Membranes

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  • Created by: alex
  • Created on: 30-04-11 16:15

Diffusion & Osmosis

Diffusion is the net moevement of particles from an area of higher concenration to an area of lower concentration.

 Osmosis is the dissusion of water molecules across a partially permeable membrane.

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The rate of  depends on:

  • the steepness of the concentration gradient
  • the thickness of the exchange surface
  • the surface area of the exchange surface

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Cells filled with water become turgid.

Cells losing water become flacid.

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Facilitated Diffusion & Active Transport

Some larger molecules can't diffuse directly through the bilayer of the cell membrane.

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Facilitated Diffusion 

  • a large molecule attaches to a carrier protein in the membrane
  • the protein changes shape
  • the molecule is released on the other side of the membrane
  • or
  • channel proteins form pores in the membrane for different molecules to dissuse down. 

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 Active Transport

  • a large molecule attaches to a carrier protein in the membrane
  • the protein changes shape
  • the molecule is released on the other side of the membrane
  • but
  • the process is moving against the concentration gradient 
  • therefor ATP is used for energy
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Endocytosis & Exocytosis

Some molecules are too large to be taken in or leave a cell through the bilayer or proteins.

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Endocytosis

  • a cell can surround a substance with a section of its plasma membrane
  • the membrane then pinches off to form a vesicle inside the cell containing the ingested substance

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Exocytosis

  • vesicles containing these substances pinch of from the sacs of the golgi apparatus and move towards to the membrane
  • the vesicles fuse with the plasma membrane and release their contents outside the cell.
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