5. Transport Across Membranes

Question 1

Why is membrane described as fluid mosaic model?

Answer 1

Fluid: phospholipid bilayer in which individual phospholipids can move relative to each other.
Mosaic: the distribution of proteins within the membarne.

Question 2

What is the structure of a phospholipid bilayer?

Answer 2

Phospholipid is 2 fatty acids, glycerol and phosphate with ester bonds.
Bilayer is 2 layers of phospholipid molecules.
Hydrophobic tails point in towards the membrane interior. Hydrophilic heads point out towards the membrane surface. Acts as a barrier to water soluble substances as the non polar fatty acid tails prevent polar molecules from passing across.

Question 3

What is the function of proteins in membrane?

Answer 3

• Channel protein: allow the movement of specific ions and polar
  molecules through the phospholipid bilayer.
• Carrier protein: transport of specific ions and polar molecules (e.g. glucose) by
  facilitated diffusion or active transport.
• Glycoproteins: made up of carbohydrate and protein, on the outer surface of the
  membrane - allowing cell recognition i.e. antigens.
• Specific receptors: molecules bind to them hormones and cause change inside
  the cell (e.g. insulin)
• Enzymes catalyse reactions

Question 4

What is the function of glycoproteins in the cell membrane?

Answer 4

made up of carbohydrate and protein, on the outer surface of the
membrane - allowing cell recognition i.e. antigens.

Question 5

Whats the function of glycolipids in cell membrane?

Question 6

Whats the function of cholesterol in cell membrane?

Answer 6

reduces the movement of other molecules in the membrane i.e. the
more cholesterol the less “fluid” the membrane.

Question 7

What is simple diffusion?

Answer 7

• passive process
• movement of small , non polar, lipid soluble molecules .e.g. O2 , CO2
• from high to low concentration
• straight through the phospholipid bilayer.

Question 8

What factors can affect the rate of simple diffusion ?

Answer 8

• conc gradient: steeper gradient= faster rate as its not limited by number of proteins.
• temp: higher temp= more ke = faster rate.
• SA: greater SA= faster rate.
• thickness: thinner membrane= faster rate due to shorter diffusion distance.

Question 9

What is facilitated diffusion?

Answer 9

• passive process
• movement of small, charged , polar molecules .e.g. NA+, K+.
• from a high to low concentration
• through channel (if ions) or carrier ( if large molecules) proteins

Question 10

What is the rate affected by in facilitating diffusion?

Answer 10

Rate increases but is limited by the number of carrier proteins.

Question 11

Why are membranes which are folded have a faster diffusion?

Answer 11

Folded into microvilli, increases SA.
Also means that have a larger number of carrier and channel proteins for faster facilitated diffusion ,until all proteins are saturated.
May also have a larger number number mitochondria to release more ATP by aerobic respiration for active transport.

Question 12

What is active transport?

Answer 12

-movement of molecules against a conc gradient
— through carrier proteins
- using ATP.
( rate limited by availability of ATP)

Question 13

What is osmosis?

Answer 13

-passive process
- water moves by osmosis down the water potential gradient
- across a partially permeable membrane
- via aquaporins.

Question 14

What has the max water potential?

Answer 14

Water= 0KPa which is the max water potential.
All othet solutions have a negative water potential.