Membranes at the surface of the cell:
- 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, osmosis or active transport.
- 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
- This makes different functions more efficient, e.g. substances needed for respiration are kept together inside the mitochondria
- Membranes of some organelles are folded, increasing their surface area and making chemical reactions more efficient.
Membrane Control continued...
- Example: the inner membrane of a mitochondrion contains enzymes needed for respiration. It has a large surface area which increases the number of enzymes present and makes respiraiton more efficient
- Can form vesicles to transport substances between different areas of the cell
- Contol which substances enter and leave the organelle, e.g. RNa leaves the nucleus via the nuclea membrane
- Also partially permeable
'Fluid Mosaic' Structure
The structure of all membranes is basically the same - composed of:
Lipids (mainly phospholipids)
Carbohydrates (usually attached to proteins or lipids)
Development of the structure:
- 1972 the fluid moasic structure 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 constantly moving.
- Protein molecules are scattered through the bilayer like tiles in a moasic. Because the bilayer is fluid, the proteins can move around within it.
- Some proteins have a polysaccharide (carbohydrate) chain attached - called glycoproteins
- Some lipids also have a polysaccharide chain attached - called glycolipids.
- Cholesterol (type of lipid) is also present in the membrane.
Membrane Properties 1 & 2
The membrane is a good barrier against most water-soluble molecules...
- Phospholipids are the major component of the membrane bilayer.
- The molecules automatically arrange themselves into a bilayer - they hydrophilic heads face out towards the water on either side and they hydrophobic tails face inwards.
- This hydrophobic centre makes it difficult for water-soluble substances, such as sodium ions and glucose, to get through.
The membrane controls what enters and leaves...
- Proteins in the membrane (e.g. carrier proteins or 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 to conduct nerve impulses.
Membrane Properties 3 & 4
The membrane allows cell communication...
- Membranes contain receptor proteins which allow the cell to detect chemicals released from other cells
- The chemicals signal to the cell to respond in some way e.g. the hormone insulin bonds to receptors in the membranes of liver cells and tells the liver to absorb glucose
- Cell communication is vital for the body to function properly
- Different cells have different receptors in their cell membranes
The membrane allows cell recognition...
- Glycoproteins and glycolipids tell white blood cells that the cell is your own
- White blood cells only attack cells that they don't recognise as self e.g. those of microorganisms like bacteria.
Membrane Properties 5
The membrane is fluid...
- Phospholipids in the plasma membrane are continually moving around
- The more unsaturated fatty acids there are in the phospholipid bilayer, the more fluid it becomes
- Cholesterol molecules fit between the phospohlipids of the bilayer, binding to their hydrophobic tails - causing them to pack more closely together.
- The more cholesterol molecules there are, the less fluid the membrane becomes
- Cholesterol is important because it makes cell membranes more rigid and prevents it from breaking up