Nutrition in plants and non-human animals

Biology · Nutrition. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Autotrophs such as green plants make their own organic food from CO₂ and water, but they also need mineral ions from the soil. A shortage of a mineral shows as a deficiency symptom, for example yellowing of leaves (chlorosis).

Heterotrophs depend on ready-made organic food. Holozoic organisms take in solid food and digest it inside the body. Saprotrophs feed on dead matter and parasites feed on a living host. Insectivorous plants are green and photosynthesise; they trap insects to obtain nitrogen.

Digestion may be intracellular (inside food vacuoles) or extracellular (in a gut cavity). Simple animals have a sac-like gut with one opening. More complex animals have a tube-like gut with a mouth and an anus.

Rules to remember

Nitrogen: part of proteins, nucleic acids and chlorophyll. Deficiency gives stunted growth and chlorosis, seen first in older leaves.
Magnesium: central atom of chlorophyll. Deficiency gives chlorosis.
Parasitic plant: Cuscuta (dodder) absorbs food through haustoria.
Insectivorous plants: Nepenthes (pitcher plant), Dionaea (Venus flytrap), Drosera (sundew).
Amoeba: food taken in by pseudopodia; digestion intracellular in a food vacuole.
Hydra and Planaria: sac-like gut with a single opening; digestion extracellular first, then intracellular. Planaria has a branched intestine.
Cockroach: tube-like gut. Crop stores food, gizzard grinds it with chitinous teeth, hepatic (gastric) caeca secrete enzymes.

Common mistake

Students think insectivorous plants are heterotrophs that eat insects for energy. They are green autotrophs and trap insects only as a source of nitrogen compounds.

Worked example

An animal has a gut with a single opening, and food particles are finally digested inside the cells lining the cavity. Is it more likely to be Hydra or a cockroach?

A single opening means a sac-like digestive system, in which the mouth also removes undigested waste.
A cockroach has a tube-like gut with a separate mouth and anus, and its digestion is extracellular.
Hydra has a gastrovascular cavity with one opening; digestion starts in the cavity and is completed inside the lining cells.

Answer: Hydra.

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Chapter 12: Nutrition

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