B2.1 Membranes and membrane transport

Question 1

The structure of phospholipids

Answer 1

• They are amphipathic (has both hydrophilic and hydrophobic parts)
• The “head”:
  – phosphate bonded to glycerol
  – hydrophilic and polar, attracting water molecules
  – faces towards water
• The “tails”:
  – two fatty acids
  – hydrophobic and non-polar, repels water molecules
  – facing one another, away from water
  – maybe unsaturated or saturated
• Phospholipids are fluid, acting as a selectively permeable membrane hence, substances can move in and out of the cell.

Question 2

How does the phospholipid bilayer function as a barrier?

Answer 2

• Hydrophobic hydrocarbon chains that form the core of the membrane have low permeability to large molecules and hydrophilic particles –> effective barriers between aqueous solutions

Question 3

What is simple diffusion across membranes?

Answer 3

When particles are passing between phospholipids in the membrane. It can only happen if the bilayer is permeable to the particles. Examples: oxygen and carbon dioxide.

Question 4

What are the types of proteins embedded in the membrane?

Answer 4

1. Integral proteins:
   - permanently attached to the whole phospholipid.
2. Peripheral proteins:
   - temporarily attached to either the phosphate or to an integral protein.

Question 5

How does water molecules move across membranes?

Answer 5

• Due to concentration gradient (osmosis)
• Aquaporins (water channels) increase membrane permeability to water

Question 6

What are channel proteins for?

Answer 6

• Transports large and hydrophilic, polar molecules to move in or out of the membrane if it is unable to diffuse naturally due to its size or nature.
• Some channels can be opened and closed.
• Via facilitated diffusion

Question 7

What are pump proteins for?

Answer 7

• Pumps uses energy from ATP to transfer specific particles across membranes and therefore, can move particles against a concentration gradient.

Question 8

What causes selectivity in membrane permeability?

Answer 8

Facilitated diffusion and active transport allow for selective permeability because channel and pump proteins are specific to particular particles.

Question 9

What is the structure and function of glycoproteins and glycolipids?

Answer 9

Glycolipids:
- carbohydrates linked with lipids
- Role in cell recognition, helps the immune system distinguish between self and non-self cells
- Carbohydrates projects outwards to the extracellular environment.

Glycoproteins:
- Conjugated proteins with carbohydrate.
- Role in cell recognition.

Both form the glycocalyx which binds adjacent cells together, preventing the tissue from falling apart.