(i) Autotrophic or holophytic nutrition: All green plants manufacture their own food from simple inorganic substances, using light energy. The two types of autotrophic nutrition are:
a. Photosynthesis: This involves the utilisation of solar energy by all green plants to manufacture food.
b. Chemosynthesis: This involves the utilisation of energy derived from the oxidation of some inorganic substances in the environment. It takes place in colourless autotrophic bacteria. Some examples of these bacteria are nitrosomonas, nitrobacter and the iron bacteria. Nitrosomonas converts ammonia to nitiries, nitrobacter converts nitirite to nitrate, while the iron bacteria changes iron II compounds to iron III compounds.
(ii) Heterotrophic nutrition: This is the type of nutrition in which the organisms involved are unable to manufacture their food but depend directly or indirectly on autotrophs. All animals, fungi, protozoa and some bacteria are heterotrophs. This type of nutrition includes holozoic nutrition and parasitic nutrition.
a. Holozoic nutrition: This is the method of nutrition in which an organism feeds on complex solid food substances which are digested before being absorbed. This method of nutrition is a characteristic of animals and those animals are said to be holozoic in their mode of nutrition.
b. Parasitic nutrition: This is the method of nutrition in which an organism depends on its host for its food and in the process, the host is being harmed by the parasite. Examples are: ectoparasites found on the skins or body surfaces of some animals, e.g., ticks, flea, aphid, bedbug and mites.
c. Endoparasites are found inside the body of the host. They have hooks and suckers for attachment, e.g., in tapeworm, mistletoes and dodders (these are plants that exist as external parasites on other plants).
(iii) Symbiotic nutrition/mutualism: This is an association in which two organisms live together for the mutual benefit of each other. A good example of symbiotic associations is found between nitrogen fixing bacteria, rhizobium in root nodules of legumes and the leguminous plants. The bacteria supply nitrates to legumes, while legumes provide carbohydrates and shelter to bacteria. Another example is the association of fungi and algae in lichens.
(iv) Saprophytic nutrition: Saprophytes are the organisms, which derive their food from dead or decaying organic matter, e.g., many fungi and bacteria. Digestive enzymes are secreted extra-cellularly on the food to digest it and the soluble products formed are absorbed. Rhizopus and mushroom are saprophytic fungi.
(v) Carnivorous plants: These are insectivorous plants with green leaves, capable of manufacturing their own food. They have special devices which are used to trap and digest insects and other small organisms. They obtain minerals and use insect as their source of nitrogen. Examples of carnivorous plants are, bladderwort (Utricularia vulgaris) a free floating aquatic plant, the venue fly trap (Dionaed) and sundew (drosera intermedia). The sundew leaf has long hairs and if a fly lands on a few of them, the rest of it curl over and cover the insect to digest it.
1. Green plants can manufacture their food during the process of photosynthesis, using light from the sun, carbon dioxide, water and chlorophyll.
2. The major product of photosynthesis is starch. Oxygen is the by-product. In addition, proteins, lipids and vitamins are also formed during photosynthesis.
3. The microelements needed by plants for growth are nitrogen, sulphur, phosphorus, calcium, iron, potassium, and magnesium. The microelements required in small quantities are manganese, copper, cobalt, boron, zinc and silicon.
4. Animals feed on already made food substances. The seven classes of food required by the body are carbohydrates, proteins, fats and oils, vitamins, mineral salts, roughage and water.
5. A balanced diet contains all classes of food in correct and adequate proportions. This is required by the body for proper growth. Absence of any class of food leads to deficiency disease.
6. Some of the common food tests are: Biuret’s test for proteins and Sudan III test for fats and oils.
7. Enzymes are organic catalysts found in living organisms.