![]() In passive transport, substances move from an area of higher concentration to an area of lower concentration. ![]() Passive transport is a naturally occurring phenomenon and does not require the cell to exert any of its energy to accomplish the movement. The most direct forms of membrane transport are passive. All cells spend the majority of their energy to maintain an imbalance of sodium and potassium ions between the interior and exterior of the cell. Red blood cells use some of their energy doing just that. Some materials are so important to a cell that it spends some of its energy, hydrolyzing adenosine triphosphate (ATP), to obtain these materials. ![]() This may happen passively, as certain materials move back and forth, or the cell may have special mechanisms that facilitate transport. Some cells require larger amounts of specific substances than do other cells they must have a way of obtaining these materials from extracellular fluids. If they were to lose this selectivity, the cell would no longer be able to sustain itself, and it would be destroyed. In other words, plasma membranes are selectively permeableâthey allow some substances to pass through, but not others. Plasma membranes must allow certain substances to enter and leave a cell, and prevent some harmful materials from entering and some essential materials from leaving.
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