Okay the next chapter that we're going to cover is the cellular level of organization and the first thing that we need to talk about our cells is the cell theories. The cell is the smallest structural and functional living unit and we have over 200 different types of human cells that differ in size, shape, sub-cellular components, and functions. Each cell maintains homeostasis at a cellular level and we couldn't examine cells until the invention of the microscope in the 17th century. But our cells have coordinated activities that allow for homeostasis at a higher organizational level and we're going to talk about how cells come from the division of pre-existing cells and the vital functions that cells produce and perform for us every second of every day within the human body. Now cells differentiate into specialized cells and you can see here the differentiation of four different tissue types from a single cell so from a single cell. We get epithelial tissue which has many different subcategories. Connective tissue again has many different subcategories. Muscle tissue has three different types we will discuss. A neural tissue and you can see that each of these cells and tissues are very different in both their shape and structure but all cells has some common structures and functions that we'll talk about. Now cell differentiation is when all cells in the body come from a single, the fertilized ovum. This ovum contains the genetic potential to become any cell in the body. As cellular division occurs, the cytoplasm of the cell is divided into smaller parcels and these parcels differ from one another because of regional differences in the position of the cytoplasm at fertilization. So this ovum has the potential to become any cell in the body and the first cell division again creates these smaller portions of the cytoplasm. Those differences in the cytoplasm might result in different daughter cells. So the cytoplasmic differences affect the DNA of the cells which can turn specific genes on or off and the daughter cell begins to develop specialized structural and functional characteristics. This process of gradual specialization is called differentiation and it's this process that produces the specialized cells that form the tissues responsible for all of our body functions. Now cells are the smallest units of life and the primary components of the cell are the plasma membrane and the cytoplasm. The plasma membrane is a bi-molecular layer of lipids and proteins that you will see in more detail coming up in a constantly-changing fluid mosaic model. It plays a dynamic role in the cellular activity but it also separates the extracellular fluid (which is sometimes referred to as the ECF) from the cytosol or intracellular fluid (ICF). The ICF and the cytoplasm as you'll see can be further subdivided into different types of organelles both non membranous organelles and membranous organelles that we will look at in more detail. So what are the functions of the plasma membrane? The plasma membrane is a physical barrier that separates the inside of the cell from the surrounding fluids. It is a selectively permeable barrier that controls the entry of ions and nutrients like glucose, the elimination of waste products, and the release of various secretions. It also provides structural support and performs most other functions via proteins that are embedded in the plasma membrane. The phospholipid bilayer is what the plasma membrane is composed of. And remember a phospholipid has a 1) phosphate head that is polar and hydrophilic and 2) a fatty acid tail that is nonpolar and hydrophobic. You can see the structure and orientation of the phospholipid bilayer here. In each half of the layer or bilayer the phospholipids lie with their hydrophilic heads facing outward and their hydrophobic tails facing inward. The hydrophobic layer in the center of the membrane isolates the cytoplasm from the extracellular fluid and again that isolation or separation is important because the composition of the intracellular fluid and the extracellular fluid is quite different. Now there's other things in the plasma membrane. You have cholesterol which increases the membrane stability and fluidity. You can also see various proteins. You have integral proteins and integral proteins are firmly inserted into the membrane. They're sometimes referred to as transmembrane proteins because they can span the length of the membrane. They function in transport as channels and carriers enzymes or receptors. You can also see peripheral proteins these are loosely attached to integral proteins and they function as enzymes, cell to cell links that may provide support, or function as motor proteins. Glycolipids are also a part of the plasma membrane and these are lipids with a polar sugar group on the outer membrane surface so the membrane proteins have a number of important functions that we will be studying throughout this course. They can function for example in transport. As transport channels or passageways for certain substances that can't get through on their own, like polar ionic charged molecules. They can also form carriers and bind a substance to transport them across the membrane. They can function as signal