Nutrition

Nutrition is the process by which an organism obtains food and the substances needed for life. Food supplies energy for movement, growth, repair, and other cellular activities; nutrients also provide raw materials for building tissues. Thus option A is correct. Nutrition does not harden the body into stone, stop breathing, or destroy water. Those choices do not describe any biological purpose of nutrition.

The governing concept is photosynthesis, the food-making process of green plants. Chlorophyll absorbs light energy, and the plant uses carbon dioxide and water to form glucose; oxygen is released as a by-product. Therefore option A is correct. Respiration releases energy from food, excretion removes wastes, and digestion breaks complex food into simpler substances.

Autotrophic nutrition means producing organic food from simple inorganic substances, usually using light energy. Green plants absorb water and carbon dioxide and use chlorophyll and sunlight to manufacture glucose by photosynthesis. Humans, fungi and mosquitoes obtain ready-made organic matter from other sources, so they are not autotrophs. Therefore option A is the correct example.

Nutrition supplies living organisms with food molecules that provide energy and raw materials. Energy is needed for movement, transport, repair, growth, and other life processes, while nutrients such as proteins and minerals help build and maintain tissues. Thus option A is correct. Body colour, sound production, and sleep are not the primary purposes of nutrition.

The governing concept is the relationship between surface area and absorption. Villi are finger-like projections in the small intestine that greatly enlarge its inner surface and contain capillaries for carrying absorbed nutrients. If their number decreases, the available surface area falls, so nutrient absorption becomes less efficient. Therefore option A is correct. The other choices concern the heart, oxygen production, or plant stomata.

The governing concept is saprophytic, or extracellular, digestion. Fungi secrete digestive enzymes onto dead and decaying organic matter. These enzymes break complex substances into simpler, soluble molecules outside the fungal body; the hyphae then absorb those molecules. Fungi do not use lungs, make food from blood, or produce chlorophyll in leaves. Thus the release of digestive juices outside the body directly explains their mode of nutrition.

Digestion breaks food into small molecules, but the body can use them only after absorption. Villi normally increase the intestinal surface area and provide thin routes into blood and lymph. If villi are damaged, less glucose, amino acids, fatty-acid products, vitamins, and minerals enter circulation, causing poor nourishment and weakness even when food is eaten. Hence option A is correct.

The governing concept is parasitic nutrition and the role of chlorophyll. Green colour generally indicates chlorophyll, the pigment needed to capture light for photosynthesis. Cuscuta has little or no chlorophyll, so it cannot manufacture sufficient food from carbon dioxide and water. It obtains nutrients from a host plant through specialised absorbing structures. Thus option A is correct; the other choices mention increased photosynthesis or organs absent from plants.

The digestive system converts food into simple nutrients that can be absorbed, using mechanical actions and digestive enzymes. Its main organs include the mouth, oesophagus, stomach and intestines. The circulatory system transports absorbed nutrients around the body, but it does not digest food. Exam tip: distinguish digestion from absorption—digestion breaks food into simpler substances, while absorption moves nutrients into the blood.

Digestion takes place in the digestive system, where food is broken down into simple, soluble nutrients that can be absorbed. The respiratory system is responsible for the exchange of oxygen and carbon dioxide, not digestion. Exam tip: If a question mentions breakdown of food or absorption of nutrients, choose the digestive system.

Photosynthesis mainly takes place in leaves because their cells contain many chloroplasts with chlorophyll. Chlorophyll absorbs sunlight and helps make food from carbon dioxide and water. Roots mainly absorb water and minerals, so they are not the principal site of photosynthesis. Exam tip: The plant part with abundant chlorophyll is usually the main site of photosynthesis.

During photosynthesis, plants take in carbon dioxide from the air through stomata. In the presence of water and sunlight, it is used to form glucose, while oxygen is released. Therefore, oxygen is a product of photosynthesis, not the main gas required. Exam tip: Remember that carbon dioxide and water are reactants in photosynthesis.

Most chemical digestion is completed in the small intestine with the help of pancreatic juice and intestinal juice. Its many villi absorb digested nutrients into the blood. The stomach mainly carries out partial protein digestion, whereas the large intestine mainly absorbs water. Exam tip: Remember that the small intestine is the main site for both digestion and nutrient absorption.