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The Transport of Substances across the Plasma Membrane

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Introduction

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 Phospholipids are fat derivatives in which one fatty acid has been replaced by a phosphate group and one of several nitrogen-containing molecules.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 phosphate group that is the tail. This gives the 'head' region of the lipid to have hydrophilic properties (water loving), where the 'tail' section has hydrophobic properties (water hating). As a result of this contrast between the two regions, when in a water environment, the 'head' region will face towards the water while the 'tail' region will be pointing away from the water. ...read more.

Middle

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 available to those molecules, however, that are a 'regulation' size; meaning that the particles must be small enough to have the ability to travel through the small pores and thus diffuse. In situations where a charged particle or large molecules are to be transported across the membrane which could be soluble or insoluble in lipids and is not able to diffuse through normal diffusion, molecules are able to do so through another form of diffusion called facilitated diffusion. ...read more.

Conclusion

Because molecules are moved against the concentration gradient, it requires energy. It is thought that active transport uses carrier proteins similar to those involved in facilitated diffusion. If very large molecules or groups of molecules need to enter or exit a cell, they do so using vesicles. The material to be transported out of the cell is surrounded by membrane. The vesicle will fuse with the cell surface membrane and the contents leave. This is called exocytosis. Materials entering the cell can do so when the plasma membrane encloses and thus surrounds the material. The membrane seals off to form a vesicle, which can then move into the cell. This is endocytosis. The transportation of substances through the plasma membrane is very specific and the procedures that facilitate diffusion are what allow the different types of d transportation to take place. The transportation of substances is fundamental to life processes and therefore to existence. ?? ?? ?? ?? Keshan Bolaky ...read more.

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