How Substances Enter and Exit a Living Cell Free Essay, Term Paper and Book Report
BASIC STRUCTURE OF A CELL MEMBRANE
The cell membrane surrounds all living cells, and is the cell's most important organelle. It controls how substances can move in and out of the cell and is responsible for many other properties of the cell as well. The membranes that surround the nucleus and other organelles are almost identical to the cell membrane. Membranes are composed of phospholipids, proteins and carbohydrates arranged in a fluid mosaic structure.
SIMPLE OR LIPID DIFFUSION
A few substances can diffuse directly through the lipid bilayer, part of the membrane. The only substances that can do this are lipid-soluble molecules such as steroids, or very small molecules, such as H2O, O2 and CO2. For these molecules the membrane is no barrier at all. Since lipid diffusion is (obviously) a passive diffusion process, no energy is involved and substances can only move down their concentration gradient. Lipid diffusion cannot be controlled by the cell, in the sense of being switched on or off. It is controlled by the concept of equilibrium. The molecules move from a r......
Word Count: 730
Page Count: 2.9 (250 words a page / double spaced)
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