AS Biology - Cell Structure - Cell Membranes

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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



at least half of this is wrong

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