Cell physiology - Osmosis

Question 1

Osmolarity definition and formula

Answer 1

Concentration of osmotically active particles in a solution

Question 2

What is the osmolarity of a 1 M NaCl solution?

Answer 2

Question 3

Number of particles (g) in a solution

• NaCl
• Glucose
• CaCl₂
• KCl

Answer 3

• NaCl = 2
• Glucose = 1
• CaCl₂ = _​3
• KCl = 2

Note: Literal number of particles

Question 4

Determine tonicity of these solutions

• 150 mM NaCl
• 150 mM CaCl3
• 350 mM mannitol
• 300 mM urea
• 300 mM mannitol

Answer 4

• 150 mM NaCl
  
  • Isotonic
• 150 mM CaCl3
  
  • Hypertonic
• 350 mM mannitol
  
  • Hypertonic
• 300 mM urea
  
  • Hypotonic
• 300 mM mannitol
  
  • Isotonic

Note: Therefore, approximately osmolarity of 300 is isotonic

Question 5

Why is 350 mM of urea hypotonic?

Answer 5

Because its reflection coefficient is zero

Question 6

Law from which calculation of osmotic pressure is based

Answer 6

van’t Hoff’s law, which states that osmotic pressure depends on the concentration of osmotically active particles

Question 7

Reflection coefficient (σ) definition and what do the values mean?

Answer 7

• Definition
  
  • A number between zero and one that describes the ease with which a solute permeates a membrane
• What if the reflection coefficient is one?
  
  • The solute is impermeable; therefore, it is retained in the original solution, it creates an osmotic pressure, and it causes water flow
• What if the reflection coefficient is zero?
  
  • The solute is completely permeable; therefore, it will not exert any osmotic effect, and it will not cause water flow

Memory aid:

• 1mpermeable

Question 8

Body substances with nearly 1 and close to 0 coefficient and its implication

Answer 8

• Serum albumin (a large solute) → reflection coefficient of nearly one
• Urea (a small solute) → reflection coefficient of close to zero → ineffective osmole

Question 9

So, what is the relationship between osmotic pressure and reflection coefficient?

Answer 9

• Effective osmotic pressure = osmotic pressure X reflection coefficient
• If the reflection coefficient is one, the solute will exert maximal effective osmotic pressure; if the reflection coefficient is zero, the solute will exert no osmotic pressure

Question 10

Solutions A and B are separated by a semipermeable membrane. Solution A contains 1 mM sucrose and 1 mM urea. Solution B contains 1 mM sucrose. The reflection coefficient for sucrose is one and the reflection coefficient for urea is zero. Compare the osmotic pressure, osmolarity and tonicity and effective osmotic pressure of the solutions.

Answer 10

• The calculated osmolarity of solution A is 2 mOsm/L
• Calculated osmolarity of solution B is 1 mOsm/L
• Therefore, solution A, which has a higher osmolarity, is hyperosmotic with respect to solution B

Actually, solutions A and B have the same effective osmotic pressure (i.e., they are isotonic) because the only “effective” solute is sucrose, which has the same concentration in both solutions