The cell membrane acts as a type of cell "skin" protecting the internal environment of the cell from the cells exterior environment. The cell membrane is selectively permeable, meaning it will allow some substances across the membrane while denying others the same transport. It does this because of a two-layer of phospholipids, with hydrophilic heads and hydrophobic tails. Hydrophilic means "water loving", while hydrophobic means "water fearing". Polar solutes, such as amino acids, nucleic acids, carbohydrates, proteins, and ions, are prevented from crossing the membrane, instead having to enter through special gateway molecules within the membrane. The membrane does allow, however, passive diffusion of hydrophobic molecules.
Hydrophobic and hydrophilic interactions can affect protein shape. Because of the polar or nonpolar nature of the constituent amino acid building blocks, as well as in carbohydrate and lipid constituents of microorganisms, molecules and sometimes whole microorganisms can assume shapes and orientations that depend on the intracellular or extracellular environment.
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