The whole point of nutrition is to obtain a source of energy and of carbon. This allows cell processes to continue and for growth and repair to take place. Obtaining this energy and carbon is done in a variety of ways by different organisms.
The following is a selection of the different types of nutrition which living things use:
Autotrophic means self-feeder, i.e. they produce their own food out of raw materials.
Heterotrophic means that they feed on other organisms that have made their own food.
- Photoautotrophic: e.g. plants and algae. Light is the primary source of energy. Carbon dioxide is the primary source of carbon.
- Chemoautotrophic: e.g. some bacteria. A chemical is the primary source of energy and the carbon source may be organic or inorganic.
- Chemoheterotrophic: e.g. animals, fungi, and most protoctista. Their energy and carbon source is often glucose.
Within the chemoheterotrophs there may be:
- Saprotrophic nutrition: e.g. fungi. They feed on the soluble organic matter from dead organisms. Enzymes are secreted onto the dead organism to digest the large molecules so digestion is external. The small molecules are absorbed and then transported within the fungus.
- Parasitic nutrition: e.g. tapeworm. They feed on already digested food (digested by the host of the endoparasite). The parasite therefore produces no enzymes and needs no gut system. Small molecules are absorbed over the body surface.
- Holozoic nutrition: e.g. many animals and carnivorous plants. They feed on solid organic matter from a living or dead organism. They therefore will need to be able to catch or obtain and produce enzymes to digest their food.
Food must be:
- Ingested: Large molecules are taken into the mouth.
- Masticated: Chewed and mixed with saliva. This increases the surface area of the food over which the enzymes can act and makes the food easier to swallow
- Digested: Begins in the mouth. It involves mechanical digestion - mixing and churning - and chemical digestion - using enzymes.
- Absorbed: Products of digestion are passed across the gut lining.
- Egested: The undigested waste is eliminated by defaecation.
Herbivores eat only plants or algae. Terrestrial plants in particular have a lot of cellulose present in their cell walls for support. Animals cannot digest cellulose and since it surrounds the cell contents, this can present a problem.
Various adaptations to solve this problem are found in the animal kingdom. Insects, for example, may have piercing mouthparts that are used to pierce the cells and suck out the cell contents or the sap in the phloem tubes.
Elephants deal with it in a much more basic way - they eat virtually all the time! If they eat enough they will receive enough nutrients to keep them alive.
Some herbivores such as sheep have good jaws and teeth to really grind the food. They regurgitate and re-chew the food (chewing the cud). They also possess a long gut so the food spends longer in there.
Many herbivores have symbiotic (mutually beneficial) bacteria in their digestive tracts. These produce cellulase, an enzyme that digests cellulose into simple carbohydrates. These carbohydrates are used by the herbivores and by the microorganisms.
Digestion of the microorganisms themselves by the herbivore gives it a protein supply - useful due to the fact that their diet provides very limited amounts of protein.