L16 - Introduction to cell membranes

Question 1

What is the role of the plasma membrane and organelle membranes?

Answer 1

Plasma membrane- provides cell boundary and prevents movement of materials in to and out of the cell

Organelle membranes - divide cytoplasm into compartments

Question 2

What are the fundamental properties of membranes?

Answer 2

• Barrier
• Flexible, self-repairing, continuous
• Selectively permeable
• Only certain molecules can pass into and out of cells

Membranes are symmetric.
Proteins always have the same orientation in the membrane and lipid composition of each of the two halves of the bilayer are different.

Question 3

What are the main components of the membrane?

Answer 3

• Lipids
• Carbohydrates
• Proteins.

Oligosaccharide chains covalently linked to protein or lipid. Made up of sugar residues linked covalently in branched chains.

Question 4

What is the role of phospholipids in the membrane structure?

Answer 4

Phospholipids are amphipathic as they have a polar hydrophilic head and a hydrophobic tail

Phospholipids move about in the bilayer – they can rotate or exchange in the lateral plane of the membrane.

Question 5

How do the number of double bonds affect the fluidity of the membrane?

Answer 5

The great the number of double bonds, the shorter the acyl chains, the less tightly packed the molecules = great fluidity.

The fewer the number of double bonds, the longer the acyl chains, the more tightly packed the molecules are = less fluidity

Question 6

Do warm or cold blooded animals have more double bonds?

Answer 6

Cold blooded and plants:

All membranes are fluid at the temperature of the organism – so plant and cold-blooded animal membranes have more double bonds and shorter acyl chains than membranes of warm blooded animals.

Question 7

What is the importance of cholesterol in the membrane?

Answer 7

Cholesterol makes the membrane LESS permeable.

Cholesterol packs between phospholipids to make the membrane less deformable at the surface - BUT does not make the membrane more rigid.

Question 8

Describe the location of integral and peripheral proteins.

Answer 8

Proteins have floating in a sea of lipids:

Integral membrane proteins directly insert in the membrane by a hydrophobic domain (span the entire width of the membrane).

Peripheral membrane proteins associate with integral membrane proteins or directly bind lipids. Covalently banded lipids which insert into the membrane (just on outside of the membrane). ——-> Small G proteins have a fatty acid modification which allows them to cycle on and off membranes - ‘Ras’ mutated in many cancers is an example of this.

Question 9

What are micro domains called rafts?

Answer 9

Where cholesterol and sphingolipids can form micro domains called rafts. The membrane is slightly thicker in the micro domain.