Plasma Membrane and Transport Processes

Hello and welcome back to this online lecture series. In the last lecture, you were taken through a tour of the cell where you were introduced to the organelles and the network of cytoskeleton components that gives organization and structure to the cells. Knowing the organelles is important to understanding how the cell functions, being able to distinguish between prokaryotes and eukaryotes, and it will definitely help in future physiology courses. You also learned about the selective barrier surrounding every cell called the plasma membrane. This lecture will dive further into the structure and function of the plasma membrane and the implications for cellular functions. We'll cover the plasma membrane, membrane proteins and functions, osmosis and tonicity, passive and active transport, including bulk transport across the plasma membrane. After this lecture, you will be able to demonstrate an understanding of the structure and functions of cell membranes and the implications for cellular processes. In particular, you'll be able to describe the structure of biological membranes, including the role of lipids and proteins, describe the function of biological membranes, and what it means that the cell membrane is selectively permeable. And you will be able to describe how various substances move across the cell membrane and distinguish between passive and active transport. As you should now know, the plasma membrane is the boundary that separates the living cell from its surroundings. Every single cell has a plasma membrane, whether they are prokaryotic or eukaryotic. Its most crucial role is as a selectively perfected cell. permeable barrier which allows some substances to cross more easily than others. This selectivity is so important to a cell's ability to regulate homeostasis and maintain controlled interactions with its environment. We will be learning all about how the cell accomplishes this, including how transport proteins are often responsible for controlling passage across cellular membranes. But first, let's review the structure of the plasma membrane. Knowing the structure of the membrane is very important to understanding how it functions. Remember back to lecture one, the structure-function relationships. Plasma membranes are comprised of a special type of lipid called a phospholipid. Recall from lecture five that a phospholipid is two fatty acids and a phosphate group attached to a glycerol. The two fatty acids are hydrophobic hydrocarbons, but the phosphate group forms a hydrophilic head. This makes the phospholipid both hydrophilic and hydrophobic, and this type of molecule is called amphipathic. Part of the molecule is water-loving, while part of it is water-fearing. Because of this, the phospholipids orient themselves so that the hydrophobic tails are sheltered inside the membrane while the hydrophilic heads are exposed to water and fluids on either side. Given the opportunity, phospholipids will automatically orient themselves like this and you can see the lipid bilayer sphere structure in this image. Notice how it's different. from the single layer lipid sphere which would not allow water to come in contact with the exposed hydrophobic tails that are on the inside of that sphere. Our membranes are phospholipid bilayers. There's two layers of phospholipids so that the intracellular environment can be water-based because after all we are 70% water. This special orientation of the phospholipids is what gives the membrane selective permeability and more on that in a little bit remember cells are the smallest units of life and must therefore perform all cellular operations that are required of life to achieve this successfully the plasma membrane possesses a unique property of fluidity whereby the phospholipids and a variety of protein components of the membrane can shift and move laterally through the bilayer. This is termed the fluid mosaic model. The image on the right is a tile mosaic, which is a pattern that's made of small regular or irregular pieces of colored stone or glass or ceramic. The membrane is a type of mosaic, but it's made of proteins bobbing in a fluid bilayer of phospholipids. However, don't mistake this to mean that the cell has no control of the fluidity or the mosaic of proteins. Proteins are not randomly distributed in the membrane. The cell is of course a finely tuned piece of organic machinery and everything is highly orchestrated to maintain homeostasis in the support of life. Movement is free sideways or laterally within the membrane, but there are rare proteins that can actually flip-flop across the membrane from one cell to another. side of the phospholipid layer to another and Scientists were very creative in naming these proteins. They're called flip ace and flop ace Let's take a look at how the cell monitors the fluidity of the membranes Recall from lecture 5 that the main type of bond holding phospholipids together in the membrane are weak hydrophobic interactions Because these bonds are weak The phospholipids have a lot of flexibility to move around within the phospholipid bilayer. Recall that the three types of lipids are fats made of saturated or unsaturated fatty acids, phospholipids, and steroids. Just like fatty oils such as olive oil are slick and fluid, so too is the phospholipid bilayer. In fact, membranes are about as fluid as salad oil. There are a few ways that the cell can tinker with the plasma membrane to affect its fluidity. This is important for the cell's ability to effectively respond to varying environmental conditions to maintain an optimal physiological environment. The first is temperature. As temperatures cool, membranes switch from a fluid state to a solid state you experience this at a macroscopic level when you heat butter to melt it the temperature at which a membrane solidifies depends on the types of lipids that are present for example membranes that are rich in unsaturated fatty acids are more fluid than those that are rich in saturated fatty acids and just look at the structure of an unsaturated fatty acid It is kinked, it's bent, which means that they can't pack in as closely, which leaves for more fluidity and movement. Whereas saturated fatty acids are usually straight and they can pack in more closely. They would be more dense. There'd be less movement and fluidity. Membranes must be fluid to work properly. Again, think of salad oil. The last thing affecting fluidity is the steroid cholesterol. which has different effects on the membrane of animal cells at different temperatures. At warm temperatures, such as 37 Celsius, which is your body temperature, cholesterol actually restrains movement of phospholipids. At cooler temperatures, it has the opposite effect. It maintains fluidity by preventing tight packing of the phospholipids. Plant cells use a different system than animal cells for controlling their fluidity. Although cholesterol is present in plant cells, they use a different set of related steroid lipids to buffer their membrane fluidity. Let's have a look at membrane proteins and their functions. Somewhat like the tile mosaic we saw earlier, a membrane is a collage of different proteins that are often clustered in groups. embedded in the fluid matrix of the lipid bilayer. Phospholipids form the main fabric of the membrane while proteins determine most of the membranes functions. There are two major categories of proteins within the plasma membrane peripheral proteins and integral proteins. Peripheral proteins are bound to the surface of the membrane. Periphery means outside the boundary of something. Integral proteins are different. Integral proteins penetrate the hydrophobic core of the membrane. Some integral proteins span the entire membrane and these are called transmembrane proteins, trans meaning across. This is an image of a transmembrane protein. Notice how it spans across the entire membrane from the extracellular side to the cytoplasmic side. Extracellular means outside the cell and of course cytoplasmic is the cytoplasm within the cell. intracellular. In the bottom image, we see there are many different forms that transmembrane proteins can take. Oftentimes, there are multiple regions winding through the membrane as seen in example two and three. The parts of the integral and transmembrane proteins that penetrate the plasma membranes are hydrophobic regions consisting of one or more stretches. of nonpolar amino acids. Recall from lecture 5 that there are 20 amino acids which are either polar, nonpolar, acidic, or basic. Any parts of the proteins that are interacting with the hydrophobic phospholipids in the membrane must also be hydrophobic. The nonpolar stretches of amino acids are often coiled into alpha helices just like the one you see here. And this protein has seven transmembrane domains. Example 3 is showing domains that are actually made of beta pleated sheets. Cell surface proteins carry out several functions important for life, including transport of molecules across the membrane. Enzymatic activity to speed up our biochemical reactions. Signal transduction to catalyze an intracellular response to an extracellular stimulus. Cell-cell recognition for immunological responses. Intercellular joining for stability and structure. And attachment to the cytoskeleton and the extracellular matrix. Carbohydrates play a very important role in cell-cell recognition. Cells are able to recognize each other by binding to molecules, which often contain carbohydrates, that are on the extracellular surface of the plasma membrane. They sort of act like tags. Membrane carbohydrates may be covalently bonded to lipids, which form glycolipids. Or more commonly, the carbohydrates are covalently bonded to proteins, and these are called glycoproteins. Notice the root words here, glyco meaning sugar. A glycolipid is a fat with a sugar on it, and a glycoprotein is a protein with a sugar on it. Carbohydrates on the extracellular side of the plasma membrane