Cell physiology Part B part 1

追加: 2026-05-09

[00:00:01]in this lecture we're going to continue our discussion of cell physiology by focusing on a very important component of the cell and that is the outer plasma membrane and as I just indicated the membrane encloses the cell and it actually defines the outer boundary for the cell but it's more than just a passive covering it is a very important component for helping the cell to maintain life by regulating what comes into the cell and what leaves the cell and that's what we refer to as the cytosol which is the interior and the extracellular environment which is exterior to the cell the key components to the cell membrane is based upon a foundation of phospholipids that we'll discuss here that I'll discuss here shortly but to give the plasma membrane important functionality are a range of different proteins glycoproteins which are proteins with carbohydrate units attached to it as well as cholesterol in fact approximately 20 percent on average of cell membranes are composed of cholesterol molecules and cholesterol is very important for cell membranes because it adds stability to the cell membrane now the structure of the cell membrane is not only effective for the entirety of the cell but is also an effective way to assemble various organelles inside the cell also so what we'll see here when we take a look at different organelles is that the outer part of organelles uses a same basic design of incl of building the that outer layer of those organelles with these foundational molecules and such examples include the mitochondrion the Golgi apparatus as well as the nucleus of the cell what we see here is a an artist's rendering of a portion of a cell membrane and we'll take a look at the different components here uh shortly generally the structure of the cell membrane is described as a thin film of lipid and protein molecules however it is not a rigid structure in fact it is a very Dynamic fluid-like structure that allows the cell to bend and to change shape and we begin with the foundation of the cell membrane which are made up of phospholipid molecules not just one layer of phospholipid molecules but two layers of phospholipid molecules where the interior of the cell membrane itself is composed of these fatty acid tails that I will describe on the next few slides we also see that the this particular membrane is embedded with a number of different structures and these are proteins and these proteins carry out a wide array wide range of functions that enables the cell to sustain life as indicated a little bit earlier some of these proteins are associated with carbohydrate units and that's what we see Illustrated on some of these proteins uh right here and when we have a carbohydrate unit attached to a protein we just simply call it a glycoprotein but as indicated on this picture you see that this particular protein does not have a carbohydrate unit attached to it uh the phospholipid bilayer as indicated on slide 3 is composed of two layers of lipid molecules uh and the entire width of that phospholipid bilayer or the membrane is approximately five nanometers so it's a very thin membrane but a very important aspect to the cell and one of those important aspects is to regulate what comes into and out of the cell the way the cell does that is by being impermeable to most water-soluble molecules so water soluble molecules are those molecules that can form reversible chemical bonds with H2O now the fact that those types of molecules cannot get into or out of the cell uh very easily uh or uh is because of that impermeable nature and that's how the cell regulates how much comes in and how much goes out but how do we get those very important water soluble molecules into the cell or out of the cell that's where the membrane proteins play an important role so the membrane protein server a number of very important functions the first function here is that the membrane protein will serve as a receptor and the receptor is a way to inform the rest of the cell the interior of the cell as to the state of the extracellular environment and the way the protein receptor does that is that it forms a chemical bond with extracellular molecules and when it forms that chemical bond that chemical bond will trigger a physiological reaction inside the cell membrane proteins function very fun play a very important role in terms of molecule transport as indicated that various molecules cannot get into the cell or cannot leave the cell through the phospholipid membrane so the membrane proteins serve as that transport mechanism between the cytosol and the extracellular environment a number of the proteins are in fact enzymes and being enzymes that when triggered by some stimulus those membrane proteins will break up an ATP molecule and harness the stored chemical energy in ATP to carry out some particular cellular function that's confined to the area of the cell membrane and another important function of these proteins is structural in nature and that structure comes by way of forming uh connections with the cytoskeleton as we'll discuss a little bit later in this unit the cytoskeleton refers to a general array of cell proteins that serves as a frame or a scaffolding that provides that structural Integrity to the cell so important are these cytoskeletons that approximately 30 percent of the animal genome the animal genome refers to the genetic code contained within the nuclei of these of cells are dedicated for making those membrane proteins so when we use the term coding coding is the term that refers to the specific set of instructions contained within the genome that will make these proteins as indicated a little bit earlier uh the phospholipid molecule or some sometimes just simply referred to as the lipid molecule is the foundation of the cell membrane and on average makes up about 50 percent of the mass of the cell membrane while the rest is protein but keep in mind that this number does vary quite a bit from cell to cell so some cells have a lower protein content in their membranes some cells have a greater protein content than the 50 number but the 50 is a rough guesstimate when we consider all the cells in the human organism and to give you an idea of the density of these phospholipid molecules we see that it's been estimated that there are approximately 5 million lipid molecules those phospholipid molecules contained with within a one micron surface area of cell membrane indicating that these are packed in quite tightly and that's driven by the nature of the chemical reactions between adjacent phospholipid molecules now the reason the phospholipid molecule is used is because it is an amphiphilic or sometimes referred to as amphipathic molecule and an amphiphilic or anthropathic molecule those terms are often used interchangeably it's a molecule that is both hydrophobic a hydrophobic molecule is one that is quote unquote water fairing so these are molecules that do not form reversible chemical bonds with H2O and are hence nonpolar in nature the hydrophobic component of the phospholipid molecule is the fatty acid tail the amphiphilic or amphipathic molecule is also hydrophilic by definition this is a water loving type of molecule quote unquote but it's a molecule that can form reversible chemical bonds with H2O and is referred to as the polar end so the entirety of the phospholipid molecule that will examine in a little bit more detail here on slide number six is such six and seven is such that uh it is both hydrophobic and hydrophilic and in this drawing here here we see one typical phospholipid molecule that we that we would find uh as the foundation for the plasma membrane so as indicated the phospholipid is made up of two components the first component is what's called a polar head and that's what we see at the very top here and this is the nature of the chemical nature of the Polar head is that it can form reversible chemical bonds with H2O making it ideal to be pointed towards the extracellular environment as well as the intracellular environment since both environments are H2O based but the interior of the cell membrane are is made up of these nonpolar Tails these fatty acids acids and each phospholipid molecule has two of these fatty acids the length of these Tails will vary uh anywhere from 14 to 24 carbon atoms in length as we see here in this drawing uh one tail is considered to be unsaturated a CIS unsaturated fatty acid while the other tail is typically a saturated fatty acid a saturated fatty acid is one that does not have any double chemical bonds but every carbon atom is fully saturated with hydrogen atoms and those types of saturated fatty acids have a very linear very straight type of appearance the unsaturated fatty acid has a typical Bend to it and that bend occurs at the point where the chemical bond is a double bond and that's because the two adjacent carbon atoms only have one hydrogen atom attached to each of it that's what gives that particular fatty acid the characteristic bend that we see so this will end the first part of our part B lecture on Cell physiology