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

How is the control of water content in the body described?

1. By active transport

2. By positive feedback

3. By negative feedback

4. By passive diffusion

The correct answer is: By negative feedback

The control of water content in the body is described by negative feedback because this mechanism helps maintain homeostasis, which is the stable and balanced internal environment necessary for proper physiological function. When there is a change in water levels, such as increased dehydration, the body responds to restore balance by signaling mechanisms that conserve water, such as the release of antidiuretic hormone (ADH) from the hypothalamus. This hormone prompts the kidneys to reabsorb more water, reducing urine output and effectively increasing water retention to counteract the initial loss. Similarly, if water levels are too high, the body can respond by decreasing the secretion of ADH, allowing more water to be excreted through urine. This process continually works to keep the water levels within a specific range, demonstrating how negative feedback loops are essential for regulating internal conditions against external changes. In contrast, active transport, positive feedback, and passive diffusion are mechanisms that do not adequately describe the regulation of water content. Active transport refers to the movement of substances against a concentration gradient and is typically involved in nutrient uptake, while positive feedback amplifies a response rather than regulating it. Passive diffusion involves the movement of molecules from an area of higher concentration to one of lower concentration without energy expenditure,