vary among species, individuals, and even cell types in an individual. Cell surface proteins are also important in the medical field. Glycoprotein and glycolipid patterns that are on a cell's surface can give many viruses an opportunity for infection. In the example that is shown here, HIV must be a virus that is not a virus. bind to the immune cell surface protein CD4, which is shown in purple, and a co-receptor CCR5 in order to infect a cell. HIV cannot enter the cells of resistant individuals who lack the CCR5 co-receptor, so drugs are being developed to mask the CCR5 protein. These viruses are able to invade these cells because the cells have binding sites on their surfaces that are specific to and compatible with certain viruses as you now know the phospholipids orient themselves into the phospholipid bilayer forming the plasma membrane but the plasma membrane isn't the only part of the cell that's made of phospholipids all of the organelles in the cell use phospholipid bilayer as their membrane This includes the nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, and vesicles used for transport within the cell. In the last lecture, we learned about the functions of each of these organelles. Take a look at the picture showing the flow of material through the cell's organelles, the ability of the ER, to bud off into a vesicle for that vesicle to fuse with the golgi and then for vesicles to be released from the golgi and then fuse with the plasma membrane for secretion this is all crucial for the endomembrane system the fact that all membranes are made of the same phospholipids allow for easy movement of materials through the cell you'll also notice from this picture that there is a sidedness to the membranes meaning they have distinct inside and outside faces. Each side has an asymmetrical distribution of proteins specific to each compartment's specialized role in the cell. We have already discussed the structure of the membranes and the resulting selective permeability. Selective permeability means the membrane allows some substances to cross more easily than others. It's very important that you understand this concept before we continue. A cell must exchange materials with its surroundings, which is controlled by the plasma membrane. Recall that plasma membranes are amphipathic. They have hydrophilic and hydrophobic regions in the phospholipids. This characteristic is what helps move some materials through the membrane and hinders the movement of others. Thank you for watching. molecules in your body are not able to cross this phospholipid bilayer. So what molecules are able to move freely through the plasma membrane? Remember the phrase, like dissolves like. Small hydrophobic nonpolar molecules can dissolve in the lipid bilayer and pass through the membrane rapidly. hydrophilic molecules including ions and polar molecules do not cross the membrane easily or at all large molecules of any type will have an issue crossing the membrane and require special transport which we will be discussing later in this lecture so to answer the question what can move freely through the plasma membrane small non-polar and lipid soluble material with a low molecular weight like oxygen and carbon dioxide can easily slip through the membrane's hydrophobic lipid core. Substances like fat-soluble vitamins, which would be vitamin A, vitamin D, vitamin E, and vitamin K, also can rapidly pass through the plasma membrane in the digestive tract and other tissues. Fat-soluble drugs and hormones also gain pretty easy entry into cells, readily transport themselves into the body's tissues and organs. Water is also able to pass through though pretty slowly since it's a polar molecule and we'll talk about another way that water can get through the membrane. This selective permeability allows for tight regulation of molecules that are trafficking into and out of the cell and this is crucial to the cells ability regulate its internal environment and maintain a favorable homeostasis which means steady-state nearly all cells in our body are in contact with the bloodstream which acts as this highway that's carrying nutrients and gases and signaling molecules and even toxins like alcohol throughout the body so it's necessary for the cell to be able to regulate what can or can't enter from the bloodstream and the cell membrane is the first and most important barrier of defense in this process and part of that defense are the transport proteins which we will focus on in this lecture transport proteins are responsible for controlling passage across cellular membranes specifically for hydrophilic polar substances polar substances present problems for the membrane. While some polar molecules can connect easily with the cells outside, they cannot readily pass through the plasma membrane's lipid core. And additionally, while small ions could easily slip through the spaces of the membrane's mosaic, their charge prevents them from doing so. Ions such as sodium and potassium, calcium and chloride need special means of penetrating the plasma membrane because of their charge. Simple sugars and amino acids also need the help of various protein channels to transport them across the plasma membrane because of their polarity. We have two major types of transport proteins. These are channel proteins and carrier proteins. Channel proteins have a hydrophilic channel that certain molecules or ions can use as a tunnel. One very specialized type of channel protein is called an aquaporin, which facilitates the passage of water molecules, hence the word aqua. Carrier proteins bind to molecules and change shape to shuttle them across that membrane, and we call this a conformational change, that shape change. But whether it is a channel or a carrier protein, all transport proteins are very specific for the substance that they're moving across the membrane. Now that we have the basics of the proteins, let's talk about the transport. We have two main types of transport in our cells, active transport and passive transport. To put it very simply, active transport uses energy, which is usually ATP, and passive transport does not use energy. Passive transport works by moving substances down their concentration gradient while active transport is using the energy to move things up or against their concentration gradient. There are examples of active and passive transport listed here and we will discuss each in detail. Before we talk further about the proteins involved in protein transport, we have to learn the concepts of diffusion and concentration gradients as this is what propels passive transport. Diffusion is defined as the tendency for molecules to spread out evenly into an available space. You encounter diffusion daily though you may not even notice it. Pouring milk into coffee or spraying perfume into the air demonstrates how the substances will move from where they are more concentrated to where they are not. to where they are less concentrated. They're spreading out evenly in their spaces over time. This diffusion requires no energy and as we'll see in a moment the gradients are a special form of energy called potential energy. Look at the image on the right which uses blue lines to follow the motion of a few molecules. This motion is called Brownian motion which is the random motion of particles suspended in a liquid or a gas caused by the collision of those particles with water molecules at the atomic level molecules are vibrating and moving due to thermal and kinetic energy and therefore they're randomly bumping into one another however although each molecule moves randomly diffusion of a population of molecules may be directional. For example, movement tends to be away from where things are more concentrated. At dynamic equilibrium, just as many molecules are crossing the membrane in one direction as in the other, and we'll encounter the concept of equilibrium a few times in this lecture. Concentration gradients play a big role in the movement of these molecules. A concentration gradient is the gradual difference in concentration over the distance between the regions of high and low concentration. Concentration is a mass or number of something per unit of volume. See how the image with the red dots has more dots per area in the high concentration than in the low concentration area? Over time the molecules will move due to Brownian motion in the direction of high to low And we call this moving down the concentration gradient Again, this requires no energy input Concentration gradients are a form of potential energy which will dissipate as the gradient is eliminated the substance will move from a high concentration to a low concentration area until the concentration is equal across a space. This is a spontaneous and irreversible process which increases entropy of a system because Particles will not spontaneously reorder themselves and we'll... learn more about entropy in the next lecture. Now that we have diffusion and concentration gradients... defined, we'll cover passive... transport and the various forms of passive transport. Let's start with a few rules for passive transport for you... to remember as we move through the rest of the lecture. Passive transport uses no energy. Molecules always move down the concentration gradient, and both channel proteins and carrier proteins may be used in passive transport. The first type of passive transport is simple diffusion. In simple diffusion, the molecules are able to diffuse across the plasma membrane. Of course, they are diffusing. down their concentration gradients from high concentration to low concentration and no energy is required we already talked about what type of molecules can or can't move across the plasma membrane small electrically neutral atoms are able to diffuse through by slipping between the spaces of the lipid molecules examples are oxygen gas and carbon dioxide gas Water is also able to move through via simple diffusion, although water is polar so it doesn't