Transport Across Membranes

Transport Across Membranes

The cell membrane prevents equal motion of water and dissolved substances from inside to outside the cell.

Many substances cannot pass through biological membranes. The transport of materials across a cell

membrane falls into two general categories.

Passive transport - does not require energy use by the cell.

a. Diffusion

· All molecules move continuously by random simple diffusion. This movement is spontaneous and does not require energy. Diffusion is the movement of particles from an area of higher concentration to an area of lower concentration.

· Heat energy causes molecules to move randomly. This motion is called Brownian motion.

· If the concentration of molecules in 2 areas is different diffusion will cause molecules to move from the area with the higher concentration to the area with the lower concentration.

· The greater the concentration difference, the more rapid the net diffusion.

· Diffusion evens out the concentrations so they are equal everywhere, i.e., equilibrium - when there is uniform concentration.

· The movement is due to collisions between particles and a few factors affect the rate

(1) Temperature - rate of diffusion increases as temperature increases.

(2) Pressure - rate of diffusion increases as pressure increases.

(3) Concentration - rate of diffusion increases as concentration increases.

(4) Size - rate of diffusion decreases as molecule size increases.

b. Osmosis

· Remember that, in a solution, the substance present in the greatest amount is called the solvent. In biology the solvent is almost always water.

· Solutes are the substances dissolved in the solvent.

· When water diffuses across a selectively permeable membrane, it is called osmosis. It occurs when a solute cannot pass through a membrane but the solvent (water) can.

· Because it is a special kind of diffusion, osmosis occurs spontaneously and requires no energy.

· Water moves from an area where there is more water to an area where there is less water. In general, water moves toward the area with a higher solute concentration because it has a lower water concentration.

· The amount of water in a solution is indirectly proportional to the amount of solute in the solution. Think of a dilute solution (call it A) as having a high water concentration (because it has few solutes) and a concentrated solution (call it B) as having a lower water concentration (because it has lots of solutes). If these two solutions were separated by a selectively permeable membrane, water would flow from high water to low water concentration. I.e., from A to B.

· 3 types of solutions

(1) Hypotonic - a solution which is more dilute (i.e., less solutes) than the cytosol. The cell gains water and

swells. This is a problem for many freshwater organisms.

(2) Hypertonic - a solution which is more concentrated (i.e., more solutes) than the cytosol. The cells loses

water and shrinks. This is a problem for many marine organisms.

(3) Isotonic - a solution which has the same concentration of solutes as the cytosol. The cell neither gains

nor loses water and remain unchanged.

(4) Osmotic swelling dilutes the cytosol and can eventually cause the cell to burst (lyse). The opposite

problem, the cell shrinking, would occur in a hypertonic solution. Cells have different ways of dealing with

these differences in concentration

(a) Cell walls of plant, fungal and bacterial cells are rigid and prevent swelling. The walls

are strong enough to allow a fairly high pressure gradient. In plants this is called turgor pressure. Plant

cells placed in a hypertonic solution will undergo plasmolysis, a condition where the plasma membrane

pulls away from the cell wall as the cell shrinks. The cell wall is rigid and does not shrink.

(b) Some simple, single-celled organisms have contractile vacuoles which store excess

water and thensquirt it out.

cell and water follows by osmosis.

(d) In complex organisms such as humans, the blood is isotonic to cytosol so that cells do not

have to face these problems

c. Facilitated Diffusion

· Some important molecules, like glucose, are helped across the cell membrane so that they move into a cell faster.

· Special protein channels help move these substances across the membrane.

· Each protein channel is specific for the molecule it is transporting.

· This process is spontaneous and does not require energy.

Active transport

a. Active transport

· Often, a cell requires substances that are at a lower concentration outside the cell than inside the cell. These substances will not move by diffusion.

· Special proteins in the membrane use energy to transport these substances into the cell. Note the similarity to facilitated diffusion.

· Enables a cell to concentrate materials inside itself that are a a low concentration in the environment.

· In humans, active transport can account for 30% of your resting energy use.

b. Endocytosis (phagocytosis and pinocytosis)

· These processes are used for macromolecules that are too big to pass through the cell membrane normally.

· The cell membrane bends inward, forming a vesicle containing extracellular fluid and other substances dissolved in it.

· Can bring in large molecules such as proteins which would not diffuse across the cell membrane.

· The vesicle is then fused with a lysosome to be digested macromolecules.

· Phagocytosis is bringing particles into the cell while pinocytosis is bringing in fluid.

· Exocytosis is the opposite - materials are excreted from a cell but the mechanism is the same.

Comparison of simple diffusion, facilitated diffusion and active transport.

Property	Simple Diffusion	Facilitated Transport	Active Transport
Requires special membrane proteins	No	Yes	Yes
Highly selective	No	Yes	Yes
Transport saturates	No	Yes	Yes
Can be inhibited	No	Yes	Yes
Uphill transport	No	No	Yes
Requires ATP energy	No	No	Yes