S2: Membrane Structure, Synthesis and Function I

Question 1

List some common features that membranes have

Answer 1

• Two molecules thick and form closed boundaries of cell
• Contain lipids and proteins to mediate distinctive cell functions
• Phospholipid bilayer
• Non covalent arrangement and are asymmetric (fluid-mosaic model)
• Membranes are electrically polarised which plays a key role in transport

Question 2

Molecular composition of lipids in the membrane

Answer 2

In general, all consist of a hydrophilic head and hydrophobic tail

• Main lipids are phospholipids with the phosphate head
• Cholesterol with OH head
• Glycolipids with a large carbohydrate head on the outside and lipid group in the membrane

Question 3

What does cholesterol do in the membrane?

Answer 3

It regulates membrane fluidity

Question 4

What does amphipathic mean?

Answer 4

They possess both hydrophilic/water loving/polar and lipophilic/fat loving properties.

e.g. phospholipids and glycolipids

Question 5

What do phospholipids and glycolipids form in aqueous media?

Answer 5

They both readily form bimolecular sheets (bilayer) in aqueous media and they ability to form liposomes might have clinical uses such as the delivery of drugs or DNA.

Question 6

Describe membrane fluidity

Answer 6

Membranes are not static, rigid structures rather they are fluid. Lipids can move across the surface (lateral movement/diffusion) rapidly. However swapping from one side to the other (transverse movement/diffusion) is slow, rarer and requires the action of the enzyme flippase. This is because it takes a lot of energy to get the hydrophilic head through the fatty membrane.

Question 7

What enzyme is needed for the transverse movement of phospholipids in the membrane?

Answer 7

Flippase

Question 8

Factors affecting the fluidity

Answer 8

• Temperature
• Fatty acid composition
• Chain length
• Degree and extent of saturation
• Cholesterol content

Question 9

What features in a fatty acid increases rigidity of membrane?

Answer 9

• Saturated fatty acyl chains increase rigidity as they pack closer together meaning stronger interactions
• Increased chain length increases rigidity as there are more interactions between chains

Question 10

How does temperature affect motility of membrane?

Answer 10

When temperature is low, the energy of molecules is low, so the motility of the membrane decreases (i.e. molecules closer together). At higher temperatures the energy associated with molecules is greater so the spaces between molecules increases and fluidity of the membrane increases.

Question 11

Describe how cholesterol in the membrane acts at different temperatures

Answer 11

Cholesterol tries to get membrane back into normal composition during changes in temperature

• At low temperatures, cholesterol (in small numbers) interferes with the interaction between phospholipids and increases membrane fluidity.
• Conversely at high temperatures, cholesterol (in greater numbers) works to stabilize the membrane and reduce membrane fluidity by bringing the phospholipids closer together.

So cholesterol has a different role at different temperatures.

Question 12

What is Spur cell anaemia?

Answer 12

In people who have spur cell anaemia their cholesterol content is increased. This high amount of cholesterol eventually decreases membrane fluidity. This leads to spikey RBC they are fragile and obstructive.

Question 13

Describe membrane synthesis

Answer 13

• New membrane is synthesised from pre-existing membrane in ER
• As it moves to golgi body it undergoes modifications
• The enzymes responsible for making the membrane are present on the inside of the ER and enzymes are only on the inside of the ER (cytosolic)

The cell will insert new phospholipid into a intracellular membranous structure. As new lipids are inserted, the bilayer starts to bow as one side gets more enlarged. Flippase then flips lipids from one side to the other so that both sides get enlarged and the membrane is no longer bowed but straight.

Question 14

What is apoptosis?

Answer 14

Cells that want to undergo programmed cell death display eat me signals for macrophages on their plasma membrane cell surface.

Question 15

What is the marker for an ‘eat me signal’ during apoptosis?

Answer 15

Phosphatidylserine is this marker.

Usually it will be on the inside of the membrane but will undergo the transverse diffusion when needed and exposed on the surface.

Question 16

Describe integral membrane

Answer 16

Integral membrane proteins span the entire membrane and it is very difficult to remove them from the membrane. Structurally they tend to be similar, usually in the transmembrane domain being alpha helix in shape with the R groups in the being hydrophobic in nature facing outwards in the membrane.
The proteins can be have a single transmembrane domain or multiple. They have strong, non-covalent bonds. IMPs interact extensively with the lipid bilayer.

Question 17

Describe

Peripheral Membrane Proteins

Answer 17

These are located on the extracellular or cytosolic membrane surface and are loosely associated with it, but not embedded just bound to a phospholipid polar head group or integral membrane protein.
They are associated by non-covalent bonds to the surface, as the association is not that strong they associate transiently with the membrane.

Question 18

What are Lipid Anchored Membrane Proteins?

Answer 18

These proteins are covalently linked to a lipid molecule such as glycerol-phosphatidylinositol.

Question 19

Describe membrane carbohydrates

Answer 19

Carbohydrates are associated with both membrane lipids and proteins, they form 2-10% of the membrane weight, in RBCs 8% of the weight is carbohydrate.
Carbohydrates on all membranes face the outside (EC domain), away from the cytosol. They are often involved in cell to cell interactions or cellular recognition.