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Transport Across Plasma Membrane

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Introduction

Transport Across Plasma Membrane The plasma membrane covers all living cells, enabling the cells' contents to be held together and controls movement of substances into and out of the cell. Plasma membranes are made of phospholipids, proteins and carbohydrates. The phospholipids are essentially made out of two fatty acid chains and a phosphate-glycerol group. They are arranged in a bilayer with the hydrophilic phosphate head facing outwards and the hydrophobic fatty acid chains facing inwards and to each other in the middle of the bilayer. This effectively provides a barrier to all but the smallest molecules. In the membrane are proteins that have a number of functions. Those that span across the entire bilayer (from hydrophobic head to hydrophobic head) act as channels and carriers to transport substances across the membrane in active or facilitated diffusion. Those on the surface act as receptors with specific binding sites where hormones and other chemicals can bind. This binding in turn triggers other events in the cell. Finally, those that are on the inside surface maintains the cell's shape and motility, such as enzymes. ...read more.

Middle

Pure water has 0 kPa, so as the concentration of solutes increases, water potential will get increasingly negative. Therefore, water will always 'move' from a low to high water potential, where 0 kPa is the highest possible water potential. Different water potentials have different effects on animal and plant cells. Since an animal cell does not possess a cell wall, it is then effectively more vulnerable to the effects osmosis. Animal cells in a hypertonic solution (higher w.p outside cell) will have water moving out of it, leaving it shriveled and crenated. Animal cells in a hypotonic solution (lower w.p. outside cell) will have water moving inwards until it eventually burst. Plant cells however, will not burst in a hypotonic solution due to the protection of its cell wall. It will instead swell and become turgid. In a hypertonic solution, the cytoplasm becomes playsmolysed. In an isotonic solution (equal w.p outside cell), all cells will stay the cell due to no net movement of water through membranes. Facilitated Diffusion This is diffusion that can only occur with the aid of trans-membrane proteins. ...read more.

Conclusion

Pumping is therefore an active process, and is the only transport mechanism that can transport substances up their concentration gradient. Endocytosis and exocytosis via vesicles. These are the only processes where larger molecules can be moved into and out of a cell. Endocytosis is the transport of substances into the cell. As the molecules gets near the cell, the membrane forms a slight dip or pit. When they are close enough, they are enclosed by a fold of the membrane, which then pinches shut to form a closed vesicle. As the vesicle moves into the cell, the molecules are digested and the product molecules released. When the materials are small and often liquid particles, endocytosis is known as pinocytosis. When the materials are large and often solid particles, the process is known as phagocytosis. Exocytosis is the transport of materials out of a cell. It is the exact reverse of endocytosis. The molecules to be transported have to be enclosed in a membrane vesicle first, usually from the Golgi apparatus and RER. It then moves to the membrane and fuses with it, forcing the substances out. These processes are controlled and require energy. ...read more.

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