VS3: Cellular homeostasis (2) Flashcards
What does the shape of cells depend on?
The cell type and its function
What is the structure and function of the cytoskeleton of a cell?
- Supports cell shape
- Enables cell movement
- Not rigid
Are cell membranes permeable or impermeable to water?
Highly permeable to water
Are water movements passive or active?
Passive
What is osmotic pressure?
The pressure generated when two compartments separated by a membrane permeable to water but not solutes (a semi-permeable membrane) contain different solute concentrations
What is Van ‘t Hoff’s Law of osmotic pressure?
π = nCRT
n = number of dissociable particles (e.g. 2 for NaCl) C = total concentration R = gas constant T = temperature in K
Differentiate between osmolarity and osmolality.
Osmolarity: number of osmoles of solute per 1 L of solvent
Osmolality: number of osmoles of solute per 1 kg of solvent
Is osmolarity or osmolality dependent on temperature?
Osmolarity
Do small or large molecules obey Van ‘t Hoff’s Law?
Small molecules
Large molecules in solution (e.g. proteins) produce a larger osmotic pressure than expected
What is oncotic pressure?
Osmotic pressure caused by proteins
What is tonicity and what is it equal to?
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
Why are hypotonic patients given saline and not pure water?
Saline draws the water out of the swollen cells whereas pure water would cause swelling of cells
Describe the process of regulatory volume decrease (RVD). When does it occur?
- K+ channels open
- Intracellular osmolality decreases
- Water moves out
- Cell shrinks
Occurs when the cell is hypertonic
Describe the process of regulatory volume increase (RVI). When does it occur?
- K+, Cl– and Na+ flood into the cell
- Intracellular osmolality increases
- Water moves in
- Cell swells
Occurs when the cell is hypotonic
What is the main buffering agent inside a cell? What is its equation?
Bicarbonate
6 CO2 + 6 H2O ⇌ 6 H2CO3 ⇌ 6 H+ + 6 HCO3–
Name an acid extruder.
Na+/H+ exchanger
Name a base extruder.
Anion exchanger (Cl–/HCO3– exchanger)
Is the NHE a form of primary or secondary active transport? Why?
Secondary because it is driven by the Na+ gradient
Is the anion exchanger a form of primary or secondary active transport? Why?
Secondary because it is driven by the Cl– gradient
How is the pH of the cell maintained?
Acidification activates the NHE and alkalinisation activates the AE
Why is it important to maintain low cytosolic concentrations of Ca2+?
- 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
How do primary active transport methods control the resting cytosolic Ca2+ concentration?
- Plasma membrane Ca2+-ATPase (PMCA)
- Sarcoplasmic/endoplasmic reticulum Ca2+-ATPase (SERCA)
Both have high Ca2+ affinity but low capacity so remove residual calcium
How do secondary active transport mechanisms control resting cytosolic Ca2+ concentrations?
- 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
How do mitochondrial Ca2+ uniporters (MCU) control the resting cytosolic Ca2+ concentration?
- Facilitated diffusion through ion channel
- Driven by mitochondrial membrane potential
- Operate at high cytosolic Ca2+ concentration
- Help to buffer potentially damaging Ca2+ concentrations
