Heterotrophic Nutrition
Senior 3 · Biology
Heterotrophic Nutrition
Lesson 1 of 5
25 min
Leave a piece of bread in a closed plastic bag in a warm kitchen for a few days. Grey-white fuzz appears, then black dots, and the bread under the fuzz goes soft and starts to disappear. The mould is eating the bread. Yet it has no mouth, no teeth and no stomach. Meanwhile a goat in the same compound eats grass with its teeth and digests it inside its body, and a roundworm in a child's intestine lives on food the child has already digested. All three are heterotrophs, but they feed in three different ways.
Definition
Heterotrophic nutrition: feeding on ready-made organic food taken from other organisms, living or dead. Animals, fungi and most bacteria are heterotrophs. (Green plants, which make their own food by photosynthesis, as you saw in Senior 2, are autotrophs.)
Holozoic nutrition: taking in solid pieces of food and digesting them inside the body. Most animals, including humans, feed this way.
Saprophytic nutrition (also called saprotrophic): feeding on dead or decaying organic matter by releasing enzymes onto it and absorbing the digested products. The organism is a saprophyte (saprotroph).
Parasitic nutrition: feeding on or in a living organism of another species, the host, which is harmed. The organism is a parasite.
A holozoic animal deals with its food in five steps. The rest of this unit follows these steps through the human body.
Figure 1: The five steps of holozoic nutrition. Egestion removes food that was never digested; it is not excretion, which removes waste made by the body's cells.
Fungi such as bread mould (Rhizopus), mushrooms and yeast, and many bacteria, feed on dead matter. A mould is made of fine threads called hyphae (singular hypha) that grow over and through the food. The hyphae secrete digestive enzymes onto the food; the enzymes break large insoluble molecules, such as starch and protein, into small soluble ones, such as glucose and amino acids; and the hyphae absorb these. Because the digestion happens outside the organism, it is called extracellular digestion.
Figure 2: Bread mould feeding. Enzymes go out, digestion happens in the food, and small soluble products come back in.
Saprophytes matter far more than spoiled bread. As decomposers they break down dead leaves, dung and dead bodies, releasing carbon dioxide and mineral salts that plants can use again (the carbon and nitrogen cycles of Unit 3). Farmers use them in compost, brewers and bakers use yeast, and mushrooms such as oyster mushrooms are grown for food on waste such as maize cobs and sawdust.
A parasite takes its food from a living host and harms it, usually without killing it quickly. Three parasites common in East Africa:
Parasites that live inside the gut have features that suit that life:
| Feature | How it helps the parasite |
|---|---|
| Hooks, suckers or cutting mouthparts | Hold on against the movement of food through the gut |
| A thick outer cuticle | Resists the host's digestive enzymes |
| Huge numbers of eggs, thousands a day | A few survive to reach a new host |
| Simple or no digestive system | The host has already digested the food |
| Feature | Holozoic | Saprophytic | Parasitic |
|---|---|---|---|
| Food source | Other organisms, eaten as solid food | Dead or decaying matter | A living host |
| Where digestion happens | Inside the gut | Outside the organism | Often already done by the host |
| Examples | Humans, goats, frogs | Moulds, mushrooms, yeast, many bacteria | Roundworm, hookworm, Striga |
Worked example 1: classifying nutrition
State the type of heterotrophic nutrition in each case, and give a reason: (a) a cow grazing; (b) a mushroom growing on a fallen tree in Nyungwe forest; (c) Striga on a sorghum plant; (d) yeast in banana juice.
Test to apply: is the food alive or dead? Is it digested inside or outside the feeder?
Worked example 2: why dried food keeps
Two slices of the same bread were kept for five days: slice A in a closed bag in a warm cupboard, slice B dried in the sun and kept in an open dish. Only slice A went mouldy. (a) Explain the difference. (b) Explain why sun-drying helps farmers store beans and maize, and fishers store isambaza.
Worked example 3: breaking the hookworm's life cycle
Hookworm eggs pass out in faeces, hatch in the soil, and the larvae enter through the skin. Suggest three ways a community can reduce hookworm infection, and explain each.
Warning
Common mistakes. (1) Calling a saprophyte a parasite: a saprophyte feeds on dead matter, a parasite on a living host. (2) Calling fungi plants that "feed on the soil": fungi have no chlorophyll; they are heterotrophs. (3) Mixing up egestion (removing undigested food) with excretion (removing waste made by cells, such as urea). Never eat mouldy food: some moulds make poisons, such as the aflatoxins that can form in badly stored maize and groundnuts.
Types of Heterotrophic Nutrition Explained, Khan Academy India. Watch for the difference between saprophytes and parasites, and note which examples digest their food outside their bodies.
Tip
Memory hook: "Holo eats whole, sapro eats the dead, para eats the living."
Practice: forms of heterotrophic nutrition
Key points
Sources and credits
Written by Ganzaa: the text, Figures 1 and 2, the tables, the worked examples and the practice questions.
