3. Membrane proteins

Question 1

What are integral membrane proteins?

Answer 1

• Integral membrane proteins are held firmly in place within the membrane.
• They are held in place by strong hydrophobic interactions with the phospholipid tails.
• Regions of hydrophobic R groups allow strong hydrophobic interactions that hold integral membrane proteins within the phospholipid bilayer.
• Some integral membrane proteins are transmembrane, meaning that they span the entire width of the membrane.

Question 2

What are peripheral membrane proteins?

Answer 2

• Peripheral membrane proteins are only loosely associated with the plasma membrane.
• 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 3

Describe how the phospholipid bilayer functions as a barrier.

Answer 3

• 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 by simple diffusion.
• However, most molecules require membrane proteins to enter or exit the cell.

Question 4

What is facilitated diffusion?

Answer 4

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

Question 5

What are channel proteins?

Answer 5

• 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 6

What are gated channel proteins?

Answer 6

• Some channel proteins are gated and change conformation to allow or prevent diffusion.
• Gated channels respond to a stimulus which causes them to open or close.

Question 7

What are ligand-gated channels?

Answer 7

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

Question 8

What are voltage-gated channels?

Answer 8

Voltage-gated channels are eletrical, and controlled by changes in ion concentrations.

Question 9

What are transporter proteins?

Answer 9

Transporter proteins bind to the specific substance to be transported and undergo a conformational change to transfer the solute across the membrane.

Question 10

What is active transport?

Answer 10

• Transport across a membrane which requires a source of metabolic energy.
• Active transport uses pump proteins that transfer substances across the membrane against their concentration gradient.
• Some active transport proteins hydrolyse ATP directly to provide the energy for the conformational change required to move substances across the membrane.

Answer 11

• All cells have an electrical potential difference (voltage) across their plasma membrane. This voltage is called the membrane potential.
• 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 12

Describe the mechanism of the sodium potassium pump.

Answer 12

• Ion pumps, such as the sodium-potassium pump, use energy from the hydrolysis of ATP to establish and maintain ion gradients.
• 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 13

Describe the process of the sodium potassium pump.

Answer 13

1. The transporter protein has high affinity for sodium ions inside the cell therefore binding occurs.
2. Phosphorylation by ATP causes the conformation of the protein to change.
3. The affinity for ions changes resulting in sodium being released outside of the cell.
4. Potassium ions from outside the cell bind to the sodium-potassium pump.
5. Dephosphorylation occurs which causes the conformation of the protein to change.
6. Potassium ions are taken into the cell and the affinity returns to the start.

Question 14

What are the practical applications of the sodium potassium pump?

Answer 14

• The sodium-potassium pump is found in most animal cells, accounting for a high proportion of the basal metabolic rate in many organisms.
• In the small intestine, the sodium gradient created by the sodium-potassium pump drives the active transport of glucose.
• The glucose transporter responsible for this glucose symport transports sodium ions and glucose at the same time and in the same direction.