Membrane Physiology - Cellular Transport PDF

Membrane Physiology & Cellular Transport Learning Outcomes Describe the characteristics of the cell membrane Describe the location and characteristics of integral and peripheral membrane proteins. Describe and compare different types of membrane transport Name the factors that...

Membrane Physiology & Cellular Transport Learning Outcomes Describe the characteristics of the cell membrane Describe the location and characteristics of integral and peripheral membrane proteins. Describe and compare different types of membrane transport Name the factors that affect substance transport through the cell membrane Describe the osmosis Explain the types of diffusion through the membrane Explain the types of active transport through the membrane Explain the differences between the diffusion and the active transport of substances through the cell membrane Explain the role of ion channels in the selective permeability of the membrane Explain how water is transported through the membrane  Cell Membrane (Plasma Membrane) The cell membrane envelops the cell and is a thin, pliable, elastic structure only 7.5 to 10 nanometers thick. It is composed almost entirely of proteins and lipids. 55% proteins, 25% phospholipids, 13% cholesterol, 4% other lipids, 3% carbohydrates. Guyton and Hall Textbook of Medical Physiology,14th ed. Cell Membrane Functions Physical Isolation A physical barrier between ECF and ICF. Regulation of exchange with the environment. Controls the entry of ions and nutrients. Communication between the Contains proteins to respond to the cell and its environment. changes in the external environment. Structural support. Cytoskeleton The structure of the cell membrane shows that it is composed mainly of a lipid bilayer of phospholipid molecules, but with large numbers of protein molecules protruding through the layer. Also, carbohydrate moieties are attached to the protein molecules on the outside of the membrane and to additional protein molecules on the inside. Guyton and Hall Textbook of Medical Physiology,14th ed.  Molecules that dissolve readily in water are said to be hydrophilic Molecules do not dissolve readily in water and are said to be hydrophobic  The basic lipid bilayer is composed of three main types of lipids— phospholipids, sphingolipids, and cholesterol. Phospholipids are the most abundant cell membrane lipids. One end of each phospholipid molecule is hydrophilic and soluble in water. The other end is hydrophobic and soluble only in fats. (Amphipathic: it has both hydrophilic and hydrophobic components) The phosphate end of the phospholipid is hydrophilic, and the fatty acid portion is hydrophobic. Because the hydrophobic portions of the phospholipid molecules are repelled by water but are mutually attracted to one another, they have a natural tendency to attach to one another in the middle of the membrane. The hydrophilic phosphate portions then constitute the two surfaces of the complete cell membrane, in contact with intracellular water on the inside of the membrane and extracellular water on the outside surface. Guyton and Hall Textbook of Medical Physiology,14th ed.  The lipid layer in the middle of the membrane is impermeable to the usual water-soluble substances: ions, glucose, urea. Conversely, it is permeable to fat-soluble substances: oxygen, carbon dioxide, alcohol Guyton and Hall Textbook of Medical Physiology,14th ed. Sphingolipids Derived from the amino alcohol sphingosine, Have hydrophobic and hydrophilic groups Present in small amounts in the cell membranes, especially nerve cells. Function: protection from harmful environmental factors, Signal transmission, adhesion sites for extracellular proteins. Merscher, Sandra & Fornoni, Alessia. (2014). Podocyte Pathology and Nephropathy – Sphingolipids in Glomerular Diseases. Frontiers in endocrinology. 5. 127. 10.3389/fendo.2014.00127.  Cholesterol They are also lipids because their steroid nuclei are highly fat-soluble. Interspersed between the phospholipid bilayer (in a sense it is dissolved) Help determine the degree of permeability (or impermeability) of the bilayer to water-soluble constituents of body fluids. Cholesterol controls much of the fluidity of the membrane as well. Integral and Peripheral Cell Membrane Proteins There are two types of cell membrane proteins, Integral proteins,, Protrude all the way through the membrane Peripheral proteins Attached only to one surface of the membrane and do not penetrate all the way through.  Integral Proteins Provides structural channels (pores) For water molecules and water-soluble substances, especially ions Have selective properties that allow preferential diffusion of some substances over others. Act as carrier proteins Transporting substances that otherwise could not penetrate the lipid bilayer. Active transport. Others act as enzymes. Serve as receptors such as peptide hormones, that do not easily penetrate the cell membrane  Peripheral Proteins Attached to integral proteins. These peripheral proteins Yang et. al, 2016. doi: 10.1016/j.chemphyslip.2016.05.003 function almost entirely as enzymes or as controllers of the transport of substances through cell membrane pores.  