AS Biology - Cell Structure - Cell Membranes

Membrane control 

Membranes at the surface of cells

• Control which substances enter and leave the cell
• Partially permeable - let some molecules through but not others
• Substances can move across the cell membrane by diffusion, active transport or osmosis
• Allow recognition by other cells e.g. cells of the immune system
• Allow cells to communicate with each other

Membranes within cells

• Membranes around organelles divide the cell into different compartments, making functions more efficient e.g. substances needed for respiration are kept together inside mitochondria)
• Membranes of some organelles are folded, increasing their surface area and making chemical reactions more efficient. E.g. the inner membrane of a mitochondria contains enzymes needed for respiration. It has a large surface area which increases the number of enzymes present and makes respiration more efficient.
• Can form vesicles to transport substances between different areas of the cell
• Control which substances enter and leave the organelle, e.g. RNA leaves the nucleus via the nuclea membrane. 
• It is also partially permeable

'Fluid mosaic' structure

Structure of all membranes is basically the same - composed of lipids (mainly phospholipids), proteins and carbohydrates (usually attached to proteins or lipids).

• 1972 the fluid mosaic model was suggested to describe the arrangement of molecules in the membrane.
• In the model, phospholipid molecules form a continuous double layer (bilayer). This is fluid because the phospholipids are continuously moving.
• Protein molecules are scattered through the bilayer like tiles in a mosaic. Because the bilayer is fluid, the proteins can move around within it.
• Some proteins have a polysaccharide (carbohydrate) chain attached - these are called glycoproteins. 
• Cholesterol (type of lipid) is also present in the membrane.

Membrane properties

(1) The membrane is a good barrier against most water-soluble molecules

• Phospholipids are the major component of the membrane bilayer.
• Molecules automatically arrange themselves into a bilayer - the hydrophilic heads face out towards the water on either side, and the hydrophilic tails face inwards.
• This hydrophobic centre makes it difficult for water-soluble substances, such as sodium ions and glucose, to get through.

(2) The membrane controls what enters and leaves

• Proteins in the membrane (carrier/channel proteins) allow the passage of large or charged water-soluble substances that would otherwise find it difficult to cross the membrane.
• Different cells have different protein channels and carrier proteins - e.g. the membrane of a nerve cell has many sodium-potassium carrier proteins (help conduct nerve impulses) and muscle cells have calcium protein channels (needed for muscle contractions).

(3) The membrane allows cell communication

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