Membranes

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.

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

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.