Osmosis

Question 1

What is osmosis?

Answer 1

net movement of water resulting in volume change (assuming sigma=1 as is the case in many biological settings).

Water moves from an area of high concentration to low concentration

Question 2

Dilution of Water by Addition of Solute- (this card illustrates the principle of osmosis with an inpermeable membrane)

Answer 2

water has a concentration:
1 liter = 1000 g/L and MW = 18g/M; [(1000g/L) / (18g/M)] = 55.5 M water (A)
• add 1 M glucose to 1 L container then add H2O and you can not add a full 1 liter of H2O, glucose takes space so only
54.5 M H2O fits (B)
• B side has higher osmolarity than A
• H2O moves from an area of low osmolarity to high osmolarity
• water moves from A to B and the B volume increases

Question 3

Does water move from low osmolarity to high osmolarity of vice-versa?

Answer 3

low to high osmolarity

Question 4

What is the definition of Osmolarity?

Answer 4

number of individual particles formed upon dissociation of a solute in a solution

1 M glucose = 1 mol / L glucose = 1 Osmole glucose
1 M NaCl = 1 mol / L Na + 1 mol / L Cl = 2 Osmoles NaCl 1MMgCl2 =1mol/LMg+2mol/LCl=3OsmolesMgCl2
1 M NaCl dissolves into: 1 osmole of Na + 1 osmole of Cl =2osmolescausedby1MNaCl

Question 5

What is the eqn for Osmotic pressure? (Van Hoff’t)

Answer 5

pi =(∆c)(R)(T)(sigma), where

sigma= reflection coefficient
if = to 1 then membrane imperm to solute
if = to 0 then membrane fully perm to solute

the greater the osmolarity difference between A and B sides, the greater the osmotic pressure = pi.
• (delta)C = difference in # of solute particles
• If reflection coefficient (sigma) = 1, membrane is impermeable to solute, only water flows across
• If reflection coefficient = 0, membrane is fully permeable to solute and pi = 0
• Hydrostatic pressure is a force exerted that will cause water to move
• When hydrostatic pressure equals osmotic pressure, no change in volume occurs.

Question 6

What is osmotic pressure?

Answer 6

is the minimum pressure which needs to be applied to a solution to prevent the inward flow of water across a semipermeable membrane. It is also defined as the measure of the tendency of a solution to take in water by osmosis.

Question 7

How will equilibrium be reached in a cell with sigma=0?

Answer 7

• Reflection coeff is 0; solutes are permeable, and solute and water move until equal concentrations on each side of membrane
• AT EQUILIBRIUM, there is an equal number of particles, thus, no osmotic pressure and no change in volume

Question 8

How will equilibrium be reached in a cell with sigma=1?

Answer 8

Reflection coeff is 1; the solutes can not move, but water can
• AT EQUILIBRIUM, the number of particles is unequal, thus, there exists an osmotic pressure and a change in volume occurs
• Osmosis = net movement of water resulting in volume change, only occurs when one solute impermeable, i.e. sigma > 0

Question 9

What is the biological relevance of sigma=1 membranes in the body?

Answer 9

Cells contain large intracellular proteins which are impermeable to the cell membrane. These increase intracellular osmolarity and leads to osmosis or a net movement of water into the cell and cell swelling. The cell’s countermeasure is to continually pump out ions thereby reducing osmolarity.

Question 10

Osmotic pressure caused by impermeable proteins is termed?

Answer 10

oncotic pressure

Question 11

Water in Capillaries or Interstitial Space. Define how equilibrium is met in relation to water flow in these systems

Answer 11

the force created by the beating heart creates a hydrostatic pressure which drives water into the interstitial space
• the proteins in capillaries (proteins are not normally present in the interstitial space) cause an osmotic pressure. Thus, water is drawn back into the capillary.
• Osmotic pressure caused by imperm proteins is termed oncotic pressure
• The balance of the hydrostatic (outward) and osmotic or oncotic (inward) pressures determines [water] in the interstitial space vs capillaries