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Fluid-Mosaic Model

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BIOL 202
Rob Beamish

Fluid-mosaic model  Plasma membrane consists of a phospholipid bilayer studded with proteins, polysaccharides, lipids  The lipid bilayer is semipermeable o Regulates passage of substances into and out of the cell o H2O and some small, uncharged, molecules (O2, CO2) can pass through  Phospholipids have two parts o "Head": hydrophilic → attracts and mixes with H2O o Two "fatty acid tails": hydrophobic Function of proteins  Carrier (change shape for different molecules) for water-soluble molecules such as glucose  Channels for ions (sodium and chloride ions)  Pumps use energy to move water-soluble molecules and ions  Adhesion molecules for holding cells to extracellular matrix  Receptors enable hormones and nerve transmitters to bind to specific cells  Recognition sites, which identify a cell as being of a particular type  Enzymes, which speed up chemical reactions at the edge of the membrane  Adhesion sites, which help some cells to stick together  E.g. glycoprotein acts as a receptor and recognition site Passive transport  Uses energy from moving particles (Kinetic Energy) Diffusion  Substances move down their conc. gradient until the conc. are in equilibrium  Microvilli are extensions of the plasma membrane o They increase the surface area of the membrane, therefore o They accelerate the rate of diffusion  Fick's law → rate of diffusion across an exchange surfaces (e.g. membrane, epithelium) depends on o surface area across within diffusion occurs (larger) o thickness of surface (thinner) o difference in concentration gradient (larger) o Fick’s law = (surface area x difference in conc gradient) / thickness of surface  Temperature increases rate of diffusion due to increasing K.E. (kinetic energy) Facilitate diffusion  Transmembrane proteins form a water-filled ion channel o Allows the passage of ions (Ca2+, Na+, Cl-) down their conc. gradient //passive - no ATP required o Some channels use a gate to regulate the flow of ions o Selective permeability - Not all molecules can pass through selective channels  How do molecules move across the membrane? o Substrate (molecule to move across the membrane) binds to carrier protein o Molecule changes shape o Release of the molecule (product) at the other side of the membrane  Example o If you want to move a muscle a nerve impulse is sent to this muscle o The nerve impulse triggers the release of a neurotransmitter o Binding of the neurotransmitter to specific transmembrane proteins o Opens channels that allow the passage of Na+ across the membrane o In this specific case, the result is muscle contraction o These Na+ channels can also be opened by a change in voltage Osmosis  Special term used for the diffusion of water through a differentially permeable cell membrane  Water is polar and abl
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