Membrane Carbohydrates—The Cell “Glycocalyx” Most membrane carbohydrates are sugars attached either to membrane proteins (glycoproteins) or to membrane lipids (glycolipids). They are found exclusively on the external surface of the cell, where they form a protective layer known as the Glycocalyx. Glycoproteins on the cell surface play a key role in the body’s immune response. For example, the ABO blood groups are determined by the number and composition of sugars attached to membrane sphingolipids. The carbohydrate moieties attached to the outer surface of the cell have several important functions: 1. Many of them have a negative electrical charge, which gives most cells an overall negative surface charge that repels other negatively charged objects. 2. The glycocalyx of some cells attaches to the glycocalyx of other cells, thus attaching cells to one another. 3. Many of the carbohydrates act as receptors for binding hormones, such as insulin. 4. Some carbohydrate moieties enter into immune reactions. Transport of Substances Through Cell Membranes  The extracellular fluid a large amount of sodium but only a small amount of potassium. The opposite is true of the intracellular fluid. The extracellular fluid a large amount of chloride ions, whereas the intracellular fluid contains very little of these ions. The concentrations of phosphates and proteins in the intracellular fluid are considerably greater than those in the extracellular fluid. These differences are extremely important to the life of the cell.  The membrane protein molecules interrupt the continuity of the lipid bilayer, constituting an alternative pathway through the cell membrane. Lipid Soluble Substances Many of these penetrating proteins can function as transport proteins. Some proteins allow free movement of water, and selected ions or molecules; these proteins are called channel proteins. Carrier proteins, bind with molecules or ions that are to be transported, and conformational changes in the protein molecules then move the substances through the interstices of the protein to the other side of the membrane. Channel proteins and carrier proteins are selective for the types of molecules or ions that are allowed to cross the membrane  Membrane Transport Mechanisms Transport through the cell membrane, either directly through the lipid bilayer or through the proteins, occurs via: Diffusion Random molecular movement of substances molecule by molecule, either through intermolecular spaces in the membrane or in combination with a carrier protein. Passive process, Movement of solutes from regions of higher concentration to regions of lower concentration (downhill) The driving force comes from the kinetic energy of the matter itself. Active transport Movement of ions or other substances across the membrane in combination with a carrier protein in such a way that the carrier protein causes the substance to move against an energy gradient, such as from a low-concentration state to a high-concentration state. Requires an additional source of energy besides kinetic energy.  Diffusion All molecules and ions in the body fluids, including water molecules and dissolved substances, are in constant motion, with each particle moving in its separate way. Motion of these particles is what physicists call “heat”—the greater the motion, the higher the temperature— and the motion never ceases under any condition except at absolute zero temperature A single molecule in a solution bounces among the other molecules—first in one direction, then another, then another, and so forth— randomly bouncing thousands of times each second. This continual movement of molecules among one another in liquids or gases is called diffusion. Diffusion Through The Cell Membrane Simple Diffusion: Simple diffusion means the kinetic movement of molecules or ions occurs through a membrane opening or through intermolecular spaces without interaction with carrier proteins in the membrane. The rate of diffusion is determined by the amount of substance available, the velocity of kinetic motion, the number and sizes of openings in the membrane through which the molecules or ions can move. Facilitated Diffusion: Requires the interaction of a carrier protein. The carrier protein aids the passage of molecules or ions through the membrane by binding chemically with them and shuttling them through the membrane in this form. Direction of Diffusion Diffusion does not occur unidirectionally. Since molecules are in constant movement, limited diffusion occurs also from low to high concentration. Diffusion of glucose between two compartments of equal volume separated by a barrier permeable to glucose. Initially, time A, compartment 1 contains glucose at a concentration of 20 mmol/L, and no glucose is present in compartment 2. At time B, some glucose molecules have moved into compartment 2, and some of these are moving back into compartment 1. The length of the arrows represents the magnitudes of the one-way movements. At time C, diffusion equilibrium has been reached, the concentrations of glucose are equal in the two compartments (10 mmol/l), and the net movement is zero. In the graph at the bottom of the figure, the blue line represents the concentration in compartment 1 (C1), and the orange line represents the concentration in compartment 2 (C2).  Direction of Diffusion The net flux of glucose between the two compartments at any instant is the difference between the two one-way fluxes. It is the net flux that determines the net gain of molecules by compartment 2 and the net loss from compartment 1. Eventually the concentrations of glucose in the two compartments become equal at 10 mmol/L. The two one-way fluxes are then equal in magnitude but opposite in direction, and the net flux of glucose is zero. The system has now reached diffusion equilibrium. No further change in the glucose concentration of the two compartments will occur, since equal numbers of glucose molecules will continue to diffuse in both directions between the two compartments.  