hello everybody my name is Iman welcome back to my YouTube channel today we're doing chapter seven for MCAT prep and this chapter is all about biological membranes now our cells are defined by a plasma membrane that surrounds the contents of said cell and it serves as a barrier or boundary the plasma membrane is the edge of Life The Boundary that separates the living cell from its surroundings and it also controls the traffic into and out of the cell now the plasma membrane exhibits selective permeability that means it allows some substances to cross more easily than others and today we specifically want to learn how cellular membranes control the passage of substances so we want to understand how membranes work we're going to examine their structures and we're going to talk a lot about different means of Transport passive and active so we have a couple of our objectives that are similar to this chapter in our biochemistry playlist but we're also going to add a little bit of more information here for the MCAT specific specifically MCAT information that you'll need for the exam so we're just going to go ahead and we're going to go get we're going to get started with our first objective our first objective is cellular membranes our fluid mosaics of lipids and membranes so let's kind of dive into what that means lipids and proteins are the stable ingredients of membranes and although carbohydrates are also important it is lipids and proteins that make up the abundance of plasma membranes the most abundant lipids in most membranes are phospholipids and the ability of phospholipids to form membranes is inherent in their molecular structure a phospholipid is an amphiopathic molecule what that means is it has a hydrophilic region and it has a hydrophobic region other types of membrane lipids can also have this property and a phospholipid bilayer can exists can exist as a stable boundary between two aqueous compartments because the molecular Arrangements can show filter the hydrophobic tails of the of the phospholipids from water while exposing the hydrophilic heads to water now like most like most like membrane lipids most membrane proteins are also have this property they're amphiopathic and these proteins can reside in the phospholipid bilayer like you see here here so a couple of examples of proteins um such proteins can reside in the phospholipid bilayer with their hydrophilic hydrophilic regions kind of coming out of the membrane so that they can interact with water and the hydrophobic regions most so shielded in the hydrophobic region of said phospholipid bilayer now the this molecular orientation really maximizes contact of the hydrophilic region of proteins with water in the cytosol and also in the extracellular fluid while providing of course their hydrophobic Parts in the non-aqueous environments now in this fluid mosaic model the membrane is a mosaic of protein molecules kind of bobbing in a fluid bilayer of phospholipids now it's important to not get carried away and think that proteins um are just randomly distributed in the membrane they're not randomly distributed in the membrane groups of proteins are often associated in long-lasting specialized patches where they carry out their functions and the lipids themselves also appear to form defined regions as well and also in some regions the membrane may be much more packed with proteins whereas other regions less so and like all models the fluid mosaic model is continually being refined as new research reveals more things about the structure so it is at this point all right that we really see the definition of fluid mosaic model in regards to our phospholipid bilayer and it's at this point that I want to make a couple more points right this fluid mosaic model accounts for the presence of lipids proteins and even sometimes carbohydrates in this Dynamic semi-solid plasma membrane that surrounds the cell right and this plasma membrane can contain things like proteins embedded within said phospholipid bilayer now another very important thing is that the membrane is not static lipids move freely in the plane of the membrane and they can also assemble things into like lipid rafts there are things like flip bases that are specific membrane proteins that maintain the bi-directional transport of lipids between the layers of the phospholipid bilayer and cells and there's also proteins and carbohydrates that can move within the membrane but of course those tend to be larger so they can be slowed down by their size all right and so what is these points that kind of Encompass this fluid mosaic model that we speak of now these points help me move into the next topic um that we're going to get into more details about the membrane components themselves now we've talked quite a bit about lipids themselves already in previous chapters um in this MCAT playlist and so it comes obviously as no surprise that lipids are the primary membrane components both by mass and by mole fraction we have things like Trisa trisoglycerols and fatty acids that act as phospholipids phospholipid um precursors we have things like glycerol phospholipids that replace one fatty acid with a phosphate group and they're often linked to other hydrophilic groups we have cholesterol that's present in large amounts and it really contributes to membrane fluidity and stability and even waxes are present in very small amounts sometimes but not always and their most prevalent in Plants they really function as a waterproofing and defense thing now we also have like we've said proteins right proteins located within the cell membrane and they can act as Transporters they can act for towards cell adhesion molecules they can act as enzymes transmembrane proteins can have one or more hydrophobic domains and they're most likely to function as receptors or channels um embedded proteins can also likely act are most likely part of a catalytic complex or they can be involved in cellular communication we can also have membrane-associated proteins that can act as recognition molecules or enzymes now in addition to the lipids and proteins which we've reiterated that they are you know some of the main components of of membranes we can have carbohydrates carbohydrates can form protective glycoprotein codes and they also can function in cell recognition and we can also have