Membrane Transport Flashcards
What are biological membranes
They act as a selective permeability barrier that block the passage (IN AND OUT) of almost all water soluble soluble molecules
they also define cells chemical environment- keeps its integrity
What kind of molecules can freely transverse the bilayer by simple diffusion
Small, uncharged,hydrophobic molecules can freely transverse the bilayer by simple diffusion down conc gradient
In contrast charged and polar molecules require specialist proteins to carry them across the membrane
What kind of substances cannot enter through lipid bilayer
Large uncharged polar molecules- glucose and sucrose
Ions- H+, Na+, Mg2+ etc
Charged polar molecules- amino acids and ATP
Name 4 different transport mechanisms
Passive Diffusion
Facilitated diffusion
Primary Active transport
Secondary Active transport
Describe Simple and Passive Diffusion
Solute will move from one side of the membrane to the other side of the membrane down its concentration gradient- NET MOVEMENT
When the concentration of this solute is equal no net movement- though the individual molecules may move through he membrane either side- random
Describe the partition coefficient
K ow = the equilibrium constant partitioning of a molecule between oil and water
In a test tube mixing water,Octonal and a molecule - splits into 2 phases
concentration in oil and in water can be investigated
K ow = the amount in octanol/ the amount in water phase
K ow= 1 means that the substance is equal as soluble in lipid and water
the higher the value for K ow, the more lipid soluble it is
How do mineral ions and hydrophilic molecules move across the membrane
through aquaporins
Describe the structure of aquaporins
All have an ion filter
some have a gate- (voltage gated, ligand gated [intra and extra], mechanically gated)
Comment on the specificity of aquaporins
Water channels are specific to water channels
Ion channels are specific to that specific ion
Why are aquaporins so specific ?
Cations have a hydration shell- whereby O d- of water is attracted to the cations ( forming H bond). energy is required to remove the H20 from the cation
The filter has amino acid side chains that point towards the aqueous channels- these side chains will have oxygens attached.
The distance between the aa side chains and cations = the distance between cation and O of the water molecule. Allows the for the protein to exchange the hydration shell with hydrated cation. Will be coordinated with oxygen on side chain
Different cations will as be different size radius and so the amino acids with oxygen will be at different distance to the centre- cannot replace the hydration shell if thee distance is different
Describe how facilitated diffusion uses a carrier protein
Binding sites are specific- when the substrate binds to the binding sites it causes a conformational change in the protein so that it opens to the inside of the cell to let the substrate in
Always down the conc gradient
describe the graph for external concentration vs the rate of uptake
Linear- as the external concentration increases the rate of uptake also increases
Comment on the graph between external concentration and the rate of uptake
0.5J max = Km
The lower the Km the higher the affinity
Describe GLUT1
GLUT 1- Lower Km (1.8 mM) than the normal Km for glucose- therefore has a higher affinity for glucose
Mediates constitutive glucose uptake in many tissues- but highly expressed in erythrocytes and in the blood brain barrier
Describe GLUT2
Low affinity- due to the Km being high ( means that it is only effective in glucose transport when the glucose concentration is very high)
Transports glucose to Pancreas B - when the blood glucose concentration is too high- in order to regulate the blood glucose concentration- this will activate pancreatic Beta cell converting glucose to glycogen
