Electron micrograph of the membranes of two adjacent cells 25 nm
1. Biomembranes are organized, sheetlike assemblies consisting mainly of proteins and lipids 1.1 The functions carried out by membranes are diverse and indispensable for life. 1.1.1 Biomembranes form boundaries around the cell and aroud distinct subcellular compartments. 1.1.2 Biomembranes are highly selective permeability barriers with specific protein channels and pumps regulating the molecular and ionic compositions of the intracellular medium (transport across membranes).
1.1.3 Biomembranes control the flow of information between cells and their environment through specific receptors on the plasma membrane (signal transduction). 1.1.4 Eukaryotic cells have an extensive internal membrane system dividing cells into various compartments (forming different organelles).
1.1.5 The two most important energy conversion processes, photosynthesis (occurring in the inner membranes of chloroplasts) and oxidative phosphorylation (ocurring in the inner membranes of mitochondria) are carried out by membrane systems. 1.1.6 Certain biosynthesis (e.g., synthesis of lipids and some proteins) occur on biomembranes.
1.2 Membranes consist mainly of lipids and proteins with mass ratio ranges between 1:4 to 4:1 (membranes with different functions have different proteins, some lipids and proteins have covalently linked carbohydrates). 1.2.1In membranes the three major classes of lipids are the glycerophospholipids ( 甘油磷脂）, the sphingolipids （鞘磷脂） and the sterols （ 固醇）.
(1)The glycerophospholipids ( 甘油磷脂） have a glycerol backbone that is attached to two fatty acid hydrocarbon chains and a phosphorylated head group. These include ： phosphatidate ( 磷脂酸） phosphatidylcholine （ 磷脂酰胆碱 ） phosphatidylethanolamine （磷脂酰胆胺） phosphatidylglycerol （磷脂酰甘油） phosphatidylinositol （ 磷脂酰肌醇） diphosphatidylglycerl （ 二磷脂酰甘油 ）
1.2.4 Membranes are cooperative noncovalent assemblies. 1.2.5 Membranes are always asymmetric with two different faces. 1.2.6 Membranes are fluid two-dimensional structures (the fluid mosaic model) with oriented proteins and lipids (which form bilayer structures).
1.3 Proteins attach to membranes in different ways. 1.3.1 Some proteins, called integral proteins, span the lipid bilayer. 1.3.2 Some proteins, called peripheral proteins, are bound to membranes loosely and reversibly. 1.3.3 Sugar residues are found only on the extracellular side of the plasma membrane attached either to lipids to form glycolipids or to proteins to form glycoproteins.
2. Proteins facilitate ions and solutes to move across the hydrophobic membranes in various ways.(E3 Membrane transport: small molecules) 2.1 Simple diffusion of ions and polar molecules in living organisms is impeded by selectively permeable biomembranes. 2.1.1 Only relatively nonpolar molecules (like O 2, N 2 ) cross biomembranes by simple diffusion (i.e., move from higher concentration area to lower one until they become evenly distributed). 2.1.2 Water, though polar, diffuses rapidly across biomembranes by mechanisms not fully understood. High concentration (55M) may be the reason.
2.2 Most ions and polar solutes move across biomembranes by carrier-mediated transport. 2.2.1 Carriers are usually proteins (also called pumps (active), transporters or permeases （ 透 ( 性 ) 酶 ） (passive)). 2.2.2 Carrier proteins are similar to enzymes, lowering the activation energy of simple diffusion process, by providing an alternative hydrophilic transmembrane pathway.
2.3 In active transport, solutes move against the concentration gradient (uphill) resulting in the accumulation of a solute on one side of a membrane. 2.3.1 Active transport occurs only when an energy source (exergonic process) is coupled.
2.3.2 In primary active transport, energy is provided directly by the hydrolysis of ATP (as with the Na + -K + ATPase on vertebrate plasma membranes), by electrons flowing down an electron transport system (as with H + pumping out of the mitochondria inner membranes), or by absorption of sunlight (as with the light-driven H + pumping of bacteriorhodopsin in halobacterium). Directly coupled to an energy source or a chemical reaction.
2.3.3 In secondary active transport, ion gradients across the membrane are used to drive the concentration uptake of other ions or metabolites. 2.3.4 Many cells contain secondary transport systems that couple the spontaneous, downhill flow of H + or Na + to the simultaneous uphill pumping of another ion, sugars, or amino acids.
2.4 Na + -K + ATPase is responsible to maintain the Na + and K + gradients across animal cell plasma membranes. 2.4.1 Most animal cells have a high concentration of K + (145 mM) and a low one of Na + (5 mM) relative the external medium (K +, 5mM; Na +, 150 mM?). (fig.) 2.4.2 The Na + -K + gradient in animal cells controls cell volume, renders nerve and muscle cells electrically excitable, and drives the active transport of sugars and amino acids.
3. Membrane fusion is central to many biological processes (E4 Membrane transport: macromolecules) Exocytosis Endocytosis Phagocytosis Pinocytosis Receptor-mediated endocytosis Clathrin-coated pits and vesicles
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