Welcome to topic 3.4, Cellular Energy. One of the characteristics of life states that all living things require energy. That energy is used by an organism for other life processes like maintaining homeostasis, growing and developing, and responding to stimuli.
Living things acquire the energy they use in different ways. Animals, as consumers, do so by consuming and ingesting organic molecules. Fungi are also consumers, but they grow directly on their food sources, absorbing energy-rich nutrients into their cells after beginning to digest them externally.
Producers, like plants, utilize light energy from the sun. And prokaryotes use a variety of energy-obtaining methods, some being consumers while others are producers. Regardless of the method by which organisms acquire energy, the same principles of energy transformations, or thermodynamics, apply in all cases. Even though energy can neither be created nor destroyed, it can be transferred and transformed as it moves from one organism to another.
Energy is broadly defined as the ability to do work or cause some sort of change to occur. and it exists in a variety of forms. Here are some examples.
Light energy, like that from a light bulb or the sun. Kinetic energy is the energy of motion, and includes examples like sound and thermal energy. Heat is the transfer of thermal energy from a warmer object to a cooler one.
There's also gravitational energy. Nuclear energy includes fusion, what stars like our sun accomplish, and fission, the process that makes nuclear reactors work. And then there's chemical energy, the form that makes batteries and glow sticks work, and fireflies light up at night. It's largely this chemical energy that we deal with in the biological sciences.
Chemical energy is what plants and other producers form via photosynthesis. Chemical energy conversions are what all consumers rely on as molecules like glucose are broken down. A molecule is said to possess chemical energy, a form of potential energy, due to its structure.
The study of the relationships between forms of energy and their transformations is called thermodynamics. Although there are four generally accepted laws of thermodynamics and a number of components within each of them, each of them, we are only going to be concerned with a few components of two of the laws. The first law is known as the principle of conservation of energy and states that energy can neither be created nor destroyed.
Since the energy of the universe is constant, that means that it can only be transferred or transformed. So a plant does not make energy, but does convert from one form, light, into another, chemical. An animal converts chemical energy from its food to other forms of chemical energy. The second law of thermodynamics has a couple of parts for our purposes.
The first states that with every energy transformation, there is less energy available to do work because those transformations are not 100% efficient. In living system, each energy transformation generates thermal energy which is lost to the surrounding environment by way of heat. Additionally, when energy transformations occur, the progression toward more disorder and more disorderly forms of energy is favored. The universal trend in thermodynamics is an increase in entropy.
Entropy, identified by the capital italicized S, is a measure of how disorderly or random a system is. In this example of a system, a building, as it's being demolished and collapses, disorder is increasing literally right before your eyes. Since the description of a system's entropy is comparative for relating one thing to another, we would describe the building as having a lower entropy than the pile of rubble, which is clearly a higher entropy state.
This collection of carbon, hydrogen, and oxygen atoms has a relatively higher entropy than the specifically constructed glucose molecule that those atoms could be used to make. So transitioning from the high entropy pile of atoms to the low entropy glucose molecule would result in a decrease in entropy, negative delta S. Conversely, deconstructing the glucose would result in an increase in entropy, positive delta S. If you consider what would have been involved in precisely arranging each of those atoms to form glucose, or all of the construction materials, like the metal and concrete and plumbing and electrical wiring, to build that building, Why the universal trend towards randomness and disorder, as stated in the second law of thermodynamics exists, should be apparent. The lower energy pile of atoms and lower energy pile of rubble represents favored states.
If we would consider a glucose molecule as having relatively low entropy, what about an entire organism? A living thing has relatively very low entropy, but in fact does not. violate the second law of thermodynamics.
As an organism lives and grows and metabolizes biological molecules, thermal energy is produced and lost to the environment in the form of heat. Also, all living things will make, at some point in the future, their final, ultimate contribution to the entropy of the universe. The second law of thermodynamics and entropy are also related to the reactions that break down or build up molecules.
Catabolic reactions are those in which large molecules are broken down into smaller products, and anabolic reactions do the reverse. Glucose and oxygen molecules can act as reactants in a catabolic reaction that produces water and carbon dioxide. This process, called cellular respiration, will be studied in a video later in this unit.
The reverse of that reaction, in which water and carbon dioxide serve as reactants to form glucose and oxygen, is photosynthesis, also appearing later in this unit. By generic definition, free energy, technically Gibbs free energy, is the energy available to a system to do work. We can modify this definition to suit our purposes and biology by stating, free energy is the energy available to a cell to carry out life processes. We represent free energy with a capital italicized G.
This could be observed as an organism's movement, or the active transport of substances across a membrane, or even powering other biochemical reactions. All chemical reactions result in a change in the amount of free energy available to a system. If the amount of free energy after a chemical reaction has occurred is greater than the initial amount, then delta G is positive.
But if the amount of energy after a chemical reaction is less than the initial amount, then delta G is negative. Whether delta G is positive or negative indicates if the reaction will proceed spontaneously or not. A reaction with a negative delta G is a spontaneous one and results in the release or use of free energy.
One with a positive delta G will not proceed spontaneously because it requires an input of additional energy to drive the reaction forward. In chemistry, one of the basic principles studied is that of equilibrium. The idea is, when the rate of the forward and reverse reactions are the same, a state of equilibrium has been achieved.
