Membrane Physiology PDF

## Chapter (3) ### MEMBRANE PHYSIOLOGY - Transport across the Cell Membrane - Passive Transport Mechanisms - Active Transport Mechanisms ### Transport Across Cell Membranes Cells of multicellular organisms are bathed in an internal sea of ECF (the internal environment). The composition...

## Chapter (3) ### MEMBRANE PHYSIOLOGY - Transport across the Cell Membrane - Passive Transport Mechanisms - Active Transport Mechanisms ### Transport Across Cell Membranes Cells of multicellular organisms are bathed in an internal sea of ECF (the internal environment). The composition of the intracellular fluid (ICF) differs greatly from that of ECF, and it varies somewhat depending upon the nature and function of the cell. The differences are due in large part to the nature of the barrier (the cell membrane) separating them. The exchange of materials occurs across the cell membrane (and also across the membrane of organelles) by the following mechanisms. 1. Passive transport mechanisms - Diffusion - Simple or free diffusion - Facilitated or carrier-mediated diffusion - Nonionic diffusion - Osmosis (Note: Filtration occurs across capillary walls). 2. Active transport mechanisms - Primary active transport - Primary active transport - Secondary active transport - Endocytosis - Exocytosis ### Diffusion Diffusion is the continuous random movement of molecules and ions from a region of higher concentration to a region of lower concentration along concentration or chemical gradient; or in the case of ions, to an area having the opposite charge (along the electrical gradient). The transfer of substances across the unit area in a given time is known as "flux". The flux into the cell is called the "influx" and the flux out of the cell is "efflux". The magnitude of diffusion across the cell membrane is governed by the properties of the substance (size, electrical charge and lipid solubility); the barrier itself (the permeability, the thickness and the cross-sectional area) and the force across the cell membrane (concentration or chemical gradient, electrical gradient). In other words, the net rate of diffusion (J) across the membrane is, directly proportionate to: - the area available for diffusion (A) - the diffusion coefficient of the membrane for the diffusing substance (D) - the concentration (or chemical) gradient (Ac) and indirectly proportionate to - the thickness of the membrane (diffusion path) (X) **That is, J = DA Ac / X** (for substances diffusing along the concentration gradient). This is called Fick's law of diffusion Thus, for example, net rate of diffusion of oxygen in the inspired air across the lungs (pulmonary membrane) into the blood will depend on the difference in partial pressures of O, between the air and the blood (the concentration or partial pressure gradient), the diffusing capacity of the lungs for O₂, the surface area of the pulmonary membrane, and the thickness of the pulmonary membrane. Any decrease in area available for diffusion due to destruction of parts of the lungs or any increase in thickness of the pulmonary membrane due to disease process will greatly reduce the oxygenation of the blood. ### Selective Permeability of Cell Membrane Cell membranes are impermeable to proteins and other organic anions such as phosphates. They constitute nondiffusible ions. If there is a nondiffusible ion on one side of a membrane, there is unequal distribution of diffusible ions (see the "Donnan effect"; Chapter 1). Lipid soluble and non-polar (hydrophobic) molecules such as O, and N, dissolve in the bilayer and cross with ease; small uncharged polar (hydrophilic) molecules such as CO, also diffuse rapidly across lipid bilayer. Diffusion of small charged particles (i.e. ions such as Na, K, Ca, Cl) and of large uncharged molecules such as glucose across the lipid bilayer is very slow. These substances utilize transport proteins to cross cell membranes. These transmembrane proteins are of various types. The limited permeability applies even to water, with simple diffusion being supplemented throughout the body with various water channels (aquaporins) ### Simple Diffusion (Free diffusion) Diffusion of substances across the lipid bilayer or diffusion of ions through the ion channels is called simple diffusion. ### Facilitated Diffusion (Carrier mediated diffusion) Diffusion of large uncharged molecules (such as glucose ) across the lipid bilayer becomes greatly facilitated when they are moved across the cell membrane along their chemical or electrical gradients by transport proteins called carriers (when these proteins bind to the substance to be transported, their configuration changes, moving the bound substance from one side of the membrane to the other). Transport of glucose by the glucose transporter, (GLUT) which moves the glucose down its concentration gradient from the ECF into the cytoplasm of the cell is an example of facilitated diffusion. ### Nonionic Diffusion Nonionic diffusion