OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

What occurs when there is a higher concentration of nutrients in the gut than in the blood?

1. Diffusion occurs naturally into the blood

2. Active transport is not required

3. Nutrients cannot enter the blood

4. Nutrients dissolve into the gut

The correct answer is: Diffusion occurs naturally into the blood

When there is a higher concentration of nutrients in the gut than in the blood, diffusion occurs naturally into the blood. This process relies on the principle that substances move from areas of higher concentration to areas of lower concentration, in order to achieve equilibrium. Since the concentration gradient favors the movement of nutrients from the gut (high concentration) to the blood (low concentration), diffusion is the mechanism that facilitates this transfer without the need for energy expenditure. Active transport is not necessary in this situation because active transport requires energy to move molecules against their concentration gradient, which is not the case here where the gradient is favorable for diffusion. Saying that nutrients cannot enter the blood contradicts the reality of diffusion, as the presence of a concentration gradient allows for their movement. Additionally, the notion that nutrients dissolve into the gut does not accurately describe the process; instead, they move into the bloodstream directly through the intestinal walls via diffusion.