Download MTC31 - Plasma Membranes and Permeability

MTC31: PLASMA MEMBRANES AND PERMEABILITY
08/10/07
LEARNING OUTCOMES
Describe the major fluid compartments of the body and the distribution within them of the main ions and solutes
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There is fluid present in the body both inside (65%) and outside (35%) of cells forming a total body
water of roughly 42 litres
As you age the percentage of body weight that is made by water drops from 80% / 75% to 50% / 45%
(male / female) making an average of 60%
Extracellular fluid (ECF) is made up of:
o Blood plasma which is contained within cardiac chambers and is made up of cellular blood
elements e.g. platelets
o Interstitial fluid which bathes the non-blood cells of the body and is made up of bone matrix
and dense connective tissue fluid
o Transcellular fluid which is trapped within spaces between epithelial cells
The ICF and ECF have equal osmolality values of 290mosmol/Kg
Cells are contained by a plasma membrane which is a fatty film based on a lipid bi-layer 5nm thick
Molecules within the bi-layer are mobile – mostly lipids and (often glycosylated) membrane proteins
The polar heads of lipids are hydrophilic and face towards fluid such that they are on the outside of
the cell on one side and towards the cytoplasm on the other; fatty acid tails are hydrophobic and face
inwards to the centre of the bi-layer
Double bonds in fatty acid chains cause a ‘kink’ such that the chain is angled sharply at the double
bond
The lipid bi-layer is a two-dimensional fluid that allows lateral diffusion (2μm/sec), rotation and
flexion but not inversion (‘flip-flop’)
Fluidity of the layer is determined by its phospholipid composition, in particular the nature and
packing of the fatty acid chains
Cholesterol molecules maintain membrane integrity by ‘filling gaps’ (e.g. caused by double bond
‘kinks’)
Generally, bi-layers are asymmetrical in that they extracellular-facing monolayer is often composed of
different lipids to the intracellular-facing monolayer
Glycolipids are only found in the outer monolayer; other specialised lipids are found in mitochondrial
membranes
There are several different forms of membrane protein:
o Trans-membrane – span the lipid bi-layer to allow molecules or charge to pass through
o Membrane-associated – anchored to the cytosolic surface
o Lipid-linked – outside of the layer but covalently linked by lipid groups
o Protein-attached – bound indirectly to one or other membrane face by inter-protein
interactions
Carbohydrates are present in the membrane as part of glycolipids and glycoproteins as well as
externally as part of glycocalyx in the carbohydrate layer (a coating forming a protective shell)
The carbohydrate layer also takes part in intercellular recognition and adhesion
Almost any molecule will diffuse down its concentration gradient across a protein-free lipid bi-layer
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Diffusion rate can be calculated as –PAΔC (permeability, area and change in concentration)
Small hydrophobic molecules such as oxygen and carbon dioxide diffuse easily and are said to have
high permeability coefficients
Charged molecules do not diffuse easily
There is a high concentration of sodium and calcium ions in the extracellular fluid and instead a high
concentration of potassium ions in the intracellular fluid
Concentrations of electrolytes in plasma and interstitial fluids are rarely equal; there are no proteins to
be found in interstitial fluid
Electrical neutrality is maintained between ICF and ECF but concentration of small diffusible ions is not
equal – the Gibbs-Donnan effect
Explain the terms osmolarity and osmolality
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Osmosis is defined as the passage of a solvent from a less concentrated to a more concentrated
solution through a semi-permeable membrane (in order to equalise concentrations)
Osmosis produces water fluxes across membranes when there are uneven solute concentrations and
the solute is osmotically active
Ions are the major solutes that produce osmosis and hence water fluxes
Osmolarity is defined as the total concentration of dissolved particles in 1 litre of solution
Differences in osmolarity produce osmotic pressure which drives osmosis
Osmolality is defined as the total concentration of dissolved particles in 1Kg of water
Define the terms iso-osmolar, hypo-osmolar, hyper-osmolar, isotonic, hypertonic, hypotonic and explain their use in
terms of the movement of water into and out of cells
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Osmolarity describes the number of osmotically active particles in a single solution
Tonicity in contrast compares one solution to another
Iso-osmolar is where extra- and intracellular osmolarity is equal
Hyperosmolar describes a solution exerting a comparatively large osmotic force such that water is
drawn into it (i.e. to dilute it) – it has a higher osmolarity; if extracellular liquid becomes hyperosmolar
than water is drawn out of the cell
Hypo-osmolar describes a solution exerting a comparatively small osmotic force such that water is
drawn away from it (i.e. to increase its concentration) – it has a lower osmolarity; if extracellular
solution becomes hypo-osmolar then water is drawn into the cell
Isotonicity is where cells can be placed into a solution without any net flow of water across the
membrane
Hypertonic is where effective osmolarity of a solution is higher; hypotonic where it is lower
SELF-STUDY NOTES
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Blood plasma is an isotonic solution to the red blood cells contained within it; osmotic pressures are
equal
This means that there is no net osmosis: they are in dynamic equilibrium
Because of this, the cells are said to have no turgor pressure – a hypotonic solution would be required
for this, such that the cell would take in water – which allows it to adapt to changes in pressure
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If bathed in a solution with a lower concentration of water, the cell becomes hypotonic and loses
water by osmosis; it is said to be crenated
If bathed in a solution with a higher concentration of water, the cell becomes hypertonic and takes on
water by osmosis; it is said to be haemolysed and lysis ultimately occurs
Phagocytosis is the engulfing of large molecules by amoebas or white blood cells