1.3 Membrane proteins

Question 1

What is the fluid mosaic model?

Answer 1

It is a model that describes the structure of the cell membrane.

Question 2

How are integral membrane proteins held within the phospholipid belayer?

Answer 2

Regions of hydrophobic R groups allow strong hydrophobic interactions that hold integral membrane proteins within the phospholipid bilayer.
Integral membrane proteins interact extensively with the hydrophobic region of membrane phospholipids.

Question 3

Transmembrane proteins

Answer 3

Some integral proteins are transmembrane proteins (this means they span the entire phospholipid bilayer).

Question 4

Peripheral membrane proteins

Answer 4

Peripheral membrane proteins have hydrophilic R groups on their surface and are bound to the surface of membranes, mainly by ionic and hydrogen bond interactions.
Many peripheral membrane proteins interact with the surfaces of integral membrane proteins.

Question 5

What can and can’t pass through the phospholipid bilayer?

Answer 5

The phospholipid bilayer is a barrier to ions and most uncharged polar molecules.
Some small molecules, such as oxygen and carbon dioxide, pass through the bilayer be simple diffusion.

Question 6

What is facilitated diffusion?

Answer 6

Facilitated diffusion is the passive transport of substances across the membrane through specific transmembrane proteins.

Question 7

How do cells perform specialised functions?

Answer 7

To perform specialised functions, different cell types have different channel and transporter proteins.

Question 8

What are channel proteins?

Answer 8

Channels are multi-subunit proteins with the subunits arranged to form water-filled pores that extend across the membrane.
Most channel proteins in animal and plant cells are highly selective.

Question 9

Gated channel proteins

Answer 9

Some channel proteins are gated and change conformation to allow or prevent diffusion.

Question 10

Ligand-gated channels

Answer 10

Ligand-gated channels are controlled by the binding of signal molecules.

Question 11

Voltage-gated channels

Answer 11

Voltage-gated channels are controlled by changes in ion concentration.

Question 12

How do transporter proteins work?

Answer 12

Transporter proteins bind to the specific substance to be transported and undergo a conformational change to transfer the solute across the membrane.
Transporters alternate between two conformations so that the binding site for a solute is sequentially exposed on one side of the bilayer, then the other.

Question 13

What does active transport use?

Answer 13

Active transport uses pump proteins that transfer substances across the membrane against their concentration gradient.
A source of metabolic energy is required for active transport.

Question 14

What are pumps the mediate active transport?

Answer 14

Pumps that mediate active transport are transporter proteins coupled to an energy source.

Question 15

Hydrolysis of ATP in active transport

Answer 15

Some active transport proteins hydrolyse ATP directly to provide the energy for the conformational change required to move substances across the membrane.
ATPases hydrolyse ATP.

Question 16

What forms the electrochemical gradient?

Answer 16

For a solute carrying a net charge, the concentration gradient and the electrical potential difference combine to form the electrochemical gradient that determines the transport of the solute.

Question 17

What creates a membrane potential?

Answer 17

A membrane potential (an electrical potential difference) is created when there is a difference in electrical charge on the two sides of the membrane.

Question 18

Where do ion pumps get their energy from?

Answer 18

Ion pumps, such as the sodium-potassium pump, use energy from the hydrolysis of ATP to establish and maintain ion gradients.

Question 19

What does the sodium-potassium pump do?

Answer 19

The sodium-potassium pump transports ions against a steep concentration gradient using energy directly from ATP hydrolysis.
It actively transports sodium ions out of the cell and potassium ions into the cell.

Question 20

How does the sodium-potassium pump work?

Answer 20

The pump has a high affinity for sodium ions inside the cell so binding occurs.
The pump is phosphorylated by ATP which changes its conformation, causing the affinity for sodium ions to decrease and releasing these ions outside the cell.
Potassium ions bind outside the cell and the pump is dephosphorylated, causing its conformation to change.
Potassium ions are taken into the cell and the affinity returns to the start.

Question 21

How many sodium and potassium ions are transported for each ATP hydrolysed?
What does this establish?

Answer 21

For each ATP hydrolysed, three sodium ions are transported out of the cell and two potassium ions are transported into the cell.
This establishes both concentration gradients and an electrical gradient.

Question 22

Where is the sodium-potassium pump found?

Answer 22

The sodium-potassium pump is found in most animal cells, accounting for a high proportion of the basal metabolic rate in many organisms.

Question 23

The sodium-potassium pump in the small intestine

Answer 23

In the small intestine, the sodium gradient created by the sodium-potassium pump drives the active transport of glucose.
In intestinal epithelial cells the sodium-potassium pump generates a sodium ion gradient across the plasma membrane.

Question 24

What does the glucose symporter do?

Answer 24

The glucose transporter responsible for this glucose symport transports sodium ions and glucose at the same time and in the same direction.

Question 25

How does the glucose symporter work?

Answer 25

Sodium ions enter the cell down their concentration gradient. The simultaneous transport of glucose pumps glucose into the cell against its concentration gradient.