slash slash slash l openstaxcollege.org disaccharide closing parenthesis to observe the formation of a disaccharide what happens when water encounters a glycosidic bond polysaccharides polysaccharides can contain a few to a thousand or more monosaccharides three are important to the body figure 2.20 starches or polymers of glucose they occur in long chains called amylose or branched chains called amylopectin both of which are stored in plant-based foods and are relatively easy to digest glycogen is also a polymer of glucose but it is stored in the tissues of animals especially in the muscles and liver it is not considered a dietary carbohydrate because very little glycogen remains in animal tissues after slaughter however the human body stores excess glucose as glycogen again in the muscles and liver cellulose a polysaccharide that is the primary component of the cell wall of green plants is the component of plant food referred to as fiber in humans cellulose fiber is not digestible however dietary fiber has many health benefits it helps you feel full so you eat less it promotes a healthy digestive tract and a diet high in fiber is thought to reduce the risk of heart disease and possibly some forms of cancer figure 2.203 important polysaccharides three important polysaccharides are starches glycogen and fiber functions of carbohydrates the body obtains carbohydrates from plant-based foods grains fruits and legumes and other vegetables provide most of the carbohydrate in the human diet although lactose is found in dairy products although most body cells can break down other organic compounds for fuel all body cells can use glucose moreover nerve cells neurons in the brain spinal cord and through the peripheral nervous system as well as red blood cells can use only glucose for fuel in the breakdown of glucose for energy molecules of adenosine triphosphate better known as atp are produced adenosine triphosphate atp is composed of a ribose sugar an adenine base and three phosphate groups atp releases free energy when its phosphate bonds are broken and thus supplies ready energy to the cell more atp is produced in the presence of oxygen o2 than in pathways that do not use oxygen the overall reaction for the conversion of the energy in glucose to energy stored in atp can be written c6h12o6 plus 602 right pointing arrow 6co2 plus six h2o plus atp in addition to being a critical fuel source carbohydrates are present in very small amounts in cell structure for instance some carbohydrate molecules bind with proteins to produce glycoproteins and others combine with lipids to produce glycolipids both of which are found in the membrane that encloses the contents of body cells 68 lipids a lipid is one of a highly diverse group of compounds made up mostly of hydrocarbons the few oxygen atoms they contain are often at the periphery of the molecule their nonpolar hydrocarbons make all lipids hydrophobic in water lipids do not form a true solution but they may form an emulsion which is the term for a mixture of solutions that do not mix well triglycerides a triglyceride is one of the most common dietary lipid groups and the type found most abundantly in body tissues this compound which is commonly referred to as a fat is formed from the synthesis of two types of molecules figure 2.21 a glycerol backbone at the core of triglycerides consists of three carbon atoms three fatty acids long chains of hydrocarbons with a carboxyl group in a methyl group at opposite ends extend from each of the carbons of the glycerol figure 2.21 triglycerides triglycerides are composed of glycerol attached to three fatty acids via dehydration synthesis notice that glycerol gives up a hydrogen atom and the carboxyl groups on the fatty acids each give up a hydroxyl group triglycerides form via dehydration synthesis glycerol gives up hydrogen atoms from its hydroxyl groups at each bond and the carboxyl group on each fatty acid chain gives up a hydroxyl group a total of three water molecules are thereby released fatty acid chains that have no double carbon bonds anywhere along their length and therefore contain the maximum number of hydrogen atoms are called saturated fatty acids these straight rigid chains pack tightly together and are solid or semi-solid at room temperature figure 2.220 butter and lard are examples as is the fat found on a steak or in your own body in contrast fatty acids with one double carbon bond are kinked at that bond figure 2.22 b these monounsaturated fatty acids are therefore unable to pack together tightly and are liquid at room temperature polyunsaturated fatty acids contain two or more double carbon bonds and are also liquid at room temperature plant oils such as olive oil typically contain both mono and polyunsaturated fatty acids figure 2.22 fatty acids shapes the level of saturation of a fatty acid affects its shape a saturated fatty acid chains are straight b unsaturated fatty acid chains are kinked 69 whereas a diet high in saturated fatty acids increases the risk of heart disease a diet high in unsaturated fatty acids is thought to reduce the risk this is especially true for the omega-3 unsaturated fatty acids found in cold water fish such as salmon these fatty acids have their first double carbon bond at the third hydrocarbon from the methyl group referred to as the omega end of the molecule finally trans fatty acids found in some processed foods including some stick and tub margarines are thought to be even more harmful to the heart and blood vessels than saturated fatty acids trans fats are created from unsaturated fatty acids such as corn oil when chemically treated to produce partially hydrogenated fats as a group triglycerides are a major fuel source for the body when you are resting or asleep a majority of the energy used to keep you alive is derived from triglycerides stored in your fat