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Heterotrophic Nutrition


Heterotrophs may be parasitic, saprophytic and holozoic.



Parasitic organisms are those which live on or inside another living organism which is called the host. They derive their food material from the host. Parasites often lack well-developed organ systems but have body parts modified for attachment to the host or for sucking food from the host body.


Liverfluke (An endoparasite)

Dodder plant attached to a host plant



Saprophytic organisms derive their food from decomposing dead organisms. They feed and grow on substances which were once part of living organisms such as stored food, wood and leather. Saprophytes secrete enzymes which are released on the substrate. These enzymes digest or break down the material on which the saprophyte is growing. The soluble end product is then absorbed back by the saprophyte. This is called extracellular digestion.


Saprophytic Fungi


Most animals and insectivorous plants are holozoic. This process involves the following steps:


Ingestion - Taking in of complex organic food. Mouth parts of different organisms are variously modified for this purpose.

Digestion - Breaking down of large, complex insoluble organic molecules into small, simple, soluble and diffusible molecules. This is achieved by enzymes.

Absorption - Utilization of soluble molecules from the digestive region across a membrane and into the blood vessels.

Assimilation - Utilization of the absorbed food molecules by the body to provide energy.

Egestion - Elimination of undigested waste matter from the body. The steps of holozoic nutrition and functions of different parts of the human alimentary canal.

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