Membrane transport Flashcards
What is passive transport?
Solutes travelling down their concentration gradient, which requires no energy
What type of solute can get across the membrane without any help?
Small hydrophobic molecules and gases
What is facilitated diffusion?
Solute goes down its concentration gradient but needs a transporter to get across
What is active transport?
A solute going against its concentration gradient. Requires energy
How do ions get through membranes?
Through ion channels or small molecules called ionophores
Why do polar solutes need a transporter to get through the membrane?
The interior of the transporter is polar and forms favourable interactions with the solute. Otherwise the water would have to be removed from the solute for it to go through the membrane, which is very unfavourable
What are the 2 classes of transport proteins?
Channels and carriers
What are channels?
Holes in the membrane that can be gated. They provide fast transport across the membrane
Can channels be saturated?
No
How good is the substrate specificity of channels?
Not as good as carriers
What is usually the structure of a channel?
A homooligomer with multiple subunits
What are carriers?
Proteins that undergo a conformational change to get the solute across the membrane
Can carriers be saturated?
Yes
How good is the substrate specificity of carriers?
Highly stereoselective
What is usually the structure of carriers?
Usually monomers
What are GLUT1 transporters?
Carrier proteins that transport glucose across the membrane of red blood cells through facilitated diffusion
What are the 2 conformations of GLUT1?
T1: open to the outside
T2: open to the inside
What is the structure of GLUT1?
12 transmembrane helices with the polar portions on the inside
What does the graph of facilitated diffusion kinetics look like?
Michaelis-Menton
What is Kt? What does it measure?
The transport constant. Measures solute affinity
What drives transport of glucose into red blood cells through GLUT1?
Glucose gets converted into glucose-6-phosphate, which locks it into the cell
What happens if there is a deficiency of GLUT1?
De vivo syndrome. The brain can’t get enough fuel and causes microencephaly and seizures
Why can we use radioactive glucose to image cancer?
Cancer cells overexpress GLUT1 and have high rates of glycolysis
Where is GLUT2 found?
Liver
Why is the high Kt of GLUT2 a good thing?
It lets the muscles get their share of glucose before the liver takes the rest and stores it as glycogen
What is GLUT5 used for?
Fructose transport
Where is GLUT4 found?
Muscle, heart, fat cells
How good is the affinity of GLUT4 for glucose?
High affinity and low Kt
What regulates the expression of GLUT4?
Insulin
What happens to GLUT4 expression when muscle cells receive an insulin signal?
Insulin signals high glucose in the blood. Vesicles with GLUT4 get brought to the membrane and they start bringing in glucose. When the insulin signal drops off, the transporters are brought back into their vesicles
What are 4 ways to get the energy for active transport?
Chemical reactions (ATP hydrolysis), oxidation, absorption of light, flow of another solute down its concentration gradient
What are the 3 types of ATPases?
P-type, F-type, V-type
How does a P-type ATPase work?
Involves reversible phosphorylation of the transporter to cause a conformation change
How does an F-type ATPase work?
Couples the energy from ATP hydrolysis to transport something against its gradient
What does a V-type ATPase do?
Acidifies vacuoles
What is SERCA?
A P-type ATPase on the ER membrane that pumps one calcium into the ER for 1 ATP
How does SERCA work?
Reversible conformation change from phosphorylation
What type of ATPase is the sodium-potassium pump?
P-type
What is lactose permease?
Symport that brings in one lactose and one proton through secondary active transport
What are ionophores?
Small cyclic peptides that coordinate a metal ion with carbonyl groups. They are specific to an ion and have a hydrophobic exterior
What do ionophores do?
They dissipate ion gradients and disrupt secondary transport
What are aquaporins?
Very narrow channels that allow water across the membrane
What are the only 2 molecules that can fit through an aquaporin?
Water and hydronium
Why doesn’t hydronium go through aquaporins?
The alpha helices of the aquaporin have their partially positive N-terminus pointing into the channel. The electrostatic repulsion keeps the hydronium out
What is the structure of a potassium channel?
Cone within a cone. It’s a homotetramer with each subunit having 2 long helices and 1 short helix. The C-terminus of the short helix points into the channel
How do potassium channels select for their substrate?
Size. Only potassium ions make optimal contacts with the backbone carbonyls, sodium is too small
How is the structure of voltage gated potassium channels different from normal potassium channels?
Still has the same cone in a cone structure, but has a few extra helices on the outside. Helix 4 had 4 arginine residues that sense the voltage change
