Cell Membrane

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Cell Surface Membranes

What do they do? - They control which substances enter and leave the cell.

-They are partially permeable

-Substances move across by either : OSMOSIS

                                                           DIFFUSION

                                           ACTIVE TRANSPORT 

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Membrane Structure

What are they made of? : Lipids, proteins and carbohydrates.

-A common way to describe membrane structure is the fluid mosaic model.

-The bilayer includes channel proteins and carrier proteins, which allow large molecules and ions to pass through.

RECEPTOR PROTEINS : Allow cell to detect chemicals released from other cells.

GLYCOPROTEINS : Proteins with a carbohydrate attatched.

GLYCOLIPIDS : Lipids with a carbohydrate attatched.

 

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Membrane components - Phospholipids

Phospholipids have a hydophyllic head (LOVES WATER) and a hydrophobic tail (HATES WATER). 

Therefore, a bilayer is formed with the heads facing outwards either side of the membrane.

Because the tails are located in the centre, the centre part of the membrane does not allow water-soluble substances to diffuse through it.

However, small non-polar substances can diffuse through it.

WATER SOLUBLE SUBSTANCES : Ions and polar molecules

EG. NON POLAR SUBSTANCE : Carbon dioxide

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Membrane components - Cholesterol

What is it used for? - It gives the membrane stability.

What is it?  A type of lipid present in all cell membranes.

Where is it found? - In between phospholipids.

How does it work? - It works by binding to the tails of a phospholipid, therefore causing a more close bunching of them. This therefore restricts the movement of them, making the membrane less fluid and more rigid.

Extra Information - Cholestrol also has hydrophobic parts therefore increasing the barrier against polar molecules. 

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Temperature and Membranes

What affect does temperature have on membranes?

-It affects how much phospholipids in the bilayer can move.

TEMPERATURES BELOW 0'C.

-Lack of energy, therefore movement is restricted.

-Channel and Carrier proteins denature, which increases the permability of the membrane.

-Possibility of ice crystals forming and piercing the membrane, therefore increasing permeability.

 

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Temperature and Membranes

TEMPERATURES BETWEEN 0 AND 45'C.

-Phospholipids are able to move around.

-Phospholipids are not packed as tightly together.

-Membrane is partially permeable.

-As the temperature increases, the permeability is increased due to the fact that phospholipids can move more because of the energy their given in the form of heat.

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Temperature and Membranes

TEMPERATURES ABOVE 45'C.

-Phospholipid bilayer starts to break down, this causes the membrane becomes more permeable.

-The membrane is put under pressure because the water inside the cell expands. 

-Channel and carrier proteins denature so therefore can not assist which substances enter and leave the cell - therefore permeability is increased.

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