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What do cell walls do in regards to stopping stuff coming through? They seem to stop organelles and things popping out and the cell lysing, but what about stuff like oxygen and how about big big stuff - do they mediate anything - can you tell me some more about them?

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2 Answers

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Membranes are made up of phospholipids as well as carbohydrates, proteins, and cholesterol (a lipid). These are the main things that we need to know about for year 12.

The plasma membrane forms a double layer of lipids. The heads face outwards and are hydrophilic and hence attract water to the membrane. The tails (made of two fatty acids per molecule) align inwards and are hydrophobic. This means for particles to pass through the membrane unassisted they either need to be lipophilic or very small. The phospholipid molecules float freely (although they maintain some structure) and so there are always tiny gaps in the membrane. This is called the fluid mosaic model. This is why molecules such as water, oxygen and carbon dioxide can diffuse passively across the membrane even though they are polar molecules.

Within the membrane protein channels are embedded (some protruding, some deeper within) and these can be used to facilitate diffusion of other polar molecules. Protein carriers also assist with diffusion of particles such as glucose.

Cholesterol within the membrane limits fluidity. It is polar as are the phospholipids and it bonds to them to help limit passage of molecules through the membrane. This also means it provides rigidity and helps to protect the membrane's integrity.

For very large particles such as solid food, bulk transport occurs and this needs ATP. When substance move in, they collide with the membrane and the membrane encloses them in a vesicle which pinches off and moves into the cell. When substances move out, secretory vesicles fuse with the plasma membrane and the substances are then released while the vesicle integrates with membrane.

Let me know if I missed anything :)

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Well, short answer is - Cell walls are composed of cellulose and aren't selectively permeable. They help to give plant cells extra protection from the environment. EG - insects, weather

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