Structure, Order and Function

Question 1

How/why are cells limited in how large they can be?

Answer 1

This is because the surface area and volume ratio does not stay the same as their size increases. Because of this, it is harder for a large cell to pass materials in and out of the membrane, and to move materials through the cell.

Question 2

What are amphiphilic substances?

Answer 2

Molecules which have both a polar and non-polar component, e.g. LCFAs and phospholipids.

Question 3

Describe the approximate volumes of the various extracellular fluid compartments.

Answer 3

Interstitial fluid ~ 10.4L
Plasma ~ 2.8L
Transcellular fluid ~ 0.8L

Question 4

Name 4 examples of where transcellular fluid is found.

Answer 4

Brain ventricles
Peritoneal cavity
Joint capsules
Vitreous humour

Question 5

Define osmotic pressure and describe the equation by which it is calculated.

Answer 5

“Osmotic pressure is a property of
solutions that is of particular importance
in medicine because of the role it plays
in the transport of molecules across
membranes.” 

Osmotic pressure (pi) = MRT

M = molarity, R = gas constant, T = thermodynamic (absolute) temperature

Question 6

Describe the total osmolality of the plasma and the respective osmolality of its constituent parts.

Answer 6

Total osmolality = 275-295 mOsmol kg-1
Principal ions (Na+, K+, Cl-, HCO3-) = 280 
Glucose and other small molecules = just under 10 
Proteins ~ 1 (less than 0.5% of total plasma osmolality)

Question 7

Explain what is meant by the terms iso-osmotic and iso-tonic.

Answer 7

Iso-osmotic - two solutions with the same osmolality.

Iso-tonic -

Question 8

Oultine the effects of hypo and hypertonic solutions on the blood plasma.

Answer 8

Hypotonic - water drawn into cells, which expand and lyse.

Hypertonic - water drawn out of cells, which shrink.