adipose tissues triglycerides also fuel long slow physical activity such as gardening or hiking and contribute a modest percentage of energy for vigorous physical activity dietary fat also assists the absorption and transport of the nonpolar fat soluble vitamins a d e and k additionally stored body fat protects and cushions the body's bones and internal organs and acts as insulation to retain body heat fatty acids are also components of glycolipids which are sugar fat compounds found in the cell membrane lipoproteins are compounds in which the hydrophobic triglycerides are packaged in protein envelopes for transport in body fluids phospholipids as its name suggests a phospholipid is a bond between the glycerol component of a lipid and a phosphorus molecule in fact phospholipids are similar in structure to triglycerides however instead of having three fatty acids a phospholipid is generated from a diglyceride a glycerol with just two fatty acid chains figure 2.23 the third binding site on the glycerol is taken up by the phosphate group which in turn is attached to a polar head region of the molecule recall that triglycerides are nonpolar in hydrophobic this still holds for the fatty acid portion of a phospholipid compound however the head of a phospholipid contains charges on the phosphate groups as well as on the nitrogen atom these charges make the phospholipid head hydrophilic therefore phospholipids are said to have hydrophobic tails containing the neutral fatty acids and hydrophilic heads containing the charged phosphate groups and nitrogen atom 70 figure 2.23 other important lipids uh phospholipids are composed of two fatty acids glycerol and a phosphate group b steriles are ring-shaped lipids shown here as cholesterol c prostaglandins are derived from unsaturated fatty acids prostaglandin e2 pge2 includes hydroxyl and carboxyl groups steroids a steroid compound referred to as a sterol has as its foundation a set of four hydrocarbon rings bonded to a variety of other atoms and molecules see figure 2.23 although both plants and animals synthesize steriles the type that makes the most important contribution to human structure and function as cholesterol which is synthesized by the liver in humans and animals and is also present in most animal-based foods like other lipids cholesterols hydrocarbons make it hydrophobic however it has a polar hydroxyl head that is hydrophilic cholesterol is an important component of bile acids compounds that help emulsify dietary fats in fact the word root chole refers to bile cholesterol is also a building block of many hormones signaling molecules that the body releases to regulate processes at distant sites finally like phospholipids cholesterol molecules are found in the cell membrane where their hydrophobic and hydrophilic regions help regulate the flow of substances into and out of the cell prostaglandins like a hormone a prostaglandin is one of a group of signaling molecules but prostaglandins are derived from unsaturated fatty acids see figure 2.23 c one reason that the omega-3 fatty acids found in fish are beneficial is that they stimulate the production of certain prostaglandins that help regulate the risk for heart disease prostaglandins also sensitize nerves to pain one class of pain relieving medications called non-steroidal anti-inflammatory drugs nsaids works by reducing the effects of prostaglandins proteins you might associate proteins with muscle tissue but in fact proteins are critical components of all tissues and organs a protein is an organic molecule composed of amino acids linked by peptide bonds proteins include the keratin in the epidermis of skin that protects underlying tissues the collagen found in the dermis of skin in bones and in the meninges that cover the brain and spinal cord proteins are also components of many of the body's functional chemicals including digestive enzymes in the digestive tract antibodies the neurotransmitters that neurons use to communicate with other cells and the peptide-based hormones that regulate certain body functions for instance growth hormone while carbohydrates and lipids are composed of hydrocarbons and oxygen all proteins also contain nitrogen in and many contain sulfur s in addition to carbon hydrogen and oxygen microstructure of proteins proteins are polymers made up of nitrogen containing monomers called amino acids an amino acid is a molecule composed of an amino group in a carboxyl group together with a variable side chain just 20 different amino acids contribute to nearly all of the thousands of different proteins important in human structure and function body proteins contain a unique combination of a few dozen to a few hundred of these 20 amino acid monomers all 20 of these amino acids share a similar structure figure 2.24 all consist of a central carbon atom to which the following are bonded a hydrogen atom and alkaline basic amino group nh2 see table 2.1 an acidic carboxyl group cooh see table 2.1 a variable group figure 2.24 structure of an amino acid notice that all amino acids contain both an acid the carboxyl group and a base the amino group amine equals nitrogen containing for this reason they make excellent buffers helping the body regulate acid base balance what distinguishes the 20 amino acids from one another as their variable group which is referred to as a side chain or an r group this group can vary in size and can be polar or nonpolar giving each amino acid its unique characteristics for example the side chains of two amino acids cysteine and methionine contain sulfur sulfur does not readily participate in hydrogen bonds whereas all other amino acids do this variation influences the way that proteins containing cysteine and methionine are assembled amino acids join via dehydration synthesis to form protein polymers figure 2.25 the unique bond holding amino acids