Cell Surface Membranes
Phospholipids
- Form the basic structure of the membrane (phospholipid bilayer)
- The tails form a hydrophobic core comprising the innermost part of both the outer and inner layer of the membrane
- Act as a barrier to most water-soluble substances (the non-polar fatty acid tails prevent polar molecules or ions from passing across the membrane)
- This ensures water-soluble molecules such as sugars, amino acids and proteins cannot leak out of the cell and unwanted water-soluble molecules cannot get in
- Can be chemically modified to act as signalling molecules by:
- Moving within the bilayer to activate other molecules (eg. enzymes)
- Being hydrolysed which releases smaller water-soluble molecules that bind to specific receptors in the cytoplasm
Cholesterol
- Cholesterol regulates the fluidity of the membrane
- Cholesterol molecules sit in between the phospholipids, preventing them from packing too closely together when temperatures are low; this prevents membranes from freezing and fracturing.
- Interaction between cholesterol and phospholipid tails also stabilises the cell membrane at higher temperatures by stopping the membrane from becoming too fluid
- Cholesterol molecules bind to the hydrophobic tails of phospholipids, stabilising them and causing phospholipids to pack more closely together
- Cholesterol also contributes to the impermeability of the membrane to ions and increases mechanical strength and stability of membranes
- Without cholesterol, membranes would break down, which would cause cells to burst
Glycolipids & glycoproteins
- Glycolipids and glycoproteins contain carbohydrate chains that exist on the surface (the periphery / extrinsically), which enables them to act as receptor molecules
- This allows glycolipids and glycoproteins to bind with certain substances at the cell’s surface
- There are three main receptor types:
- signalling receptors for hormones and neurotransmitters
- receptors involved in endocytosis
- receptors involved in cell adhesion and stabilisation (as the carbohydrate part can form hydrogen bonds with water molecules surrounding the cell
- Some act as cell markers or antigens, for cell-to-cell recognition (eg. the ABO blood group antigens are glycolipids and glycoproteins that differ slightly in their carbohydrate chains)
Proteins
- Transport proteins create hydrophilic channels to allow ions and polar molecules to travel through the membrane. There are two types:
- channel (pore) proteins
- carrier proteins
- Each transport protein is specific to a particular ion or molecule
- Transport proteins allow the cell to control which substances enter or leave
- The number of transport proteins in a section of membrane helps to control the rate of transport as required by that cell
Examiner Tip
Membranes become less fluid when there is:
- An increased proportion of saturated fatty acid chains as the chains pack together tightly and therefore there is a high number of intermolecular forces between the chains
- A lower temperature as the molecules have less energy and therefore are not moving as freely which causes the structure to be more closely packed
Membranes become more fluid when there is:
- An increased proportion of unsaturated fatty acid chains as these chains are bent, which means the chains are less tightly packed together and there are less intermolecular forces
- At higher temperatures, the molecules have more energy and therefore move more freely, which increasing membrane fluidity