Direction of Diffusion The net flux is the most important component in diffusion since it is the net amount of material transferred from one location to another. Although the movement of individual mole cules is random, the net flux always proceeds from regions of higher concentration to regions of lower concentration. Diffusion of Lipid-Soluble Substances Through the Lipid Bilayer The lipid solubilities of oxygen, nitrogen, carbon dioxide, and alcohol are high, and all these substances can dissolve directly in the lipid bilayer and diffuse through the cell membrane in the same manner that diffusion of water solutes occurs in a watery solution. Diffusion of Water and Other Lipid-Insoluble Molecules Through Protein Channels Even though water is highly insoluble in the membrane lipids, it readily passes through channels in protein molecules that penetrate all the way through the membrane. Many of the body’s cell membranes contain protein “pores” called aquaporins that selectively permit rapid passage of water through the membrane. The aquaporins are highly specialized, and there are at least 13 different types in various cells of mammals. Other lipid-insoluble molecules can pass through the protein pore channels in the same way as water molecules if they are water-soluble and small enough. Selective Permeability And “Gating” Of Channels The protein channels have two important characteristics: 1. They are often selectively permeable to certain substances; 2. Many of the channels can be opened or closed by gates that are regulated by electrical signals (voltage-gated channels) or chemicals that bind to the channel proteins (ligand-gated channels). Thus, ion channels are flexible dynamic structures, and subtle conformational changes influence gating and ion selectivity. Selective Permeability of Protein Channels Many protein channels are highly selective for the transport of one or more specific ions or molecules. This selectivity results from specific characteristics of the channel, such as its diameter, shape, and the nature of the electrical charges and chemical bonds along its inside surfaces.  Potassium Channels Permit passage of potassium ions across the cell membrane about 1000 times more readily than they permit passage of sodium ions. A tetrameric structure consisting of four identical protein subunits surrounding a central pore was obtained. At the top of the channel pore are pore loops that form a narrow selectivity filter. Lining the selectivity filter are carbonyl oxygens. When hydrated potassium ions enter the selectivity filter, they interact with the carbonyl oxygens and shed most of their bound water molecules, permitting the dehydrated potassium ions to pass through the channel  Sodium Channels 0.3 to 0.5 nanometer in diameter. The narrowest part of the sodium channel’s open pore, the selectivity filter, is lined with strongly negatively charged amino acid residues. These strong negative charges can pull small dehydrated sodium ions away from their hydrating water molecules into these channels, although the ions do not need to be fully dehydrated to pass through the channels. Once in the channel, the sodium ions diffuse in either direction according to the usual laws of diffusion. Thus, the sodium channel is highly selective for the passage of sodium ions. Gating of Protein Channels Gating of protein channels provides a means of controlling the ion permeability of the channels. The opening and closing of gates are controlled in two principal ways: Voltage Gated Chemical (Ligand Gated)  Voltage Gated In the case of voltage gating, the molecular conformation of the gate or its chemical bonds responds to the electrical potential across the cell membrane. When there is a strong negative (-) charge on the inside of the cell membrane, this presumably could cause the outside sodium gates to remain tightly closed; Conversely, when the inside of the membrane loses its (-) charge, these gates would open suddenly and allow tremendous quantities of sodium to pass inward through the sodium pores  Chemical (ligand) Gating Some protein channel gates are opened by the binding of a chemical substance (a ligand) with the protein; This causes a conformational change in the protein that opens /closes the gate This is called chemical gating or ligand gating Acetylcholine channel Acetylcholine (Ach) opens the gate of this channel, providing a negatively charged pore about 0.65 nanometer in diameter that allows uncharged molecules or positive ions smaller than this diameter to pass through This gate is exceedingly important for the transmission of nerve signals from one nerve cell to another and from nerve cells to muscle cells to cause muscle contraction  Facilitated Diffusion Requires membrane carrier proteins That is, the carrier facilitates the diffusion of the substance to the other side In this type of diffusion, substances are transported from the membrane in the direction of concentration gradient However, transport is mediated by a carrier molecule Charged or large particles Most glucose and amino acids are transported by facilitating diffusion  Facilitated Diffusion Facilitated diffusion differs from simple diffusion in the following important way: Although the rate of simple diffusion through an open channel increases proportionately with the concentration of the diffusing substance, in facilitated diffusion the rate of diffusion approaches a maximum, called Vmax, as the concentration of the substance increases the facilitated diffusion rate does not increase more than Vmax. Effect of concentration of a substance on the rate of diffusion through a membrane by simple diffusion and facilitated diffusion. This graph shows that facilitated diffusion approaches a maximum rate, called the Vmax. Guyton and Hall Textbook of Medical Physiology,14th ed.  