Biological membranes

what are the functions of a biological membrane?

• Separating cell contents from outside environment. 

• Separating cell components from the cytoplasm.  

• Cell recognition and cell signaling. 

• Holding the components of some metabolic pathways in place e.g. mitochondria and chloroplasts. 

• Regulating the transport of materials into and out of cells: cell surface membranes are selectively permeable. this is because of the channel and carrier proteins. 

what makes up the basic structure of the membrane?

Phospholipid molecules.

what’s a phospholipids structure?

• It is made up of a glycerol molecule, 2 hydrophobic fatty acid tails and a hydrophilic head. 

• One of the fatty acid tails is saturated, the other is unsaturated and has a C=C double bond.  

• Amphipathic = dual nature 

• modified triglycerides, where one of the fatty acids is replaced with a phosphate group. 

• Inorganic phosphate ions are negatively charged, so the phosphate end of the molecule ('the head') is hydrophilic (will interact with water).  

• The fatty acid chains ('tails') are non-polar and therefore repelled by water – hydrophobic

why is it important membranes aren’t just made of a phospholipid

• A phospholipid bilayer on its own would be too fragile to function as a biological membrane. 

• Membranes therefore have other components, as well as the phospholipid bilayer, to make sure that they can function properly.  

 

what’s the fluid mosaic model?

• The fluid mosaic model was proposed by Singer and Nicolson in 1972. 

• This model was put forward because: 
  ^the phospholipids are free to move within the layer (fluid). 

^the proteins embedded in the bilayer vary in shape size and position (mosaic). 

why do the phospholipids create a bilayer?

 

• At the boundary between 2 aqueous environments phospholipids will form a bilayer with the hydrophobic tails face inwards.  

• they rotate this way to avoid or get closer to the water.

how are phospholipids surfactants?

• Because of their hydrophilic/hydrophobic structure, phospholipids  will naturally form a layer on the surface of water – they are surfactants. 

what are intrinsic proteins?

• Proteins that spans the phospholipid bilayer. 

• 3 different types in membranes: channel, carrier and glycoproteins. 

what do channel proteins do and how?

• Provide a hydrophilic channel that allows the passive movement (diffusion) of polar molecules and ions down a concentration gradient through membranes.  

• They are held in position by interactions between the hydrophobic core of the membrane and the hydrophobic R-groups on the outside of proteins.  

what do carrier proteins do and how?

• Have an important role in both passive transport (down a concentration gradient) into cells. 

• This often involves the shape of the protein changing. 

what do glycoproteins do and how?

• Embedded in surface cell membrane with attached carbohydrate (sugar) chains of varying lengths and shapes.  

• They also play a role in cell adhesion (when cells join together to form tight junctions in certain tissues) and as receptors for chemical signals.  

• When the chemical binds to the receptor, it elicits a response from the cell.  

• This may cause a direct response or set off a cascade of events inside the cell signalling.  

• Some examples include: 
  - receptors for neurotransmitters such as acetylcholine at nerve cell synapses. The binding of the neurotransmitters triggers or prevents an impulse in the next neurone.  

       -receptors for peptide hormones, including insulin and glucagon, which affect the uptake and storage of glucose by cells.  

• Some drugs act by binding to cell receptors. 

what are extrinsic proteins?

• Extrinsic proteins or peripheral proteins are present in one side of the bilayer. 

• They normally have hydrophilic R-groups on their outer surfaces and interact with polar heads of the phospholipids or with intrinsic proteins.  

• They don’t span the phospholipid bilayer. 

• They can be present in either layer and some move between layers. 

what is cholesterol and what does it do?

• Cholesterol is a lipid with a hydrophilic end and a hydrophobic end, like a phospholipid.  

• It regulates the fluidity of membranes.  

• Cholesterol molecules are positioned between phospholipids in a membrane bilayer, with the hydrophilic end interacting with the heads and the hydrophobic end interacting with the tails, pulling them together.  

• In this way cholesterol molecules prevent the membranes becoming too solid by stopping the phospholipid molecules from grouping too closely and crystallising.  

what are glycolipids and what do they do?

• Similar to glycoproteins. 

• They are lipids with attached carbohydrate chains.  

• These molecules are called cell markers or antigens and can be recognised by the cells of the immune system as self (of the organism) or non-self (of cells belonging to another organism).