Which process involves the movement of small molecules against their concentration gradient and requires an expenditure of cellular energy?

The process that involves the movement of small molecules against their concentration gradient and requires an expenditure of cellular energy is called Active Transport.

Here are the steps involved in this process:

1. Active transport usually happens across the cell membrane. Small molecules that the cell needs are identified.

2. These molecules are against their concentration gradient, which means they move from an area of lower concentration to an area of higher concentration. This is the opposite of what would happen naturally.

3. Because this process goes against the natural flow, it requires energy. The cell uses ATP (Adenosine Triphosphate), the main energy currency of the cell, to power this process.

4. The ATP provides the energy to change the shape of the transport protein that sits within the cell membrane.

5. This change in shape allows the small molecule to be moved across the cell membrane, from outside to inside, or vice versa.

6. Once the molecule has been transported, the ATP is converted back to ADP (Adenosine Diphosphate), and the process can start again when more ATP is available.

This process is vital for many cellular functions and allows cells to maintain conditions necessary for life.