Membrane Structure and Function II

• Plasma membrane - Barrier, transport, signal transduction • Golgi - Post-translational modification, processing for secretion • Nuclear membranes - Attachment of chromatin • Lysosomes - Hydrolytic enzymes • Peroxisomes - Fatty acid oxidation

Inner membrane - Energy transduction Outer membrane - Barrier

RER - Translation protein processing SER - Synthesis of complex lipids

Different functions of membranes (Erythrocyte and hepatocyte -> More protein for more interactions in the blood) Inner mitochondrial membrane - More protein needed (75%) needs to synthesise ATP

• Large uncharged or polar molecules e.g. glucose • Ions

Nonmediated simple diffusion through the membrane and nonmediated diffusion through a channel protein

Uniporters

Symporter and antiporter In the symporter, A and B molecules are transported in the same direction. It requires both molecules A and B to function In the antiporter, A will move in one direction while B will move in the opposite direction

Simple diffusion - Linear graph, constant rate of transport (straight line) Carrier mediated facilitative diffusion - As concentration of solute increases, response gets saturated (plateaus) as carriers get filled up. Both do not require energy. Carrier mediated is specific. It is also faster than simple diffusion

GLUT1 and GLUT3 Kt -> 1 millimolar This corresponds to the circulating conc of glucose (resting) 4mM, so all tissues would have a supply of glucose at all concentrations under physiological conditions.

Measure of affinity of the transporter for the molecule that it transports

GLUT2 - Liver and pancreatic beta cells Produce insulin, which stimulates the lowering of glucose conc. The liver takes up glucose to store as glycogen or as fatty acids.

GLUT2 - Liver and pancreatic beta cells Produce insulin, which stimulates the lowering of glucose conc. The liver takes up glucose to store as glycogen or as fatty acids. Kt - 15-20 mM

• Glucose binds to transporter, then undergoes conformational changes. • Glucose passes across membrane, into cell • After glucose is released, transporter returns to its original conformation. • Continues until glucose conc gradient is diminished • To continue this, the cell also removes its own glucose inside by converting it into another molecule, e.g. glucose 6 phosphate.

Increase the number of transporters on the cell surface to overcome saturation

From the gut into the bloodstream, when Na+ is present, glucose is transported into the cell through SGLUT-1 through co-transport, as sodium is in a higher conc outside of the cell. (symporter)

Sodium removed from cell by sodium potassium ATPase (active transport)

Re-hydration therapy - 1litre of water, 1 tsp of salt, 8tsp sugar. Sodium and glucose are absorbed by the gut, glucose then used as an energy source. Sodium will draw in water and rehydrate the individual.

• Intracellular membranes create local environments and separate reactions • Enables electrochemical gradients to be established • Brings enzymes and reactants together and/or keep them apart

The mitochondria outer membrane is a barrier, but the inner membrane is to generate ATP. The inner membrane has more proteins to generate a H+ gradient and then ATP, as the gradient dissipates. The impermeability also allows it to form ATP

Lysosome peroxisome - Proton pump to pump H+ ions into the cells, (requires hydrolysis of ATP) to maintain lower pH Peroxisome contains many enzymes, pH similar to cytosol

Small sequence (2-3 amino acids) directs particular protein to particular organelle. E.g. cytosol protein, add small nuclear localising sequence, protein then moves to nucleus.