Membranes

Question 1

Describe the ion makeup of intracellular solutions.

Answer 1

High potassium, low sodium and very low calcium ions.

Question 2

Describe the ion makeup of extracellular solutions.

Answer 2

Low calcium ions (but much higher than intracellular), low potassium and high (about 145mM) sodium ions.

Question 3

List the factors affecting solute diffusion across membranes.

Answer 3

• Lipid solubility of the substance
• Presence of specific membrane proteins
• Transmembrane voltage gradients
• Molecular weight of the diffusing substance
• Diffusion distance
• Membrane surface area

Question 4

What is simple diffusion and when does it occur?

Answer 4

• Solutes are passed down their concentration gradient across a partially permeable membrane.
• Happens directly through the lipid bilayer if the diffusing substance is lipid soluble e.g. oxygen, steroid hormones
• Passive process

Question 5

What is facilitated diffusion and when does it occur?

Answer 5

• Non-energy requiring transporters including channels/carriers.
• Polar ions and molecules are hydrophilic so require specific membrane proteins to form a diffusion pathway.
• Occurs down concentration/electrochemical gradient.

Question 6

What are 3 key differences between simple and facilitated diffusion?

Answer 6

1. FD is specific
2. FD is limited by the number of carrier proteins as they can become saturated
3. FD can be affected by competitive inhibition (something else binds to carrier protein)

Question 7

What is osmosis?

Answer 7

• Water will move across a membrane from a region of low solute concentration (high water concentration) to a region of high solute concentration (low water concentration).
• Largely depends on aquaporins

Question 8

What is osmolarity and what are the units?

Answer 8

The number of particles per litre of solution (mosmol L−1)

Question 9

What is osmotic pressure?

Answer 9

Exact amount of pressure required to STOP osmosis.

Question 10

What are isosmotic solutions?

Answer 10

Both have the same concentration of dissolved solutes (the same osmostic pressure).

Question 11

What is hyperosmotic?

Answer 11

Increased osmotic pressure (due to increased dissolved solutes; more concentrated).

Question 12

What is hypo osmotic?

Answer 12

Decreased osmotic pressure (due to decreased dissolved solutes; more dilute).

Question 13

What is tonicity?

Answer 13

The ability of a solution to cause a cell to gain or lose water.

Question 14

What is the saturation difference between channels and carriers?

Answer 14

A “carrier” will saturate, but a “channel” will not.

Question 15

What is active transport?

Answer 15

• Always requires carrier proteins
• Uses an energy source e.g. ATP to move molecules against their gradients.

Question 16

What is primary active transport and name an example.

Answer 16

• Carrier protein hydrolyses ATP and utilizes part of the released energy for the
  transport.
• Proton pump in the stomach transports H+ from the blood to the
  stomach against a huge electrochemical gradient.

Question 17

What is secondary active transport and name an example.

Answer 17

• Uses the potential energy stored by a concentration difference of an ion. The
  energy-requiring transport is coupled to simultaneous transport of an ion
  (most commonly Na+) down its electrochemical gradient.
• The movement of glucose and amino acids into the intestine.

Question 18

What is endocytosis?

Answer 18

• Uptake of extracellular substances by vesicles.
• Requires energy.

Question 19

What is exocytosis?

Answer 19

• Process by which a cell transports secretory products through the cytoplasm to the plasma membrane via vesicles.
• Requires energy

Question 20

What is transcytosis?

Answer 20

Transports substances from one extracellular fluid compartment to another;
the endocytic vesicle is transported to the opposite side of the cell and content released by exocytosis.

Question 21

Outline the pattern of K+ ions across the cell membrane and what movement this causes.

Answer 21

The concentration of K+ is higher inside the cell that outside and the membrane is more permeable to K+, so K+ diffuses out.

Question 22

Define equilibrium potential.

Answer 22

The potential at which an ions chemical driving force in one direction is equal to the electrical force driving it in the other direction and bulk ion movement is thus in equilibrium.

Question 23

Explain the term resting membrane potential.

Answer 23

• The membrane potential when a cell is at rest.
• This occurs because of large (membrane impermeable) charged molecules within the cell, the pumps and channels create an imbalance of ions inside and outside the cell.
• At rest the membrane is more permeable to K+ than Na+ - K+ will move out and no Na+ can move in (membrane impermeable).

Question 24

Define hyperpolarisation.

Answer 24

The membrane becomes more -ve.

Question 25

Define depolarisation.

Answer 25

The membrane becomes less –ve (more +ve).

Question 26

What properties do all ion channels have?

Answer 26

• Gate (open or close)
• Sensor e.g. voltage-gated, pH, mechanical activity
• Membrane bound proteins