Membrane Transport

Question 1

Why are membrane proteins important?

Answer 1

Without them, the cell would be effectively impermeable to polar molecules/ions or macromolecules. The proteins are specialised for a particular molecule or ion, allowing for very tight regulation of transport.

Question 2

Why are membranes important in the cell

Answer 2

They allow the flow of molecules in and out of cells and organelles to be regulated, permitting different compartments to have different properties.

Question 3

Briefly Describe the cell membrane.

Answer 3

Cell membranes are constructed of a lipid bi-layer, (largely impermeable, and liquid at body temperature) and are studded with proteins (specialised transporters). These proteins will generally ‘float’ across the surface of the membrane on lipid rafts.

Question 4

Phosphatidylcholine is a common phospholipid in cell membranes. Briefly describe it.

Answer 4

It consists of two hydrophobic fatty acids forming a tail and bonded to a glycerol spine. One of the glycerol’s terminal carbons is bonded to phosphate, with choline attached for that, giving significant polarity. (need pic?)

Question 5

Why is the proportion of saturated and unsaturated fatty acids in cell membranes important?

Answer 5

It will influence the consistency of the membrane. As saturated fatty acids are straight, they are generally solid at room temperature and will be less liquid (than unsaturated) at body temperature.
Unsaturated fatty acids are kinked, so are liquid at room temperature and at body temperature.

Question 6

What is the most obvious visual difference between channels and transporters in the cell membrane?

Answer 6

While channels may cross the membrane (sub-membrane domain) once or twice, transporters will generally have 8-14 sub-membrane domains.

Question 7

Briefly describe an electrochemical gradient.

Answer 7

The electrochemical gradient is the net combination of the concentration gradient and the membrane potential. Thus, it takes into account both concentration of solutes and the relative voltage across the gradient. Solutes will move down the electrochemical gradient, but equilibrium is unlikely to be 50:50 (in particles) because of the relative charge).

Question 8

How do the electrochemical properties of the cell differ from the ECF?

Answer 8

Cells maintain low Na+ and low Cl-, but high K+ compared to the ECF. This allows the passage of Na+ or K+ to generate a charge in the cell relative to the ECF.

Allowing Na+ to travel into the cell down its concentration gradient generates a net positive charge in the cell, and thus a membrane potential and an electrochemical gradient.
Allowing K+ to travel out of the cell generates a net negative charge in the cell, membrane potential and electrochemical gradient.

Question 9

What is simple diffusion in the cellular context?

Answer 9

Small and non-polar molecules can cross the lipid bilayer without the assistance of proteins. This includes CO2, O2, benzene, some H2O, and ethanol.

Question 10

How many cell membrane transport proteins are accounted for in the human genome?

Answer 10

Around 400 channels and 800 transporters.

Question 11

What are the two types of transport (and transporter) across the cell membrane?

Answer 11

Active transporters - move ions and molecules against their electrochemical gradients. This requires energy and does not occur spontaneously.

Passive transporters - move ions and molecules with their electrochemical gradients. This occurs spontaneously (thus releases energy) but is still regulated. Passive transport is also known as facilitated diffusion.

Question 12

Briefly describe channel transport across the cell membrane.

Answer 12

Channels are passive transporters that form pores within the membrane, thus allowing the regulated passage of particular ions or molecules. They are regulated to be closed or open, and allow the passage of 10^7-10^8 ions/s.

Channels are specific to their particular ion or molecule.

Question 13

What is the key difference between a channel (membrane transport protein) and a transporter.

Answer 13

While channels form a pore in the membrane, transporters span the membrane and move molecules or ions through a conformational change. As a result, they are orders of magnitude slower than channels.

Question 14

What are the four basic types of transporters (membrane transport proteins)

Answer 14

Pumps - active transporters powered directly by ATP.

Uniporters - Passive transporters that can transport one solute at a time (hence uni) through a conformational change.

Symporters - active transporters that transport multiple solutes in the same direction (e.g. into the cell) with each conformational change - one with, and one against, its concentration gradient.

Antiporters - active transporters that operate like symporters, but move the solutes in opposite directions (i.e. into and out of the cell).

Question 15

What are the key qualities of transport pumps in the cell?

Answer 15

Pumps are active transporters, directly powered by ATP and so often called primary active transporters. They move ions or molecules against their electrochemical gradient at a rate between 10^0 and 10^3 ions/s.

Pumps can be used to establish an electrochemical gradient, as they can pump ions against their concentration gradient.

Question 16

What are the key qualities of uniporters in the cell?

Answer 16

Uniporters are passive transporters that port one solute at a time (hence uni), and can shift between 10^2 and 10^4 solutes/s.
Aside from speed, the main difference between a uniporter and a channel is that, while a channel forms a pore through the membrane, a uniporter undergoes a conformational chance - closing one side while opening the other.

Question 17

What are the key qualities of symporters in the cell?

Answer 17

Symporters are active transporters that move multiple solutes in the same direction (i.e. into or out of the cell) with each step, through a conformational change. One of these solutes will move down its electrochemical gradient, providing the power to move the other solute against its electrochemical gradient. They operate between 10^2 and 10^4 solutes/s.

Symporters (and antiporters) are often called secondary active transporters, because they utilise energy liberated from another transported solute to power their operations.

Question 18

What are the key qualities of antiporters in the cell?

Answer 18

Antiporters are active transporters that move multiple solutes in opposite directions (i.e. one into the cell, one out) through a conformational change. One of these solutes will be moving with its electrochemical gradient, providing power for the other solute to move against its electrochemical gradient.

Antiporters (and symporters) are often called secondary active transporters, because they utilise energy liberated from another transported solute to power their operations.