BIL 150 notes exam 2 - Membrane structure and function The...

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Membrane structure and function The plasma membrane is the boundary that separates the living cell from its surroundings Membranes contain phospholipids, proteins, glycol (sugar) groups, and cholesterol. Phospholipids are the most abundant lipid in the plasma membrane Cellular membranes are fluid mosaics of lipids and proteins The fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in a sea of lipids The fluidity of membranes Phospholipids in the plasma membrane can move within the bilayer (near neighbor exchange: 10 7 /sec) Most of the lipids, and some proteins, drift laterally Rarely does a molecule flip-flop transversely across the membrane (usually catalyzed) Once per month The steroid cholesterol has different effects on membrane fluidity at different temperatures At warm temperatures (such as 37 C),
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At Cold temperatures, SEARCH FOR THIS Roles of membrane proteins Transport by forming channels through which molecules can get through or by pumping them through using ATP Enzymatic activity Signal transduction and sending signals into the cell or a change in the biochemistry of the cell Cell-cell recognition Intercellular joining Attachment to the cytoskeleton and extracellular matrix (ECM) Role of membrane carbohydrates in cell-cell recognition Cells recognize each other by binding to membrane molecules, often carbohydrates Membrane carbohydrates may be covalently bonded to lipids (forming glycolipids) or to proteins (forming glycoprotein) Membrane structure results in selective permeability Plasma membranes are selectively permeable, regulating the cell’s molecular traffic Permeability of lipid bilayer
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Hydrophobic (nonpolar) molecules can pass through the membrance rapidly Active vs. passive transport Passive transport requires no energy, but may involve proteins. It allows atoms and molecules to diffuse down their concentration gradients Diffusion is the tendency for molecules to spread out evenly into the available space Dynamic equilibrium, rate of transfer is equal Substances diffuse down their concentration gradients by random, thermal motion Effects of osmosis on water balance Osmosis is the diffusion of water across a membrane Water diffuses from the low concentration of solute (salt, chemical) to the side of high concentration until it reaches equilibrium Active transport requires energy and can pump atoms and molecules up their concentration gradients Osmotic pressure is that pressure at which the flow of water stops Water balance of cells
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Hypotonic solution: solute conc. Is less than that inside the cell; cell gains water Isotonic solution: solute conc. Is the same no net movement Hypertonic solution: solute conc. (outside of cell) greater than inside; cell loses water Diffusion of charged particles Side 1: water plus KCl Side 2: water Both K+ and Cl- permeable.
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