move very rapidly. In this image we see two sides that are separated by a permeable membrane. We begin with a concentration gradient of blue molecules with the higher concentration in the extracellular space. In the middle panel the molecules have begun to diffuse. and they'll continue until equilibrium is reached. In the rightmost panel we see the solution has reached equilibrium and equilibrium does not mean equal concentration. Equilibrium means equal rates of diffusion. In other words, just as many blue molecules are moving into the cell as are moving out. As I mentioned, water can also diffuse across a selectively permeable membrane and this has a special term called osmosis. Pay close attention to this explanation. Water diffuses across a membrane from a region of lower solute concentration to a region of higher solute concentration until the solute concentrations equal on both sides. Take a look at the image on the right. These beakers are filled with water and solute. at different concentrations separated by a selectively permeable membrane. There are two components here, water and solute, each with a different concentration gradient. The membrane is only permeable to water, yet everything wants to reach equilibrium. Because water is the only thing that can cross, it's going to want to diffuse down its own concentration gradient. which means it will move from an area of higher water concentration to lower water concentration this means it's actually moving from lower solute concentration to higher solute concentration the left and right sides of the tube begin with equal amounts of water but in equal solute concentrations which means in equal water concentrations the left has a higher water concentration than the right, so water will move from the left towards the right, which is from lower solute concentration to higher solute concentration. The osmotic pressure is so great that the water level on the right exceeds the water level on the left. Osmosis allowed for equilibrium with similar concentration of solute on both sides of the membrane and equal rates moving across the membrane. If you've ever heard of reverse osmosis, it's a water purification process that uses a partially permeable membrane to separate out ions and unwanted molecules and larger particles from your drinking water. Now that you know the basics of diffusion and osmosis, we can apply this to our cells. As I keep mentioning, homeostatic balance is crucial to all aspects of cellular physiology. The proper balance of water in the cell and in the body is included in that. Tonicity is the ability of a surrounding solution to cause a cell to gain or lose water. The tonicity of that solution depends on its concentration of solutes that cannot cross the membrane relative to the inside of the cell. What this does is create a solute concentration gradient which will drive osmosis from or into the cell. Organisms have highly adept mechanisms for controlling osmosis. and we call this osmoregulation. Osmoregulation is the control of solute concentrations and water balance. As examples, unicellular eukaryotes like paramecium are hypertonic relative to their pond water environment. So it has a contractile vacuole that can pump water. Bacteria and archaea that live in excessively salty environments have their cellular mechanisms to ensure that they don't lose too much water to their environment. There are three types of tonic solutions, isotonic, hypertonic, and hypotonic. Let's start with isotonic, as it's the easiest one to remember. Isotonic solutions have a solute concentration that is the same as inside the cell. Therefore, there's no net movement across the plasma membrane. The center panel shows an isotonic red blood cell which is able to maintain its proper shape and therefore function. Notice there is water diffusion into and out of the cell. Osmosis is therefore in equilibrium here, again with no net change in movement. If you've ever had a saline drip intravenously at a hospital, that saline IV would have to be a concentration of salts and nutrients that are isotonic to your blood. Otherwise, the saline IV would create a net diffusion of water either into or out of your cells, which would either dehydrate or burst your cells. The animal cell likes isotonic environments. The next type of tonicity is a hypertonic solution. In a hypertonic solution, the site... concentration is greater than inside the cell. So the cell loses water. The top panel shows cells in... hypertonic solutions. When the solution has a site... concentration that's greater than that within the cell, water wants to create equal concentrations. Therefore, water will diffuse from the area of the lower solute concentration inside the cell to the area of the higher solute concentration outside the cell. This causes the red blood cell to shrivel, a process that's termed crenation. The last type of tonicity is hypotonic. In a hypotonic solution, the concentration of solutes is less than that inside the cell. Therefore, the cell will gain water. The bottom panel shows cells in hypotonic solutions. The concentration of solute in this solution is less than the concentration within the cell. Once again, water wants to move from an area of lower solute concentration to higher solute concentration. Therefore, water will rush into the cell and will cause it to swell until it bursts or lices. Plant cells are a bit different because they have sturdy cell walls to help maintain water balance. Remember, plants have both a plasma membrane and a cell wall, and they like when the membrane is pressed firmly against the cell wall. This is seen in a hypotonic environment. The plant cell is thriving in the hypotonic solution. Water rushes into the cell, but it does not burst because it has the strong cell wall to maintain its structure. This cell is said to be turgid and is the optimal form for plant cells. This is why plants love water. Water has far less concentration of solutes and nutrients than plant cells, and therefore osmotic pressure will cause the diffusion of water into plant cells and keep them turgid. The plant cell in the middle panel is in an isotonic solution where there is no net movement into the cell. This cell becomes flaccid or limp, which is not ideal. In the hypertonic solution, the plant has lost water and the membrane pulls away from the cell wall, causing the plant to wilt. The cell walls do not buckle, but the plasma membranes have shriveled and pulled away from the cell walls here. This is called plasmolysis, which is potentially lethal to plants. Hypertonic solutions are detrimental to both animal and plant cells. Now you've had a complete review of simple diffusion. The movement of molecules in diffusion is driven solely by concentration gradients. Simple diffusion is also unique in that it uses no transport proteins. It only has to do with movement of water or solutes across that selectively permeable membrane. Now we will learn the second type of passive transport called facilitated diffusion. This type of diffusion does use proteins to aid in transport, but still does not require energy. It is still a type of diffusion after all. In facilitated diffusion, transport proteins speed the process of passive movement of molecules across the plasma membrane. Transport proteins are the two types we discussed earlier in the lecture, channel proteins and carrier proteins. Channel proteins have a hydrophilic channel that allows charged molecules or ions to use as a tunnel. One special type of channel protein is the aquaporin which facilitates diffusion of water. There are also basic ion channels which to facilitate the transport of ions. Carrier proteins are different. When they bind to their molecules, they change shape to shuttle them across the membrane. Transport proteins are always, always, always highly specific for the substance that they move. Let's look at channel proteins, specifically the aquaporin. Aquaporins provide corridors for water to rapidly cross the membrane. Notice how it's a transmembrane protein with the hydrophilic domains exposed to the intracellular and extracellular fluids and a hydrophilic channel. through the membrane core that provides a hydrated opening through the hydrophobic membrane layers. Passage through the channel allows polar compounds to avoid the plasma membrane's nonpolar inner layer that would otherwise slow down or prevent their entry into the cell. Here's a model of an aquaporin transporting water molecules across that membrane. The yellow molecule is a water molecule that's highlighted to help you follow the flow of molecules. This movement is driven by diffusion and is passive transport. Aquaporins are most prevalent in the kidneys for water reabsorption. They help to maintain fluid homeostasis in several different tissues, including the kidney, lung, the GI tract, and brain. Here's another type of channel protein, the gated. ion channel. Ion channels facilitate the transport of ions across the membrane. Some of these ion channels are gated, meaning they open or close in response to a particular stimulus. Examples of stimuli are voltage and ligand binding. A prime example is in nerve cells where sodium ion channels open in response to electrical stimulus to conduct an action potential. and you'll learn more about that in physiology or in your psychology courses check your understanding and retention match the term in the left column to the correct answer in the right column pause the video then play to check your answers Now we will cover active transport. Let's set a few rules for active transport for you to remember as we move through the lecture. Active transport requires energy from ATP hydrolysis. Molecules always move up or against a concentration gradient. All proteins involved in active transport are carrier proteins, and active transport creates important electrochemical gradients. This is a good time to review ATP hydrolysis as this is how our cells harness energy. ATP hydrolysis