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| allowing some substances to cross more easily than others |
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| lipids(most abundant being phospholipids), proteins, some carbs, |
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| both hydrophobic and hydrophilic regions (e.g. phospholipids in the cell membrane) |
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| In 1895, Charles Overton hypothesized that membranes are made of lipds because substances that dissolve in lipids enter cells faster than those that are insoluble. |
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| In 1925, using artificial membranes, Gorter and F. Grendel reasoned that cell membranes must be a phospholipid bilayer, two molecules thick. |
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| hydrophobic fatty acid tails are sheltered from water while the hydrophilic phosphate groups interact with water |
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| In 1935, Davson and Danielli proposed a sandwich model in which the phospholipid bilayer lies between two layers of globular proteins. |
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| In 1972, Singer and Nicholson presented a revised model, the fluid mosaic model, which proposed that the membrane ptns are dispersed and individually inserted into the phospholipid bilayer |
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| what influences membrane fluidity |
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| temperature, cholesterol (warm temp cholesterol reduces fluidity, cool temp cholesterol maintains fluidity) and phospholipid concentration (Membranes rich in unsaturated fatty acids are more fluid that those dominated by saturated fatty acids because the kinks in the unsaturated fatty acid tails prevent tight packing) |
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| not embedded in the lipid bilayer, often connected to the integral proteins |
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| penetrate the hydrophobic core of the lipid bilayer |
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| what does permeability depend on? |
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the chemical properties of the molecule passing through the membrane
Hydrophobic & non-polar molecules dissolve in the lipid bilayer and cross easily.
Hydrophilic, polar molecules, and ions do not cross as easily. They require transport proteins for assistance: |
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| diffusion: driven by kinetic energy of molecules, net movement of a solute across a membrane/barrier |
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| passive transport of water |
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| higher concentrations of solute |
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| solution with lower concentrations of solute |
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| equal solute concentrations |
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| fuctions as a bilge pump to force water out of a cell |
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| a plant in a hypotonic solution will swell until the elastic wall opposes further uptake |
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| too much water in a cell causing somewhat of an explosion |
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| If a plant cell and its surroundings are isotonic, there is no movement of water into the cell and the cell is flaccid and the plant may wilt (bad!) |
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| The passive movement of molecules down the concentration gradient via a transport protein |
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| have specific binding sites, can become saturated,catalyze a physical process, two types :Channel and Carrier proteins |
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| provide corridors allowing a specific molecule or ion to cross the membrane |
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alternate between two conformations moving a solute across the membrane as the protein changes shape
can move things against their concentration gradient |
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| involves the movement of solutes against their concentration gradient |
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| when a transport vesicle buds from the golgi apparatus is moved by the cytoskeleton into the plasma membrane |
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| a cell brings in macromolecules and particulate matter by forming new vesicles from the plasma membrane |
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