Membranes

Question 1

What does the fluid mosaic model suggest? (4)

Answer 1

Proteins are at a low concentration
Proteins fit perfectly into the bilayer
The bilayer has constant thickness
The bilayer is symmetrical

Question 2

What does the fluid-mosaic model not show? (3)

Answer 2

Many proteins form complexes with themselves or lipids.
Membranes are patchy and asymmetrical
The bilayer deforms to accommodate proteins and lipids

Question 3

What are the basic properties of lipids? (3)

Answer 3

• Hydrophilic head and hydrophobic fatty acid tail
• Spontaneously self-associate due to the hydrophobic effect and van der Waals interactions
• Lipids are amphipathic

Question 4

What is cholesterol?

Answer 4

A molecule important in controlling membrane fluidity and lipid packing. It’s polar OH group gives it weak ampipathic properties

Question 5

Why is membrane asymmetry important?

Answer 5

The two outer monolayers have different lipid compositions as different proteins will bind to different lipids on each side of the membrane

Question 6

In what ways can the lipids in the bilayer move around? (4)

Answer 6

• can flip-flop from one side of the membrane to the other
• can move/diffuse laterally
• can rotate freely along their long axis (opposite of flip flop)
• general hydrocarbon chain movement

Question 7

What is the transition temperature of a membrane?

Answer 7

The temp at which bilayers change from fluid to rigid gel.
Lateral diffusion is greatly reduced in this state.
Is made lower by shorter chains and double bonds

Question 8

What are the properties of peripheral/extrinsic membrane proteins? (4)

Answer 8

• bind to membrane surface only
• bind via membrane proteins or lipid head regions
• can be easily dissociated from the membrane
• often soluble in aqueous buffers

Question 9

What are the properties of integral membrane proteins? (5)

Answer 9

• embedded in the bilayer
• tightly bound to the membrane by hydrophobic forces (can only be separated by disrupting the membrane)
• can pass through the membrane in a single pass or a multi pass
  Insoluble in aqueous buffers
  -alpha helical or beta barrel linked by loops that aren’t in the membrane
• contain lots of hydrophobic amino acids to allow them to stay in the membrane

Question 10

What is a transmembrane alpha-helix?

Answer 10

• most common transmembrane secondary structure
• 20-25 amino acids long
• thermodynamically stable as all hydrogen bonds that can be formed are formed.
• Helices can be predicted from the amino acid sequence by searching for segments high in hydrophobic residues

Question 11

What are transmembrane beta-sheets?

Answer 11

• almost always antiparallel
• alternating polar and hydrophobic amino acids make them hard to predict
• hydrophilic interior can create a pore to allow molecules through the membrane

Question 12

What is the importance of the lipid membrane being semi-permeable?
What can and can’t pass through?

Answer 12

• Allows generation of difference like ion gradients that make movement across the membrane easier
• hydrophobic molecules can move through easily
• Uncharged polar molecules need the help of transport molecules or systems
  Ions can’t move throug without a channel

Question 13

What is the difference between a transporter and a channel?

Answer 13

Transporters move things down or against the concentration gradient (passive and active)
Channels only allow movement down the gradient (passive)

Question 14

What so solute transporters do?

Answer 14

• move small molecules across lipid membranes (sugars, a.acids, peptides)
• often require co-transport

Question 15

What is a uniport transporter?

Answer 15

Transports one molecule down its concentration gradient

Question 16

What is a symport transporter?

Answer 16

One molecule going down its concentration gradient allows another to move up its concentration gradient in the same direction

Question 17

What is an antiport transporter?

Answer 17

One molecule going down its concentration gradient allows another molecule to move up its concentration gradient in the opposite direction

Question 18

How does Na+/K+ ATPase maintain ion gradients across the plasma membrane?

Answer 18

• hydrolysis of 1 ATP drives the export of 3Na+ and the import of 2K+
• this generates a chemical and electrical difference
  This provides free energy to drive other transporters

Question 19

What is the protonmotive force?

Answer 19

• in the bacterial membrane, electron transport generates an electrochemical proton gradient
• the PMF drives ATP synthase and other bacterial transporters