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The Transport of Substances Across the Plasma Membrane Essay

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The Transport of Substances Across the Plasma Membrane

The plasma membrane or, the cell surface membrane, is made almost entirely of protein and lipid. The plasma membrane controls the movement of substances into and out of a cell. It is partially permeable so some substances cross more easily than others.

The lipids found in the membrane are known as phospholipids. Phospholipids are fat derivatives in which one fatty acid has been replaced by a phosphate group and one of several nitrogen-containing molecules. The phospholipids’ structure is such that it appears to have a ‘head’ attached to a ‘tail’. The head section of the lipid is made of a glycerol group which is then attached to an ionised
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The plasma membrane, as well as lipids, includes several proteins; the proteins that are within the membrane are found buried or embedded into the lipid bilayer. These proteins include enzymes, receptors and antigens.

There are four methods of transportation both in and out of a cell; diffusion, facilitated diffusion, osmosis, and active transport. Fick's law is used to measure the rate of diffusion:

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Diffusion is the movement of atoms, molecules or ions from a region of high concentration to a region of low concentration (down the concentration gradient²). The energy for this to occur comes directly from the particle itself; this is defined as passive (not requiring energy). The reason for it not requiring energy from another substance is the fact that the particle is in constant movement and is able to use its kinetic energy for diffusion. Diffusion occurs in the cell membrane where the membrane is fully permeable. In the case of the plasma membrane, the membrane is permeable to non-polar substances such as steroids and glycerol, as well as oxygen and carbon dioxide in solution. Diffusion can also occur through pores in the membrane. Water diffuses through these protein lined pores or can diffuse through spaces between the phospholipid molecules. This situation is only
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