together is called a peptide bond a peptide bond is a covalent bond between two amino acids that forms by dehydration synthesis a peptide in fact is a very short chain of amino acids strands containing fewer than about 100 amino acids are generally referred to as polypeptides rather than proteins 72 figure 2.25 peptide bond different amino acids join together to form peptides polypeptides or proteins via dehydration synthesis the bonds between the amino acids are peptide bonds the body is able to synthesize most of the amino acids from components of other molecules however nine cannot be synthesized and have to be consumed in the diet these are known as the essential amino acids free amino acids available for protein construction are said to reside in the amino acid pool within cells structures within cells use these amino acids when assembling proteins if a particular essential amino acid is not available in sufficient quantities in the amino acid pool however synthesis of proteins containing it can slow or even cease shape of proteins just as a fork cannot be used to eat soup and a spoon cannot be used to spear meat a protein shape is essential to its function a protein shape is determined most fundamentally by the sequence of amino acids of which it is made figure 2.260 the sequence is called the primary structure of the protein 73 figure 2.26 the shape of proteins ah the primary structure is the sequence of amino acids that make up the polypeptide chain b the secondary structure which can take the form of an alpha helix or a beta pleated sheet is maintained by hydrogen bonds between amino acids in different regions of the original polypeptide strand c the tertiary structure occurs as a result of further folding and bonding of the secondary structure d the quaternary structure occurs as a result of interactions between two or more tertiary subunits the example shown here is hemoglobin a protein in red blood cells which transports oxygen to body tissues although some polypeptides exist as linear chains most are twisted or folded into more complex secondary structures that form when bonding occurs between amino acids with different properties at different regions of the polypeptide the most common secondary structure is a spiral called an alpha helix if you were to take a length of string and simply twist it into a spiral it would not hold the shape similarly a strand of amino acids could not maintain a stable spiral shape without the help of hydrogen bonds which create bridges between different regions of the same strand see figure 2.26 b less commonly a polypeptide chain can form a beta pleated sheet in which hydrogen bonds form bridges between different regions of a single polypeptide that has folded back upon itself or between two or more adjacent polypeptide chains the secondary structure of proteins further folds into a compact three-dimensional shape referred to as the proteins tertiary structure see figure 2.26 c in this configuration amino acids that had been very distant in the primary chain can be brought quite close via hydrogen bonds or in proteins containing cysteine via disulfide bonds a disulfide bond is a covalent bond between sulfur atoms in a polypeptide often two or more separate polypeptides bond to form an even larger protein with a quaternary structure see figure 2.26 d the polypeptide subunits forming a quaternary structure can be identical or different for instance hemoglobin the protein found in red blood cells is composed of four tertiary polypeptides two of which are called alpha chains and two of which are called beta chains when they are exposed to extreme heat acids bases and certain other substances proteins will denature denaturation is a change in the structure of a molecule through physical or chemical means denatured proteins lose their functional shape and are no longer able to carry out their jobs an everyday example of protein denaturation as the curdling of milk when acidic lemon juice is added the contribution of the shape of a protein to its function can hardly be exaggerated for example the long slender shape of protein strands that make up muscle tissue is essential to their ability to contract shorten and relax lengthen as another example bones contain long threads of a protein called collagen that acts as scaffolding upon which bone minerals are deposited these elongated proteins called fibrous proteins are strong and durable and typically hydrophobic 74 in contrast globular proteins are globes or spheres that tend to be highly reactive and are hydrophilic the hemoglobin proteins packed into red blood cells are an example see figure 2.26 d however globular proteins are abundant throughout the body playing critical roles in most body functions enzymes introduced earlier as protein catalysts are examples of this the next section takes a closer look at the action of enzymes proteins function as enzymes if you were trying to type a paper and every time you hit a key on your laptop there was a delay of six or seven minutes before you got a response you would probably get a new laptop in a similar way without enzymes to catalyze chemical reactions the human body would be non-functional it functions only because enzymes function enzymatic reactions chemical reactions catalyzed by enzymes begin when substrates bind to the enzyme a substrate is a reactant in an enzymatic reaction this occurs on regions of the enzyme known as active sites figure 2.27 any given enzyme catalyzes just one type of chemical reaction this characteristic called specificity is due to the fact that a substrate with a particular shape and electrical charge can bind only to an active site corresponding to that substrate due to this jigsaw puzzle-like match between an enzyme and its substrates enzymes are known for their specificity in fact as an enzyme binds to its substrates