Cell Membrane

Cell membrane
From the Book: Biology Principles & Explorations and the book: Biology life on earth.

The cell membrane is also known as plasma membrane because it is a liquid. It is the envelopethat makes up the outer surface of a cell, and the site of much of the cell’s activity, which isolates the cell while allowing communication with its surroundings.
Each cell is surrounded by a thinplasma membrane, which can be considered a gatekeeper, allowing only specific substances in or out and passing chemical messages from the external environment to the cell’s interior. As a gatekeeper,the plasma membrane must perform three general functions: Selectively isolate the cell’s contents from the external environment, regulate the exchange of essential substances between the cell’s contentsand the external environment, and communicate with other cells.
The cell membrane is composed of two main groups of organic molecules: phospho-lipids and proteins. The phosphor-lipid is a moleculeshaped like a head with two tails, the head is polar but the two tails are nonpolar. The head attracts water molecules because of both polarity, but water cannot join with the nonpolar tails so ittends to push them away. The result of this is a double layer of phospholipids called a lipid bilayer. The individual phospholipid molecules, arranged side by side, float within the bilayer. In mostcells, the phospholipid bilayer of membranes also contains cholesterol molecules. Cholesterol affects the membrane structure and its function by making it stronger, more flexible, less fluid, and lesspermeable to water-soluble substances such as ions or monosaccharides.
The important thing of the bilayer is the center, where is the nonpolar zone, this is the true barrier that separates the cellfrom its surroundings. Most polar particles cannot go across this zone, because they´re repelled by the nonpolar interior of the bilayer. But if they cannot pass through it, how does the cell absorb...