VS3: Cellular homeostasis (2)

Question 1

What does the shape of cells depend on?

Answer 1

The cell type and its function

Question 2

What is the structure and function of the cytoskeleton of a cell?

Answer 2

• Supports cell shape
• Enables cell movement
• Not rigid

Question 3

Are cell membranes permeable or impermeable to water?

Answer 3

Highly permeable to water

Question 4

Are water movements passive or active?

Answer 4

Passive

Question 5

What is osmotic pressure?

Answer 5

The pressure generated when two compartments separated by a membrane permeable to water but not solutes (a semi-permeable membrane) contain different solute concentrations

Question 6

What is Van ‘t Hoff’s Law of osmotic pressure?

Answer 6

π = nCRT

n = number of dissociable particles (e.g. 2 for NaCl)
C = total concentration
R = gas constant
T = temperature in K

Question 7

Differentiate between osmolarity and osmolality.

Answer 7

Osmolarity: number of osmoles of solute per 1 L of solvent

Osmolality: number of osmoles of solute per 1 kg of solvent

Question 8

Is osmolarity or osmolality dependent on temperature?

Answer 8

Osmolarity

Question 9

Do small or large molecules obey Van ‘t Hoff’s Law?

Answer 9

Small molecules

Large molecules in solution (e.g. proteins) produce a larger osmotic pressure than expected

Question 10

What is oncotic pressure?

Answer 10

Osmotic pressure caused by proteins

Question 11

What is tonicity and what is it equal to?

Answer 11

The effective osmolality

Equal to the sum of the concentrations of the solutes which have the capacity to exert an osmotic force across the membrane

Question 12

Why are hypotonic patients given saline and not pure water?

Answer 12

Saline draws the water out of the swollen cells whereas pure water would cause swelling of cells

Question 13

Describe the process of regulatory volume decrease (RVD). When does it occur?

Answer 13

1. K⁺ channels open
2. Intracellular osmolality decreases
3. Water moves out
4. Cell shrinks

Occurs when the cell is hypertonic

Question 14

Describe the process of regulatory volume increase (RVI). When does it occur?

Answer 14

1. K⁺, Cl^– and Na⁺ flood into the cell
2. Intracellular osmolality increases
3. Water moves in
4. Cell swells

Occurs when the cell is hypotonic

Question 15

What is the main buffering agent inside a cell? What is its equation?

Answer 15

Bicarbonate

6 CO₂ + 6 H₂O ⇌ 6 H₂CO₃ ⇌ 6 H⁺ + 6 HCO₃^–

Question 16

Name an acid extruder.

Answer 16

Na⁺/H⁺ exchanger

Question 17

Name a base extruder.

Answer 17

Anion exchanger (Cl^–/HCO₃^– exchanger)

Question 18

Is the NHE a form of primary or secondary active transport? Why?

Answer 18

Secondary because it is driven by the Na⁺ gradient

Question 19

Is the anion exchanger a form of primary or secondary active transport? Why?

Answer 19

Secondary because it is driven by the Cl^– gradient

Question 20

How is the pH of the cell maintained?

Answer 20

Acidification activates the NHE and alkalinisation activates the AE

Question 21

Why is it important to maintain low cytosolic concentrations of Ca²⁺?

Answer 21

• To maintain the steep calcium concentration between the ECF and the cytosol
• Small changes in cytosolic Ca²⁺ concentration play an important role in cell signalling, e.g.:
  
  • Calcium-dependent enzyme activity
  • Vesicular release
• Large changes in cytosolic Ca²⁺ concentration are toxic to cells

Question 22

How do primary active transport methods control the resting cytosolic Ca²⁺ concentration?

Answer 22

• Plasma membrane Ca²⁺-ATPase (PMCA)
• Sarcoplasmic/endoplasmic reticulum Ca²⁺-ATPase (SERCA)

Both have high Ca²⁺ affinity but low capacity so remove residual calcium

Question 23

How do secondary active transport mechanisms control resting cytosolic Ca²⁺ concentrations?

Answer 23

• Na⁺–Ca²⁺ exchanger (NCX)
• Low Ca²⁺ affinity but high capacity so effective removal of most calcium
• Electrogenic: 3 positive charges enter cell for 2 positive charges leaving
• Activity is membrane potential-dependent

Question 24

How do mitochondrial Ca²⁺ uniporters (MCU) control the resting cytosolic Ca²⁺ concentration?

Answer 24

• Facilitated diffusion through ion channel
• Driven by mitochondrial membrane potential
• Operate at high cytosolic Ca²⁺ concentration
• Help to buffer potentially damaging Ca²⁺ concentrations