Biology

The correct answer is: B.To move small substances through channels against a concentration gradient.

Explanation:

In active transport, transport proteins play a crucial role in moving substances against their concentration gradient (from an area of low concentration to an area of high concentration). This process requires energy, usually in the form of ATP.

There are two main types of transport proteins involved:

1.Carrier proteins: These proteins undergo conformational changes to transport substances across the membrane.

2.Pump proteins: These actively move ions or molecules against their gradient, like the sodium-potassium pump.

Why the other options are incorrect:

A.To move water across a membrane: This describes osmosis, which is a type of passive transport, not active transport. Wate...