Membrane structure and function I

Question 1

What are some common features that membranes have?

Answer 1

• They’re two molecules thick and form the closed boundaries of the cell.
• They consist of lipids (phospholipids) and proteins that carry out specific functions.
• They are non - covalent assemblies (held together by weak forces).
• They are asymmetric (aka the fluid mosaic model)
• They are electrically polarised which plays a key role in transport.

Question 2

What are the three main lipids and what do they consist of?

Answer 2

1. Triglycerides - with a glycerolmolecule attached to 3 fatty acids.
2. Phospholipids - a phosphate head attached to a glycerol with 2 fatty acids attached to it.
3. Cholesterol - a hydroxyl group (OH) attached to a hydrocarbon ring and a hydrocarbon tail.

Question 3

What two of the lipids are amphipathic and what does that mean?

Answer 3

Phospholipids and glycolipids are amphipathic meaning they posses both hydrophilic/ water-loving/ polar and lipophilic/ fat-loving properties).

Question 4

What are the factors that affect membrane fluidity?

Answer 4

1. Temperature :- as temperature rises the energy associated with the phospholipid also drops.
2. Fatty acid composition:- if you have a saturated fatty acid the molecules can pack closely together. If you have a mixture of saturated and unsaturated fatty acids they cant pack as closely together and this can affect the membrane fluidity.
3. Chain length:- A long molecule has a greater opportunity of interaction.
4. Degree and extent of saturation.
5. Cholesterol content

Question 5

Describe how cholesterol and temperature interact to affect the membrane fluidity.

Answer 5

1. Cholesterol molecules are present between the phospholipids.
2. At low temperature, the cholesterol inserts itself between some phospholipid molecules and increases the distance between the phospholipids. This prevent them from forming a good tight association. So the membrane fluidity will increase.
3. At high temperatures, cholesterol will cause the phospholipids to come in closer and bond with the cholesterol and therefore reduce membrane fluidity.

Just cholesterol effect - Hydrogen bonds form between cholesterol and phospholipids making the membrane more rigid. So as cholesterol content increases membrane fluidity decreases.

Just temperature effect - as the temperature increases the molecules gain more energy and so the spaces between the molecules increases and the fluidity of the membrane increases. And reverse for low temperatures.

Question 6

What happens in spur cell anaemia?

Answer 6

• In people who have spur cell anaemia their cholesterol content is increased by 25-65% as compared to a healthy individual.
• This causes the membrane to become rigid and less permeable.
• The cells that normally circulate and be able to squeeze through small gaps in capillaries can no longer do it.
• This causes a decreased functionality of RBCs.

Question 7

What are the two types of membrane movement?

Answer 7

There are two scouts of membrane movement:-

1. Lateral movement of lipids in the membrane is rapid.
2. Transverse movement (movement across the membrane) is slow and requires the action of three enzymes.

Question 8

Describe how membranes synthesise lipids.

Answer 8

1. New phospholipids are synthesised on the cytosolic surface of the ER. So synthesis only happens on one side of the bilayer.
2. As new lipids are inserted, the bilayer starts to bend as once side gets more enlarged.
3. Flippase then flips the phospholipids from the cytosolic side of the bilayer to the lumen side of the ER. This requires ATP.

(The reason enzyme flippase is needed is because the hydrophilic head cannot pass through the hydrophobic middle region of the membrane so it has to be flipped instead).

Question 9

What are the three transmembrane lipid transporter proteins?

Answer 9

1. Flippase - flips phospholipids from the cytosolic side (outer side) to the lumen side (inner side). This requires ATP.
2. Floppase - flips the phospholipids from the lumen side (inner side) to the cytosolic side (outer side). This requires ATP.
3. Scarmblase - Can flip the phospholipids between both sides (in to out, vice versa). [Bidirectional movement].

Question 10

What is apoptosis?

Answer 10

Apoptosis is programmed cell death.

Question 11

How does apoptosis occur?

Answer 11

1. Cells that want to under go apoptosis express phosphatidylserine on their plasma membrane.
2. Macrophages will recognise this signal and trigger apoptosis.
3. Enzymes break down the cell cytoskeleton.
4. The cytoplasm becomes dense, with organelles closely packed.
5. The cell surface membrane changes and small pieces called ‘blebs’ form.
6. Chromatin condenses and the nuclear envelope breaks, DNA breaks into fragments.
7. The cell breaks into vesicles that are taken up by phagocytosis.
8. The cellular debris is disposed of and does not damag any other cells or tissues.

Question 12

Why is apoptosis important?

Answer 12

It marks the cell for phagocytosis and the removal of these dying cells occurs in an orderly way without triggering inflammation which is important.

Question 13

Describe integral membrane proteins.

Answer 13

• They can be single or multi pass - it determine how many times the protein passes through the membrane.
• They have strong non-covalent bonds.
• In order for the proteins to interact with the membrane they also have hydrophobic and hydrophilic regions. the hydrophobic reactions of the molecules is usually associated with an alpha helix. And the majority of the amino acids which are present in the external surface of these proteins will be hydrophobic in nature. The hydrophilic portion can be in the pores of these proteins.
• Trans-membrane domain often an alpha-helix.

Question 14

Describe peripheral membrane proteins.

Answer 14

• There are located on the extracellular or cytosolic membrane.
• These proteins are weakly associated with the membrane by non-covalent bonds. Therefore they are easier to extract.
• They can be associated with lipids or proteins.

Question 15

Describe lipid anchored membrane proteins.

Answer 15

These proteins are covalently linked to a lipid molecule such as a glycerophosphatidylinositol.

Question 16

Describe membrane carbohydrates.

Answer 16

• 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.
• Carbohydrate on all membranes face the outside (away from the cytosol).
• They are often involved in cell-cell interactions or cellular recognition.