is the diffusion of some weak acids and bases in the undissociated (nonionic) form. If in ionic form, diffusion becomes difficult e.g. it is NH, and not NH that readily diffuses across the renal tubular cell membrane (refer to renal handling of H+ ions). ### Osmosis If 2 solutions of equal volumes but of unequal strengths are separated by a membrane permeable to the solvent (water) but not to the solute, water will move across the membrane from the side with lower concentration of the solute (i.e. with higher concentration of water) to the other side. This type of solvent movement is known as osmosis. Essentially, it is diffusion of water along its concentration gradient. The pressure necessary to apply on the more concentrated solution to prevent solvent movement is the effective osmotic pressure of the solution. Osmotic pressure depends on the number rather than the type of particles in a solution. The concentration of osmotically active particles is expressed in osmoles **Osmotic pressure = drawing force exerted by solute particles (Na, K, Ca, glucose, urea, protein!) oncotic pressure = drawing force of plasma proteins only** A hyperosmolar solution (strength stronger than that of plasma) of NaCl is also hypertonic since Nat is practically impermeable to the cell membrane (due to Na pump) and therefore exerts osmotic effect. But the hyperosmolar solution of urea is not hypertonic since urea is permeable to the cell membrane and therefore does not exert osmotic effect. Osmosis is important because it is the major mechanism by which water moves across biological membranes. ### Filtration Filtration is the process by which fluid is forced through a membrane or barrier (a capillary wall) due to the difference in hydrostatic pressure on the two sides. The amount of fluid filtered is proportionate to the difference in pressure (hydrostatic pressure gradient), the surface area and the permeability of the membrane. Molecules smaller than the pores of the membrane pass through with the fluid, and larger molecules are retained. Filtration through capillary wall is termed ultrafiltration since not only particulate matter like blood cells but also colloids like proteins are retained. **Amount of filtration = K x Pressure gradient** **Kp = filtration coefficient (area x permeability)** When the filtering membrane is very permeable, the amount of fluid flowing in one direction becomes very large (bulk flow), and the solvent tends to drag along some molecules of solute along with it. This is called solvent drag. In most situations in the body, its effects are very small but is quite appreciable in glomerular filtration where the glomerular capillaries are 50 times as permeable as those in muscle. ### Active Transport Mechanisms Active transport requires metabolic energy and the movement is usually uphill i.e. against concentration or electrical gradients. #### Primary Active Transport Active transport is carried out by "protein pumps" in the cell membranes and the energy is supplied by adenosine triphosphate (ATP) generated by the metabolism of cells ("metabolic energy"). 1. **The sodium-potassium pump (Na - K+ - activated ATPase)** - It is made up of alpha and beta subunits. The alpha subunit has a Na+ K+ activated ATPase which catalyzes the hydrolysis of ATP to ADP and uses the liberated energy to extrude 3 Nat from the cell and take 2 K+ into it (coupling ratio = 3 : 2) for each mole of ATP hydrolyzed. Thus the pump is electrogenic (i.e. it causes electrical charge difference across the cell membrane by producing net movement of a positive charge out of the cell). - When 3 Nat bind to sites on the alpha subunit accessible only from the inside of the membrane, hydrolysis of ATP by ATPase occurs and the protein changes its conformation so that 3 Nat are extruded into the ECF. In the process, phosphate released trom ATP is also bound to the alpha subunit (phosphorylation). In the second conformation of the protein, 2 K bind to sites accessible from the outside of the membrane, dephosphorylating the alpha subunit which returns to the original conformation and releases 2 K+ into the cell. - The Na* K* pump is found in almost all cells. Active transport of Na* and K* is one of the major energy-using processes in the body accounting for a large part of the basal metabolism. (Note : foon nomusiq of energy. App p. c? ) - extrudes H* from the cells in exchange for K*; found in acid - secreting cells in the gastric mucosa and the renal tubules. 2. **H - K ATPase** 3. **Ca-ATPase** - pumps Ca2+ out of cytoplasm into the endoplasmic reticulum in skeletal and cardiac muscle cells. This leads to muscle relaxation. - **SERCA pump** (Smooth ER ca2+ ) 4. **Proton-ATPases (V-ATPases)** - pump protons (H+) out of the cytoplasm into organelles such as lysosomes and parts of the Golgi complex. 