Membrane Transport

Question 1

What is diffusion?

Answer 1

Movement of atoms, molecules or ions as a result of random motion

Question 2

What is net diffusion?

Answer 2

Movement of substance down the concentration gradient.

Question 3

What is Flick’s first law of diffusion?

Answer 3

The rate of diffusion across a planer surface is directly proportional to the area of the plane and the concentration gradient.

Question 4

Give the Flick’s first law equation.

Answer 4

J= - DA(dc/dx)
J = net rate of diffusion in moles or grams per unit in time
A = area of the plane
dc/dx = concentration gradient across the plane
D = constant of proportionality, diffusion coefficient.

Question 5

How is diffusion coefficient calculated?

Answer 5

Diffusion coefficient is directly proportionate to absolute temperature and inversely proportionate to radius of molecule and viscosity of the medium.

Question 6

How do time for diffusion differ over long distances?

Answer 6

Time for diffusion is proportionate to the square of the distance.

Question 7

Describe the rate of diffusion across a barrier when molecules don’t interact with barrier.

Answer 7

The net rate of diffusion is directly proportionate to the diffusion coefficient, area of the plane, and the concentration gradient across the barrier. It is inversely proportionate to the thickness of the barrier.

Question 8

Give the equation for diffusion across a barrier.

Answer 8

J = - DA(deltaC/deltaX)
Delta C = concentration gradient across the barrier
Delta X = the thickness of the barrier.

Question 9

What is lipid solubility?

Answer 9

It is the most important factor determining how fast a substance move across a membrane.

Question 10

How does lipid solubility determine rate of diffusion?

Answer 10

• The more lipid soluble a substance is, the faster the diffusion rate.
• With same lipid solubility, the smaller the substance is the faster it diffuse across a membrane.

Question 11

How is lipid solubility determined?

Answer 11

Lipid solubility of a substance (partition coefficient) is the ratio between concentration in oil phase and concentration in water phase.

Question 12

Give the equation for partition coefficient.

Answer 12

Bi = [i] oil/ [i] water

Question 13

How do different membranes behave with different substance?

Answer 13

Different substances will be more or less readily dissolve in the membrane depending on the lipid solubility of the substance.

Question 14

How does the concentration gradient determined in diffusion across a membrane?

Answer 14

Gradient is given by the difference between the concentration at the inner edge and the outer edge of the membrane.

Question 15

How do the inner and outer edge concentration relate to the concentration in the extracellular fluid and the cytoplasm?

Answer 15

Cm,o=BCo

Cm,i=BCi

Question 16

Give the equation for diffusion of a substance across a membrane.

Answer 16

J= - PA(Ci-Co)

Question 17

How do gases, water and small uncharged water-soluble molecules cross the membrane?

Answer 17

They readily pass through the membrane.

Question 18

How do size affect the permeability of the molecule?

Answer 18

Increase in size will decrease permeability

Question 19

What is the size limit for molecules to pass through the membrane?

Answer 19

Water soluble molecules with MW>200 are impermeable.

Question 20

How do ions pass through the membrane?

Answer 20

• Ions are not permeable through the lipid membrane.

- Ions can only move across membranes with the help of integrated membrane proteins.