Membrane transport

Question 1

a. discuss factors that determine the movement of solutes by diffusion, including driving forces (electrical and chemical gradients) and membrane permeability

Answer 1

• The driving force is the electrochemical gradient
• The permeability of the membrane depends on the electric gradient and the chemical gradient, if there is a gradient the solute will diffuse across the membrane
• Once equilibrium is reached the permeability remains the same, but dynamic equilibrium is reached (so equal amount of diffusion in/out)The Nernst equation only calculates the equilibrium potential of one ion

Question 2

b. distinguish between facilitated diffusion and active transport

Answer 2

Facilitated diffusion is a type of passive transport and along the gradient. Think of a pipe connecting a full tank of water to an empty one. Active transport is against the gradient.

e.g Facilitated - solute carriers, uniporters, symporters and anti porters

Active transport - ATP dependent ion transpoeters

Question 3

c. explain primary and secondary active transport mechanisms with examples
d. describe the role of voltage gated ion channels

Answer 3

Primary active transport - transport directly coupled to ATP hydrolysis
Secondary - energy in the electrochemical gradient of one molecule is used to move another molecule against its electrochemical gradient

e.g sodium and glucose symporter –due to the electrochemical gradient of sodium there is stored energy on the outside of the cell. As sodium is actively transported out of the cell in the sodium potassium ion pump the stored energy is used for the glucose to couple with the sodium as it diffuses through the symporter

Question 4

ion channels and permeability

Answer 4

Different ion channels have different levels of selectivity, conductance and gating
Factors that control gating (permeability) are

• Membrane voltage
• Extracellular agonists or antagonists (Ach)
• Intracellular messengers (ATP)
• Mechanical stretch of the plasma membrane

Question 5

Ionotropic receptors

Answer 5

Activation of receptor causes a pore to open through which ions can pass, ions will move from one side of the membrane to the other through the poreEg –sodium gated ion channels

Question 6

Metabotropic receptors

Answer 6

Activation of receptor initiates an intracellular signalling mechanism (ions do not pass through the receptor protein)This signalling mechanism is then used to open an ionotropic (directly linked to the channel) receptor

Question 7

Movement of water

Answer 7

Water enters the cell via aquaporins
The amount of water influx is regulated by altering the number of Aquaporins in the membrane and changing the permeability (channel gating)

Question 8

Solute carriers

Answer 8

Help molecules get from one side of the membrane to the other (aka facilitated diffusion)

Question 9

Uniporters

Answer 9

Transport a single molecule across a membraneFor example the glucose transporter GLUT2

Question 10

Symporters

Answer 10

Couple the movement of two or more/ions across the membrane

The molecules are transported in the same direction via co-transport

Question 11

Antiporters

Answer 11

Couple movement of two or more molecules/ions across the membrane in opposite directionsEg the Na+-H+antiporte

Question 12

ATP dependent ion transporters

Answer 12

Sodium potassium ion pump is an example of active transportThree subunits are present (α, β, and γ) α subunithas binding sites for: Na+, K+, ATP and Ouabain (inhibitory)