Active transport across a membrane is directional and requires an input of energy to move substances against their concentration gradients. Active transport allows a cell to maintain small molecules and ions at concentrations very different from those in the surrounding environment.
learning outcomes
You should be able to:
Compare and contrast diffusion and active transport.
Explain why active transport requires energy.
Describe and give examples of the different types of proteins that carry out active transport.
Differentiate between primary and secondary active transport.
Why is energy required for active transport?
Substances tend to diffuse and reach equilibrium, where they are evenly distributed in the environment. Energy is needed to overcome this natural tendency.
Why is the sodium–
An antiporter transports two substances in opposite directions. In the case of the Na+–K+ pump, Na+ is transported across the cell membrane outward, and K+ is transported across the cell membrane inward.
What are the differences between primary active transport and secondary active transport?
In primary active transport, ATP hydrolysis supplies the energy needed for transport against a concentration gradient. In secondary active transport, the energy comes from a gradient set up by a different (secondary) active transport.
What are the similarities and differences between active transport and facilitated diffusion?
Both active transport and facilitated diffusion speed up transport across a membrane by using a protein that binds to the substance transported. However, active transport is an energy-
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We have examined a number of passive and active ways in which ions and small molecules can enter and leave cells. But what about large molecules such as proteins? Many proteins are so large that they diffuse very slowly, and their bulk makes it difficult for them to pass through the phospholipid bilayer. A completely different mechanism is needed to move intact large molecules across membranes.