Movement of Molecules across the Membrane

Question 1

What molecules can diffuse directly through the phospholipid bilayer?

Answer 1

Small uncharged molecules such as oxygen and carbon dioxide

Question 2

Describe Channel Proteins and give an example

Answer 2

Channel proteins allow molecules to diffuse through passively (facilitated diffusion) and each type of channel protein is specific for the ion or molecule that it allows through. An example of a channel protein is aquaporin which allows the passage of water.

Question 3

What are the two types of gated channel proteins and describe how they work

Answer 3

1) Ligand gated channels- these channels are controlled by ligands that bind to the transmembrane proteins resulting in the opening or closing of a channel.
2) Voltage gated channels- the channels are activated by changes in the electrical membrane potential near the channel

Note: There are also ungated channels

Question 4

Describe transporter proteins and give an example

Answer 4

Transporter proteins can be involved in both facilitated diffusion and active transport. Unlike channel proteins the transporter proteins bind to the ions or molecules, causing the protein to undergo a conformational change- the proteins usually pass the ions or molecules across the membrane rather than just providing a route through. Transporter proteins can also take part in coupled transport whereby the movement of one material ‘downhill’ along its concentration gradient is used to pull another material ‘uphill’ against its concentration gradient. This is seen in the glucose symporter where sodium ions are transported down their concentration gradient out of the cell and glucose is moved into the cell against its concentration gradient.

Question 5

Give four examples of signal transduction

Answer 5

1) The activation of an enzyme of a G protein
2) A change in the uptake of secretion molecules
3) The re-arrangement of the cytoskeleton
4) The activation of proteins that regulate gene transcription

Question 6

What does the Na/K ATPase pump do?

Answer 6

It pumps 3 sodium ions out of the cells while pumping 2 potassium ions into the cells both against their concentration gradients.

Question 7

State the stages the Na/K ATPase pump goes through

Answer 7

1) Cytoplasmic Na+ binds to the sodium potassium pump.
2) The Na+ binding stimulates phosphorylation by ATP.
3) Phosphorylation causes the protein to change its shape and Na+ is expelled to the outside.
4) K+ binds on the extracellular side and triggers release of the phosphate group.
5) The loss of the phosphate restores the protein’s original shape.
6) The K+ is released and the cycle begins again.

Question 8

Why is the movement of sodium and potassium ions into and out of the cell important?

Answer 8

Maintaining the osmotic balance in animal cells.
Generation of the sodium ion gradient for glucose symport in small intestine.
Generation of sodium ion gradient in kidney tubules.
Generation and maintenance of ion gradients for resting potential in neurons.

Question 9

Give the steps in nerve transmission

Answer 9

1) Neurotransmitter binds to receptor protein in ligand gated sodium channel allowing Na+ ions to diffuse into the neuron.
2) Na+ movement causes depolarisation and when the threshold level is reached all voltage gated channels will open.
3) An action potential will then be generated and an impulse will be fired.
4) Once +40mV is reached, Na+ channels close and the K+ channels open causing K+ ions to move out of the cell in repolarisation.
5) K+ channels then close, the sodium-potassium pump restarts, restoring the normal distribution of Na+ and K+ ions and the resting potential.