transduction. they can also function as 1) attachments for the cytoskeleton or 2) enzymatic activity catalyzing chemical reactions or 3) causing cell-to-cell linkages and 4) cell-to-cell recognition. now membranes have varying levels of permeability. So the plasma membrane as noted is an effective barrier where the conditions inside the cell are different from the conditions outside the cell. The activities of the plasma membrane must be coordinated. Plasma membranes do demonstrate permeability. Some things are freely permeable which means any substance can pass through the plasma membrane without difficulty. Other things are selectively permeable where the plasma membrane only permits the passage of some materials and prevents the passage of others. Other things are impermeable where nothing can get through. Now, for things that can't get through the membrane, there are particular types of membrane transport mechanisms and we will be examining these different types of membrane transport mechanisms. There's basically two broad categories of membrane transport: 1) passive processes where no cellular energy which remember is ATP is required and a substance generally moves down its concentration gradient going from a higher concentration to a lower concentration. 2) There's also active processes where we must expend some energy or ATP. So passive processes can be further subdivided into diffusion or carrier mediated. and Active processes are also carrier mediated or can be different types of vesicular transport. So let's look at the simplest form of passive process: simple diffusion. This is the movement of solutes through a selectively permeable membrane from an area of high solute concentration to an area of low solute concentration where the solute is the dissolved particle in a solution while the solvent is the substance that the solute dissolves into. And remember water is generally our universal solvent. Simple diffusion might be an example of a fat soluble molecule that goes directly through the phospholipid bilayer. Nonpolar lipid soluble which are hydrophobic substances diffuse directly through the phospholipid bilayer and you can see they go from an area of high concentration to low concentration. Facilitated diffusion is also known as carrier-mediated and this is where a protein carrier specific for one chemical allows the binding of a substrate which causes a change in the shape of the transport protein allowing the substance to get across certain lipophobic molecules like amino acids might use a carrier or channel they exhibit specificity so they're very selective for what can get across and another important concept to note is they are saturable the rate is determined by the number of carriers or channels if you only have three carriers or channels then at any one time you can only get three substances across and the binding of the substrate causes a change in the shape of the carrier thereby allowing it to cross the membrane as you can see in the diagram now osmosis is a special type of diffusion that refers to water it is the movement of water the solvent across a selectively permeable membrane water diffuses through the plasma membrane or lipid bilayer generally through channels called aquaporins and water goes from an area of low solute to high solute concentration as shown here in the diagram or water goes from an area of high water concentration to low water concentration so because water can freely cross the semipermeable membrane water the water concentration is determined by the solute concentration so water wants to go from the side of the beaker with low solute to high solute until the concentration of solute particles per water is equivalent on both sides of the membrane now as Moses occurs water enters or leaves a cell and tonicity is the ability of a solution to cause a cell to shrink or swell in an isotonic solution the solution has the same solute concentration as that of the cytosol in a hypertonic solution and we're talking about the solution that the cells in this diagram are being bathed in the solution has a greater solute concentration than the cytosol so it has a higher solute concentration which means it has a lower water concentration water moves out of the cell making it shrink and die this is called cremation in a hypotonic solution again the bathing solution the solution has less solute a less solute concentration than that of the cytosol so higher water lower solute concentration and water moves into the cell making it swell and burst this is known as hemolysis now the passive processes that we have talked about are simple diffusion facilitated diffusion and osmosis now we're going to examine active transport active transport processes require energy and there is two types of active transport processes that we will discuss primary active transport secondary active transport and then vesicular transport both use eight or all of these use ATP to move solutes across a living plasma membrane and generally membrane proteins are used as Kerik carriers again in this scenario we are moving solutes against their concentration gradient now the sodium potassium pump is primary active transport the sodium potassium pump is located in all