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Transcript
Cell Membrane The cell membrane regulates what enters and leaves the cell and also provides protection and support. If a substance can diffuse across a membrane the membrane is said to be permeable to that substance. If a substance cannot pass across a membrane the membrane is said to be impermeable to that substance. Most membranes are selectively permeable – they allow some substances to cross, but not others. Concentration – the mass of solute in a given volume of solution. The cytoplasm of a cell is at a certain concentration. The fluid surrounding the cell is at another concentration. Diffusion – movement of particles from an area of greater concentration to an area of lesser concentration. When the particles in the two areas have moved to where both areas are at the same concentration, the environment is said to be at equilibrium. Depends on random particle movements. Done without requiring energy. Osmosis – the diffusion of water through a selectively permeable membrane. Isotonic solutions – occurs when two solutions have reached equilibrium or a solution with the same concentration as water. Hypertonic solution – a solution with a higher concentration of solute than water. Hypotonic solution – a solution with a lower concentration than water. These solutions drive osmosis because osmosis does not require energy. Osmotic Pressure – the pressure exerted on a cell membrane due to a concentration gradient (different concentrations on either side of the cell membrane.) Facilitated Diffusion – larger or highly charged molecules that would normally not pass through the lipid bilayer on their own move through protein channels and gain access to the inside of the cell. The protein channels are very specific to the molecule they allow through. Active Transport – the movement of molecules against a concentration gradient. Requires energy. Small molecules and ions are carried across by protein “pumps.” Example: Na+-K+ pump. Larger molecules and solid clumps of material are transported across the membrane by movements of the cell membrane. o Endocytosis – the process of taking material into the cell by means of infoldings or pockets of the cell membrane. The pocket then breaks loose from the membrane and forms a vacuole within the cytoplasm. o Phagocytosis – “cell eating”, extensions of cytoplasm surround a particle and package it within a food vacuole. Then the cell engulfs it. o Pinocytosis – the process of taking liquid into the cell by means of a tiny pocket formed along the cell membrane, filled with liquid and pinched off to form vacuoles within the cell. o Exocytosis – the release of large amounts of material from the cell by means of the membrane of a vacuole containing the material fusing with the cell membrane and forcing the contents out of the cell.