Question

what is the function of active transport in moving small molecules and ions across cell membrane? what is an example?

Answer 1

The movement of materials against a concentration difference is known as active transport. Active transport requires energy. In active transport, energy is expended to transport the small molecules, but they are not chemically altered.

Example:E.coli-ABC transporters have been found in all living species with 80 identified in the E. coli genome and 48 in the human genome.

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Answer 2

You want a specific example why it is good to use active transport of ions?

Answer 3

Anyway a more specific example why we use active transport could be to consider the following case:
As you you might neurons fire electric impulses down their axon. The amplitude of the impulse is mostly the same, but the frequency isn't.

Now the way, the neuron make this impulse is to move ions across their membrane and then returns in ions back again. Now consider we are in a high stress and the ions don't get time to diffuse back into the neuron. We therefor need to get our ion concentrations resorted some other way. A way to do so is to use active transport, among them the well known Na/K-ATPase (Na/K-pump). This ensure we get our concentration of K and Na restored and as it is a unequal distribution of ions we get to create our rest-potential.