Sources of food

What this heading covers

CTET's Paper II Science syllabus puts Food into three headings: sources of food, components of food, and cleaning food. This one is the first: where food comes from, what part of a plant you are actually eating, and the different ways living things obtain food.

The paper sets these at the level of classes VI to VIII, so the questions are rarely "name a source". They are more often "which of these is a stem?", "which plant is a parasite?", or a matching grid. The detail is what is tested, so the detail is what this page gives you.

Everything traces back to a green plant

Food comes from plants or animals — but the two are not independent. A green plant makes its own food; every animal, directly or at one remove, lives on what a plant made. Milk is an animal product, but the cow got it from grass.

That is why a food chain always begins with a green plant, and why plants are called producers and animals consumers. If a question asks what would happen to a food chain if producers vanished, this is the reasoning it wants.

Which plant part are you eating?

This is the single most examinable table in the topic.

Part eaten	Examples
Root	Carrot, radish, beetroot, turnip, sweet potato
Stem	Potato, ginger, turmeric, sugarcane, onion
Leaf	Spinach, cabbage, coriander, lettuce, curry leaves
Flower	Cauliflower, broccoli, banana flower, clove
Fruit	Mango, tomato, brinjal, cucumber, lady's finger
Seed	Rice, wheat, gram, mustard, groundnut

The examiner's interest is almost entirely in the ones that look like something they are not:

• Potato is a stem, not a root. It grows underground, but it is a tuber — it carries buds, the "eyes", and buds belong to stems. Sweet potato, by contrast, really is a root.
• Onion is a bulb — a very short underground stem with thick fleshy scale leaves around it. Counting it as a leaf or a root both lose the mark.
• Ginger and turmeric are rhizomes, which are stems that creep underground.
• Tomato, brinjal, cucumber and lady's finger are fruits, botanically, because each develops from a flower and carries seeds. That they are cooked as vegetables is a kitchen fact, not a botanical one.
• Cauliflower is a cluster of unopened flowers, and clove is a dried flower bud.

Food from animals

Milk, eggs, meat, fish and honey are the direct animal sources; curd, paneer, butter and ghee are products made from milk rather than separate sources.

Honey is worth a moment because it is the one common food an insect makes: bees gather nectar from flowers and convert it. A question that asks for "a food obtained from an insect" is asking about honey.

The five ways living things get their food

Two broad modes, and the second has four named types. CTET sets questions on the names.

Autotrophic — the organism makes its own food. Green plants photosynthesise in the chloroplasts of their leaves:

6CO₂ + 6H₂O  →  C₆H₁₂O₆ + 6O₂     (in sunlight, in the presence of chlorophyll)

Carbon dioxide enters through stomata, water arrives from the roots, and chlorophyll captures the light. Remove any one and the process stops — a standard experiment question.

Heterotrophic — the organism takes food made by another:

• Saprotrophic. Feeds on dead and decaying matter by releasing digestive juices onto it and absorbing what dissolves. Fungi, mushrooms, bread mould.
• Parasitic. Lives on a living host and takes its food from it. Cuscuta (amarbel) is the standard example — a yellow, twining plant with no chlorophyll, so it cannot feed itself at all.
• Insectivorous. Pitcher plant, Venus flytrap, sundew. These plants are green and do photosynthesise. They trap insects for nitrogen, which the marshy soil they grow in cannot supply. Treating them as non-green or as fully heterotrophic is the commonest error in this whole topic.
• Symbiotic. Both partners gain. A lichen is an alga and a fungus living together — the alga makes food, the fungus provides shelter and water. Rhizobium bacteria live in the root nodules of pea, gram and other legumes, fixing nitrogen from the air while the plant feeds and houses them.

How animals feed

By what they eat: herbivore (deer, cow), carnivore (lion, frog), omnivore (crow, human), and scavenger (vulture, hyena) — which eats animals that are already dead, and is not the same as a carnivore that hunts.

By how they take it in — the part that turns up as a matching question:

Animal	Mode of feeding
Housefly	Sponging
Butterfly, honeybee, humming bird	Sucking or siphoning
Mosquito	Piercing and sucking
Cockroach, grasshopper	Biting and chewing
Human infant	Suckling
Snake	Capturing and swallowing
Amoeba	Engulfing with pseudopodia
Starfish	Everting its stomach onto the prey

A question of the kind the paper sets

Match the animal with its mode of feeding. (a) Housefly (b) Humming bird (c) Cockroach (d) Infants (i) Biting and chewing (ii) Sucking (iii) Sponging (iv) Suckling

Work it from the mouthpart, not from memory. A housefly has a spongy pad and no biting jaws, so it mops up liquid food — (iii). A humming bird takes nectar through a long slender beak — (ii). A cockroach has mandibles that cut and grind — (i). An infant feeds on milk — (iv).

Answer: a–iii, b–ii, c–i, d–iv.

Mistakes that cost marks here

• Calling potato a root or onion a leaf.
• Saying insectivorous plants cannot photosynthesise. They can; they are after nitrogen.
• Calling Cuscuta a saprophyte. It feeds on a living host, so it is a parasite.
• Calling Rhizobium a parasite. Both sides gain, so it is symbiotic.
• Using "scavenger" and "carnivore" interchangeably.

In one line

Every food chain starts at a green plant, and everything else in this topic is a variation on how an organism gets at the food a plant made.

In the classroom

This topic rewards objects over diagrams. A handful of real vegetables sorted by the part of the plant they came from settles the potato-and-onion confusion in one lesson far better than a labelled drawing, and it is the kind of activity CTET's pedagogy questions treat as good practice.