Membrane structure and Synthesis

Question 1

Examples of membranes

Answer 1

Plama membrane, Nuclear membrane, Mitochondrial membrane

Question 2

Why do biological membranes form bilayers?

Answer 2

Because this arrangement is energetically favourable as the hydrophobic heads will face the aqueous environments at each surface of the bilayer while the hydrophobic tails will be shielded from these aqueous environments

Question 3

Why are biological membranes stabalised by non-covalent forces?

Answer 3

It makes the membrane very dynamic as non-covalent forces are quite weak

Question 4

What are the 3 main components of a biological membrane?

Answer 4

1. Phospholipids
2. Cholesterol
3. Glycolipids

Question 5

Basic structure of a phospholipid

Answer 5

Hydrophobic tail - 2 Fatty acids
Glycerol molecule
Hydrophilic head - Phosphate group attached to other groups e.g. Choline, Serine

Question 6

What is the only part of the cholesterol molecule is present within the hydrophilic part of the membrane?

Answer 6

OH group of cholesterol molecules

Question 7

What is the reason for the composition of the inside and outside of a cell being different?

Answer 7

Because the phospholipids (as well as other components within the membrane) aren’t distributed evenly within it - makes composition of the membrane asymmetrical

Question 8

What are the factors that affect membrane fluidity?

Answer 8

1. Temperature
2. Fatty acid composition - saturated vs unsaturated
3. Chain length
4. Degree and extent of saturation
5. Cholesterol content

Question 9

How does fatty acid composition affect membrane fluidity?

Answer 9

If membrane contains mainly saturated fatty acids then molecules can pack closely together thus decreasing membrane fluidity whereas if membrane made up of saturated and unsaturated fatty acids membrane fluidity increases as molecules can’t pack as closely

Question 10

How does Cholesterol affect membrane fluidity?

Answer 10

At low temperatures cholesterol interacts with the molecules of the phospholipids and prevents them from forming a tight association and crystallising
This causes an increase in membrane fluidity

At high temperatures cholesterol reduces fluidity of membrane by stabilising the membrane and increasing its melting point

Question 11

In spur cell anaemia by what percentage does the cholesterol content of the red blood cell membrane increase by?

Answer 11

25-65%

Question 12

What are the consequences of the increase in cholesterol within the red blood cell membrane?

Answer 12

Leads to decreased membrane fluidity meaning that the membrane becomes rigid. This results in certain molecules not being able to diffuse through the cell membrane of the red blood cell

Question 13

What are the different ways that a molecule can move through a membrane?

Answer 13

Lateral movement - sideways movement

Transverse movement - movement across a membrane

Question 14

Is lateral movement of phospholipids slow or rapid?

Answer 14

Lateral movement of lipids in the membrane is rapid

Question 15

Is transverse movement of phospholipids slow or rapid?

Answer 15

Transverse movement is slow and therefore requires the action of three enzymes

Question 16

What are the 3 enzymes involved in facilitating transverse movement of phospholipids?

Answer 16

Floppase - moves phospholipids from the cytosolic side to the extracellular side (requires ATP)

Flippase (flipase) – moves phospholipids from the extracellular side to the cytosolic side (requires ATP)

Scramblase – bidirectional movement (requires calcium but doesn’t require ATP)

Question 17

What is the main process that these enzymes are involved in? why is this?

Answer 17

Synthesis of new lipids

Enzymes that synthesise new membrane are present on cytosolic surface of the Golgi apparatus
This means that new membrane synthesis only takes place on cytosolic side of the membrane
To synthesise new membrane that is structurally correct however, there needs to be transverse movement of phospholipids from cytosolic side to luminal side
This movement is facilitated by the ABC family of enzymes

Question 18

Why is the movement of phospholipids and other molecules across a membrane so important?

Answer 18

It maintains the asymmetry of the membrane

Question 19

What is Apoptosis?

Answer 19

programmed cell death

Question 20

During Apoptosis how does the distribution of Phosphotidylserine change?

Answer 20

Phosphatidylserine mainly deposited on inner surface of the plasma membrane

During process of Apoptosis distribution of Phosphatidylserine changes significantly as it then expressed on the outer surface of the plasma membrane

This is caused by the inhibition of the enzyme aminophospholipid translocase which transports Phosphatidylserine rom the luminal side to the cytosolic side of the membrane

Question 21

What causes the distribution of phosphotidylserine to change?

Answer 21

This is caused by the inhibition of the enzyme aminophospholipid translocase which transports Phosphatidylserine from the luminal side to the cytosolic side of the membrane

Question 22

What occurs as a result of the change in distribution of Phosphotidylserine?

Answer 22

Movement recognised as a signal for a cell undergoing apoptosis by macrophages which will then ‘consume” the cell via phagocytosis

Question 23

What is the advantage of cells being destroyed by apoptosis?

Answer 23

Apoptosis allows cells to be turned over without having to induce an inflammatory response

Question 24

What are the 3 types of protein that can be associated within a membrane?

Answer 24

1. Integral membrane proteins
2. Peripheral membrane proteins
3. Lipid anchored membrane proteins

Question 25

Features of integral membrane proteins

Answer 25

Strongly associated with phospholipids within the membrane

Categorized as single or multi pass

Contain hydrophobic and hydrophilic regions

Hydrophobic region usually associated with an alpha-helix

Hydrophillic region usually within the extracellular and cytosolic part of the protein

Question 26

Features of peripheral membrane proteins

Answer 26

More weakly associated with the phospholipids than the integral membrane proteins

Located on the extracellullar or cytosolic membrane

Associated by non-covalent bonds

Can be associated with integral membrane proteins or be associated directly with the phospholipids

Question 27

Features of lipid anchored membrane proteins

Answer 27

Covalently linked to a lipid molecule

Includes proteins with lipid component embedded directly into membrane or proteins that are associated with a glycolipid or proteins

Question 28

Features of membrane carbohydrates (Glycolipids)

Answer 28

Carbohydrates are associated with both membrane lipids and proteins

Carbohydrate on all membranes faces away from the cytosol

They are often involved in cell-cell interactions or cellular recognition