Membrane Transport

Objectives

When you have completed this section, you should be able to

  • explain what is meant by a selectively permeable membrane;
  • describe the various mechanisms for transporting material through the plasma membrane; and
  • define osmolarity and tonicity and explain their importance.

The plasma membrane is both a barrier and gateway between the cytoplasm and extracellular fluid (ECF). It is selectively permeable—it allows some things through, such as nutrients and wastes, but usually prevents other things, such as proteins and phosphates, from entering or leaving the cell.

The methods of moving substances into or out of a cell can be classified in two overlapping ways: as passive or active mechanisms and as carrier-mediated or not. Passive mechanisms require no energy (ATP) expenditure by the cell. In most cases, the random molecular motion of the particles themselves provides the energy. Passive mechanisms include filtration and diffusion (including a special case of diffusion, osmosis). Active mechanisms, however, require the cell to consume ATP. These include active transport and vesicular transport. Carrier-mediated mechanisms use a membrane protein to transport substances from one side of the membrane to the other. We will first consider the mechanisms that are not carrier-mediated (filtration and simple diffusion) and then the carrier-mediated mechanisms (facilitated diffusion and active transport).

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