Session 2.1: Water Movement Across Membranes

Question 1

Why can water quite easily move across hydrophobic membrane bilayer?

Answer 1

Small
Uncharged (despite dipole of H-O)

Question 2

How does water move through the membrane bilayer

Answer 2

Diffusion

Driven by osmotic gradient of solutes

Question 3

How is water distributed ina. 70kg man

Answer 3

42L - total body water
2/3 of which, 28L - intracellular fluid - mostly in tissues, 2% in blood cells
1/3 of which, 14L - extracellular fluid, 75% of which is in interstitial water, 10.5L, 25% in plasma
Blood volume = blood cells + plasma, 2+3.5=5.5L

Question 4

What is the ionic distribution like between intra cellular fluid and extracellular fluid

Answer 4

Intracellular - Na+, Cl- (lower in ICF), K+ (higher in ICF)
Comparison to ECF
Sodium has concentration gradient inwards (higher on outside), 145mM to 12mM
Potassium has concentration gradient outwards (lower on outside), 4mM to 140mM
Calcium has concentration gradient inwards (higher on outside), 1.5mM to x10-7mM
Chloride has concentration going inwards (higher on outside), 123mM to 4.2mM

Question 5

What is isotonic

Answer 5

Equal solute to equal water

Question 6

What is hypotonic

Answer 6

Less solute, more water

Question 7

What is hypertonic

Answer 7

More solute. Less water

Question 8

How will water move dependent on tonicity

Answer 8

Water moves from where there is the most water (hypotonic) to isotonic to where there is less water (hypertonic)

Question 9

What happens if animal cells placed in hypotonic solution

Answer 9

More water on outside, so move into cell
Gain water
Swell
Burst (cytolysis)

Question 10

What happens if animal cells placed in hypertonic solution

Answer 10

More water on inside so leaves
Lose water
Shrivel
Shrink
Lyse

Question 11

Why is there osmotic equilibrium among different fluid compartments

Answer 11

Because water can move freely through the cell membrane and blood capillary wall
The osmolality of plasma, interstitial fluid, and intracellular fluid is roughly the same, around 280-310mOsm/kg or mmol/L

Question 12

What is osmolality determined by

Answer 12

In ECF - Na and Cl (80%)

Question 13

How can serum osmolality be estimated in clinical practice

Answer 13

By doubling serum sodium

Concentration x 2 (as 2 ions) x osmotic coefficient

Question 14

How can osmotically effective concentration be determined

Answer 14

Osmotic coefficient x number of ions formed by dissociation(eg: NaCL =2) x molar concentration of solute

Question 15

What’s the definition of osmole

Answer 15

Measure of a solution’s ability to create osmotic pressure and thus affect the movement of water

Question 16

What is osmole proportional to and give 3 examples

Answer 16

Proportional to the number of osmotic particles in solution
1mol of glucose - 1osmolar solution in 1L water
1 mol of NaCl - 2 osmolar solution in 1L water
1 mol of CaCl2 - 3 osmolar solution in 1L water

Question 17

What is osmolality

Answer 17

When the concentration of a solution is expressed in osmoles per kilogram of water, the osmolar concentration of a solution is referred to as its osmolality
1osmole/Kg of water = 1osmole/L = 1000milliosmoles/L = 1000mOSM = 1000mmol/L

Question 18

What is osmosis

Answer 18

The diffusion of water from a hypotonic solution to a hypertonic solution across a semi permeable membrane

Question 19

How can the movement of water be prevented

Answer 19

Applying pressure to the piston against direction of flow = osmotic pressure of solution, pressure required to stop flow of water.
If cell inflating = pressure increases, if cell shrivelling = pressure decreases

Question 20

How else is water able to move across lipid bilayer

Answer 20

Aquaporin channels
Not ion channels, integral membrane proteins
Permit rapid water diffusion (must faster that passive diffusion)
There are 13 or more distinct isoforms - different functions and roles

Question 21

How does water move through aquaporin channels

Answer 21

Depends on the solute concentration gradient
3 billion water molecules per second pass through channel
Single file passage
Positively charged residues within the pore which prevent ions from moving through the channel, only water. So Hydrogen ion gradients not disrupted and neither is pH

Question 22

Aquaporin family members are

Answer 22

Widely distributed

Lots in kidney