is the metabolic reaction to release chemical energy in high energy bonds. ATP is adenosine triphosphate, which is an adenine attached to a ribose sugar attached to three phosphate groups. The bonds between the phosphate groups are the phospho and hydride bonds. When ATP is broken down, we create ADP, which is adenosine diphosphate, and an inorganic phosphate. Inorganic simply means it is not attached to a molecule that contains carbon. It was an organic phosphate when attached to the ATP, but once cleaved, it is an inorganic phosphate. Hydrolysis of the terminal phosphoenhydride bond is highly exergonic meaning it releases a lot of energy the gibbs free energy is negative 34 kilojoules per mole and we'll learn what this is in the next lecture chemical energy present within the bonds of that molecule is released by splitting the bonds and harnessed for mechanical energy to do work Energy is neither created nor destroyed, so we are simply transforming one form of energy into another, chemical energy into mechanical energy. ADP can be further hydrolyzed to give energy. AMP is created here, which is adenosine monophosphate, and we get another inorganic phosphate. We're about to get into the first type of active transport, which is electrogenic pump. But first I think it's helpful to explain what... electrogenic means. Very simply, electrogenic pumps create electrochemical gradients. So what's an electrochemical gradient? First we have to understand the concept of membrane... potential. Remember earlier I mentioned... that gradients are a form of potential energy. The membrane potential is the voltage across a membrane. You know what voltage is maybe related to energy or a battery. Voltage is created by difference in the distribution of positive and negative ions across a membrane. Look at the cell membrane image to illustrate this. In this image, the yellow circles are positive potassium ions. The purple hexagons are positive sodium ions. And the green circles are negative chloride ions. All of these ions are distributed differently across the two sides of the membrane. And overall, the intracellular space is more negative than the extracellular space. And this is true of all cells. They are more negative on the inside. And this is voltage. When we measure neurons, they are negative 70 millivolts. There's an imbalance in both charge and the individual ion concentrations. These two forces make up the electrochemical gradient, which drives the diffusion of ions across the membrane. The two forces are called the chemical force and the electrical force. The chemical force is the ion concentration gradient. So for example, There is a higher concentration of sodium outside the cell than inside the cell. That is the chemical force. The electrical force is the effect of the membrane potential, the charge on the ion's movement. So there is a higher concentration of negative ions inside the cell. So chloride is a negatively charged ion. And It would want to move down that concentration gradient if we were looking just at the electrical force. But in reality, both of these forces, the chemical force and the electrical force, are factored into how ions will move across the membrane. And look, these two forces make up the gradient name, electrochemical gradient. So back to electrogenic pumps. Electrogenic pumps create these electrochemical gradients. They are transport proteins that generate voltage across the membrane. They do this by using energy, ATP, to pump substances against their concentration gradient. This ability for active transport proteins to move solute up their concentration gradients allows cells to maintain concentration gradients that differ from their surroundings. The sodium-potassium pump is the major electrogenic pump in animal cells, and the proton pump is prevalent in plants, fungi, and bacteria. Electrogenic pumps'big job is to help store energy that can be used for cellular work. Again, this would be potential energy. So the first type of active transport is the electrogenic pump. Here we have the sodium potassium pump. The sodium potassium pump uses ATP and carrier proteins to move substances against their gradients. An animal cell has a higher potassium K plus and a much lower sodium which is Na plus. concentration compared to its surroundings. This is controlled by the sodium potassium pump. Let's walk through the basic steps of how the sodium potassium pump functions. The overall goal is to move three sodium ions out of the cell and two potassium ions into the cell. Make sure you note that three sodium out, two potassium in. The carrier proteins begin open facing the interior of the cell. Three orange sodium ions bind. In step two, the energy molecule ATP is hydrolyzed, resulting in the transfer of that phosphate group onto the carrier protein. This activates the protein for movement. The carrier protein has a conformational change, shape change, and releases the sodium ions to the outside. Then it's open for the two potassium ions to bind and the phosphate group is removed, resulting in the final conformational shape change as seen in step four. The process then repeats and each cycle requires the hydrolysis of one ATP molecule. Next we have another electrogenic pump. the proton pump. Here an integral membrane protein pump builds up a proton gradient across a membrane. In the image we see ATP being used to drive hydrogen ions to the outside where there is a higher proton concentration already and a corresponding low pH. Proton pumps are prevalent in the stomach to create an acidic environment for our digestion. And you may also find proton pumps as a part of co-transport. Co-transport is another type of active transport, and it's also called secondary active transport. Secondary active transport does not require direct ATP usage. Instead, it is the movement of material due to the electrochemical gradient that's established by primary active transport using ATP of another substance. In this process, the diffusion of an actively transported solute down its concentration gradient is coupled with the transport of a second substance that will be going against its own concentration gradient. There are two types of proteins that are involved in co-transport. We have antiporters and symporters. Antiporters move two substances across the membrane in opposite directions and simporters move two substances across in the same direction. The sodium-potassium pump is actually an antiporter and many amino acids and glucose actually enter the cell via co-transport. In the center image, the yellow simporter is moving both sodium and glucose into the cell in the same direction. Notice sodium's concentration gradient. Remember the sodium gradient was created by the sodium potassium pump that we just learned about, which utilized ATP energy. It's favorable for sodium to then move down its concentration gradient into the cell via diffusion without any energy. However, glucose is more concentrated in the cell. So moving from outside to inside is a... against its gradient and would need energy. The symporter allows glucose to take advantage of sodium's gradient and hitch a ride into the cell. So in this way, glucose. is indirectly using ATP to move into the cell, which is why this is called secondary active transport, and that is cotransport. The final type of active transport is bulk transport. Through this lecture, we've discussed how small molecules and water enter or leave the cell through the lipid bilayer or via transport proteins. However, large molecules present a problem because they don't fit through the membrane or through transport proteins. Large molecules such as polysaccharides and polypeptide proteins cross the membrane in bulk via vesicles. We have two basic forms, exocytosis and endocytosis. Exo means outside, so exocytosis is things exiting the cell. Well, endo means inside, so endocytosis is things entering the cell. Of course, moving large things across a membrane is an arduous endeavor, so bulk transport requires ATP energy. In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents outside the cell. Remember, vesicles are usually coming off from the... Golgi apparatus and with some finished products after having gone through the endomembrane system. Vesicles are of course surrounded by phospholipid bilayer, so they easily fuse with the plasma membrane. Many secretory cells use exocytosis to export their products to the extracellular matrix or nearby cells or through ducts. In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane. Endocytosis is a reversal of exocytosis involving different proteins. There are three types of endocytosis. Phagocytosis, meaning cellular eating. Penocytosis, meaning cellular drinking. And receptor-mediated endocytosis. We learned of phagocytosis in the previous lecture when we learned about vacuoles. In phagocytosis, or cellular eating, a cell engulfs a particle or another cell in a special vacuole called a food vacuole. In penocytosis, or cellular drinking, molecules that are dissolved in droplets are taken up when extracellular fluid is gulped into tiny vesicles. And in receptor-mediated endocytosis, binding of a specific solute to its receptor, which is sitting on the outside of the cell, will trigger vesicle formation and taking in of that triggering molecule. Human cells use receptor-mediated endocytosis to take in cholesterol. which is carried through our bloodstream in particles called LDLs or low-density lipoproteins. Individuals with the disease familial hypercholesterolemia have missing or defective LDL receptor proteins. This leaves individuals with a higher risk of heart disease and a greater risk of early heart attack. And that will conclude this lecture on membrane transport. I know that was a lot. So I've created this schematic to help you mentally map out all that we've covered. Use it as inspiration for your own notes and add details where needed to help you learn the material. And I will see you next lecture.

