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Ap Bio Essays Cell Membrane

Cell membranes are made up of a phospholipid bilayer and various proteins. Phospholipds are amphipathic: one region is hydrophobic (the fatty acid tails) and one is hydrophilic (the cycerole/phosphate head). In an aqueous environment, they congregate into a bilayer to protect the hydrophobic tails. Peripheral and integral proteins can be embedded all around and within the bilayer in what is known as the “fluid mosaic model,” and cholesterol molecules can sit within the layer to improve its fluidity. The membrane proteins can function as enzymes, improving adhesion to neighboring cells, transporting molecules into and out of the cell (actively or passively), acting as receptors for cell signaling, and recognizing other molecules. Transport can occur by simple or facilitated diffusion via a transport protein due to osmotic pressure. Active transport occurs via a pump powered by ATP or cotransport. Endocytosis (pinocytosis and phagocytosis) and exocytosis (receptor mediated) are other methods of exchange.

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