A cell consists of three parts: the cell membrane, the nucleus, and, between the two, the cytoplasm. The cell membrane encloses the cytoplasm of a cell. It forms a barrier between the cytoplasm and the environment outside the cell. Its function is to protect and support the cell. It also controls what enters or leaves the cell. It allows only certain substances to pass through. It keeps other substances inside or outside the cell.     The structure of the cell membrane explains how it can control what enters and leaves the cell. The membrane is composed mainly of two layers of phospholipids.... Show more

A cell consists of three parts: the cell membrane, the nucleus, and, between the two, the cytoplasm. The cell membrane encloses the cytoplasm of a cell. It forms a barrier between the cytoplasm and the environment outside the cell. Its function is to protect and support the cell. It also controls what enters or leaves the cell. It allows only certain substances to pass through. It keeps other substances inside or outside the cell.    

The structure of the cell membrane explains how it can control what enters and leaves the cell. The membrane is composed mainly of two layers of phospholipids. Each phospholipid molecule has a head and two tails. The heads are “water loving” (hydrophilic), and the tails are “water fearing” (hydrophobic). The water-loving heads are on the outer surfaces of the cell membrane. They point toward the watery cytoplasm within the cell or the watery fluid that surrounds the cell. The water-fearing tails are in the middle of the cell membrane.    

Hydrophobic molecules “like” to be near other hydrophobic molecules. They “fear” being near hydrophilic molecules.    
The opposite is true of hydrophilic molecules. They “like” to be near other hydrophilic molecules. They “fear” being near hydrophobic molecules. These “likes” and “fears” explain why some molecules can pass through the cell membrane while others cannot.

Hydrophobic molecules can pass through the cell membrane. That's because they like the hydrophobic interior of the membrane and fear the hydrophilic exterior of the membrane. Hydrophilic molecules can't pass through the cell membrane. That's because they like the hydrophilic exterior of the membrane and fear the hydrophobic interior of the membrane.   

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