03_membranes and transport

03_membranes and transport - Membranes and Membrane...

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Membranes and Membrane Transport -1- MEMBRANES and MEMBRANE TRANSPORT CONCEPTS CHECK LIST MEMBRANES The plasma membrane is a selectively permeable barrier that separates a cell’s cytoplasm from the external environment. The basic structure of a biological membrane is a fluid phospholipid bilayer with associated proteins. Because of the selectively permeable nature of membranes, gradients of concentration, electrical charge, or both can be formed and used to do work such as active transport. Membranes are interfaces at which many of a cell’s activities occur. Our current model of membranes is the fluid mosaic model of Singer and Nicholson (1972) with relatively few modifications. The two leaflets of a membrane are asymmetrical with respect to their distribution of lipids, peripheral proteins, and integral proteins. The oligosaccharide side chains of membrane glycolipids and glycoproteins are found almost exclusively at the noncytosolic face (e-face) since they become attached within the ER or Golgi The orientation of membrane leaflets is maintained after membrane fusion events. Membranes must be fluid-like (above their phase transition temperature) to function. Membrane fluidity is required for adequate rates of simple diffusion and many other functions of membranes in which its molecules must undergo a change in shape. The fluidity of a membrane is affected by its lipid composition and temperature. The hydrophobic core of membranes makes them very permeable to hydrophobic molecules, less permeable to hydrophilic molecules, and virtually impermeable to charged molecules of any type. SIMPLE DIFFUSION The net direction in which a molecule moves across a lipid bilayer by simple diffusion is from the side of highest concentration to the side of lowest concentration and its rate of passage is determined in part by the strength of the concentration gradient. At equilibrium, molecules that can move across a lipid bilayer by simple diffusion continue to do so, but there is not net passage across the bilayer. Simple diffusion exhibits nonsaturable, linear kinetics. During osmosis, the net passage of water across a lipid bilayer is in a direction against the solute concentration gradient. During osmosis, the net passage of water across a lipid bilayer occurs toward the compartment with the highest osmotic pressure. The osmotic pressure of pure water equals zero. The osmolarity of a solution refers to its osmotic effects on an ideal or nonbiological membrane. The tonicity of a solution refers to its osmotic effects on a biological membrane. Most animal cells require an isotonic environment to thrive, whereas most plant cells require a hypotonic environment to thrive.
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Membranes and Membrane Transport -2- PASSIVE TRANSPORT The passive transport of a molecule across a membrane does not require an additional input of energy but does require the assistance of a membrane protein: either a transporter protein or a channel protein.
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This note was uploaded on 01/15/2011 for the course BIOL 190A taught by Professor Dr.davidcreasey during the Fall '09 term at University of Victoria.

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03_membranes and transport - Membranes and Membrane...

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