Osmolarity: Partition of fluids across membranes

Question 1

Classify the types of transport across cell membranes in the body

Answer 1

PASSIVE TRANSPORT
No Energy --> Down concentration gradients)
1. Diffusion 
2. Osmosis
3. Facilitated diffusion

ACTIVE TRANSPORT
Energy –> against concentration gradients
1. Carrier mediated transport
2. Vesicular transport

Question 2

How does the amount of water in the body vary with age and sex

Answer 2

Decreases with increasing age

Females contain less body water

Question 3

List the predominant intracellular and extracellular ions

Answer 3

Intracellular
K+ which balances the neg charge of organic solutes (A-)

Extracellular
Na + and Cl-

Question 4

Why is water an excellent solvent?

Answer 4

1. Has a high dielectric constant (reduces electrostatic forces between molecules by 80 times when compared to air)
2. Water ‘dipoles’ are strongly attracted to ions and charged solutes –> coating them with an insulating layer of water molecules –> this allows the ions to exist in solution, without reacting with other ions nearby.

Question 5

What barriers need to be crossed in order for solutes to be successfully transported to the cell?

Answer 5

1. The capillary endothelium
2. Basement membrane (collagen matrix)
3. Interstitial fluid layer
4. Plasma membrane of cell

Question 6

Define diffusion

Answer 6

The process by which the molecules of a substance transfer through a layer or area such as the surface of a solution.

Question 7

Define Ficks law of diffusion

Answer 7

The rate of diffusion of a substance across unit area (such as a surface or membrane) is proportional to the concentration gradient

Question 8

What is ‘ion trapping’.

Give two examples of where this occurs

Answer 8

Weak acids and bases exist in both ionized and non-ionized (undissociated) forms. the undissociated form may travel intracellularly by non-ionic diffusion. The unionized form then dissociates intracellularly in the interior of the cell, where it remains.

1. NH3 unionized passes through tubular cells into the urine where it combines with H+ ions to form NH4+ which is trapped in the urine
2. Obstetric anaesthesia using local anaesthetic drugs: foetus has lower blood pH than the mother

Question 9

Define osmosis

Answer 9

The diffusion of a solvent across a membrane that is impermeable to the solute, from a region of low to high solute concentration, so as to equalize the concentration of solute molecules

Question 10

Differentiate and define osmolarity and osmolality. Why can these terms be used interchangeably within the human body

Answer 10

Osmolarity
Sum of individual molarities of ions in the body: mmol/L
It is affected by temperature and the actual concentration of the solute

Osmolality
A measure of solute concentration independent of temperature and the actual concentration of the solute.
It is the number of osmoles per kilogram of solute i.e. mOsmol/kg.

Interchangeable use:
In the body the solvent is water, which has a density of 1g/1ml and solutions are dilute. Therefore, osmolarity and osmolality are essentially the same at body temperature so can be used interchangeably.

Question 11

What is the formula used to calculate the osmotic pressure generated across a semi-permeable membrane by a solute?

Answer 11

P = Sum (c1 - c2) RT

(similar symbols to universal gas law PV = nRT)

P = osmotic pressure
Sum (c1 - c2) = the sum of the difference in concentrations of the different ions each side of the membrane, moles/L
R = Universal gas constant (8.31 J/K/mole)
T = absolute temperature (in Kelvin)

Question 12

How many moles of a solute, at what volume and at what temperature generate a pressure of 101.325 kPa (1atm)

Answer 12

1 mole of ANY SOLUTE in 22.4 L at 0 degrees Celsius

Question 13

What is the osmotic force between plasma and interstitial fluid created by?

Answer 13

Proteins as the endothelium is permeable to water and electrolytes but not protein

Question 14

Define the Starling filtration equation

Answer 14

Net fluid movement across a capillary wall:

= Kf (Pc - Pi) - δ( π c - π i)

Kf - filtration co-efficient
(constant related to the permeability of the capillary wall)

δ - reflection co-efficient, a constant that represents the permeability of the capillary to proteins

P = hydrostatic pressure
π = interstitial pressure
subscript c - capillary
subscript i - interstitium

Question 15

Define the universal gas equation

Answer 15

PV = nRT
P = pressure (kP
V = volume (L)
n = a number of moles (moles)
R = Universal gas constant (8.31 J/K/mole)
T = absolute temperature (K)

Question 16

How does 1 mole of a solute in 1L of solvent affect the freezing point and boiling point. Does the type of solute matter?