plasma membranes the sodium potassium pump is important for pumping sodium out of the cell and potassium into the cell and we create a gradient a higher concentration of sodium outside the cell then inside the cell and this is important because we can use that gradient that we've established to then transport other things either into or out of the cell again we must expend energy in order to do this secondary active transport depends on that ion gradient that we just established via primary active transport and in this case the energy stored in those ionic gradient differences can be used indirectly to drive the transport of other solutes so primary active transport is a direct expenditure of energy secondary active transport is indirect we tap into the energy gradient previously established co-transport allows substances to be transported we can transport more than one substance at a time sometimes we can transport substances in the same direction which is known as symport or sometimes we can transport substances in opposite directions which is anti port now the signaler transport there is different types exocytosis where transporting something out of a cell exit exocytosis is a good way to remember that endocytosis we're transporting something into the cell or we can transport something into a cross and then out of the cell which is trans saito and the secular transport requires cellular energy in the form of ATP it often involves the formation of protein coded vesicles they're often receptor-mediated and very selective endocytosis has three different types phagocytosis pinocytosis and receptor mediated cytosis phagocytosis is where cells generally engulf large particles or solids and bring them into the cell's interior our macro Fache and some white blood cells use phagocytosis as you will see throughout your study of anatomy and physiology fluid-filled endocytosis is called pinocytosis the plasma membrane in folds bringing extracellular fluid and solutes into the interior of the cell nutrient absorption in the small intestine is an example of this an receptor mediated endocytosis is where we have these pits that have been coated and they provide the main route for endocytosis when we take up enzymes or low-density lipoproteins insulin those are all examples of this form of endocytosis exocytosis is much like endocytosis in Reverse and this is where a vesicle fuses with the plasma membrane and the contents are released into the extracellular fluid examples of this in our cells are hormone secretion new neurotransmitter release or just when we want to get rid of metabolic waste products so the active processes that we have talked about are primary active transport secondary active transport exocytosis and then phagocytosis pinocytosis and receptor mediated endocytosis now let's examine some of the cellular organelles here is a typical human cell and it contains the primary organelles that we will discuss and some of the internal structures you can see the cytoplasm various organelles and the plasma membrane now the endoplasmic reticulum is one cellular structure and the endoplasmic reticulum is a network of intracellular membranes connected to the nuclear envelope which surround the nucleus there is two types of endoplasmic reticulum rough ER and smooth ER rough ER is denoted as rough because of the large numbers of ribosomes that are attached to the surface rough ER functions as what's known as a workshop and it is the site where newly constructed proteins are chemically modified and packaged for export to the golgi apparatus so the rough ER manufactures all secreted proteins smooth ER consists of no ribosomes and the tubules are generally arranged in a looping work smooth ER also has numerous functions including synthesizing phospholipids and cholesterol steroid hormones synthesizing and storing glycerides synthesizing and storing glycogen the Golgi apparatus is a series of stacked and flattened membranous sacs the Golgi apparatus modifies concentrates and packaged packages proteins and lipids transport vesicles from the endoplasmic reticulum fuse with the Golgi apparatus proteins then pass through the Golgi apparatus and secretory vesicles can leave the Golgi apparatus and move to other designated parts of the cell there are two specialized types of vesicles formed by the Golgi apparatus lysosomes are spherical membrane sacs containing digestive enzymes lysosomes digest in ingested bacteria viruses and toxins they may degrade non-functional organelles break down and release glycogen break down bone to release calcium or destroy cells an injured tissue for example peroxisomes are another type of special vesicle and these are membranous sacs containing oxidases and catalyzes peroxisomes detoxify harmful or toxic substances they neutralize dangerous free radicals which are highly reactive chemicals with unpaired electrons so you can see that there's a continuous movement an exchange of materials Wien organelles and vesicles this allows for the cells to grow mature and respond to the environment and here is a picture of the peroxisome that I mentioned previously now mitochondria are another type of cellular organelle mitochondria are a double mains double membrane