Membrane Transport Processes

Learning objectives

1. Students will predict the direction of water transport across the membrane under different conditions of salt and osmolarity.
2. Students will distinguish among the types of transport (simple diffusion, facilitated diffusion, and active transport), based on their kinetics and energy requirements.

Membrane Permeability and Osmosis

The lipid bilayer is “semi-permeable,” meaning that some molecules can diffuse rapidly across the membrane, while other molecules cross only very slowly or not at all. In general, small uncharged molecules like O2 and CO2 can diffuse across freely, while charged molecules (Na+, H+) or polar molecules (glucose) cannot. Even water molecules diffuse only slowly across cell membranes (osmosis). Diffusion results in net movement of molecules down their concentration gradient, from an area of high concentration to an area of low concentration.

Membrane Proteins and Transport

How do cells transport molecules like glucose across the membrane? Membranes have dedicated transport proteins with transmembrane domains. The transmembrane domains form channels in the membrane that are specific for various molecules like glucose, phosphate, Na+, H+, and even H2O. Water transport is mediated by highly conserved proteins called aquaporins, which are present in all 3 domains of life.

Click the link to download and view the molecular movie showing transport of a labeled water molecule through an aquaporin channel. The movie encompasses about 12 nanoseconds:
waterpermeation (same movie as in the aquaporin video above)
The movement of molecules through protein channels is called facilitated diffusion. The protein channels are highly specific for the molecule, and again results in net transport down a concentration gradient, from a region of high concentration to a region of low concentration.
What if the cell wants to move molecules against a concentration gradient? Even when phosphate concentrations outside the cell are very low, cells can transport phosphate into the cell, where the cytoplasmic concentration of phosphate may be much higher. Active transport of molecules against their concentration gradient requires expenditure of energy, often in the form of hydrolysis of ATP.

Kinetics of Transport

Active transport is easy to distinguish because it requires energy, and goes against the concentration gradient. But is it possible to distinguish facilitated diffusion from simple diffusion?
Because facilitated diffusion is mediated by protein channels, and because the number of protein channels in a cell membrane is limited, facilitated diffusion shows saturation kinetics.

As the concentration difference across the membrane becomes greater, the rate of diffusion increases, for both facilitated and simple diffusion. However, the protein channels will reach a point where the flux of molecules through them reaches a limit; all protein channels are being traversed as rapidly as possible. Further increases in the concentration gradient cannot drive faster transport. Simple diffusion will rarely reach such a limit, because the entire area of the membrane is available.

To summarize membranes and transport, check out this animation: http://www.johnkyrk.com/cellmembrane.html – one mistake is that the animation incorrectly shows sphingolipids on both sides (leaflets) of the plasma membrane, whereas sphingolipids are located only on the outer (extracellular side) leaflet.
To test your understanding of transport processes, look at this teaching tidbit on Cell Membrane Permeability.

Put it all together

Case study on Cystic Fibrosis