You also learned about the selective barrier surrounding every cell called the plasma membrane. This lecture will dive further into the structure and function of the plasma membrane and the implications for cellular functions. We'll cover the plasma membrane, membrane proteins and functions, osmosis and tonicity, passive and active transport, including bulk transport across the plasma membrane.

After this lecture, you will be able to demonstrate an understanding of the structure and functions of cell membranes and the implications for cellular processes. In particular, you'll be able to describe the structure of biological membranes, including the role of lipids and proteins, describe the function of biological membranes, and what it means that the cell membrane is selectively permeable. And you will be able to describe how various substances move across the cell membrane and distinguish between passive and active transport.

As you should now know, the plasma membrane is the boundary that separates the living cell from its surroundings. Every single cell has a plasma membrane, whether they are prokaryotic or eukaryotic. Its most crucial role is as a selectively perfected cell. permeable barrier which allows some substances to cross more easily than others.

This selectivity is so important to a cell's ability to regulate homeostasis and maintain controlled interactions with its environment. We will be learning all about how the cell accomplishes this, including how transport proteins are often responsible for controlling passage across cellular membranes. But first, let's review the structure of the plasma membrane. Knowing the structure of the membrane is very important to understanding how it functions.

Remember back to lecture one, the structure-function relationships. Plasma membranes are comprised of a special type of lipid called a phospholipid. Recall from lecture five that a phospholipid is two fatty acids and a phosphate group attached to a glycerol. The two fatty acids are hydrophobic hydrocarbons, but the phosphate group forms a hydrophilic head.

This makes the phospholipid both hydrophilic and hydrophobic, and this type of molecule is called amphipathic. Part of the molecule is water-loving, while part of it is water-fearing. Because of this, the phospholipids orient themselves so that the hydrophobic tails are sheltered inside the membrane while the hydrophilic heads are exposed to water and fluids on either side. Given the opportunity, phospholipids will automatically orient themselves like this and you can see the lipid bilayer sphere structure in this image.

Notice how it's different. from the single layer lipid sphere which would not allow water to come in contact with the exposed hydrophobic tails that are on the inside of that sphere. Our membranes are phospholipid bilayers.

There's two layers of phospholipids so that the intracellular environment can be water-based because after all we are 70% water. This special orientation of the phospholipids is what gives the membrane selective permeability and more on that in a little bit remember cells are the smallest units of life and must therefore perform all cellular operations that are required of life to achieve this successfully the plasma membrane possesses a unique property of fluidity whereby the phospholipids and a variety of protein components of the membrane can shift and move laterally through the bilayer. This is termed the fluid mosaic model. The image on the right is a tile mosaic, which is a pattern that's made of small regular or irregular pieces of colored stone or glass or ceramic. The membrane is a type of mosaic, but it's made of proteins bobbing in a fluid bilayer of phospholipids.

However, don't mistake this to mean that the cell has no control of the fluidity or the mosaic of proteins. Proteins are not randomly distributed in the membrane. The cell is of course a finely tuned piece of organic machinery and everything is highly orchestrated to maintain homeostasis in the support of life. Movement is free sideways or laterally within the membrane, but there are rare proteins that can actually flip-flop across the membrane from one cell to another.

side of the phospholipid layer to another and Scientists were very creative in naming these proteins. They're called flip ace and flop ace Let's take a look at how the cell monitors the fluidity of the membranes Recall from lecture 5 that the main type of bond holding phospholipids together in the membrane are weak hydrophobic interactions Because these bonds are weak The phospholipids have a lot of flexibility to move around within the phospholipid bilayer. Recall that the three types of lipids are fats made of saturated or unsaturated fatty acids, phospholipids, and steroids.

Just like fatty oils such as olive oil are slick and fluid, so too is the phospholipid bilayer. In fact, membranes are about as fluid as salad oil. There are a few ways that the cell can tinker with the plasma membrane to affect its fluidity.

This is important for the cell's ability to effectively respond to varying environmental conditions to maintain an optimal physiological environment. The first is temperature. As temperatures cool, membranes switch from a fluid state to a solid state you experience this at a macroscopic level when you heat butter to melt it the temperature at which a membrane solidifies depends on the types of lipids that are present for example membranes that are rich in unsaturated fatty acids are more fluid than those that are rich in saturated fatty acids and just look at the structure of an unsaturated fatty acid It is kinked, it's bent, which means that they can't pack in as closely, which leaves for more fluidity and movement. Whereas saturated fatty acids are usually straight and they can pack in more closely.

They would be more dense. There'd be less movement and fluidity. Membranes must be fluid to work properly. Again, think of salad oil. The last thing affecting fluidity is the steroid cholesterol.

which has different effects on the membrane of animal cells at different temperatures. At warm temperatures, such as 37 Celsius, which is your body temperature, cholesterol actually restrains movement of phospholipids. At cooler temperatures, it has the opposite effect.

It maintains fluidity by preventing tight packing of the phospholipids. Plant cells use a different system than animal cells for controlling their fluidity. Although cholesterol is present in plant cells, they use a different set of related steroid lipids to buffer their membrane fluidity. Let's have a look at membrane proteins and their functions.

Somewhat like the tile mosaic we saw earlier, a membrane is a collage of different proteins that are often clustered in groups. embedded in the fluid matrix of the lipid bilayer. Phospholipids form the main fabric of the membrane while proteins determine most of the membranes functions. There are two major categories of proteins within the plasma membrane peripheral proteins and integral proteins. Peripheral proteins are bound to the surface of the membrane.

Periphery means outside the boundary of something. Integral proteins are different. Integral proteins penetrate the hydrophobic core of the membrane.

Some integral proteins span the entire membrane and these are called transmembrane proteins, trans meaning across. This is an image of a transmembrane protein. Notice how it spans across the entire membrane from the extracellular side to the cytoplasmic side. Extracellular means outside the cell and of course cytoplasmic is the cytoplasm within the cell. intracellular.

In the bottom image, we see there are many different forms that transmembrane proteins can take. Oftentimes, there are multiple regions winding through the membrane as seen in example two and three. The parts of the integral and transmembrane proteins that penetrate the plasma membranes are hydrophobic regions consisting of one or more stretches.

of nonpolar amino acids. Recall from lecture 5 that there are 20 amino acids which are either polar, nonpolar, acidic, or basic. Any parts of the proteins that are interacting with the hydrophobic phospholipids in the membrane must also be hydrophobic. The nonpolar stretches of amino acids are often coiled into alpha helices just like the one you see here. And this protein has seven transmembrane domains.

Example 3 is showing domains that are actually made of beta pleated sheets. Cell surface proteins carry out several functions important for life, including transport of molecules across the membrane. Enzymatic activity to speed up our biochemical reactions.

Signal transduction to catalyze an intracellular response to an extracellular stimulus. Cell-cell recognition for immunological responses. Intercellular joining for stability and structure.

And attachment to the cytoskeleton and the extracellular matrix. Carbohydrates play a very important role in cell-cell recognition. Cells are able to recognize each other by binding to molecules, which often contain carbohydrates, that are on the extracellular surface of the plasma membrane.

They sort of act like tags. Membrane carbohydrates may be covalently bonded to lipids, which form glycolipids. Or more commonly, the carbohydrates are covalently bonded to proteins, and these are called glycoproteins. Notice the root words here, glyco meaning sugar. A glycolipid is a fat with a sugar on it, and a glycoprotein is a protein with a sugar on it.