structure with cristae or folds inside mitochondria provide most of the cell's ATP via aerobic cellular respiration and mitochondria contain their own DNA and RNA mitochondria produce ATP through three processes glycolysis the citric acid cycle and the electron transport chain which you will learn about later on the cytoskeleton consists of various components microtubules microfilaments and intermediate filaments microfilaments are actin strands that are attached to the cytoplasmic side of the plasma membrane microfilaments are involved in cell motility change in shape and also possibly of different types of vesicular transport like endocytosis and exocytosis microtubules are hollow tubes and they determine the overall shape of the cell and distribution of organelles intermediate filaments are tough insoluble rope-like protein fibers and they resist pulling forces on the cell and here you can see the microtubules in the centrioles cilia is a type of cellular extension cilia along with flagella are whip-like motile extensions found on the surface of certain cells they contain microtubules cilia can help move substances across cell surfaces flagella which is found in the tail of the sperm propels the whole cell ribosomes are another type of cellular structure ribosomes are responsible for protein synthesis and there are constructed of two subunits one large one small now we've already examined the fixed ribosomes attached to the endoplasmic reticulum but we also have free ribosomes that are found throughout the cytoplasm and they help to manufacture proteins the nucleus is the genetic library that contains the blueprint for nearly all cellular proteins the nucleus responds to signals and dictates the kinds and amounts of proteins to be synthesized within the cell most cells have only one nucleus red blood cells don't have a nucleus they are a nucleus and other types of cells like skeletal muscle cells bone destruction cells and some liver cells are multinucleate now the nucleus has a double membrane structure that contains pores the outer layer is continuous with the rough endoplasmic reticulum and bares ribosomes the inner lining maintains the shape of the nucleus the pores regulate transport of molecules into and out of the nucleus the nucleoli is a dark staining spherical body within the nucleus and it's involved in ribosomal RNA synthesis and ribosome subunit assembly now here's the structure of DNA and DNA consists of chromatin chromatin are thread-like strands of DNA that also contain his stone proteins and RNA and they can condense into bodies called chromosomes when the cell starts to divide and you can see the base pairing of DNA as DNA coils together and becomes double-stranded now the process of protein synthesis is with DNA replication and there's various steps transcription and translation transcription is within the cell nucleus and translation is in the cell cytoplasm now the steps of protein synthesis are outlined here the DNA template is used to make the RNA strands immature RNA is modified to form which messenger RNA messenger RNA leaves the nucleus through the nuclear pores and then translation occurs in the cytoplasm ribosomal RNA binds to the messenger RNA and reads the codons or triplets transfer RNA transfers the appropriate amino acids to the ribosome and proofreads it its anticodon amino acids are then covalently bonded together by peptide bonds to form a polypeptide and then the polypeptide can take on the different levels of shape that we talked about previously so let's look at the process of transcription in a little bit more detail so transcription is where we go from DNA to messenger RNA now DNA is bound together in a helix and the helix needs to unwind and there is an enzyme that does that helicase it untwist the DNA double helix and exposes the complementary base chains there is a y-shaped site of replication known as the replication fork and then each nucleotide strand can serve as a template for building a new complementary strand DNA polymerase reads the strands there is a continuous reading of the leading strand where it is synthesized and then the lagging strand is synthesized in short segments that are then spliced together later on be a DNA ligase this process is called semi-conservative replication because two DNA molecules are formed from the original and then we have to process the RNA remember we have messenger RNA and messenger RNA is spliced out removing the introns which the non-coding regions of DNA DNA has both coding regions and non-coding regions so in transcription RNA polymerase oversees the synthesis of mRNA that you saw previously and then in translation is the next step after RNA has been processed to produce a spliced form of RNA containing only the coding regions which are the exons translation converts that base sequence of nucleic acids into the amino acid sequence of proteins and this involves messenger RNA transfer RNA and ribosomal RNA messenger RNA attaches to a smart small ribosomal subunit that moves along the messenger RNA to the start codon the large ribosomal unit attaches forming now what we know is a functional ribosome the anticodon of transfer RNA binds to its complimentary codon and adds the