Cell Membrane Transport I - Diffusion and Osmosis Flashcards
Learning outcomes
- Understand the random movement of molecules down a
concentration gradient - Understand the importance of molecular weight, lipid solubility and
charge on membrane permeability - Define the concept of flux and its relationship to membrane
permeability - Contrast the diffusion of solutes through a membrane and through a
pore/channel - Understand the concepts of osmolarity and tonicity
Some key concepts: molecules diffuse
down their own concentrations
- Systems spontaneously evolve towards
thermodynamic equilibrium (2nd law) - Mass action means more molecules
move from the area of high
concentration to low concentration than
those moving from low to high
concentration
Why should I learn about cell
membrane transport?
- Almost all biomedical and biological disciplines
study it - 1/3 of the genome is devoted to membrane
transport - Critical to organism survival:
–Nutrient transport: organisms must ‘eat’!
–Waste management: organisms must ‘excrete’!
–Salts & Water: must maintain cell structure/volume
–Messengers: most co-ordinate function within & between cells
–Charge: cells use voltage to communicate, co-ordinate & control
–Motility: Organisms power and control motility by altering transport - Impaired cell membrane transport underlies the
pathology of many diseases: cystic fibrosis, myotonia
congenita - It is in your exam (sorry!)
What stuff do we need to transport?
We need different transport mechanisms for
different types of molecules
* Macromolecules: complex carbohydrates, proteins,
long-chain fatty acids, nucleic acids etc.
* Small solutes: neutral and non-polar molecules
(small sugars, lipids etc.)
* Polar and charged molecules or ions: H2O, Na+
, K+
,
Cl-
, amino acids, Mg2+ etc.
Some key concepts: molecules in
solution are in constant random motion
- An inefficient process: constant changes in direction
- Fast movement over very short distances but slower
to move long distances
Some key concepts: Fick’s law of
diffusion
- Flux (J) = number of molecules passing through a certain area in a
given amount of time (mol.cm-2
.s-1
) - In simple diffusion, the flux of an uncharged substance through
membrane lipid is directly proportional to its concentration gradient
(ΔC) and inversely proportional to membrane thickness (Δx)
Some key concepts: Fick’s law of
diffusion
Some key concepts: Lipid solubility
- Substances with a greater lipid solubility will cross the
membrane easier and more rapidly - Lipid solubility can be indicated by the oil-water partition
coefficient
Some key concepts: polarity
- Non-polar = higher membrane permeability
- Polar = lower membrane permeability
- Small non-polar molecules such as O2 and
CO2 diffuse easily across lipid bilayers - Some polar compounds such as water and
urea can diffuse across membranes but very
slowly
–But: some of these compounds have specialized channels
to facilitate diffusion
Some key concepts: electrical charge
- Ions are the most abundant dissolved solutes
and maintenance of intracellular and
extracellular concentrations is vital for cell
function - Charged compounds cannot diffuse through
the lipid membranes
A (mini) summary
Osmosis: some key things to remember
- Water also diffuses down its concentration gradient
- Osmolality: total concentration of dissolved particles/kg of water
- Osmolarity: total concentration of dissolved particles/litre of solution
–Roughly the same thing in effect in dilute solutions
Remember the difference between concentration (mol/L) and osmolarity (Osm/L) - Some compounds dissolve further in solution
- e.g. Glucose (does not dissolve further in solution): 300 mM glucose solution = 300 mOsm/L
- e.g. NaCl (dissolves further in solution into Na+ and Cl-
): 150 mM NaCl solution = 300 mOsm/L - Osmolarity of typical cells = 300 mOsm/L
A higher osmolarity means a lower water concentration
How does water move easily through plasma
membranes? It is a polar compound…
Aquaporins!!!
* Small, highly specific
channels which allow
bidirectional diffusion
(osmosis) of H2O
* Highly specific: pore is lined
with hydrophilic amino acids
that provide transient
hydrogen bonds for water
molecules
* Each aquaporin passes
~109 (1 billion!) water
molecules per second
Water will move from an area of high solute
concentration to low solute concentration
Why do I care about hydrostatic pressure?
Answer: tonicity
Tonicity is different to osmolarity
* Osmolarity = concentration of solute in a
particular solution
* Tonicity = The effect that a solution has
on cell volume
* Related, but not the same
Hydrostatic pressure can cause the
membrane to move
Approximate water distribution in adult
humans
think of it as the water bs trying to match the solute concentration of the cell to the environment
Fluid compartments of the body
Hydrostatic and oncotic pressure are
important for regulating fluid flow!
- Fluid is filtered out of capillaries
on the arterial side and
reabsorbed into the capillaries on
the venous side
Hydrostatic and oncotic pressure are
important for regulating fluid flow!
Regulation of fluid balance is extremely
important
Summary
- Solute transport across membranes is essential for cell function
- Passive diffusion occurs from random movement of molecules down their
concentration gradient - Membrane permeability is influenced by molecular weight, lipid solubility, polarity
and charge - Water movement (osmosis) across membranes is important for cell volume
- Osmolarity = total concentration of dissolved particles/litre of solution
- Higher osmolarity means lower water concentration –water will diffuse down its
concentration gradient - Aquaporins allow rapid passage of polar H2O molecules through the membrane
- Regulation of fluid balance in the body is important