Carbohydrates on the extracellular side of the plasma membrane vary among species, individuals, and even cell types in an individual. Cell surface proteins are also important in the medical field. Glycoprotein and glycolipid patterns that are on a cell's surface can give many viruses an opportunity for infection. In the example that is shown here, HIV must be a virus that is not a virus. bind to the immune cell surface protein CD4, which is shown in purple, and a co-receptor CCR5 in order to infect a cell.

HIV cannot enter the cells of resistant individuals who lack the CCR5 co-receptor, so drugs are being developed to mask the CCR5 protein. These viruses are able to invade these cells because the cells have binding sites on their surfaces that are specific to and compatible with certain viruses as you now know the phospholipids orient themselves into the phospholipid bilayer forming the plasma membrane but the plasma membrane isn't the only part of the cell that's made of phospholipids all of the organelles in the cell use phospholipid bilayer as their membrane This includes the nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, and vesicles used for transport within the cell. In the last lecture, we learned about the functions of each of these organelles.

Take a look at the picture showing the flow of material through the cell's organelles, the ability of the ER, to bud off into a vesicle for that vesicle to fuse with the golgi and then for vesicles to be released from the golgi and then fuse with the plasma membrane for secretion this is all crucial for the endomembrane system the fact that all membranes are made of the same phospholipids allow for easy movement of materials through the cell you'll also notice from this picture that there is a sidedness to the membranes meaning they have distinct inside and outside faces. Each side has an asymmetrical distribution of proteins specific to each compartment's specialized role in the cell. We have already discussed the structure of the membranes and the resulting selective permeability.

Selective permeability means the membrane allows some substances to cross more easily than others. It's very important that you understand this concept before we continue. A cell must exchange materials with its surroundings, which is controlled by the plasma membrane. Recall that plasma membranes are amphipathic.

They have hydrophilic and hydrophobic regions in the phospholipids. This characteristic is what helps move some materials through the membrane and hinders the movement of others. Thank you for watching.

molecules in your body are not able to cross this phospholipid bilayer. So what molecules are able to move freely through the plasma membrane? Remember the phrase, like dissolves like.

Small hydrophobic nonpolar molecules can dissolve in the lipid bilayer and pass through the membrane rapidly. hydrophilic molecules including ions and polar molecules do not cross the membrane easily or at all large molecules of any type will have an issue crossing the membrane and require special transport which we will be discussing later in this lecture so to answer the question what can move freely through the plasma membrane small non-polar and lipid soluble material with a low molecular weight like oxygen and carbon dioxide can easily slip through the membrane's hydrophobic lipid core. Substances like fat-soluble vitamins, which would be vitamin A, vitamin D, vitamin E, and vitamin K, also can rapidly pass through the plasma membrane in the digestive tract and other tissues.

Fat-soluble drugs and hormones also gain pretty easy entry into cells, readily transport themselves into the body's tissues and organs. Water is also able to pass through though pretty slowly since it's a polar molecule and we'll talk about another way that water can get through the membrane. This selective permeability allows for tight regulation of molecules that are trafficking into and out of the cell and this is crucial to the cells ability regulate its internal environment and maintain a favorable homeostasis which means steady-state nearly all cells in our body are in contact with the bloodstream which acts as this highway that's carrying nutrients and gases and signaling molecules and even toxins like alcohol throughout the body so it's necessary for the cell to be able to regulate what can or can't enter from the bloodstream and the cell membrane is the first and most important barrier of defense in this process and part of that defense are the transport proteins which we will focus on in this lecture transport proteins are responsible for controlling passage across cellular membranes specifically for hydrophilic polar substances polar substances present problems for the membrane. While some polar molecules can connect easily with the cells outside, they cannot readily pass through the plasma membrane's lipid core. And additionally, while small ions could easily slip through the spaces of the membrane's mosaic, their charge prevents them from doing so.

Ions such as sodium and potassium, calcium and chloride need special means of penetrating the plasma membrane because of their charge. Simple sugars and amino acids also need the help of various protein channels to transport them across the plasma membrane because of their polarity. We have two major types of transport proteins.

These are channel proteins and carrier proteins. Channel proteins have a hydrophilic channel that certain molecules or ions can use as a tunnel. One very specialized type of channel protein is called an aquaporin, which facilitates the passage of water molecules, hence the word aqua.

Carrier proteins bind to molecules and change shape to shuttle them across that membrane, and we call this a conformational change, that shape change. But whether it is a channel or a carrier protein, all transport proteins are very specific for the substance that they're moving across the membrane. Now that we have the basics of the proteins, let's talk about the transport. We have two main types of transport in our cells, active transport and passive transport. To put it very simply, active transport uses energy, which is usually ATP, and passive transport does not use energy.

Passive transport works by moving substances down their concentration gradient while active transport is using the energy to move things up or against their concentration gradient. There are examples of active and passive transport listed here and we will discuss each in detail. Before we talk further about the proteins involved in protein transport, we have to learn the concepts of diffusion and concentration gradients as this is what propels passive transport. Diffusion is defined as the tendency for molecules to spread out evenly into an available space.

You encounter diffusion daily though you may not even notice it. Pouring milk into coffee or spraying perfume into the air demonstrates how the substances will move from where they are more concentrated to where they are not. to where they are less concentrated.

They're spreading out evenly in their spaces over time. This diffusion requires no energy and as we'll see in a moment the gradients are a special form of energy called potential energy. Look at the image on the right which uses blue lines to follow the motion of a few molecules.

This motion is called Brownian motion which is the random motion of particles suspended in a liquid or a gas caused by the collision of those particles with water molecules at the atomic level molecules are vibrating and moving due to thermal and kinetic energy and therefore they're randomly bumping into one another however although each molecule moves randomly diffusion of a population of molecules may be directional. For example, movement tends to be away from where things are more concentrated. At dynamic equilibrium, just as many molecules are crossing the membrane in one direction as in the other, and we'll encounter the concept of equilibrium a few times in this lecture.

Concentration gradients play a big role in the movement of these molecules. A concentration gradient is the gradual difference in concentration over the distance between the regions of high and low concentration. Concentration is a mass or number of something per unit of volume. See how the image with the red dots has more dots per area in the high concentration than in the low concentration area?

Over time the molecules will move due to Brownian motion in the direction of high to low And we call this moving down the concentration gradient Again, this requires no energy input Concentration gradients are a form of potential energy which will dissipate as the gradient is eliminated the substance will move from a high concentration to a low concentration area until the concentration is equal across a space. This is a spontaneous and irreversible process which increases entropy of a system because Particles will not spontaneously reorder themselves and we'll... learn more about entropy in the next lecture. Now that we have diffusion and concentration gradients... defined, we'll cover passive...

transport and the various forms of passive transport. Let's start with a few rules for passive transport for you... to remember as we move through the rest of the lecture.

Passive transport uses no energy. Molecules always move down the concentration gradient, and both channel proteins and carrier proteins may be used in passive transport. The first type of passive transport is simple diffusion.

In simple diffusion, the molecules are able to diffuse across the plasma membrane. Of course, they are diffusing. down their concentration gradients from high concentration to low concentration and no energy is required we already talked about what type of molecules can or can't move across the plasma membrane small electrically neutral atoms are able to diffuse through by slipping between the spaces of the lipid molecules examples are oxygen gas and carbon dioxide gas Water is also able to move through via simple diffusion, although water is polar so it doesn't move very rapidly. In this image we see two sides that are separated by a permeable membrane.

We begin with a concentration gradient of blue molecules with the higher concentration in the extracellular space. In the middle panel the molecules have begun to diffuse. and they'll continue until equilibrium is reached. In the rightmost panel we see the solution has reached equilibrium and equilibrium does not mean equal concentration. Equilibrium means equal rates of diffusion.

