Final answer:
Transporting a solute against its concentration gradient requires energy, such as ATP, because it is an active transport process that moves substances against the natural direction of diffusion.
Step-by-step explanation:
Transporting a solute against its concentration gradient is an active process known as active transport. This process requires energy, typically in the form of adenosine triphosphate (ATP), and is facilitated by specific membrane proteins that function as pumps. Unlike passive transport mechanisms such as passive diffusion and facilitated diffusion, which move substances along their concentration gradients without the need for cellular energy, active transport moves substances from regions of lower concentration to regions of higher concentration, counteracting the natural diffusion process.
Therefore, the answer to the student's question is 1) energy. This energy is used to change the shape of the transport protein, allowing the substance to bind, be transported, and ultimately released on the other side of the membrane against its concentration gradient.