Excretion

The governing concept is removal of metabolic waste. Cellular reactions produce substances such as carbon dioxide, nitrogenous wastes, and excess water or salts; if they accumulate, they can harm the body. Their removal is called excretion, making option A correct. Taking in food is ingestion, pumping blood is circulation, and making oxygen is not the meaning of excretion.

The governing concept is the human excretory system. Kidneys continuously filter the blood, remove nitrogenous wastes such as urea, regulate water and salts, and form urine. Therefore option A is correct and the kidneys are considered the main excretory organs. The heart pumps blood, the stomach digests food, and a hair is not an excretory organ, although skin can remove small amounts of sweat.

The governing concept is excretion, the removal of metabolic waste products from an organism. Cellular activities produce substances such as carbon dioxide, excess water and nitrogenous wastes, which can disturb the internal balance if they accumulate. The body therefore removes them through organs such as kidneys, lungs and skin. Option A is correct. Nutrition supplies materials, transport moves substances, and respiration releases energy; none of these terms specifically means waste removal.

Excretion is the biological process by which metabolic waste products are removed from an organism. Cellular activities produce substances such as carbon dioxide, excess water, and nitrogenous wastes; their accumulation can be harmful. Therefore, option A gives the correct meaning. Taking in food is ingestion, whereas the other choices are not definitions of excretion.

Plants remove much of their excess water by transpiration. Water reaches the leaves through xylem, evaporates from moist leaf cells, and then diffuses out mainly through stomata as water vapour. This process also supports cooling and helps maintain the upward movement of water. Therefore option A is correct; digestion, blood circulation, and breathing are not the appropriate plant process here.

The governing concept is the distinction between nutrition and waste removal. Digestion is a nutritional process that mechanically and chemically breaks complex food into simpler, absorbable substances such as glucose and amino acids. Excretion removes wastes produced by metabolism, including nitrogenous wastes, excess salts, water, and carbon dioxide through appropriate organs. Therefore option A is correct; digestion does not itself mean urine formation, and the other choices confuse separate processes.

A nephron is the structural and functional unit of the kidney. It filters blood, reabsorbs useful substances, and secretes certain wastes to form urine. A neuron conducts nerve impulses, so it is not involved in urine formation. Exam tip: Associate the kidney and urine formation with the nephron.