the enzyme structure changes slightly to find the best fit between the transition state a structural intermediate between the substrate and product and the active site just as a rubber glove molds to a hand inserted into it this active site modification in the presence of substrate along with the simultaneous formation of the transition state is called induced fit overall there is a specifically matched enzyme for each substrate and thus for each chemical reaction however there is some flexibility as well some enzymes have the ability to act on several different structurally related substrates figure 2.27 steps in an enzymatic reaction according to the induced fit model the active site of the enzyme undergoes conformational changes upon binding with the substrate substrates approach active sites on enzyme b substrates bind to active sites producing an enzyme substrate complex c changes internal to the enzyme substrate complex facilitate interaction of the substrates d products are released and the enzyme returns to its original form it's ready to facilitate another enzymatic reaction binding of a substrate produces an enzyme substrate complex it is likely that enzymes speed up chemical reactions in part because the enzyme substrate complex undergoes a set of temporary and reversible changes that cause the substrates to be oriented toward each other in an optimal position to facilitate their interaction this promotes increased reaction speed the enzyme then releases the products and resumes its original shape the enzyme is then free to engage in the process again and will do so as long as substrate remains other functions of proteins advertisements for protein bars powders and shakes all say that protein is important in building repairing and maintaining muscle tissue but the truth is that proteins contribute to all body tissues from the skin to the brain cells also certain proteins act as hormones chemical messengers that help regulate body functions for example growth hormone is important for skeletal growth among other roles as was noted earlier the basic and acidic components enable proteins to function as buffers in maintaining acid-base balance but they also help regulate fluid electrolyte balance proteins attract fluid and a healthy concentration of proteins in the blood the cells and the spaces between cells helps ensure a balance of fluids in these various compartments moreover proteins in the cell membrane help to transport electrolytes in and out of the cell keeping these ions in a healthy balance like lipids proteins combined with carbohydrates they can thereby produce glycoproteins or proteoglycans both of which have many functions in the body 75 the body can use proteins for energy when carbohydrate and fat intake is inadequate and stores of glycogen and adipose tissue become depleted however since there is no storage site for protein except functional tissues using protein for energy causes tissue breakdown and results in body wasting nucleotides the fourth type of organic compound important to human structure and function are the nucleotides figure 2.28 a nucleotide is one of a class of organic compounds composed of three subunits one or more phosphate groups a pinto sugar either deoxyribose or ribose a nitrogen containing base adenine cytosine guanine thymine or uracil nucleotides can be assembled into nucleic acids dna or rna or the energy compound adenosine triphosphate figure 2.28 nucleotides uh the building blocks of all nucleotides are one or more phosphate groups a pinto sugar and a nitrogen containing base b the nitrogen containing bases of nucleotides c the two pinto sugars of dna and rna nucleic acids the nucleic acids differ in their type of pinto sugar deoxyribonucleic acid dna is nucleotide that stores genetic information dna contains deoxyribose so called because it has one less atom of oxygen than ribose plus one phosphate group and one nitrogen containing base the choices of base for dna are adenine cytosine guanine and thymine ribonucleic acid rna is a ribose containing nucleotide that helps manifest the genetic code as protein rna contains ribose one phosphate group and one nitrogen containing base but the choices of base for rna are adenine cytosine guanine and uracil the nitrogen containing bases adenine and guanine are classified as purines a purine is a nitrogen-containing molecule with a double ring structure which accommodates several nitrogen atoms the basis cytosine thymine found in dna only and uracil found in rna only are pyrimidines a pyrimidine is a nitrogen containing base with a single ring structure bonds formed by dehydration synthesis between the pinto sugar of one nucleic acid monomer and the phosphate group of another form of backbone from which the components nitrogen-containing bases protrude in dna two such backbone 76 attach at their protruding bases via hydrogen bonds these twist to form a shape known as a double helix figure 2.29 the sequence of nitrogen containing bases within a strand of dna form the genes that act as a molecular code instructing cells in the assembly of amino acids into proteins humans have almost 22 000 genes in their dna locked up in the 46 chromosomes inside the nucleus of each cell except red blood cells which lose their nuclei during development these genes carry the genetic code to build one's body and are unique for each individual except identical twins figure 2.29 dna in the dna double helix two strands attach via hydrogen bonds between the bases of the component nucleotides in contrast rna consists of a single strand of sugar phosphate backbone studded with bases messenger rna mrna is created during protein synthesis to carry the genetic instructions from the dna to the cell's protein manufacturing plants in the cytoplasm the ribosomes adenosine triphosphate the nucleotide adenosine triphosphate