Most modern biology textbooks will tell you that low-molecular-weight molecules like CO2, NH3 (ammonia), and urea cross cell membranes by simple diffusion. Some scientists postulate, however, that specific pores mediate this transport. A recent study conducted to further this hypothesis examines expression of the water channel aquaporin-1 (AQP-1) and its enhancing affect on CO2 permeation into cells.
Why would scientists challenge the traditional simple diffusion theory? Two strong points of evidence led to this experiment:
First, many cell membranes have a very low permeability to small molecules: The outer membranes of some gastric glands allow very little CO2 to enter the cell, while most frog egg cells have membranes that act as virtual barriers to the harsh materials present in their environment. If the simple diffusion theory is correct, diffusion of small molecules into cells should be an unregulated process. If diffusion is an unregulated process, how could some cells have a lower permeability to small molecules than others?
Second, scientists have already found membrane proteins that facilitate the entrance of water and urea: While the UT2 protein enables transport of urea past the cell membrane, an entire class of membrane proteins called the aquaporins enable the permeation of water into cells. If small molecules enter the cell by diffusion, why would the cell have extra mechanisms to facilitate their entry?
Q) What's this diffusion thing you keep talking about?
A) Particles randomly flow from areas of high concentration (a lot of particles) to areas of low concentration (not many particles) until they are evenly dispersed.
If you've ever put a normal cell in a concentrated salt solution, you know that it quickly shrivels up. This is because the particles inside the cell want to have the same salt concentration as the medium surrounding it- since the cell membrane does not allow salt to enter the cell, water quickly pushes its way out of the cell, causing it to shrink.
On a similar note, most biologists believe that diffusion is what causes small molecules to pass across cell membranes. According to this theory, small molecules can push their way through the plasma membrane into or out of cells simply because they are diffusing into an area of lower concentration.
Q) What is a Xenopus laevis, and why do I need to know?
A) The Xenopus laevis (pictured at the top of this page) is a species of frog from Southern Africa- it is a claw-toed (platanna), smooth-skinned aquatic frog, roughly 13cm (5in) long when full grown. This species and its eggs are used widely in scientific studies, including this one.
Q) Aquaporin-1 (AQP-1)? Isn't that an aftershave?
A) No, you're thinking of Aquavelva tm. Aquaporin-1 (AQP-1) is a water channel located on cell membranes. The aquaporins are a family of proteins known for facilitating water transport, and AQP-1 (first called CHIP28)...