extracellular ligands that can bind to membrane receptors that will function as either channels or enzymes and second messenger Pathways and so we have quite a bit of things that contribute to our membrane components composition and as we all know composition plays a role in the structure which plays a role in function and so we could see here main cell components lipids proteins carbohydrates and some of the main bio membrane structures we can see are bilayers vesicles we can see different kinds of of configurations and of course structure has a role in function things like signaling process signal processing molecular recognition adhesion cell trafficking Etc and so those are all important things to consider and it allows us to discuss our second objective that membrane structure results in selective permeability all right we also want to talk about membrane receptors a little more after we cover this point and that will help us transition into discussing different transport methods now the biological membrane it's an Exquisite example of a a structure that has molecules ordered into higher levels of organizations and it has emergent properties Beyond those of the individual molecules because of the ordering and the remainder of this review is going to really focus on those properties and then how they're used in their ability to regulate transport again across cellular boundaries and how they um can act as a ability to regulate function That's essential to the cell's existence and we're going to see time and time again that form fits function that structure relates to the kind of function you know there's there's a relationship between structure and function in other words of course our fluid mosaic model it helps us explain how membranes regulate the cell's molecular traffic it helps us visualize that there is this traffic of small molecules and ions that move across the plasma membrane in both directions and the cell is able to take up some small molecules and ions and exclude others and so sort of the first thing we want to cover in regards to this objective here is that permeability of this lipid bilayer non-polar molecules like hydrocarbon CO2 O2 these are hydrophobic they can therefore dissolve in the lipid bilayer of the membrane and cross it easily without the aid of membrane proteins now the hydrophobic interior of the membrane does impede direct Passage through the membrane for things like ions and polar molecules which tend to be hydrophilic polar molecules like glucose and other sugars they can only pass slowly through the lipid bilayer and even water which is a very small polar molecule can't cross the lipid bilayer very rapidly and then it charged atoms or molecules are even less likely to penetrate the hydrophobic interior of the membrane and so you can think of this lipid bilayer this phospholipid bilayer as a gatekeeper all right it's responsible for the cell's selective permeability it lets some things easily in and some things not so much and some of these more polar molecules these larger charged ions they can cross through regulation of things like transmembrane proteins proteins that are built into the membrane are going to play key roles in regulating transport and so that allows us to talk about things like transport proteins right specific ions in a variety of polar molecules they can't move through this cell membrane on their own but these Hydro hydrophilic substances they can avoid contact with the lipid bilayer how by passing through transport proteins these transport proteins they span the membrane they're also called channel proteins and they function by having a hydrophilic channel that certain molecules or ions can use as a tunnel through the membrane so for example the passage of water molecules through the membrane in certain cells is really sometimes facilitated by channel proteins called aquaporins and so then a trans protein helps it is it's specific for the substance it moves and it only allows again certain substances to cross so there's a lot of Regulation here right there's regulation imparted by the phospholipid bilayer that only allows small hydrophobic molecules to pass readily and then there's membrane proteins that are embedded in the phospholipid bilayer that will allow that can help hydrophilic molecules polar charge molecules pass but they're also regulated right they're not letting anything in right there's some sort of Regulation some mechanistic regulation that is occurring within the protein itself that allows certain things to move and other things not to for example aquaporins these are our membrane proteins that are just going to help mostly in the passage of water molecules and then there's other membrane proteins for other things of course and they're all regulated that's the important takeaway they're regulated now what about membrane receptors right some of the Transporters Transporters for facilitated diffusion and even active transport which we're going to talk next they can be activated or deactivated right and they're usually activated or deactivated by membrane receptors which tend to also be transmembrane proteins so now we can think about cells within a tissue they can form cohesive layers through intracellular Junctions and these Junctions they can provide direct Pathways of communication between neighboring cells or between cells and the extracellular Matrix and so these cell cell Junctions are generally comprised of cell adhesion molecules short and desk cam right cell adhesion molecules and which are proteins that allow cells to recognize each other and then they contribute to proper cell differentiation and developments and there's actually three types of cell cell Junctions that you should know for the MCAT all right first one is a gap Junction Gap Junctions allow for direct cell to cell communication and they're often found in small bunches together Gap Junctions can also be called connexons and I'm going to spell that connect songs in case I happen to mispronounce it you have the spelling and they are formed by the alignment and the interaction of of pores that are composed of um connects and molecules they permit movement of water and some solutes directly between cells however proteins those are generally not transferred through Gap Junctions all right now there's also tight junctions