In other words, just as many blue molecules are moving into the cell as are moving out. As I mentioned, water can also diffuse across a selectively permeable membrane and this has a special term called osmosis. Pay close attention to this explanation.

Water diffuses across a membrane from a region of lower solute concentration to a region of higher solute concentration until the solute concentrations equal on both sides. Take a look at the image on the right. These beakers are filled with water and solute. at different concentrations separated by a selectively permeable membrane. There are two components here, water and solute, each with a different concentration gradient.

The membrane is only permeable to water, yet everything wants to reach equilibrium. Because water is the only thing that can cross, it's going to want to diffuse down its own concentration gradient. which means it will move from an area of higher water concentration to lower water concentration this means it's actually moving from lower solute concentration to higher solute concentration the left and right sides of the tube begin with equal amounts of water but in equal solute concentrations which means in equal water concentrations the left has a higher water concentration than the right, so water will move from the left towards the right, which is from lower solute concentration to higher solute concentration.

The osmotic pressure is so great that the water level on the right exceeds the water level on the left. Osmosis allowed for equilibrium with similar concentration of solute on both sides of the membrane and equal rates moving across the membrane. If you've ever heard of reverse osmosis, it's a water purification process that uses a partially permeable membrane to separate out ions and unwanted molecules and larger particles from your drinking water.

Now that you know the basics of diffusion and osmosis, we can apply this to our cells. As I keep mentioning, homeostatic balance is crucial to all aspects of cellular physiology. The proper balance of water in the cell and in the body is included in that.

Tonicity is the ability of a surrounding solution to cause a cell to gain or lose water. The tonicity of that solution depends on its concentration of solutes that cannot cross the membrane relative to the inside of the cell. What this does is create a solute concentration gradient which will drive osmosis from or into the cell.

Organisms have highly adept mechanisms for controlling osmosis. and we call this osmoregulation. Osmoregulation is the control of solute concentrations and water balance. As examples, unicellular eukaryotes like paramecium are hypertonic relative to their pond water environment.

So it has a contractile vacuole that can pump water. Bacteria and archaea that live in excessively salty environments have their cellular mechanisms to ensure that they don't lose too much water to their environment. There are three types of tonic solutions, isotonic, hypertonic, and hypotonic.

Let's start with isotonic, as it's the easiest one to remember. Isotonic solutions have a solute concentration that is the same as inside the cell. Therefore, there's no net movement across the plasma membrane. The center panel shows an isotonic red blood cell which is able to maintain its proper shape and therefore function. Notice there is water diffusion into and out of the cell.

Osmosis is therefore in equilibrium here, again with no net change in movement. If you've ever had a saline drip intravenously at a hospital, that saline IV would have to be a concentration of salts and nutrients that are isotonic to your blood. Otherwise, the saline IV would create a net diffusion of water either into or out of your cells, which would either dehydrate or burst your cells. The animal cell likes isotonic environments. The next type of tonicity is a hypertonic solution.

In a hypertonic solution, the site... concentration is greater than inside the cell. So the cell loses water. The top panel shows cells in...

hypertonic solutions. When the solution has a site... concentration that's greater than that within the cell, water wants to create equal concentrations.

Therefore, water will diffuse from the area of the lower solute concentration inside the cell to the area of the higher solute concentration outside the cell. This causes the red blood cell to shrivel, a process that's termed crenation. The last type of tonicity is hypotonic.

In a hypotonic solution, the concentration of solutes is less than that inside the cell. Therefore, the cell will gain water. The bottom panel shows cells in hypotonic solutions. The concentration of solute in this solution is less than the concentration within the cell. Once again, water wants to move from an area of lower solute concentration to higher solute concentration.

Therefore, water will rush into the cell and will cause it to swell until it bursts or lices. Plant cells are a bit different because they have sturdy cell walls to help maintain water balance. Remember, plants have both a plasma membrane and a cell wall, and they like when the membrane is pressed firmly against the cell wall.

This is seen in a hypotonic environment. The plant cell is thriving in the hypotonic solution. Water rushes into the cell, but it does not burst because it has the strong cell wall to maintain its structure.

This cell is said to be turgid and is the optimal form for plant cells. This is why plants love water. Water has far less concentration of solutes and nutrients than plant cells, and therefore osmotic pressure will cause the diffusion of water into plant cells and keep them turgid. The plant cell in the middle panel is in an isotonic solution where there is no net movement into the cell.

This cell becomes flaccid or limp, which is not ideal. In the hypertonic solution, the plant has lost water and the membrane pulls away from the cell wall, causing the plant to wilt. The cell walls do not buckle, but the plasma membranes have shriveled and pulled away from the cell walls here.

This is called plasmolysis, which is potentially lethal to plants. Hypertonic solutions are detrimental to both animal and plant cells. Now you've had a complete review of simple diffusion.

The movement of molecules in diffusion is driven solely by concentration gradients. Simple diffusion is also unique in that it uses no transport proteins. It only has to do with movement of water or solutes across that selectively permeable membrane. Now we will learn the second type of passive transport called facilitated diffusion.

This type of diffusion does use proteins to aid in transport, but still does not require energy. It is still a type of diffusion after all. In facilitated diffusion, transport proteins speed the process of passive movement of molecules across the plasma membrane.

Transport proteins are the two types we discussed earlier in the lecture, channel proteins and carrier proteins. Channel proteins have a hydrophilic channel that allows charged molecules or ions to use as a tunnel. One special type of channel protein is the aquaporin which facilitates diffusion of water.

There are also basic ion channels which to facilitate the transport of ions. Carrier proteins are different. When they bind to their molecules, they change shape to shuttle them across the membrane. Transport proteins are always, always, always highly specific for the substance that they move. Let's look at channel proteins, specifically the aquaporin.

Aquaporins provide corridors for water to rapidly cross the membrane. Notice how it's a transmembrane protein with the hydrophilic domains exposed to the intracellular and extracellular fluids and a hydrophilic channel. through the membrane core that provides a hydrated opening through the hydrophobic membrane layers.

Passage through the channel allows polar compounds to avoid the plasma membrane's nonpolar inner layer that would otherwise slow down or prevent their entry into the cell. Here's a model of an aquaporin transporting water molecules across that membrane. The yellow molecule is a water molecule that's highlighted to help you follow the flow of molecules.

This movement is driven by diffusion and is passive transport. Aquaporins are most prevalent in the kidneys for water reabsorption. They help to maintain fluid homeostasis in several different tissues, including the kidney, lung, the GI tract, and brain. Here's another type of channel protein, the gated.

ion channel. Ion channels facilitate the transport of ions across the membrane. Some of these ion channels are gated, meaning they open or close in response to a particular stimulus.

Examples of stimuli are voltage and ligand binding. A prime example is in nerve cells where sodium ion channels open in response to electrical stimulus to conduct an action potential. and you'll learn more about that in physiology or in your psychology courses check your understanding and retention match the term in the left column to the correct answer in the right column pause the video then play to check your answers Now we will cover active transport.

Let's set a few rules for active transport for you to remember as we move through the lecture. Active transport requires energy from ATP hydrolysis. Molecules always move up or against a concentration gradient. All proteins involved in active transport are carrier proteins, and active transport creates important electrochemical gradients.

This is a good time to review ATP hydrolysis as this is how our cells harness energy. ATP hydrolysis is the metabolic reaction to release chemical energy in high energy bonds. ATP is adenosine triphosphate, which is an adenine attached to a ribose sugar attached to three phosphate groups.

