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Membranes160-page2 - upper legs to retain more fluidity in...

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Membrane Structure and Function - 2 Phospholipid Bilayer A phospholipid has both polar and non-polar regions. The fatty acid "tails" of the two phospholipid layers are oriented towards each other so that the hydrophilic "heads", which contain the phosphate portion, face out to the environment as well as into the cytoplasm of the cell's interior, where they form hydrogen bonds with surrounding water molecules. Because the individual phospholipid molecules are not bonded to each other, a membrane is flexible (or “fluid”), something which is pretty important to its functions. The fluidity of a membrane is crucial to its function. In caribou, circulation is reduced in the lower legs to prevent excess heat loss during cold winters. The membranes of the lower legs have more unsaturated fatty acids than those of the
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Unformatted text preview: upper legs to retain more fluidity in reduced temperatures. Brain cell membranes in ground squirrels become more solid during hibernation. Phospholipids containing more polyunsaturated fatty acids are more fluid than those with fewer polyunsaturated fatty acids. Cholesterol in membranes reduces fluidity as well. There are times when membranes need more or less fluidity, and molecular composition provides for sure membrane flexibility. Phospholipid Movement Unsaturated/Saturated With Cholesterol Many materials that enter or leave the cell are water-soluble; the fatty acid layers serve as a barrier to their free entry. Proteins in the membrane are required to move these substances through the membrane. Lipids generally pass through the membrane more easily....
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