that's the second kind of cell cell Junction that you should know these prevent solutes from leaking into the space between cells via a paracellular route and so these tight junctions they're found in things like epithelial cells and they function as a physical link between the cells as they form a single layer of tissue now tight junctions they can limit permeability enough to create some sort of trans epithelial voltage difference based on different concentration of ions on either side of that epithelium to be effective though tight junctions have to form a continuous band around the cell otherwise fluids could leak through spaces between those tight junctions and then the third sort of cell to cell Junction are desmosomes these um bind adjacent cells by anchoring to the cytoskeleton and they're formed by interactions between transmembrane proteins associated with intermediate filaments inside those adjacent cells they're primarily found in the interface between two layers of epithelial cells um and so that's that's the third type of cell to cell Junction we can we can visualize these better here as well right we can have tight junctions these are impermeable Junction they provide molecules from passing through intracellular space these are the lining of your digestive tract right you'd also have Gap Junctions these allow for intracellular communication they allow ions and small molecules to pass through channels called channels formed by connexon protein cylinders and then you have decimals these are anchoring Junctions that bind to adjacent cells like Velcro so now we are well equipped to talk about a really important discussion and that's membrane transport we're going to talk about passive and active transport and then we're going to end this section with a discussion on endo and exocytosis all right now to set this up before we start with the details of each the cell membrane like we've said functions to control movement of substances into and out of the cell however it's going to vary in its set a selectivity for different substances for example transport of small nonpolar molecules can occur rapidly through diffusion while ions and larger molecules are gonna require more specialized transport processes um and these processes can happen through things like active or passive transport and they're they're kind of driven by concentration gradients or intracellular energy stores storages all right like we said transport processes they can be classified as either active or passive depending um depending on the thermodynamics so spontaneous processes that don't require energy that have a negative Delta G right they're going to proceed through passive transport while things that are non-spontaneous that require energy that have a positive Delta G those are going to proceed through active transport now diffusion facilitated diffusion and osmosis those generally increase in rate as temperature increases on the other hand things like active transport they may or may not be affected by temperature that of course depends on the enthalpy of the process so in that case the primary thermodynamic motivator can be can be other things all right so we're going to first start by talking about passive transport first all right passive transport is diffusion of a substance across a membrane with no energy investment all right these are processes that do not require intracellular energy but rather they utilize things like concentration gradients to supply the energy for particles to move the most basic of all membrane traffic processes is just a simple diffusion in which you'll have substrates that move down their concentration gradient directly across the membrane only particles that are freely permeable to the membrane are going to be able to undergo simple diffusion there is potential energy in a chemical gradient some of this energy is dissipated as the gradient is utilized during simple diffusion we can let like in this process to a ball rolling down a hill right there's potential energy in the ball when it sits at the top of the hill and then has the ball spontaneously rolls down the hill some of that energy is dissipated you can think of simple diffusion in the same way there's also osmosis we could talk about osmosis um this is a specific kind of diffusion that concerns water water not water water will move from a region of low solute concentration to one of high solute concentration so in essence it's going to move from a region of higher water concentration down its gradient to a to a region of lower water concentration usually until equilibrium is reached osmosis is important in several places most notably when the solute itself is impermeable to the membrane and in that case water will move to bring solute concentrations to equal molarity now let's take this as an example right let's move here down here for a little bit if the concentration of solutes inside the cell is higher than the surrounding solution the solution um is said to be hypotonic all right if the concentration of solutes inside this cell is higher than the surrounding solution then the solution is said to be hypotonic and this kind of solution will cause a cell to swell as water rushes into reach equal molarity right sometimes even to the point of bursting all right so a hypotonic solution it's going to have more solutes inside and if there's molar solutes inside then there's less water inside too and so water is going to rush into into the cell all right sometimes it could be so much that the cell swells and even bursts all right now A solution that is more concentrated than the cell is termed a hypertonic solution and water will actually move out of the cell so if there is more water here all right water's gonna want to move out of the cell all right sometimes this causes the cell to shrivel up and then if the solution inside and outside are equal molar then they're said to be isotonic all right now here's an important thing here's something that people sometimes mess up a key Point here is that being having an isotonic solution does not prevent movement all right it just prevents net movement of particles so water is coming in and out kind of at an equal rate you will think right water molecules will continue to move it's just that the cell will have neither gain nor loss of water overall all