transport across membranes

Question 1

Explain the arrangement of phospholipids in a cell membrane.

Answer 1

-Bilayer, with water present on each side
-Hydrophobic fatty acid tails repelled from water so point away from water.
-Hydrophillic phosphate heads attracted to water so point to water.

Question 2

Explain the role of cholesterol in cell membranes.

Answer 2

-Restricts movement of other molecules making up the membrane
-So decreases fluidity + permeability

Question 3

Suggest how cell membranes are adapted for other functions.

Answer 3

-Phospholipid bilayer is fluid therefore membrane can bend for vesicle formation
-Glycoproteins / glycolipids act as receptors + are involved in cell signalling / recognition.

Question 4

Describe how movement across membranes occurs by simple diffusion.

Answer 4

-Lipid-soluble or very small substances move from an area of high conc. to low conc., down a conc. gradient
-across the phospholipid bilayer
-passive so doesn’t require energy from ATP

Question 5

Explain the limitations imposed by nature of the phospholipid bilayer.

Answer 5

-Restricts movement of water soluble (polar) + larger substances
-Due to hydrophobic fatty acid tails in interior of bilayer

Question 6

Describe how movement across membranes occurs by facilitated diffusion.

Answer 6

-Water-soluble (polar) substances move down a conc. gradient
-Through specific channel / carrier proteins
- Passive so doesn’t require energy from ATP