What is it that limits the rate of facilitated diffusion? The rate at which molecules can be transported by this mechanism can never be greater than the rate at which the carrier protein molecule can undergo change back and forth between its two states. This mechanism allows the transported molecule to move—that is, diffuse—in either direction through the membrane. Carrier protein with a pore large enough to transport a specific molecule partway through. It also shows a binding receptor on the inside of the protein carrier. The molecule to be transported enters the pore and becomes bound. Then, in a fraction of a second, a conformational or chemical change occurs in the carrier protein, so that the pore now opens to the opposite side of the membrane. Because the binding force of the receptor is weak, the thermal motion of the attached molecule causes it to break away and be released on the opposite side of the membrane.  Factors That Affect net Rate of Diffusion What is important is the net rate of diffusion of a substance in the desired direction. This net rate is determined by several factors Effect of concentration difference on net diffusion through a membrane Effect of membrane electrical potential on diffusion of ions—The “Nernst Potential.” Effect of a pressure difference across the membrane 1-Effect of Concentration Difference: Figure A in the right shows a cell membrane with a high concentration of a substance on the outside and a low concentration of a substance on the inside The rate of net diffusion into the cell is proportional to the concentration on the outside minus the concentration on the inside in which C o is the concentration outside and C i is the Effect of concentration difference ( A ), concentration inside the cell. electrical potential difference affecting negative ions ( B ), and pressure difference ( C ) to cause diffusion of molecules and ions through a cell membrane. C o , concentration outside the cell; C i , concentration inside the cell; P 1 pressure 1; P 2 pressure 2 Guyton and Hall Textbook of Medical Physiology,14th ed.  Membrane Electrical Potential and Diffusion of Ions—The “Nernst Potential.” If an electrical potential is applied across the membrane, the ions pass through the membrane due to their charge even though there is no concentration difference. In this case, while (-) ions are equal on both sides of the membrane, when a positive charge is applied to the right side of the membrane, an electrical difference is created Positive charges attract negative charges. Hence the net diffusion from left to right. After a while, the concentration difference will develop for this ion in the opposite direction of the electrical potential difference. At this moment, the concentration difference forces the ion to move to the left, while the electrical difference pushes it to the right. If the difference in concentration is high enough, the two effects balance each other.  At normal body temperature (37°C), the electrical difference that will balance a given concentration difference of univalent ions—such as Na+ ions—can be determined from the following formula, called the Nernst equation: in which EMF is the electromotive force (voltage) between side 1 and side 2 of the membrane, C 1 is the concentration on side 1, and C 2 is the concentration on side 2. This equation is extremely important in understanding the transmission of nerve impulses  Effect of a Pressure Difference Across the Membrane Occasionally, a significant difference in pressure develops between the two sides of a permeable membrane. For example, this condition occurs in the capillary membrane. There is a pressure difference of 20 mm Hg on the inside of the capillaries with respect to the outside. Pressure is the sum of the forces of different molecules striking a unit area at a given time. In most instances, more molecules hit one side of this membrane than the other. As a result, molecules move from the high pressure side to the low one.  Osmosis Across selectively permeable membranes—“Net Diffusion” of Water the most abundant substance that diffuses through the cell membrane is water. Enough water ordinarily diffuses in each direction through the red blood cell membrane per second to equal about 100 times the volume of the cell itself Yet, normally, the amount that diffuses in the two directions is balanced so precisely that zero net movement of water occurs Therefore, the volume of the cell remains constant For osmosis: -The amount of solute on both sides of the membrane should be different -Membrane should not be permeable to the solute  Osmosis If concentration differences for water exists, net movement of water does occur across the cell membrane, causing the cell either to swell or to shrink, depending on the direction of the water movement. This process of net movement of water caused by a concentration difference of water is called osmosis  Osmosis Water molecules easily pass through the cell membrane, whereas Na and Cl ions pass through with difficulty The concentration of water molecules on the side of NaCl ions will be lower