Crossing membranes Flashcards
What property allows cells to control what enters and leaves?
Impermeability
2 Different types of transport
Passive
Active
Types of passive transport
Simple diffusion
Channel mediated
Carrier mediated
Simple diffusion
Gases and small uncharged molecules
Diffusion from a high to low concentration
What does the rate of simple diffusion depend on?
Diffusion gradient
Hydrophobicity of molecules
Rate limiting step in simple diffusion
Transition from aqueous to hydrophobic interior of the membrane
Isotonic media
No net flow of water
Hypotonic media
Water flows into a cell causing swelling
Hypertonic media
Water flows out and the cell shrinks
Named example of a water channel
Aquaporin
Aquaporin
Tetramer - has 4 channels
Protein has several membrane embedded alpha helices which are connected by loops. Each subunit forms a channel and channels are lined with polar amino acid side chains so hydrogen bonds can form with water.
What are the four types of channels/transporters?
Uniporter
Symporter
Antiporter
ATP-powered pump
Uniporter
Movement is gradual due to difference in concentration gradient which is accelerated by conformational changes in the protein
Symporter
Movement of two molecules in the same direction where one of the molecules is moving against its concentration gradient. Facilitated by the movement of the molecule that is moving with the concentration gradient.
Antiporter
Movement of two molecules in opposite directions where one of the molecules is moving against its concentration gradient. This is facilitated by the movement of the molecules that is moving with the concentration gradient.
Glucose uptake
Symporter imports 2 Na+ and 1 glucose molecule from the intestinal lumen into the cytosol. An Na+/K+ ATPase pump exports Na+ into the blood and K+ into the cytosol. Glucose is then transported out of the cell into the blood by GLUT2 uniporter.
Glut 1
Found in the blood barrier and transports glucose from high to low concentrations.
Movement of glucose as facilitated by Glut 1
Glucose binds inducing a conformational change so Glut1 changes from having an outward facing conformation to inward facing. Facilitated diffusion and glucose has moved down its concentration gradient into the cytosol of the cell.
How do uniporters (Glut1) exhibit simple enzyme kinetics?
Bind affinity determines the rate of movement Km
How is transport controlled in uniporters (Glut1)?
Limited number so can reach a point of full saturation
Blocked by competitive and non-competitive inhibitors
Transport can be controlled by controlling the number of transporters
Glucose transport by Glut4
Controlled by insulin-responsive exocytosis
Insulin binds to a receptor to create a signalling cascade that stimulates glucose transporters to fuse with the plasma membrane so glucose can enter a cell
Glut 4 glucose and insulin movement explained
In the absence of insulin glucose cannot enter a cell
Insulin present and binds to receptor, signal transduction cascade, GLUT 4 transporters inserted into plasma membrane, glucose enters the cell.
Symporters and antiporters compared
Both couple the movement of two molecules where one molecule is moving with its concentration gradient and one molecule is moving against its concentration gradient (energetically unfavourable). The energetically favoured reaction drives the unfavoured reaction.
In symporters both molecules are moving in the same direction, in antiporters the molecules are moving in opposite directions
Movement of sodium and glucose into the small intestine
2 Na+ molecules move into a symporter as well as one glucose molecule. The symporter has an occluded conformation. Conformational change is induced so the symporter changes from having an outward to an inward facing conformation. Na+ and glucose released into the cell. Na+ has moved with its concentration gradient and glucose against. The outward facing conformation of the symporter is re-gained
