Given a scientific question, generate a correct answer to it.

Input: Consider Input: What occurs when substances move from areas of lower to higher concentration or when very large molecules are transported?

Output: active transport

Input: Consider Input: What is the primary energy source for cells?

Output: glucose

Input: Consider Input: Convex lenses are thicker in the middle than at the edges so they cause rays of light to converge, or meet, at a point called what?
Output: focus