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The Distribution and Function of Membranes of Cells.

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The Distribution and Function of Membranes of Cells Membrane components may: Be protective: The proteins on cell membranes are very important. Some of them form junctions for transport. However, others help to identify the cell. All the cells in the body can be recognised by the proteins and glycoproteins they contain on the membrane. If you receive a transplanted organ, your immune system recognizes that the new cells do not belong to your body due to the different proteins and glycoproteins on the membrane and attempt to destroy the cells by producing antibodies. Regulate transport in and out of cell: * Diffusion - Small molecules are able to move through the membrane by diffusion. This is a process in which molecules randomly move from an area where there is a high concentration into an area where there is a low concentration along the concentration gradient. Diffusion is a slow process since there is no energy used to push the molecules, and it only works if the molecules are small enough to pass through tiny pores in the membrane. How the molecules pass through the membrane is also affected by whether they are fat-soluble or water-soluble. Some of the membrane proteins form channels that help water-soluble molecules pass through the hydrophobic lipid interior of the membrane. ...read more.


It is connected by glycerol to two fatty acid tails. One of the tails is a straight chain fatty acid (saturated). The other has a kink in the tail because of a double bond (unsaturated). The lipid bilayer gives the membranes its fluid characteristics. The picture on the left shows that at low temperatures, the bilayer is in a gel state and tightly packed. At higher (body temperatures) the bilayer actually melts and the interior is fluid allowing the lipid molecules to move around, rotate, exchange places. This also allows movement of other components of the membrane. Membrane Cholesterol Cell Membranes Another type of lipid in the membrane is cholesterol. The amount of cholesterol may vary with the type of membrane. The cholesterol molecule inserts itself in the membrane with the same shape as the phospholipid molecules. The picture below left shows phospholipid molecules with a cholesterol molecule in between. Cholesterol molecules have several functions in the membrane: * They immobilize the first few hydrocarbon groups of the phospholipid molecules. This makes the lipid bilayer more stronger and decreases its permeability to small water-soluble molecules. Without cholesterol (such as in a bacteria cell) a cell would need a cell wall. ...read more.


The molecules in the electron transport chain are found as a cluster organised in the cristae. These membrane shelves may be more numerous in mitochondria that are more active in the production of ATP so they may increase the density of these membranes as the need arises. The outer membrane of the mitochondria contains a protein. This forms an aqueous channel through which proteins can pass and go into the intermembrane space. The small molecules actually equilibrate between the outer membrane and the cytosol. However, most proteins cannot get into the matrix unless they pass through the inner membrane. This membrane contains cardiolipin that makes it virtually impermeable so it requires transport mechanisms across the membrane that are more organized and regulated. Chloroplasts Chloroplasts have a double membrane. The fluid inside this double-membrane organelle is called the stroma. The stroma is, according to the endosymbiont theory, the cytoplasm of the prokaryotic endosymbiont. Floating in the stroma are tiny membrane sacs. These are called thylakoids. The sacs are stacked in groups. Each group is called a granum. There are many grana in each chloroplast. The thylakoid membranes are the site of the photosynthetic light reactions. The thylakoids have intrinsic and extrinsic proteins, some with special prosthetic groups, allowing for electrons to be moved from protein complex to protein complex. Chaitanya Sanapala 1 ...read more.

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