These notes summarise all of the 2.5 topic on biological membranes, from the structure and function of the membrane to different types of transport across the membrane and how each molecules moves across it.
It also contains useful information on bulk transport.
Biological Membranes
- Cell membranes form a barrier and separate the cell contents from the cell’s exterior
environment, or separate organelles from cytoplasm.
- They need to allow some molecules through, in or out of the cell
Permeability refers to the ability to let substances pass through.
- Some very small molecules simply diffuse through the cell membrane, in between
structural molecules
- Some substances dissolve in the lipid layer and pass through
- Others pass through special protein channels or are carried by carrier proteins
- Because the membranes do not let all molecules through they are described as partially
permeable barriers.
Roles of membranes around cells Roles of membranes within cells around
(cell surface membrane) organelles.
A barrier between cell and environment - Divide the cell into different compartments,
controlling what can enter and leave the cell acting as a barrier between organelle and
cytoplasm.
Allow recognition by other cells - Have Can form vesicles to transport substances
antigens, so that the organisms immune between different areas of the cell
system can recognise the cell as being good
and not attack it
Allow cell communications (also called cell They control which substances enter and
signalling) - contain receptors for chemical leave the organelle, eg. RNA leaves the
signals nucleus via nuclear membrane.
May be the site of chemical reactions. Site of chemical reactions, eg. inner
membrane of mitochondria contains enzymes
for respiration.
May contain enzymes involved in specific
metabolic pathways.
Fluid mosaic model of cell membrane:
- In 1972 Singer and Nicolson proposed the fluid mosaic model. Which explained how cell
membranes could be more dynamic and interact more with the cells’ environment.
- It proposed that the membrane was made of a phospholipid bilayer with proteins floating
in it, making up a mosaic pattern.
- The phospholipid bilayer contains two different types of molecules
- The hydrophilic heads are in contact with the watery exterior or cytoplasm
- The hydrophobic tail regions are in the centre of the membrane away from water.
, What can’t travel through the bilayer?
Polar molecules like ions cannot travel through the bilayer.
Large molecules cannot travel through the bilayer
Only small and non polar molecules can travel through the bilayer through diffusion.
Water is dipolar so can go through the bilayer but not easily so travel through a water filled
channel.
Structure:
Phospholipids: form a partially permeable bilayer.
Proteins: peripheral proteins (on the edge) can act as enzymes, antigens or receptor sites.
Carrier proteins (integral) are active transport sites that can flip and transport small molecules in
and out. Channel proteins have a hole enabling small polar ions that cannot travel through the
bilayer.
Glycoproteins: a carbohydrate chain attached to a protein molecule. Can act as receptor sites
for reactions, like for receptors. Can be involved in cell to cell recognition.
Glycolipids: stabilisation, receptor molecules and cell to cell recognition.
Cholesterol: helps regulate fluidity of the membrane, maintain the mechanical stability and
resits the effect of temperature changes on the structure of the membrane.
Other membranes:
- Nerve cells (neurones), the protein channels and carriers in the plasma membrane
covering the long axon allow entry and exist of ions to bring about the conduction of
electrical impulses along their length.
- Neurones have a myelin sheat formed by flattened cells wrapped around them several
times, giving several layers of cell membrane.
- The plasma membranes of white blood cells contain special protein receptors that
enable them to recognise the antigens on foreign cells usually from invading pathogens
but also from tissue or organ transports.
- Root hair cells in plants have many carrier proteins to actively transport nitrate ions from
the soil into the cells
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