12. Membrane structure and function

Question 1

Describe a membrane (1pts)

Answer 1

The membrane defines boundaries between cells and within walls

Question 2

Describe membrane properties (3pts)

Answer 2

1. Flexible
2. Self-sealing
3. Selectively permeable

Question 3

Describe membrane function (4pts)

Answer 3

1. Involved in transport
2. Cell recognition
3. Cellular communication
4. Metabolic regulation

Question 4

Describe a biological membrane (4pts)

Answer 4

1. Biological membranes are composed largely of lipids and proteins.
2. The lipids are amphipathic. They have a hydrophilic and hydrophobic part.
3. Phospholipids are composed of a hydrophilic phosphate head which is directed towards water and a hydrophobic fatty acid tail which is directed outside of water.
4. It assembles spontaneously- made up of non-covalent bonds.

Question 5

Describe the Lipid composition in the biological membrane? (4pts)

Answer 5

1. Contains a phospholipid. Attached to the phosphate head can be any of the choline, serine, ethanolamine and inositol. These groups are important in cell signalling.
2. Contains cholesterol which maintains the fluidity of the membrane.
3. Contains a glycolipid. They have fatty acid tails attached to a large carbohydrate group. They are important for cell recognition and stability of the membrane.
4. Contains ceramide and Sphingomyelin.

Question 6

Describe membrane Assymetry? (1pts)

Answer 6

The outside layer of the membrane has a different composition to the inside layer which means that the membrane has asymmetry in terms of composition. Cholesterol is relatively evenly spread.

Question 7

Describe membrane synthesis in lipids? (3pts)

Answer 7

1. Membrane is synthesized in the ER.
2. As the membranes move from the ER to the Golgi they mature
3. The membranes are then distributed to the cell surface.

• synthesis takes place on the cytoplasmic surface of the membrane

Question 8

Describe ABC transporters? (1pts)

Answer 8

ATP-binding casside transporters enable the movement of highly charged head groups across a hydrophobic region in the core of the membrane. This requires energy and enzymes such as Floppase, Flippase and Scramblase.

Question 9

Describe the enzyme Floppase?

Answer 9

Floppase moves phospholipids from the inner to the outer leaflet. Requires ATP.

Question 10

Describe the enzyme Flippase? (1pts)

Answer 10

Flippase moves phospholipids from the outer to the inner leaflet. Requires ATP.

Question 11

Describe the enzyme Scramblase? (1pts)

Answer 11

Bidirectional movement

Question 12

Name the 5 factors that alter membrane fluidity (5pts)

Answer 12

1. Temperature
2. Fatty acid composition
3. Chain length
4. Degree and extent of saturation. The higher the quantity of unsaturated fatty acids the higher the membrane fluidity.
5. Cholesterol content

Question 13

Describe the effect of temperature and cholesterol on membrane fluidity? (4ts)

Answer 13

1. Low temperature= Low fluidity.
2. High temperature= High fluidity.
   At high temperatures the molecules have higher kinetic energy and will therefore be more motile
3. At low temperatures cholesterol increases the fluidity of the membrane as it reduces the interactions between the phospholipid fatty acid chain.
4. At high temperatures cholesterol decreases the fluidity of the membrane.

Question 14

Describe Spur cell Anaemia? (3pts)

Answer 14

1. In individuals with sickle cell anaemia the plasma membrane cholesterol content is increased by 25-65% leading to decreased membrane fluidity.
2. The spur anaemic blood cell has spurs coming out of it.
3. These red blood cells get trapped as they pass through capillaries. These red blood cells then rupture leading to spur cell anaemia.

Question 15

Describe the fluid mosaic model (1pts)

Answer 15

The movement of proteins is restricted

Question 16

Describe integral membrane proteins (6pts)

Answer 16

1. Proteins can be single or multi-pass
2. Proteins contain strong non-covalent bonds.
3. Contain a trans-membrane domain which is often an a-helix.
4. The structure can be predicted from the sequence.
5. Mostly hydrophobic interactions
6. Integral proteins interact extensively with the lipid bilayer.

Question 17

Describe Peripheral membrane proteins ? (4pts)

Answer 17

1. located on both the extracellular cytosolic membrane
2. Associated by non-covalent bonds
3. There is direct interaction between the peripheral membrane proteins and
   the phospholipid component.
4. Peripheral proteins interact with integral proteins or lipid polar head groups.

Question 18

Describe Lipid anchored membrane proteins? (1pts)

Answer 18

Covalently linked to a lipid molecule

Question 19

Describe membrane carbohydrates (5pts)

Answer 19

1. Carbohydrates are associated with both membrane lipids and proteins
2. Carbohydrates form 2-10% of the membrane weight.
3. The majority of glycolipids and glycol proteins are externally facing.
4. They are often involved in cell-cell interactions or cellular recognition.
5. in red blood cells 8% of the weight is carbohydrate.