MEMBRANES

We often think of replication, the ability to reproduce as the essential property of life. However, replication is impossible without cells and cells cannot survive without membranes. So lipid membranes are as essential to life as nucleic acids.

Cell membranes in all organisms are composed mainly of lipids Lipids have a water-loving polar head and water hating fatty acid tails. Mixed with water they naturally form a bilayer with two rows of head groups facing water on each side and the tails in the middle, away from the water. So that is all that is necessary to set up a membrane around a cell or separating cell compartments.

There are no bonds sticking the lipid molecules together, they are free to move and the structure has no mechanical strength, just a kind of stability resulting from its amphipathic properties. The rate of movement of molecules across the membrane will be influenced by size, charge and affinity for the different phases represented in the system .

The direction of passive movement is controlled simply by concentration differences across the membrane. So with this simple lipid membrane cells would have little control of the flow of molecules in or out.

Control of solute flux across membranes is achieved through special kinds of proteins. Proteins are a major component of cell membranes and many of them are involved in transport of ions and molecules across the membrane.

Two kinds of protein do little more than"help" molecules across membranes -carriers and channels - they move specific molecules but they can not move them against a concentration gradient (e.g. from soil solution to root cell).

That takes energy, usually ATP. So the energy from the hydrolysis of ATP by an ATPase can be used to move ions and molecules"uphill" - this is active transport. 

Alternatively, concentration differences across membranes can be used to drive energy formation.  This happens in respiration and photosynthesis.

Copyright © Michael Knee
The Ohio State University
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