Lecture 2: Diffusion Flashcards
What percentage of women is made up of water?
about 55%
What percentage of men is made up of water?
about 60%
What percentage of total body water is made up of ICF?
66%
What is ICF and where is it?
intracellular fluid
this is inside cells
What percentage of total body water is made up of ECF?
33%
What is the ECF and where is it?
extracellular fluid
fluid not inside cells
What are the three components of ECF and what percentage of ECF do they make up?
Interstitial fluid: 75% ECF
Plasma: 20%
Transcellular: 5%
Where is interstitial fluid?
in between cells
What is plasma?
it is a solution in blood
Where is transcellular fluid?
inside the epithelial lined space
What are the three main ions in the ECF and ICF that determine membrane potentials?
Na+
K+
Cl-
What is the osmolarity of plasma (ICF)?
285 mOsmolL-1
What is the osmolarity of muscle (ICF)?
285 mOsmolL-1
What is the main ion in the ICF and what is its concentration?
K+
150 mmolL-1
What is the main ion in the ECF and what is it’s concentration?
Na+
150 mmolL-1
What is the range for osmolarity?
285 - 300 mOsmolL-1
Define diffusion
Spontaneous movement of individual molecules from a region of high concentration to a region of low concentration
When is the rate of diffusion very fast?
over very short distances
Rate of diffusion decreases rapidly with what?
distance
Solute molecules move independently of what?
solvent molecules and other solute molecules
What does flux describe?
the magnitude and direction of solute movement
What does the net diffusional flux result from?
two unidirectional fluxes
What is flux?
movement of the solute molecules
What is the net flux?
net combination of the flux in one direction and the flux in the other direction
If the concentration of the two solutions is different, the net flux is positive/negative/zero?
positive
If the concentration of the two solutions is the same the net flux is positive/negative/zero? What happens here?
zero
there is two unidirectional fluxes but they are equal and opposite
What is Fick’s Law?
Jnet ∝ dn/dt = -D.A (dc/dx)
What do we simplify Fick’s Law to be and why is this?
Jnet = PΔC
This is because D, A and Δx are all constant for a given membrane and are incorporated into a single experimentally determined permeability coefficient, P
Define permeability
a measure of how fast solute crosses the membrane
Permeability is specific for what?
a given membrane and solute
Permeability gives an indication of what?
the rate of movement (in cms-1)
What can permeability be used to define? Give an example
The selectivity of a membrane. For a given cell membrane, PK»_space; PNa
Permeability makes no assumptions about the mechanisms involved. What does this mean?
It doesn’t tell you anything about how the solutes get through eg. through diffusion or through a transporter
What is another way of writing Jnet = PΔC? What does this tell us?
Jnet = P (Co - Ci)
This means the steeper the concentration, the faster the rate of diffusion
The steeper the concentration, the faster the rate of diffusion but only if..
the cell is permeable to that solute
How is permeability obtained?
From the modification of Fick’s Law:
the concentration is given by the difference in concentration inside and outside the cell
What is the cell plasma membrane?
a physical and chemical barrier that separated the inside from the outside of the cell
The plasma membrane is a __________ _________ embedded with ________
lipid bilayer
proteins
In the plasma membrane, the phosphate-rich heads are on the inside/outside
outside
In the plasma membrane the hydrophobic lipid tails are in the inside/outside
inside
What is the plasma membrane impermeable to? What does this exclude? Why is this?
it is impermeable to water-soluble molecules except water because it is polar
P allows for the fact that there are different what?
mechanisms for different solutes to cross cell membranes
What does solubility diffusion refer to?
the movement of most non-electrolytes
Why can solubility diffusion be described by Fick’s law?
because it is a pure diffusional process and the solute dissolves into the lipid bilayer and diffuses across the membrane
There is a positive/negative relationship between solubility and permeability. What are the exceptions?
positive
water, urea
The more lipid soluble the solutes are, the more/less permeable they are
more
For solutes with similar lipid solubilities, the smaller the solute, the more/less permeable the solute`
more
Permeability correlates with size. The smaller the molecule the faster/slower the rate of diffusion
faster
Permeability correlates with lipid solubility. The more soluble the solutes, the more/less permeable they are
more
What is interesting about adding acetates to molecules and their fat solubility?
adding more acetate increases fat solubility although it increases the size
Solubility diffusion does not account for what?
Permeability of electrolytes. Often the permeability is much greater than predicted from their lipid solubility
What can the permeability for a given ion vary from and with?
it can vary from time to time and can vary with time
What does the temporary combination with membrane proteins account for?
the movements of electrolytes and anomalous non-electrolytes
Why is combination with membrane proteins is not described by Fick’s law?
because it shows
- selectivity
- saturation kinetics
- competition between similar species
- inhibition
Even though protein mediated transport is not described by Fick’s law, __________ is still a useful measure
permeability
What does protein-mediated transport mean?
transport aided by proteins such as channels and transporters
Transporters may not be saturated over what?
physiological concentration range
Even when saturated at physiological concentrations, the rate of transport can be varied by what two things?
- varying kinetics of transporter
- altering the number of transporters
As the ΔC or driving force increases, what happens to Jnet?
it increases at a decreasing rate and then levels off as the transporters become saturated
At low concentrations, there is a smaller/larger influx
larger
