Physiological functions of transporters

Question 1

Cell shape

Answer 1

• Highly variable

- Depends on cell type and its function

Question 2

Cytoskeleton

Answer 2

• Supports cell shape
• Enables cell movement
• But is not rigid (shape can be changed by external factors)

Question 3

Osmotic pressure

Answer 3

Pressure generated when two compartments separated by a semi-permeable membrane contain different solute concentrations.

Question 4

van’t Hoff’s law

Answer 4

Osmotic pressure   π=nCRT
n: no. of dissociable particles, e.g. 2 for NaCl
C: total conc
R: gas constant
T: temp in K

Question 5

Osmolarity

Answer 5

Number of osmoles of solute per 1 L of solvent (Osm/L).

Question 6

Osmolality

Answer 6

Number of osmoles of solute per 1 kg of solvent (Osm/kg).

Question 7

Osmole

Answer 7

Dissolved particle

Question 8

Why is osmolality a better measure than osmolarity?

Answer 8

It is temperature independent as volume varies with temp but mass does not.

Question 9

Oncotic pressure

Answer 9

• Osmotic pressure that is caused by proteins
• Proteins (and other large molecules) in solution produce larger osmotic pressure than expected
• van’t Hoff’s law only applies to small molecules

Question 10

Tonicity

Answer 10

The effective osmolality and is equal to the sum of the concentrations of the solutes which have the capacity to exert an osmotic force across the membrane.

Question 11

How is the cell regulatory volume controlled

Answer 11

• Regulatory volume decrease (RVD)- mainly loss of KCl (cell conc of these ions is very high)
• Regulatory volume increase (RVI)- mainly NaCL uptake, but Na+ exchanged for K+ via Na+/K+ ATPase (3:2 ratio)

Question 12

What membrane transport proteins maintain cellular pH

Answer 12

• Na/H exchanger (acid extruder)

- Anion exchanger (base extruder)

Question 13

Na+/H+ exchanger (NHE)

Answer 13

• Antiport- exchanges extracellular Na+ for intracellular H+ -> alkalises cytoplasm
• Secondary active transport driven by Na+ gradient
• Also regulates cell volume

Question 14

Anion exchanger (AE)

Answer 14

• Antiport- exchanges extracellular Cl- for intracellular HCO3- -> acidifies cytoplasm
• Secondary active transport driven by Cl- gradient

Question 15

Co-ordination of acid and base extrusion

Answer 15

Rates of acid and base extrusion from cytoplasm are pH sensitive:

• Acidification activates Na+/H+ exchanger
• Alkalisation activates anion exchanger
• Only works with small disturbances

Question 16

Cellular calcium homeostasis

Answer 16

• Steep calcium concentration gradient: ~4-5 orders of magnitudes
• Small changes in cytosolic [Ca2+] play important role in cell signalling e.g. calcium-dependent enzyme activity & vesicular release (e.g. synapses)
• Large changes in cytosolic [Ca2+] are toxic to cells -> important to maintain low cytosolic [Ca2+]

Question 17

Sources of Ca2+ to increase cytosolic [Ca2+]

Answer 17

• Extracellular membrane via Ca2+ channels

- Release from intracellular stores

Question 18

Removal of excess cytosolic Ca2+ via active transport

Answer 18

• Plasma membrane Ca2+ ATPase (PMCA)
• Sarco/endoplasmic reticulum Ca2+ ATPase (SERCA)
• High Ca2+ affinity, but low capacity
• Remove residual calcium

Question 19

Removal of excess cytosolic Ca2+ via secondary active transport

Answer 19

• Na+Ca2+ exchanger
• Low Ca2+ affinity, but high capacity
• Removal of most Ca2+

Question 20

Removal of excess cytosolic Ca2+ via mitochondria

Answer 20

• Mitochondrial Ca2+ uniporters
• Facilitated diffusion
• Operate at high [Ca2+]cyt
• Mitochondria -ve charged -> driven via electrical gradient against conc gradient