The cell membrane, or plasma membrane, is made up of a lipid bilayer in which hydrophilic heads face outward and hydrophobic tails face inward. This arrangement within the cell membrane allows it to A. control the movement of substances in and out of the cell. B. keep out all viral and bacterial infections. C. fuse with ribosomes during DNA replication. D. absorb water from the cytoplasm whenever necessary.

The correct answer is A. control the movement of substances in and out of the cell.

Explanation:

All cell membranes are made up of lipids and proteins. Lipids are organized in two closely adjoining sheets that form a lipid bilayer. This bilayer provides the basic structure of the membrane and acts as a permeability barrier (impermeable) for most water-soluble molecules. Cells live and grow by exchanging molecules with the surrounding environment and the plasma membrane acts as a barrier that controls the transit of molecules into and out of the cell. As the interior of the membrane (the lipid bilayer) is hydrophobic, the membrane tends to block the passage of almost all water-soluble molecules (which dissolve in water). Protein molecules, which are normally “dissolved” in the lipid bilayer, act as mediators or facilitators of almost all membrane functions, either by transporting specific molecules through it or by catalyzing membrane-associated reactions, such as synthesis of ATP.

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