appropriate amino acid to the now growing protein new amino acids are added by other transfer RNAs as the ribosome moves along ribosomal RNA until we reach a stop codon and you can see that shown here now the genetic code consists of a three base sequence of DNA each three base sequence is represented by a codon a codon is a complimentary three base sequence on messenger RNA so you can see that we have the template strand of DNA through transcription we make RNA and then through translation we make the proteins here you can see the genetic code one of the things about the genetic code is that remember in DNA we have finding and in RNA we have uracil so we have our DNA triplet template strand the coding strand of DNA that matches up with that if the base was adenine adenine adenine would be 3 bases of thymine now when we make our messenger RNA messenger RNA does not contain it contains uracil so your cell would be the appropriate mRNA codon binding from the DNA template strand because thymine is not present the transfer RNA anticodon that would match up with messenger RNA is 3 bases of adenine and this would produce the amino acid phenylalanine so just be sure that you know that an mRNA it's uracil and in DNA is thymine and the bases are always going to match up with your a with T a with u and cytosine and guanine always match up together now the last part of this chapter is the cell cycle and there's two major phases of the cell cycle mitosis which is cell division and interphase when the cell is growing and performing normal functions interphase is further subdivided into what we call g1 s and g2 phases so g1 is also called gap one and that's where vigorous growth amitab is occurring s is a synthetic phase where DNA replication is occurring and g2 is preparation for division it's important that the cell cycle occurs appropriately because cell division if it's interrupted at all could lead to errors in cell replication or it could lead to tumor growth as you'll see later on so the mitotic phase that's shown here of the cell cycle is essential for body growth and tissue repair most of our cells have the ability to divide it does not occur in some cells like nervous tissue and it does include distinct events or stages prophase metaphase anaphase and telophase and cytokinesis is the final stage of replication now DNA replication some of the points that I made earlier are shown here where you have the unwinding of the strands you have the replication fork remember helicase unwinds the strands so that each strand is exposed the leading strand is synthesized continuously and that's because of the direction that DNA polymerase works in the lagging strand is synthesized in short segments and then DNA ligase bonds them together later on it is semi-conservative because each original strand served as a template for the previous strand to be replicated from so here are the different phases that I referred to in mitosis prophase metaphase anaphase telophase and cytokinesis for each of these phases there is distinct events that are occurring so that we can get two identical daughter cells produced let's look at each of these phases in a little more detail mitosis prophase is the first phase of mitosis this is where the chromosomes become visible the centrosomes separate and start to migrate towards the opposite poles the mitotic spindle forms which is going to set the stage for the cell division nuke the nuclear envelope begins to break down in fragment and the microtubules are forming and being drawn to the equator of the cell in preparation for division metaphase the centromeres of the chromosomes are aligned along the equator of the cell this plane is midway between the two poles and is also known as the metaphase plate anaphase is the shortest phase the centromeres of the chromosomes split each chromatid now becomes a chromosome as they're pulled to opposite poles telophase the final phase of mitosis begins when the chromosome movement stops the nuclear membrane forms around each chromatin chromatin mass the nucleoli begins to reappear the spindle apparatus disappears and cytokinesis two daughter cells are pinched apart each containing a nucleus identical to the original now cancer cells or body cells that have lost their ability to regulate cell division and cancer cells can form when some part of the cell cycle goes arrey cancer cells can form from tumors tumors can either be benign or malignant a tumor is basically just a mass or swelling produced by abnormal cell growth or division benign tumors the cells remain within the tissue where they originated and they don't spread to other tissues these tumors are not usually threatening to an individual's life and can generally be surgically removed malignant tumors the cells divide very rapidly they release chemicals that stimulate the growth of blood vessels into the area and then they begin to migrate into surrounding tissues and nearby blood vessels and this process is known as metastasis and can produce secondary tumors throughout the body this type of tumor is more difficult to treat and can be deadly as the tumors continue spread throughout the body this concludes our look at chapter 3 the cellular level of organization