5. **F-ATPases** - pump protons from mitochondrial matrix into the intracristal space, setting up the proton gradient essential for ATP synthesis. #### Secondary Active Transport When the transport of a substance is coupled to active transport of Nat, it is termed secondary active transport. The Nat - K pump maintains the Na gradient, and the energy of this gradient drives the carrier (co-transporter) which transports both that substance (either into or out of the cell) and the Na (into the cell). Metobolic energy is not used directly. The name of secondary active transport, therefore, refers to the indirect utilization of ATP as an energy source. ##### Sodium-dependent glucose transport: - **Intestine-glucose absorption** - **renal glucose reabsorption** The cell membranes of intestinal and renal tubular cells contain a cotransport protein (symport SGLT) that transports glucose into the cells only if Na* binds to it. Na moves down its electrochemical gradients and glucose also moves into the cells. The Nat gradient is maintained by the active transport of Nat out of the cells. Another examples: sodium dependent amino acid transport, and sodium dependent potassium and chloride transport in intestinal and renal tubular cells. ##### Sodium - dependent Ca** transport : - **Ceardiac and smooth muscle relaxation)** Cardiac and smooth muscle cell membranes contain an exchange protein (antiport: Ca²+ Na+ exchange) which extrudes one Ca²+ ion (against the electrochemical gradient) for three Na* ions into the cell.Na* moves down its electrochemical gradients and Ca²+ also moves out of the cells. The Na* gradient is maintained by the active transport of Nat out of the cells. The rate of this exchange is proportional to the concentration gradient for Nat across the cell membrane which in turn depends on the activity of the sodium pump. Another examples: sodium dependent H+ transport in renal tubular cells. ### Endocytosis Endocytosis is the process by which proteins and large molecules enter the cell without disruption of the cell membrane. The cell membrane folds inwards and pinches off to form a tiny sphere of membrane (called the vesicle) which encloses the ECF and the substance/s that are being transported. With endocytosis, there is loss of membrane enveloping the cell. There are 2 types: 1. **Phagocytosis (cell eating)** It is the process by which large particulate matter (i.e. not in solution in the body fluids) such as bacteria, dead tissue, or other bits of material visible under the microscope are transported into the cell. Only certain cells have capability of phagocytosis, most notably the tissue macrophages and some of the white blood cells. **Mechanism of phagocytosis:** The particle makes contact with the cell membrane which then invaginates and extends pseudopodia around the sides of the particle ("engulfment") that fuse to enclose the particle within a vacuole ("phagocytic vacuole"); usually, this vacuole fuses with lysosomes, forming a digestive vacuole. (Note: after absorption of the products of digestion in the digestive vacuole into the cytoplasm, the undigestible materials are extruded out of the cell by exocytosis. This is referred to as cellular defaecation ). 2. **Pinocytosis (cell drinking)** - It is the process by which large molecules in solution e.g. proteins, are transported into the cell. - The process involves the following steps: contact of the material with cell membrane; invagination of the cell membrane; pinching off of the invagination; and formation of pinocytic vacuole which may pass through the cell unaltered, as in the case of capillaries and the intestines, or may combine with intracellular organelles such as lysosomes to form digestive vacuoles. ### Exocytosis (Reverse Endocytosis or Cell Vomiting) Exocytosis is the extrusion process by which cellular secretions (proteins and large molecules like chylomicra) are liberated to the exterior. e.g. secretion of protein hormones and enzymes. Proteins that are secreted by cells move from the endoplasmic reticulum to the Golgi apparatus, and from the trans Golgi they are packed into secretory granules or vesicles. The granules and vesicles move to the cell membrane. Their membrane then fuses with the cell membrane and the area of fusion breaks down. This leaves the contents of the granules or vesicles outside the cell and the cell membrane intact. Exocytosis adds to the total amount of membrane enveloping the cell. The process requires Ca2+ and energy, and "docking proteins" that dock the secretory granules or vesicles to the cell membrane. Proteins may be exocytosed with little or no prior processing or storage (constitutive pathway) or after processing in the secretory granules (e.g. conversion of prohormones to mature hormones) (nonconstitutive pathway). ### Vesicular Transport (Transcytosis or Cytopempsis) Small amounts of protein are transported out of capillaries across endothelial cells by endocytosis followed by exocytosis on the interstitial side of the cells. The transport system makes use of coated vesicles and is called vesicular transport (transcytosis or cytopempsis).