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chap7studyguide - Chapter 7 Membrane Structure and Function...

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Chapter 7 Membrane Structure and Function Study Guide Overview: Life at the Edge The plasma membrane separates the living cell from its nonliving surroundings. This thin barrier, 8 nm thick, controls traffic into and out of the cell. Like all biological membranes, the plasma membrane is selectively permeable, allowing some substances to cross more easily than others. Concept 7.1 Cellular membranes are fluid mosaics of lipids and proteins The main macromolecules in membranes are lipids and proteins, but carbohydrates are also important. The most abundant lipids are phospholipids. Phospholipids and most other membrane constituents are amphipathic molecules. ° Amphipathic molecules have both hydrophobic regions and hydrophilic regions. The arrangement of phospholipids and proteins in biological membranes is described by the fluid mosaic model. Membrane models have evolved to fit new data. In 1972, S. J. Singer and G. Nicolson presented a revised model that proposed that the membrane proteins are dispersed and individually inserted into the phospholipid bilayer. ° In this fluid mosaic model, the hydrophilic regions of proteins and phospholipids are in maximum contact with water, and the hydrophobic regions are in a nonaqueous environment within the membrane. A specialized preparation technique, freeze-fracture, splits a membrane along the middle of the phospholipid bilayer. When a freeze-fracture preparation is viewed with an electron microscope, protein particles are interspersed in a smooth matrix, supporting the fluid mosaic model. Membranes are fluid. Membrane molecules are held in place by relatively weak hydrophobic interactions. Most of the lipids and some proteins drift laterally in the plane of the membrane, but rarely flip-flop from one phospholipid layer to the other. The lateral movements of phospholipids are rapid, about 2 microns per second. A phospholipid can travel the length of a typical bacterial cell in 1 second. Many larger membrane proteins drift within the phospholipid bilayer, although they
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move more slowly than the phospholipids. ° Some proteins move in a very directed manner, perhaps guided or driven by motor proteins attached to the cytoskeleton. ° Other proteins never move and are anchored to the cytoskeleton. Membrane fluidity is influenced by temperature. As temperatures cool, membranes switch from a fluid state to a solid state as the phospholipids pack more closely. Membrane fluidity is also influenced by its components. 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 at the locations of the double bonds prevent tight packing. The steroid cholesterol is wedged between phospholipid molecules in the plasma
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chap7studyguide - Chapter 7 Membrane Structure and Function...

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