Structure & function of plasma membrane

Question 1

What shows the existence of the plasma membrane

Answer 1

1) Cell swelling (hypotonic) and shrink 2) Escape of cell contents if mechanically ruptured
3) Compartmentalisation needed for biochemical activity of cells.

Question 2

Describe the structure of the plasma membrane:

Answer 2

• Lipid molecules make up 50% of the mass of most animal cell membranes
• Are asymmetrically distributed
• Are amphiphilic: hydrophobic and hydrophilic regions

Question 3

How do lipid molecules maintain favourable position?

Answer 3

• They spontaneously aggregate to keep their hydrophobic tails inside and hydrophilic heads to water

Question 4

What does aggregation style depend on?

Answer 4

Depends on shape:

• Cone shaped - Lipid molecules (single chained) form micelles
• Cylinder shape - Lipid molecules (double chained) form bilayers

Question 5

What are the 2 types of phospholipids?

Answer 5

Glycerol-based (glycolipid) - Phosphoglyceride
Sphingosine based (sphingolipid)

Question 6

What are the main phosphoglycerides in the plasma membrane?

Answer 6

• Phosphatidylcholine
• Phosphatidylethanolamine
• Phosphatidylaserine
• Phosphatidyllinositol

Question 7

State the phospholipid structure

Answer 7

• Polar head (usually choline, ethanolamine, serine or inositol)
• Glycerol or sphingosine
• Lipid tail(s)

Question 8

How are glycolipids formed?

Answer 8

By the addition of carbohydrate group(s) to lipids

Question 9

What are glycospingolipids?

Answer 9

Combination of glycerol and spingosine based.

Example: Cerebrosides & gangliosides

Question 10

Structure of glycolipid

Answer 10

• Carbohydrate head group
• Lipid backbone
• Glycerol or sphingosine based

Question 11

Example of glycolipid?

Answer 11

Blood group antigens

Question 12

What do sterols have large amounts of?

Answer 12

Cholesterol

Question 13

What are the functions of Sterols?

Answer 13

• Affects PM fluidity
• Increased permeability properties of PM
• Maintains stability and integrity of PM

Question 14

Describe the 3 types of movement a phospholipid molecule can carry out?

Answer 14

• Rotation about its long axis
• Lateral diffusion by exchanging places with near molecules in the same layer
• Transverse diffusion (flip-flop) from one monolayer to the other (rare)

Question 15

What happens to membrane fluidity when temperature is altered?

Answer 15

Temp increases - MF (lateral diffusion) increases

Temp decreases - MF decreases

Question 16

What buffers fluidity changes over a temperature range?

Answer 16

Cholesterol - Supports membrane fluidity

Question 17

How can composition of phospholipids affect fluidity?

Answer 17

• Shorter chain length: tails do not interact as often

- Cis-double bonds - produce kinks in HC chains. Difficult to pack together.

Question 18

What are the 3 classes of membrane proteins?

Answer 18

• Integral
• Peripheral
• Lipid anchored

Question 19

Examples of membrane proteins

Answer 19

• Transporters
• Receptors
• Cell-cell interaction proteins
• Enzymes

Question 20

Describe integral proteins

Answer 20

Embedded within bilayer
Alpha helix areas are 20 amino acids long
Hydrophobic segments have affinity for hydrophobic interior of bilayer
Hydrophilic regions extend outward from membrane into aqueous phase

Question 21

Describe peripheral proteins

Answer 21

Located on surface of PM

Attached through ionic interactions (not intrinsic)

Question 22

Describe lipid anchored proteins

Answer 22

Modified proteins attached to glycolipid

Lipid part of glycolipid anchor is embedded in hydrophobic region of the membrane

Question 23

Describe lipid rafts

Answer 23

Clusters of lipids and proteins within membrane
Rich in cholesterol and saturated fatty acid tails.
Associated with signalling units. May be transient.