Membrane Transport

Membrane Transport Mechanisms

There are four primary means that water and other small molecules cross into or out of cells. They are reviewed here since they play such an important role in body fluids and many other physiological functions.

Diffusion

	Powered by random movement of molecules in a solution
	Net movement is from regions of high concentration to low.
	Does not saturate as the concentration or gradient changes
	Diffusion of different substances do not interfere with each other (no competition).
	Net flux (amount of movement) is proportional to the concentration difference and the permeability of any barrier like a membrane.
	Substances can cross membranes by diffusion if they can dissolve in the oily interior of the membrane (hydrophobic)
	Diffusion can occur through tight junctions or within bulk solutions.
	Diffusion of water down its concentration gradient is called osmosis.

You're missing a great java demo of diffusion

Anions and cations shown moving at random, as via diffusion in a solution. Put the cursor over the box above to start the simulation. Click within the box to reset all the particles to a single point.

For a more extensive review of Diffusion, Osmosis, and Nernst Potentials, see my separate chapter on that subject.

Facilitated Diffusion

	Proteins act as carriers or pores permit flux of substances that cannot diffuse directly through the membrane.
	Movement is still passive (like diffusion), from high concentration to low.
	Occurs across cell membranes only.
	Saturates when substance reaches high concentrations due to lack of available protein.
	Related substances can compete for the same carrier or pore.
	Maximum rate of transport (fully saturated) is called Tm, the transport maximum.

Primary Active Transport

Proteins in the membrane can also act as pumps.

Move ions or small molecules from low concentration to high concentration (i.e. up their gradients).

Require cellular energy, usually as ATP

Saturates when substance reaches high concentrations due to lack of available protein.

Example: Na-K ATPase

	Present in nearly every cell in the body
	Pumps 3 Na ions out in exchange for 2 K ions pumped in (cost=1 ATP)

Other pumps include the Ca-ATPase, and the H-ATPase.

Secondary Active Transport

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	Uses proteins similar to those for facilitated diffusion.
	Couples the movement of several different molecules in each cycle.
	Saturates when substance reaches high concentrations due to lack of available protein.
	Cotransport moves 2 or more molecules in the same direction across the membrane.
	Counter transport moves molecules in opposite directions.
	The gradient for one molecule can cause the other to move against its own diffusion gradient.
	Normal active transport (Na-K ATPase) makes a strong Na gradient, which in turn powers many secondary active transport mechanisms.
	Example: Na-Glucose cotransport.

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Copyright 1999, Joe Patlak, Department of Physiology, University of Vermont.
For comments, problems or questions regarding this web contact Joe Patlak.
Last updated: November 01, 2000.