The bonds between the phosphate groups are the phospho and hydride bonds. When ATP is broken down, we create ADP, which is adenosine diphosphate, and an inorganic phosphate. Inorganic simply means it is not attached to a molecule that contains carbon. It was an organic phosphate when attached to the ATP, but once cleaved, it is an inorganic phosphate. Hydrolysis of the terminal phosphoenhydride bond is highly exergonic meaning it releases a lot of energy the gibbs free energy is negative 34 kilojoules per mole and we'll learn what this is in the next lecture chemical energy present within the bonds of that molecule is released by splitting the bonds and harnessed for mechanical energy to do work Energy is neither created nor destroyed, so we are simply transforming one form of energy into another, chemical energy into mechanical energy.

ADP can be further hydrolyzed to give energy. AMP is created here, which is adenosine monophosphate, and we get another inorganic phosphate. We're about to get into the first type of active transport, which is electrogenic pump. But first I think it's helpful to explain what... electrogenic means.

Very simply, electrogenic pumps create electrochemical gradients. So what's an electrochemical gradient? First we have to understand the concept of membrane... potential. Remember earlier I mentioned...

that gradients are a form of potential energy. The membrane potential is the voltage across a membrane. You know what voltage is maybe related to energy or a battery. Voltage is created by difference in the distribution of positive and negative ions across a membrane. Look at the cell membrane image to illustrate this.

In this image, the yellow circles are positive potassium ions. The purple hexagons are positive sodium ions. And the green circles are negative chloride ions. All of these ions are distributed differently across the two sides of the membrane.

And overall, the intracellular space is more negative than the extracellular space. And this is true of all cells. They are more negative on the inside.

And this is voltage. When we measure neurons, they are negative 70 millivolts. There's an imbalance in both charge and the individual ion concentrations.

These two forces make up the electrochemical gradient, which drives the diffusion of ions across the membrane. The two forces are called the chemical force and the electrical force. The chemical force is the ion concentration gradient.

So for example, There is a higher concentration of sodium outside the cell than inside the cell. That is the chemical force. The electrical force is the effect of the membrane potential, the charge on the ion's movement.

So there is a higher concentration of negative ions inside the cell. So chloride is a negatively charged ion. And It would want to move down that concentration gradient if we were looking just at the electrical force.

But in reality, both of these forces, the chemical force and the electrical force, are factored into how ions will move across the membrane. And look, these two forces make up the gradient name, electrochemical gradient. So back to electrogenic pumps.

Electrogenic pumps create these electrochemical gradients. They are transport proteins that generate voltage across the membrane. They do this by using energy, ATP, to pump substances against their concentration gradient. This ability for active transport proteins to move solute up their concentration gradients allows cells to maintain concentration gradients that differ from their surroundings. The sodium-potassium pump is the major electrogenic pump in animal cells, and the proton pump is prevalent in plants, fungi, and bacteria.

Electrogenic pumps'big job is to help store energy that can be used for cellular work. Again, this would be potential energy. So the first type of active transport is the electrogenic pump. Here we have the sodium potassium pump. The sodium potassium pump uses ATP and carrier proteins to move substances against their gradients.

An animal cell has a higher potassium K plus and a much lower sodium which is Na plus. concentration compared to its surroundings. This is controlled by the sodium potassium pump. Let's walk through the basic steps of how the sodium potassium pump functions. The overall goal is to move three sodium ions out of the cell and two potassium ions into the cell.

Make sure you note that three sodium out, two potassium in. The carrier proteins begin open facing the interior of the cell. Three orange sodium ions bind. In step two, the energy molecule ATP is hydrolyzed, resulting in the transfer of that phosphate group onto the carrier protein. This activates the protein for movement.

The carrier protein has a conformational change, shape change, and releases the sodium ions to the outside. Then it's open for the two potassium ions to bind and the phosphate group is removed, resulting in the final conformational shape change as seen in step four. The process then repeats and each cycle requires the hydrolysis of one ATP molecule.

Next we have another electrogenic pump. the proton pump. Here an integral membrane protein pump builds up a proton gradient across a membrane.

In the image we see ATP being used to drive hydrogen ions to the outside where there is a higher proton concentration already and a corresponding low pH. Proton pumps are prevalent in the stomach to create an acidic environment for our digestion. And you may also find proton pumps as a part of co-transport. Co-transport is another type of active transport, and it's also called secondary active transport. Secondary active transport does not require direct ATP usage. Instead, it is the movement of material due to the electrochemical gradient that's established by primary active transport using ATP of another substance.

In this process, the diffusion of an actively transported solute down its concentration gradient is coupled with the transport of a second substance that will be going against its own concentration gradient. There are two types of proteins that are involved in co-transport. We have antiporters and symporters. Antiporters move two substances across the membrane in opposite directions and simporters move two substances across in the same direction.

The sodium-potassium pump is actually an antiporter and many amino acids and glucose actually enter the cell via co-transport. In the center image, the yellow simporter is moving both sodium and glucose into the cell in the same direction. Notice sodium's concentration gradient.

Remember the sodium gradient was created by the sodium potassium pump that we just learned about, which utilized ATP energy. It's favorable for sodium to then move down its concentration gradient into the cell via diffusion without any energy. However, glucose is more concentrated in the cell. So moving from outside to inside is a... against its gradient and would need energy.

The symporter allows glucose to take advantage of sodium's gradient and hitch a ride into the cell. So in this way, glucose. is indirectly using ATP to move into the cell, which is why this is called secondary active transport, and that is cotransport. The final type of active transport is bulk transport.

Through this lecture, we've discussed how small molecules and water enter or leave the cell through the lipid bilayer or via transport proteins. However, large molecules present a problem because they don't fit through the membrane or through transport proteins. Large molecules such as polysaccharides and polypeptide proteins cross the membrane in bulk via vesicles.

We have two basic forms, exocytosis and endocytosis. Exo means outside, so exocytosis is things exiting the cell. Well, endo means inside, so endocytosis is things entering the cell. Of course, moving large things across a membrane is an arduous endeavor, so bulk transport requires ATP energy. In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents outside the cell.

Remember, vesicles are usually coming off from the... Golgi apparatus and with some finished products after having gone through the endomembrane system. Vesicles are of course surrounded by phospholipid bilayer, so they easily fuse with the plasma membrane. Many secretory cells use exocytosis to export their products to the extracellular matrix or nearby cells or through ducts. In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane.

Endocytosis is a reversal of exocytosis involving different proteins. There are three types of endocytosis. Phagocytosis, meaning cellular eating.

Penocytosis, meaning cellular drinking. And receptor-mediated endocytosis. We learned of phagocytosis in the previous lecture when we learned about vacuoles. In phagocytosis, or cellular eating, a cell engulfs a particle or another cell in a special vacuole called a food vacuole. In penocytosis, or cellular drinking, molecules that are dissolved in droplets are taken up when extracellular fluid is gulped into tiny vesicles.

And in receptor-mediated endocytosis, binding of a specific solute to its receptor, which is sitting on the outside of the cell, will trigger vesicle formation and taking in of that triggering molecule. Human cells use receptor-mediated endocytosis to take in cholesterol. which is carried through our bloodstream in particles called LDLs or low-density lipoproteins. Individuals with the disease familial hypercholesterolemia have missing or defective LDL receptor proteins.

This leaves individuals with a higher risk of heart disease and a greater risk of early heart attack. And that will conclude this lecture on membrane transport. I know that was a lot. So I've created this schematic to help you mentally map out all that we've covered. Use it as inspiration for your own notes and add details where needed to help you learn the material.

And I will see you next lecture.

Transcript for:Plasma Membrane and Transport Processes

Transcript for:
Plasma Membrane and Transport Processes