A physical space in which there is a different concentration of a single substance is said to have a concentration gradient. In passive transport, substances move from an area of higher concentration to an area of lower concentration in a process called diffusion. Passive transport is a naturally occurring phenomenon and does not require the cell to expend energy to accomplish the movement. The most direct forms of membrane transport are passive. The structure of the plasma membrane contributes to these functions, but it also presents some problems. 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. This may happen passively, as certain materials move back and forth, or the cell may have special mechanisms that ensure transport. Some cells require larger amounts of specific substances than do other cells they must have a way of obtaining these materials from the 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 permeable-they allow some substances through but not others. Plasma membranes must allow certain substances to enter and leave a cell, while preventing harmful material from entering and essential material from leaving. Define tonicity and describe its relevance to passive transport.Understand the processes of osmosis and diffusion.Explain why and how passive transport occurs.By the end of this section, you will be able to:

Concentration gradient