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Molecular Machines in Cell Membranes
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19651 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
10 / 1991 |
2,921 Words |
| Author
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Marco Colombini
Marco Colombini is a zoology professor at the University of
Maryland at College Park. |
It moved! It was just the leg of a frog. The head and upper body had been removed, but when Luigi Galvani touched it with a small electrical charge, it moved! The muscle of the leg shortened, causing the leg to kick outward.
Why did the muscle respond to the electrical charge? How did the muscle shorten? Was the muscle still alive even though the frog was not? Many such questions must have rushed through Galvani's head!
Now, two hundred years after the Italian physiologist Galvani published his observations, we can answer, at least in a descriptive way, all these questions and more. Although today our knowledge and understanding at the molecular level generally is incomplete, we should, by the end of the century, be able to understand in detail the operational processes of the molecular machines that are responsible for events such as the one just described.
The muscle cells that shorten when frogs jump are specialized cells. Yet their basic structure is much the same as all other living cells. For one thing, each cell is enveloped by a surface membrane that separates the cell from its surroundings, detects and responds to those surroundings, and helps maintain an environment within the cell that allows the processes of life to proceed.
The membranes of living cells are fascinating structures. They form the boundary that is the surface of the cell. They also form the boundaries of specialized compartments (organelles) within the cell; the nucleus, containing the DNA that stores most of the cell's information; the mitochondria, that produce most of the cell's energy; and other compartments used for tasks as diverse as protein export, digestion of engulfed bacteria, and collection of energy from light. The membranes surrounding both the cell and its various organelles are all extremely thin liquids that are confined to motion on the surface of the enclosed unit. Membranes are constantly moving, essentially two-dimensional mosaics of lipids (oil-like molecules) and proteins (precisely folded amino acid chains). The lipids form the impermeable barrier while the proteins are the machines that perform the specialized tasks, including rendering membranes selectively permeable. Biological membranes are somewhat like house walls with the lipids forming the bricks and the proteins forming the doors, windows, doorbells, and lights. However, imagine all of these elements moving in the plane of the wall in a random fashion. Despite the movement the wall always remains the same thickness and no holes are
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