cell physiology Flashcards
how do we create homeostasis?
Each cell contributes by the exchange of nutrients to and from the intra- and extra-cellular fluid in specialised ways. This ability is determined by the cell membrane.
The cell membrane
is a phospholipid bilayer studded with proteins
phospholipid bilayer
ECF > lipid bilayer > ICF
polar heads - hydrophilic
nonpolar tails - hydrophobic
polar heads - hydrophilic
* Structural - gives cells shape
* Flexible - allows cells to change shape
* Barrier - prevents water-soluble substances passing through
The phospholipid bilayer is:
semi-permeable
Permeable to lipid-soluble molecules (e.g. steroids)
Permeable to small uncharged molecules
(e.g. O2, CO2, urea can move freely between phospholipids)
Impermeable to ions & large molecules
(e.g. sodium, proteins, glucose)
H2O? Permeable yes, but a bit more tricky
Anything which passes directly through the bilayer
does so by simple diffusion
Movement through the phospholipid bilayer
Anything passing directly through the lipid bilayer does so by simple diffusion……
…thus driven by concentration gradient.
* bigger the gradient, the more goes through
diffusion
Diffusion occurs if the substance can permeate the membrane
no diffusion occurs if the membrane is impermeable to a substance
Fick’s Law
tells us all the factors that affect simple diffusion through a membrane:
Net rate of diffusion (Q) =( ∆C . P . A) / ∆X . MW
Where: ∆C = concentration gradient
P = permeability of membrane to the diffusing substance
A = is surface area available for diffusion
∆X = membrane thickness
MW = molecular weight of substance
* Rapid process over short distances:
Osmosis
Water also moves down its concentration gradient
When water moves down its activity gradient
we call it ‘osmosis’. - simply the diffusion of water
what happens when no activity gradient
Try thinking of water molecules as being just like any other molecules:When there is no activity gradient there is no osmosis
Water ‘activity’
Water ‘activity’ (i.e. concentration) is reduced by the presences of solutes (eg ions, molecules)….…but osmosis only occurs with an activity gradient to drive it. Membrane penetrating solutes reduce water activity equally on both sides of the membrane.
A water activity gradient
A water activity gradient can exist if non-penetrating solutes occur on one side of the membrane!
* Moves from area of high water concentration > low concentration
Therefore Water moves from a compartment with low total solute concentration, into a compartment with high total solute concentration.
Q: Do all solutes behave the same in terms of influencing water activity, and thus osmosis?
A: No, they can differ in two ways.
- Whether they are (as already considered) …. membrane penetrating (e.g. urea), or …. non-penetrating (e.g. glucose and ions).
- Whether they dissociate in solution.e.g. compare solutions of glucose and sodium chloride (NaCl)
How do we describe the difference between solutions that contain solutes that dissociate in solution and those that do not?
As ‘osmolarity’ of the solution (and we can calculate it)
Osmolarity of a solution =
∑ {(solute concentration) x (#dissociated species)}
N.B.
Water activity is inversely proportional to osmolarity (i.e. water activity is low when osmolarity is high)
Higher osmolarity = less water
Osmosis depends
Osmosis depends on the water activity gradient which is determined by the osmolarity gradient.
Higher water activity,
because osmolaritylower.