In a closed system, in which no new reactants are introduced or no products are removed like a beaker or flask, the reaction halts. This illustration represents that principle. Initially, on the left, the higher water level represents the reactants and the lower level, on the right, the products.
In a state of disequilibrium, the water flows, lighting up the light bulb, which represents the reaction. Once equilibrium has been achieved, however, no reaction occurs and there's no light. But in a living system, like a cell or organism, there's a constant intake of matter and energy, as well as the release of waste products.
Therefore, and this is critical to understanding biochemical reactions, equilibrium is never achieved in a living system. As a matter of fact, we've seen a number of examples previously in which disequilibrium is purposefully maintained through energy requiring active processes. The chemical reactions taking place within a cell are not standalone, but rather exist in steps in much more complex metabolic pathways.
Metabolic pathways are linked together so that progress from one reaction to another can occur. products of one reaction end up as the reactants for the next one. This 8,000 by 6,000 pixel illustration maps out just some of the metabolic pathways within a human cell.
Zooming in on just one small section, we can see the very beginning stages of cellular respiration in which glucose is broken down into other metabolites. So far we've seen a collection of mutually exclusive opposing terms related to energy and entropy and chemical reactions. Let's now take a look at how all of those terms are related to one another. In this table on the left will be a collection of terms or phrases that are all related to the breakdown of molecules, and on the right, those related to building them up.
A catabolic reaction involves relatively larger and fewer reactants being broken down into more numerous, smaller products. That breakdown increases entropy, where building it up would decrease it. Catabolic reactions are exergonic, releasing energy, and anabolic ones, being endergonic, store it.
Because exergonic reactions release energy, they would have a delta G that is negative, and endergonic reactions would have a positive delta G. Reactions classified on the left would occur spontaneously, not. requiring a net input of energy to proceed, but the opposite is true of the non-spontaneous reactions on the right.
Chemical reactions all involve breaking and or building of chemical bonds. Breaking a bond requires an input of energy, whereas building a bond releases it. Since electrons are the metaphorical glue that hold atoms together in a molecule, A transfer of electrons from reactants to products occurs as a chemical reaction progresses.
The atom or group of atoms receiving electrons are said to be reduced since electrons carry a negative charge. The atom or atoms providing the electrons are being oxidized. This two-part process of reduction and oxidation, also known as redox, cannot occur independently of one another. There can't be a single atom in the atom.
be a reduction without an oxidation, or vice versa. If a chemical reaction involves breaking bonds, which requires an input of energy, and then building new bonds, which results in a release of energy, what determines the overall net result for delta G? Whether a reaction ultimately releases or stores energy is the result of the bond energies in both the reactant and product molecules. Although calculating and quantifying bond energies for a given molecule are far beyond the scope of this class, understanding it conceptually is important. Take this double replacement reaction for example.
There are two reactant molecules and two product molecules. As the reaction proceeds, the original bonds between A and B and C and D are weakened, then broken with an input of energy. The formation of the products A and C and B and D results when new bonds are built and energy is released.
If this model was representing a reaction with a negative delta G, that would mean that the bond energies of the reactants were higher than the bond energies of the products. The excess energy would have been released into the surroundings, perhaps in the form of sound, or kinetic, or thermal energy. On the other hand, if this was a representation of a positive delta G, it would be the products with higher bond energies than the reactants.
But in order for this to be the case though, there must have been energy from the surroundings put into the system. A reaction like this can be exemplified by photosynthesis, which requires an input of light energy. Finally, let's take a look at what is perhaps a cell's most important energetic molecule.
ATP. ATP is the molecule that fuels many processes in living cells such as nerve impulses, active transport, biochemical pathways, and muscle contraction. ATP is similar to an RNA adenine nucleotide.
It possesses adenine, ribose, and three phosphate groups, alpha, beta, and gamma. ATP can be hydrolyzed into ADP, adenosine diphosphate, and a free inorganic phosphate ion. When that occurs and the gamma phosphate is released, the liberated ion is bonded to another reactant in a process called phosphorylation. The construction of this new bond during phosphorylation is releases energy, allowing work to happen.
You probably recall from a previous biology class that you were told that it's the breakdown of ATP that releases energy. In reality though, this is a bit misleading because it's actually the phosphorylation process that releases energy. Of course, that phosphorylation wouldn't have been possible had ATP not been broken down in the first place.
So rather than saying it was misleading, perhaps saying that it was incomplete would be more fair. Some examples of the hydrolysis of ATP and phosphorylation of some other molecule include the sodium-potassium pump, using energy to expel sodium ions and bring in potassium ions. The phosphorylation of glucose is the very first step in its breakdown. We'll explore that and the rest of cellular respiration later in this unit.
Signal transduction a topic in Unit 4, is a process by which chemical messages received by cells result in some action by that cell, relying on a series of phosphorylation events. The contraction and relaxation cycle of muscle involves the phosphorylation and dephosphorylation of protein fibers within muscle cells. Even the formation of ATP is the result of ADP being phosphorylated. Phosphorylation is one of the most common processes occurring in cells and is ubiquitous in living things. That concludes this video on topic 3.4.
Thank you for watching and until next time, take care.