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Membrane Structure Biological membranes are external boundaries of cells and regulate the molecular traffic across that boundary. Their selective permeability makes them retain certain compounds and ions within specific cellular compartments while excluding others. The structure and functions of biological membranes are ascribed to their protein and polar lipid components as well as carbohydrates present as part of glycoproteins and glycolipids. Membranes are 5 to 8 nm (50 to 80 Å) thick and appear trilaminar when viewed in cross section with the electron microscope [1]. The structure of biological membrane is a typical of a fluid mosaic model as the phospholipids form a bilayer (the basic structural element of membranes) and the integral/transmembrane proteins (e.g. porins, glycophorin, bacteriorhodopsin etc.) hold strong association with lipid bilayer, held by hydrophobic interactions with their nonpolar amino acid side chains, and cannot easily become detached. While the peripheral membrane proteins hold weak interactions with the surface of the bilayer and can easily be detached from the membrane. [1, 2, 3]  Image: Biological membrane
Source: saylordotorg.github.io [5] Membrane Dynamics The flexibility i.e., the ability to change shape without losing integrity and becoming leaky is a remarkable feature of all biological membrane. This feature depends on the kinds of lipid present and change with temperature. Transbilayer movement of lipids also occurs which may be uncatalyzed diffusion of lipid molecule from one leaflet of the bilayer to the other called a flip-flop diffusion or can be facilitated by several family proteins such as flippases, floppases and scramblases. Flippases (ATP dependent) catalyze the translocation of amino phospholipids from the extracellular to the cytosolic leaflet, Floppases (ATP dependent) move plasma membrane phospholipids from the cytosolic to extracellular leaflet and Scramblases are proteins that moves any membrane phospholipids across the bilayer down its concentration gradients, activated by Ca 2+ . Membrane curvature occurs due to the presence of cavolin forcing membrane to curve inward to form caveolae, probably involved in membrane transport and signaling. This membrane curvature mediates the fusion of two membranes, which accompanies process such as endocytosis, exocytosis and viral invasion. [1, 3] Membrane Transport Few nonpolar compounds can dissolve in the lipid bilayer and cross membrane unassisted, but for transmembrane of any polar compound or ion, a membrane protein or ion channel is important. Membrane proteins aid the diffusion of solute down its concentration gradient. Transport can also occur against a gradient concentration, electrical charge or both. All process requires energy from either ATP hydrolysis or energy release by solute moving down its electrochemical gradient to move another solute up its gradient [1]. Membrane transport can be; PASSIVE TRANSPORT: this is driven by the kinetic energy of the molecules being transported or by membrane transporters by facilitate crossing e.g. simple diffusion, facilitated diffusion and osmosis. ACTIVE TRANSPORT: this depends upon the expenditure of cellular energy in the form of ATP hydrolysis. There are two major active transport; Primary active transport: solute accumulation is coupled to an exergonic chemical reaction such as the conversion of ATP to ADP + Pi Secondary active transport: here, endergonic (uphill) transport of one solute is coupled to the exergonic (downhill) flow of a different solute that was originally pumped uphill by primary active transport [1, 4]. Authors I. T. Adebayo, ScoreBooster Online Classroom J. K. Adewole, National University of Science & Technology Literature Cited 1. David L. N., Michael M. C. Lehninger Principles of Biochemistry W.H. Freeman and Company New York; Fifth Edition, 2008. 2. Biological Membrane: https://www.sciencedirect.com/topics/materials-science/biological- membrane 3. Biological Membrane: https://en.wikipedia.org/wiki/Biological_membrane 4. Biology for 4ISC: https://biology4isc.weebly.com/cell-membranes.html 5. Membrane and Membrane Lipids: https://www.google.com/url?sa=i&url=https%3A%2F%2Fsaylordotorg.github.io%2Ftext_the- basics-of-general-organic-and-biological-chemistry%2Fs20-03-membranes-and-membrane- lipids.html&psig=AOvVaw2RrydTx4w4VUwMVmaxOXn_&ust=1695219196468000&source=imag es&cd=vfe&opi=89978449&ved=0CBAQjRxqFwoTCIjE4OjttoEDFQAAAAAdAAAAABAE membranetechbrief November 2023
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