A Level Biology Practice Exam

What defines a toxin in biological terms?

1. An edible substance

2. A non-harmful chemical

3. A substance that is poisonous to living cells

4. An environmental nutrient

The correct answer is: A substance that is poisonous to living cells

A toxin is defined as a substance that is poisonous to living cells. This definition captures the essential characteristic of toxins, which is their ability to cause harm or disrupt normal biological functions within an organism. Toxins can lead to cell damage, malfunction, or even cell death, depending on their nature and concentration. This harmful effect is typically the result of the interaction between the toxin and specific cellular structures or metabolic processes. In contrast, an edible substance indicates something that can be consumed without harmful effects, which does not align with the concept of toxicity. A non-harmful chemical, by definition, implies a lack of toxicity, thus it cannot be classified as a toxin. Finally, an environmental nutrient refers to substances that are beneficial and necessary for the growth and survival of organisms, which also does not meet the criteria for being a toxin. Therefore, the distinction of a toxin lies in its capacity to pose a danger to biological systems, making the correct answer a clear choice.