Answer 16

1 mole of solute (any type) in 1 L of water:

1. Depresses freezing point by 1.86 deg C
2. Elevates boiling point 1.86 deg C

Question 17

How is serum osmolarity calculated and how and why does this differ from measured serum osmolarity

Answer 17

Estimated Osmolarity = 2[Na] + [Glucose] + [Urea]
= ± 300 mOsmol/L

Measured (direct) osmolarity = ± 285 - 295 mOsmol/L

Differs from measured osmolarity due to interactions between ions which make the activity less than 100%

If the direct and estimated osmolarities differ, deductions concerning the presence of foreign substances or abnormalities of ECF volume can be made.

Question 18

Define tonicity

Answer 18

Tonicity describes the behaviour of cells when bathed in a solution and refers to the comparison of the osmolarity of the plasma with that solution.

Hypertonic - water moves out of cells
Isotonic - equal movement of water in and out
Hypotonic - Water moves into cells

Question 19

Define facilitated diffusion

Answer 19

Carrier proteins that facilitate movement of molecules across a the membrane

No energy required
Substances move down conc. gradient in either direction.
The transmembrane proteins are specific to the substance

Question 20

Differentiate primary active transport from secondary active transport

Answer 20

Primary active transport - energy is directly linked to the movement across the membrane.

Secondary active transport
The ‘downhill’ movement of one substance can provide the energy for the ‘uphill’ movement of another: Co-transport and countertransport.

Question 21

Describe the function of the Na-K-ATPase pump and what type of transport is this

Answer 21

Primary active transport

Driven by the enzyme ATPase

3 Na bind cytoplasmic side of transmembrane protein –> change of protein conformation –> the molecule becomes phosphorylated at the expense of an ATP –> which leads to further conformational change and transfer of 3Na ions across the membrane –> new conformation has a high affinity for K ions on the extracellular side which bind –> further conformational change –> release of 2 K ions intracellularly

Question 22

How many different ATPase systems are there and what are these

Answer 22

1. Calcium
2. Hydrogen
3. Na/K pump

Question 23

Give examples of cotransport and countertrasnsport secondary active transport systems

Answer 23

Cotransport
Intestinal wall
1. Na - Glucose
2. Na - amino acid

Countertransport
Throughout
1. Ca/Na
2. H/Na
3. K/H

Question 24

List common substances that affect the function of the Na/K ATPase pump

Answer 24

Increase pump function

1. Thyroxine
2. Aldosterone
3. Insulin

Decrease pump function

1. Cardiac glycosides (digoxin)
2. Dopamine

Magnesium is required for the normal functioning of the Na/K ATPase pump

Question 25

Why does glucose cross the cell membrane slowly. what system is in place to assist with glucose transport into cells and how does this differ from glucose absorption in the small intestine

Answer 25

Large size and large atomic weight (180).

A family of glucose transporters: GLUT 1 - 4 aids glucose uptake into cells.

SKELETAL MUSCLE AND FAT
Insulin –> increases numbers of GLUT 4 transporters in cell membrane –. accelerating uptake of glucose in skeletal muscle and fat.

GIT
1. Across intestinal brush border:
Secondary active transport: Na - Gluc symport (driven by conc. gradient formed by the Na/K ATPase pump

2. Across intestinal cell membrane into interstitial fluid –> down concentration gradient:
   a) Facilitated diffusion (carrier protein = GLUT 2)

Question 26

What is a patch clamp technique

Answer 26

The technique allows a single ion channel to be isolated from the membrane and its function characterized.

Question 27

What are aquaporins and where are they stored

Answer 27

Aquaporins are a family of 5 water channels that are stored in the endosomes of cells

Question 28

Classify and describe the distribution of aquaporins

Answer 28

Kidney
- Aquaporin 1, 2 and 3 in the collecting duct inserted into the luminal membrane in response to ADH

Brain
- Aquaporin 4

Salivary glands / Lacrimal glands / RSP tract
- Aquaporin 5

Question 29

With regard to gated channels, what are the 3 types

Answer 29

Voltage gated
Ligand gated
Mechanically gated (e.g. stretch)

Question 30

What is endocytosis and what are the three types

Answer 30

The invagination of the plasma membrane enclosing extracellular material, sealing it and budding off from the membrane, without leaving a gap

1. Phagocytosis
2. Pinocytosis
3. Receptor-mediated endocytosis

Question 31

What three proteins are important in the process of endocytosis

Answer 31

Caveolin
Clathrin
Dynamin

Question 32

What is exocytosis

Answer 32

Reverse of endocytosis whereby proteins are released into the ECF - requires ATP and is triggered by an increase in calcium concentration