Concept 8.2 Membrane structure results in selective permeability A steady traffic of small molecules and ions moves across the plasma membrane in both directions. For example, sugars, amino acids, and other nutrients enter a muscle cell, and metabolic waste products leave. The cell absorbs oxygen and expels carbon dioxide. It also regulates concentrations of inorganic ions, such as Na + , K + , Ca 2+ , and Cl − , by shuttling them across the membrane. However, substances do not move across the barrier indiscriminately; membranes are selectively permeable. The plasma membrane allows the cell to take up many varieties of small molecules and ions and exclude others. Substances that move through the membrane do so at different rates. Movement of a molecule through a membrane depends on the interaction of the molecule with the hydrophobic core of the membrane.
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