Marc Doucet, a 35 year old male from Bathurst, New Brunswick asks on February 20, 1998,Are the pores of the cell membrane composed of protein molecules or are they simply holes in the membrane where the membranes touch? How then can sugar molecules get through the membrane by osmosis?
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The pores in membranes (known as channels) are in fact channels that are formed by a very large protein. The calcium channel protein is 2000 aa long and has many trans-membrane domains. These form a circle of protein that surround a central channel (pore) which is specially designed to allow only the specific ion or molecule for which it is designed, into the cell.
Most substances are actively moved across the plasma membrane by energy-requiring pumps. These pumps are driven by ATP hydrolysis or by the diffusion gradient of another substance. Ones that use a diffusion gradient of H+ are called proton syn-porters or anti-porters (depending on whether the "pumped" substance is going with the protons or against their gradient). Most sugars are pumped this way.
Water has a special "water channel" protein; dissolved substances do not enter with the water through this channel (to the best of my knowledge).
A good reference is The Molecular Biology of the Cell by Alberts, Bray, Lewis, Raff, Roberts, and Watson (3rd edition, 1994).
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