than pure water As a result, on the left side with pure water, the number of water molecules will be greater than on the right Thus, net water movement from left to right occurs, osmosis is seen from pure water to NaCl solution Osmosis at a cell membrane when a sodium chloride solution is placed on one side of the membrane and water is placed on the other side  Tonicity: The ability of an extracellular solution to make water move into or out of a cell by osmosis Isotonic: equal solute concentration of solution, relative to the inside of the cell, cell is healty Hypotonic: Low solute concentration of solution, relative to the inside of the cell, water enters the cell → cell swells Net water entry into erythrocytes → hemolysis Hypertonic: High solute concentration of solution, relative to the inside of the cell, water leaves the cell → cell shrinks  Active Transport Moves solutes against their electrochemical gradient (uphill) across the membrane When a cell membrane moves molecules or ions uphill against a concentration gradient (or uphill against an electrical or pressure gradient), the process is called active transport. It is a very important event for life. Because many substances must be taken into the cell (K+) despite high intracellular concentrations or taken out of the cell (Na+) despite high extracellular concentrations Sodium, potassium, calcium, iron, hydrogen, chloride, iodide, urate ions, various sugars and amino acids are actively transported through cell membranes.  Some substances such as K+ ions must be kept at high levels inside the cell. Conversely, some other ions such as Na+ must be kept in high concentrations outside the cell. Neither of these two effects can occur by simple diffusion. If ions move against the concentration gradient across the cell membrane, this process is called active transport.  Primary Active Transport and Secondary Active Transport Active transport is divided into two types, primary and secondary active transport, according to the source of energy used in transport In primary active transport, the energy is derived directly from the breakdown of adenosine triphosphate (ATP) In secondary active transport, The electrochemical gradients established by primary active transport store energy, which can be released as the ions move back down their gradients. Secondary active transport uses the energy stored in these gradients to move other substances against their own gradients. In both cases, transport depends on carrier proteins penetrated through the cell membrane.  Primary Active Transport Sodium-Potassium Pump: Sodium-Potassium Pump transports Na+ ions out of cells and K+ ions into cells Na+, K +, Ca +2, H + and Cl- are transported by primary active transport The Na+ -K+ pump transports Na+ ions out of the cell, K+ ions into the cell This pump is responsible for maintaining the sodium and potassium concentration differences across the cell membrane Carrier protein consists of two protein α- subunit (large) β- lower unit (small) α- subunit has three specific features 1. It has 3 binding sites for Na+ ions on the portion of the protein that protrudes to the inside of the cell. 2. It has 2 binding sites for K+ ions on the outside 3. The inside portion of this protein near the sodium binding sites has adenosine triphosphatase (ATPase) activity https://www.youtube.com/watch?v=xweYA-IJTqs&t=6s  The Na+-K+ Pump Is Important for Controlling Cell Volume. One of the most important functions of the Na+-K+ pump is to control the cell volume. Without function of this pump, most cells of the body would swell until they burst.  Secondary Active Transport When Na+ ions are transported out of the cell by primary active transport a large concentration gradient develops for Na+ with a high concentration outside the cell and a low concentration inside This gradient represents a storehouse of energy, because the excess Na+ outside the cell is always attempting to diffuse to the interior Under appropriate conditions, this diffusion energy of Na+ can pull other substances along with Na+ through the cell membrane Postulated mechanism for sodium co- transport of glucose. This phenomenon, called co-transport, is one form of secondary active transport Active transport is also divided into 1. Uniport: Only one substance and ion are transported (Hydrogen) 2. Cotransport: Two different substances are transported simultaneously and in the same direction or in the opposite direction (glucose, amino acids). is divided into two: Antiport: Two different substances are transported at the same time but in different directions (Na, Ca) Simport: Two different substances move in the same direction  Na+-K+ ATPase antiporter: Contrary to the electrochemical gradient in the cell membrane, Na and K ions are transported in the opposite direction. Ca2+ -ATPase uniporter: Transport of Ca+2 ions in the membrane in the opposite direction to the gradient Sodium counter-transport of calcium and H+-K+ ATPase simporter: hydrogen ions Transport of K+ and H+ ions in the parietal cell membranes of the stomach Textbooks Hall, J. E. (2021). Guyton and hall textbook of medical physiology (14th ed.). W B Saunders.. Boron, W.F. and Boupaep, E.L. (2016) Medical Physiology. 3rd Edition, Elsevier Publisher, Philadelphia. Barrett K.E., & Barman S.M., & Brooks H.L., & Yuan J.J.(Eds.), (2019). Ganong's Review of Medical Physiology, 26e. McGraw Hill. Widmaier, Eric P. (2019). Vander, Sherman, & Luciano's human physiology : the mechanisms of body function. Boston :McGraw-Hill Higher Education.