Plasma membranes act not only as a barrier, but also as a gatekeeper. They must allow needed substances to enter and cell products and waste to leave the cell. They must also prevent harmful material from entering the cell and they must keep essential cell materials (i.e. nutrients) from leaving the cell. In other words, plasma membranes are selectively permeable—they allow some substances through but not others. If the membrane were to lose this selectivity, the cell would no longer be able to maintain homeostasis, or to sustain itself, and it would be destroyed.
Cells need ways to move specific materials into and out of the cell. This may happen passively, as certain materials move back and forth, or the cell may have special mechanisms that ensure transport. Most cells expend most of their energy, in the form of adenosine triphosphate (ATP), to create and maintain an uneven distribution of ions on the opposite sides of their membranes. The structure of the plasma membrane contributes to these functions.
Recall that plasma membranes have hydrophilic and hydrophobic regions. This characteristic helps the movement of certain materials through the membrane and hinders the movement of others. Lipid-soluble material (hydrophobic molecules) can easily slip through the hydrophobic lipid core of the membrane. Substances such as the fat-soluble vitamins A, D, E, and K readily pass through the plasma membranes in the digestive tract and other tissues. Fat-soluble drugs also gain easy entry into cells and are readily transported into the body’s tissues and organs. Small non-polar molecules such as oxygen and carbon dioxide have no charge and can pass directly through the membrane.
Polar substances, with the exception of water, present problems for the membrane. While some polar molecules connect easily with the outside of a cell, they cannot readily pass through the hydrophobic core of the plasma membrane. Additionally, whereas small ions (charged particles) could easily slip through the spaces in the mosaic of the membrane, their charge prevents them from doing so. Ions such as sodium, potassium, calcium, and chloride must have a special means of penetrating plasma membranes. Simple sugars and amino acids (which are relatively large and polar) also need help with transport across plasma membranes.
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Text adapted from: OpenStax, Concepts of Biology. OpenStax CNX. May 18, 2016 http://firstname.lastname@example.org