Organisms which feed on complex ready made organic compounds are referred to as heterotrophs. The main types of heterotrophic nutrition are holozoic, saprozoic, saprophytic, mutualistic and parasitic.

Most animals fall into this group. The process by which they capture and use their food materials varies widely but the process as a whole can be summarised in the following steps:-

1. Ingestion is the taking in of complex organic substances into the digestive tract or apparatus.
2. Digestion is the breakdown of large complex insoluble molecules into small, simple soluble molecules which are able to diffuse into the organisms body.
3. Absorption is the uptake of the soluble products of digestion into the body tissues.
4. Assimilation is the utilisation of the absorbed molecules by the organism.
5. Egestion is the removal of undigested materials from the digestive structures.

The following terms are applied to the different forms of holozoic organisms:- herbivores consume plant material, carnivores consume other animals, omnivores have a diet consisting of both plant and animal materials. If the food is consumed in the form of tiny particles the organism is said to be microphagous, if it is liquid then they are fluid feeders.

Many fungi, bacteria and slime moulds feed by saprozoic or saprophytic means. The cells of the organism secrete enzymes onto the food material and the soluble products are absorbed by diffusion or active transport into the cytoplasm.

 If the organism is very small and its food is large then a parasitic relationship may develop. The feeding process may involve invasion of and physiological adaptation to the host’s body. The distinction between a predator and a parasite becomes very blurred in these cases. For example which of these would you consider a parasite: A Bantu warrior (they feed on the blood and milk of their cows), a vampire bat, a mosquito, a head louse, a microfilarial worm living in the human bloodstream? If the head louse is a parasite then is a greenfly also one?

When two organisms share a close relationship in which they both gain an advantage we refer to them as mutualistic. For example certain forms of plants produce root nodules when they are invaded by a bacterium called Nitrobacter. This bacterium can fix nitrogen to form nitrate which the plant receives for the manufacture of amino acids. The bacterium receives a supply of glucose from the plant which enables it to perform its nitrogen fixation. Both organisms are benefiting from this relationship and therefore it is mutualistic.