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Structure and Function of Biomolecules > Membrane transport > Flashcards

Membrane transport Flashcards

1
Q

What is passive transport?

A

Solutes travelling down their concentration gradient, which requires no energy

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2
Q

What type of solute can get across the membrane without any help?

A

Small hydrophobic molecules and gases

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3
Q

What is facilitated diffusion?

A

Solute goes down its concentration gradient but needs a transporter to get across

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4
Q

What is active transport?

A

A solute going against its concentration gradient. Requires energy

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5
Q

How do ions get through membranes?

A

Through ion channels or small molecules called ionophores

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6
Q

Why do polar solutes need a transporter to get through the membrane?

A

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

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7
Q

What are the 2 classes of transport proteins?

A

Channels and carriers

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8
Q

What are channels?

A

Holes in the membrane that can be gated. They provide fast transport across the membrane

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9
Q

Can channels be saturated?

A

No

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10
Q

How good is the substrate specificity of channels?

A

Not as good as carriers

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11
Q

What is usually the structure of a channel?

A

A homooligomer with multiple subunits

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12
Q

What are carriers?

A

Proteins that undergo a conformational change to get the solute across the membrane

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13
Q

Can carriers be saturated?

A

Yes

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14
Q

How good is the substrate specificity of carriers?

A

Highly stereoselective

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15
Q

What is usually the structure of carriers?

A

Usually monomers

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16
Q

What are GLUT1 transporters?

A

Carrier proteins that transport glucose across the membrane of red blood cells through facilitated diffusion

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17
Q

What are the 2 conformations of GLUT1?

A

T1: open to the outside
T2: open to the inside

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18
Q

What is the structure of GLUT1?

A

12 transmembrane helices with the polar portions on the inside

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19
Q

What does the graph of facilitated diffusion kinetics look like?

A

Michaelis-Menton

20
Q

What is Kt? What does it measure?

A

The transport constant. Measures solute affinity

21
Q

What drives transport of glucose into red blood cells through GLUT1?

A

Glucose gets converted into glucose-6-phosphate, which locks it into the cell

22
Q

What happens if there is a deficiency of GLUT1?

A

De vivo syndrome. The brain can’t get enough fuel and causes microencephaly and seizures

23
Q

Why can we use radioactive glucose to image cancer?

A

Cancer cells overexpress GLUT1 and have high rates of glycolysis

24
Q

Where is GLUT2 found?

A

Liver

25
Q

Why is the high Kt of GLUT2 a good thing?

A

It lets the muscles get their share of glucose before the liver takes the rest and stores it as glycogen

26
Q

What is GLUT5 used for?

A

Fructose transport

27
Q

Where is GLUT4 found?

A

Muscle, heart, fat cells

28
Q

How good is the affinity of GLUT4 for glucose?

A

High affinity and low Kt

29
Q

What regulates the expression of GLUT4?

A

Insulin

30
Q

What happens to GLUT4 expression when muscle cells receive an insulin signal?

A

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

31
Q

What are 4 ways to get the energy for active transport?

A

Chemical reactions (ATP hydrolysis), oxidation, absorption of light, flow of another solute down its concentration gradient

32
Q

What are the 3 types of ATPases?

A

P-type, F-type, V-type

33
Q

How does a P-type ATPase work?

A

Involves reversible phosphorylation of the transporter to cause a conformation change

34
Q

How does an F-type ATPase work?

A

Couples the energy from ATP hydrolysis to transport something against its gradient

35
Q

What does a V-type ATPase do?

A

Acidifies vacuoles

36
Q

What is SERCA?

A

A P-type ATPase on the ER membrane that pumps one calcium into the ER for 1 ATP

37
Q

How does SERCA work?

A

Reversible conformation change from phosphorylation

38
Q

What type of ATPase is the sodium-potassium pump?

A

P-type

39
Q

What is lactose permease?

A

Symport that brings in one lactose and one proton through secondary active transport

40
Q

What are ionophores?

A

Small cyclic peptides that coordinate a metal ion with carbonyl groups. They are specific to an ion and have a hydrophobic exterior

41
Q

What do ionophores do?

A

They dissipate ion gradients and disrupt secondary transport

42
Q

What are aquaporins?

A

Very narrow channels that allow water across the membrane

43
Q

What are the only 2 molecules that can fit through an aquaporin?

A

Water and hydronium

44
Q

Why doesn’t hydronium go through aquaporins?

A

The alpha helices of the aquaporin have their partially positive N-terminus pointing into the channel. The electrostatic repulsion keeps the hydronium out

45
Q

What is the structure of a potassium channel?

A

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

46
Q

How do potassium channels select for their substrate?

A

Size. Only potassium ions make optimal contacts with the backbone carbonyls, sodium is too small

47
Q

How is the structure of voltage gated potassium channels different from normal potassium channels?

A

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

Structure and Function of Biomolecules (33 decks)

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