right now one method of quantifying the driving force behind osmosis is osmotic pressure and this is a colligative property so if you go back to your general chemistry knowledge you'll remember the osmotic pressure is one of four colligative properties things like you know boiling point elevation vapor pressure depression and and freezing point depression now osmotic pressure is one of those four colligative properties and that's a physical property of a solution that is dependent on the concentration of dissolved particles but not on the chemical identity of said dissolved particles and here we can see an equation for osmotic pressure again throwback to General chemistry osmotic pressure is equal to um m is the molarity of the solution R is the ideal gas constant T is the absolute temperature in kelvins and I is the vamphoft van Hoff factor which is simply the number of particles obtained from the molecule one in solution fantastic now there's also facilitated diffusion right we've talked about simple diffusion osmosis there's also facilitated diffusion facilitated diffusion is just simple diffusion for molecules that are impermeable to the membrane so these are large polar or uncharged the energy barrier is too high for these molecules to cause fear freely but facilitated diffusion requires integral membrane proteins to serve as the transporter or the channel for these substances and it still happens without the need for a great input energy with that that's passive energy now we can move into active that was passive transport now we can move into active transport now despite the help of um transport proteins we said facilitated diffusion is considered a passive transport because the solute is essentially moving down its concentration gradient right a process that requires no energy all right now some some other transport proteins however they can move solutes against their concentration gradient across the plasma membrane from the side where they are less concentrated to the side where they are more concentrated and this is gonna require energy to pump a solute across a membrane against its gradient is going to require work it's going to require energy right and therefore this type of membrane traffic this is called active transport the trans proteins that move solutes against their concentration ingredients they're all carrier proteins rather than channel proteins here right so that's one point of difference and this makes sense because when channel proteins are open they just allow solutes to diffuse down their concentration gradients all right that's not the case for Carrier proteins they're going to open up and they're going to there's going to be a very um kind of like conscious conscious decision to move things against their concentration gradients all right active transport enables a cell um to maintain internal concentration uh concentrations of small solutes that differ from the concentration in its environment so for example compared with its surrounding an animal cell is going to have a much higher concentration of potassium and a much lower concentration of sodium the plasma membrane helps maintain these steep gradients how well by pumping sodium out of the cell potassium into the cell all right and this is one transport system that works this way is called the sodium potassium pump it exchanges sodium for potassium across the plasma membrane of an animal cell now you and we're going to go over that right here we can see how that happens the sodium potassium pump but you might ask initially well how do ion pumps maintain a membrane potential right all cells have voltages across their plasma membrane and a voltage by the way is just electrical potential energy which is a fancy way of saying that there is a separation of opposite charges now the cytoplasmic side of the membrane is negative in charge relative to the extracellular side why because of an unequal distribution of anions and cations on the two sides the voltage across the membrane call is called a membrane potential and it can range anywhere between minus 50 to minus 200 millivolts all right and that minus sign indicates that the inside of the cell is negative relative to the outside all right so taking all this in some membrane proteins that actively transport ions are going to contribute to that membrane potential and a good example like we've said is the sodium potassium pump notice that the pump does not translocate sodium and potassium one for one instead it pumps three sodium ions out of the cell for every two potassium ions it pumps into the cell all right with each crank of that pump there's a net transfer of one positive charge from the cytoplasm into the extracellular fluid and that kind of process it stores energy as a voltage all right so that's an important thing to keep in mind fantastic now what we can do is talk about our final topic endocytosis and exocytosis bulk transport across the plasma membrane it can occur through exocytosis or endocytosis endocytosis occurs it's demonstrated here when the cell membrane kind of takes in and engulfs material to bring it into the cell the material is encased in a vesicle which is important because cells will sometimes ingest toxic substances all right we have pinocytosis this is the endocytosis of fluids and dissolved particles you can also have phagocytosis this is the ingestion of large solids like bacteria some straight binding to specific receptors embedded within the plasma membrane are going to initiate this process of endocytosis now we can also have exocytosis that occurs when um when we have vesicles that um secrete secrete secret secretion vesicles that fuse with the membrane that release materials from inside the cell to the extracellular environment now exocytosis is important in the nervous system and in intracellular signaling so for example exocytosis of neurotransmitters from synaptic vesicles there's a really crucial aspect of neuron physiology and so in essence again bulk transport is made possible whether it's into the cell or out of the cell through exocytosis and endocytosis all right that's all I have for you in regards to review next time we'll do some practice problems but let me know if you have any questions down below other than that good luck happy studying and have a beautiful beautiful day future doctors