Library · Physiology
Transport across the cell membrane
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Substances cross the cell membrane, the cell's outer envelope, in two main ways. This movement is called transport across the membrane, and it can be passive transport or active transport.
In passive transport, molecules move from where there are more of them to where there are fewer, down what is called the concentration gradient, and no energy needs to be spent. The most common form is diffusion. Osmosis is also passive: water crosses toward the side with more solute, which is whatever is dissolved in it, when that solute cannot get through the membrane.
Active transport does the opposite: it moves against the gradient, and that does use energy. It is like coasting downhill on a bike versus pedaling uphill. One example is the sodium-potassium pump, which uses ATP, the molecule that gives the cell energy, to move three sodium ions out and two potassium ions in.
In brief
Transport across a membrane has two main modes: passive transport, mostly by diffusion, moves from higher to lower concentration without using energy, and active transport uses energy to move against the gradient.
Like coasting downhill on a bike, or pedaling uphill.
Osmosis, a form of passive transport, moves water toward the side with more solute when that solute cannot cross the membrane.
The sodium-potassium pump, a form of active transport, uses ATP to move three sodium ions out of the cell and two potassium ions in.
Sources (1)
- National Center for Biotechnology Information, StatPearls PublishingPhysiology, Active Transport, StatPearlsPublished
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