Transport Across Membranes 2.3

Question 1

Name 5 factors that affect the rate of diffusion

Answer 1

1. Temperature (increased Kinetic energy = faster rate of diffusion)
2. Diffusion distance
3. Size of molecule
4. Surface area
5. Difference in concentration (gradient)
   (6. Amount of carrier proteins embedded in membrane, limiting factor for facilitating diffusion as all binding sites full = rate of diffusion plateaus)

Question 2

What is the function of Channel proteins?

Answer 2

Channel proteins are tertiary globular lipoproteins - Hydrophilic intrinsic channels act as pores by binding specific SMALL ions/water molecules (acting as a pathway as one side of the protein closes as the other opens.
In simple and facilitated diffusion channel proteins move ions from a high concentration to a low concentration across a partially/selectively-permeable membrane.

Question 3

What is the function of Carrier proteins?

Answer 3

Carrier proteins are tertiary globular glycoproteins -
Intrinsic hydrophilic carrier proteins embedded in membrane bind (in complementary shaped binding site) to a large, charged, soluble, complementary molecule and undergo a reversible conformational change in shape to release ions or polar molecules on the other side of the bilayer membrane.

In Facilitated Diffusion = passive process
In Active Transport = requires energy from ATP hydrolysis

Question 4

Define Intrinsic and Extrinsic

Answer 4

Intrinsic proteins are embedded all the way through the surface-plasma membrane (tertiary carrier/channel proteins, glycoproteins)

whereas

Extrinsic proteins are only half-embedded (loosely bound) in the membrane (receptor proteins, glycolipids)

Question 5

What is cholesterol in the phospholipid bilayer and it’s function? (1)

Answer 5

Cholesterol is an extrinsic lipoprotein (type of lipid) which;
1. decreases permeability and fluidity of the plasma membrane.

(More cholesterol = less fluid & permeable membrane)

Question 6

State Fick’s Law

Answer 6

Surface area x difference in concentration
_______________________________
diffusion distance

Question 7

☆ Describe the processes involved in the absorption of the products of starch digestion (5)

Answer 7

1. a-Glucose moves in with sodium (into epithelial cell);
2. Via (carrier/channel) protein/symport;
3. Sodium removed (from epithelial cell) by active transport/sodium potassium pump;
4. Into blood;
5. Maintaining low concentration of sodium (in epithelial cell) / maintaining sodium concentration gradient (between lumen and epithelial cell);
6. Glucose moves into blood;
7. By (facilitated) diffusion.

Question 8

Explain why phospholipids form a bilayer in plasma membranes (4)

Answer 8

1. Phospholipids have a charged phosphate hroup which is part of the hydrophilic polar head.
2. Fatty acid tails (hydrocarbon chains) are not charged/not polar and are hydrophobic.
3. Both tissue fluid and cytoplasm are aqueous solutions
4. Phospholipids form two layers called a bilayer with the hydrophobic fatty acid tails facing inward towards eachother and the hydrophilic phosphate groups facing outward interacting with the aqueous (cytoplasmic) environment.

Question 9

Describe receptor proteins

Answer 9

Tertiary globular proteins (intrinsic, embedded in P.L bilayer) can be receptors for hormones, with a (specific) complementary shaped binding site

Question 10

Why can molecules like O2 and CO2 simply diffuse through the bilayer?

Answer 10

They are small non-polar and non-charged molecules.

Question 11

What are glycoproteins involved in?

Answer 11

Cell-cell recognition

Question 12

What is the term for water channel proteins and what do they do?

Answer 12

Aquaporins
- allow water to diffuse across the membrane at a very fast rate.
(Ion channel proteins allow ions to diffuse across the membrane)