Of all the processes that continue from cell to cell as life goes on, none is more fundamental than those which provide life's energy.
A cell requires something to keep its insides in and the outside out. That is the role of the cell membrane, a flexible film made largely of lipids. These are smallish tadpole-shaped molecules with heads that are comfortable in water and twin tails that shun it. When put into a watery solution they naturally form double layers in which the water-tolerant heads are on the outside and the water-wary bits on the inside.
The world’s prokaryotic cells vastly outnumber their eukaryotic cousins. Your own body has roughly as many single-celled prokaryotes living on and inside it as it has eukaryotic cells making up muscles, nerves, bones, blood and so on. Some parts of Earth’s biosphere, such as the ocean floors, contain more or less nothing but prokaryotic life.
Even prokaryotic cells, though, are big compared with the molecules they contain. A bacterium two millionths of a metre long encompasses around 3m protein molecules as well as thenecessary to make use of those descriptions and the various smaller molecules that proteins stick together and break apart in the course of their duties . The membrane of such a bacterium, moreover, contains around 20m lipid molecules.
Of all the processes that continue from cell to cell as life goes on, none is more fundamental than those which provide life’s energy. These are completely dependent on the membranes in cells. Conditions on the two sides of a membrane will almost always be different; different molecules will be present in different concentrations. The laws of thermodynamics, though, take a dim view of different concentrations of something being next to each other.
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