Nutrition in Plants Class 7 Science — Revision Notes

TL;DR: Plants are autotrophs that produce their own food through photosynthesis using sunlight, water, and carbon dioxide. Understanding plant nutrition is f…

Plants are autotrophs that produce their own food through photosynthesis using sunlight, water, and carbon dioxide. Understanding plant nutrition is fundam

Key Points to Remember

• Photosynthesis is the process by which plants produce glucose using sunlight, carbon dioxide, and water
• Chlorophyll is the green pigment in chloroplasts that captures light energy
• The leaf is the food factory of the plant; food made there is carried to every other part, including the roots
• Each stoma is bounded by two guard cells, which open and close the pore
• Carbon dioxide is absorbed through stomata, pores on plant leaves
• Water is absorbed by roots and transported to leaves through the xylem
• Glucose produced in photosynthesis is used for plant growth and respiration
• Heterotrophs are organisms that depend on autotrophs for food
• Saprophytes are organisms that feed on dead organic matter through decomposition
• Parasites feed on living host organisms, harming them in the process
• Nitrogen, phosphorus, and potassium are essential mineral nutrients for plants
• Root nodules in legumes contain bacteria that fix atmospheric nitrogen

Important Formulas

• Photosynthesis (word equation): Carbon dioxide + Water → Carbohydrate + Oxygen, in the presence of sunlight and chlorophyll
• Sunlight and chlorophyll are conditions and are written on the arrow, not as reactants — neither is used up

Key Terms

• Photosynthesis: Process by which plants produce glucose from carbon dioxide and water using light energy
• Chlorophyll: Green pigment in chloroplasts that absorbs light energy for photosynthesis
• Autotroph: Organism that produces its own food from inorganic substances
• Heterotroph: Organism that depends on other organisms for food
• Stomata: Pores on plant leaves through which gas exchange and transpiration occur
• Chloroplast: Organelle in plant cells where photosynthesis takes place

What nutrition actually means in this chapter

Nutrition is the whole business of taking in food and using it — not just eating. The food supplies nutrients, which are the substances a living body needs to build itself, repair damage and release energy. The five nutrient groups named in the chapter are carbohydrates, proteins, fats, vitamins and minerals, and water and roughage are added to that list as essential components of a balanced diet.

Every living thing needs nutrients, but they do not all obtain them the same way. That single question — where does the food come from — splits all organisms into two camps, and the entire chapter is organised around that split. Get this straight and the rest of the chapter is just examples.

Autotrophs and heterotrophs — the split the whole chapter rests on

Autotrophic nutrition is making your own food from simple inorganic substances. The word is built from Greek: auto meaning self, and trophos meaning nourishment. Green plants are the classic autotrophs, because they build carbohydrate out of carbon dioxide and water. Algae are autotrophs too — they are green because they contain chlorophyll, and they photosynthesise exactly as leaves do.

Heterotrophic nutrition is depending on others for food, either directly or indirectly. Animals and most fungi are heterotrophs. Note the word indirectly: a tiger eats a deer, the deer ate grass, and the grass made the food. Trace any food chain far enough back and it ends at an autotroph, which is why plants are called producers.

A trap worth naming now: some plants are not purely autotrophic. Cuscuta makes no food at all, and the pitcher plant photosynthesises but still traps insects. Being a plant does not automatically make an organism an autotroph.

Photosynthesis — the food-making process

Photosynthesis is the process by which green plants synthesise food from carbon dioxide and water in the presence of sunlight and chlorophyll. The name says it: photo means light, synthesis means putting together.

• The word equation CBSE expects: carbon dioxide + water, in the presence of sunlight and chlorophyll, gives carbohydrate plus oxygen. Write the conditions on the arrow, not as reactants — this is a common marking point.
• Chlorophyll is the green pigment in leaves. Its job is to trap energy from sunlight; without it the reaction does not run, which is why photosynthesis happens in the green parts of a plant.
• The carbohydrate first made is glucose; the plant stores the extra as starch. Starch is made of carbon, hydrogen and oxygen — exactly the elements available in carbon dioxide and water.
• Oxygen is released as a by-product. Almost all the oxygen in the air is there because of photosynthesis, which is why the process matters far beyond the plant itself.
• Leaves are called the food factories of the plant. Food made in the leaves is carried to every other part, including the roots, which is why a root can store food it never made.

The three things a leaf needs, and how each one arrives

A leaf cannot photosynthesise unless carbon dioxide, water and sunlight all reach it. Each arrives by a different route, and CBSE likes asking about the route rather than the list.

• Carbon dioxide enters through stomata — tiny pores mostly on the lower surface of the leaf. Each stoma is guarded by two guard cells that open and close it, so the plant controls the exchange rather than leaving it open.
• Water and minerals are absorbed from the soil by the roots, specifically by root hairs, and are carried up to the leaves through vessels running through the root and stem. These vessels form a continuous pipeline from root tip to leaf.
• Sunlight is simply absorbed by the chlorophyll in the leaf. This is why leaves are broad and flat and are arranged so they shade each other as little as possible.
• Large amounts of water are lost through the same stomata as vapour, so the stomata are doing two jobs at once — letting gas in and letting vapour out.

Other modes of nutrition in plants

Not every plant feeds itself. The chapter names four alternative arrangements, and the exam almost always asks for the mode plus one named example.

• Parasitic — the plant takes ready-made food from another living plant. Cuscuta, called amarbel, is the standard example: a yellow tubular vine with no chlorophyll that twines around a host and draws nutrients from it. The plant it grows on is the host, and the host is harmed.
• Insectivorous — the plant photosynthesises normally but traps and digests insects to obtain nitrogen, because it grows in soil poor in that nutrient. The pitcher plant is the named example: the leaf forms a pitcher with a lid, an insect entering is trapped by hairs pointing downward, and digestive juices break it down.
• Saprotrophic — the organism secretes digestive juices onto dead and decaying matter, breaks it down outside the body, and then absorbs the dissolved food. Fungi such as mushrooms and bread mould feed this way, which is why they appear on damp bread, pickles and rotting wood in the rainy season.
• Symbiotic — two organisms live together and both benefit. Lichens are the textbook case: an alga and a fungus living together, where the alga makes the food and the fungus provides shelter, water and minerals.

Nitrogen, proteins and putting nutrients back into the soil

Carbohydrates need only carbon, hydrogen and oxygen, all of which the plant gets from carbon dioxide and water. Proteins additionally need nitrogen, and that is where the difficulty lies.

• Air is rich in nitrogen, but plants cannot use nitrogen gas directly from the air. They take it from the soil, in a soluble form.
• The bacterium Rhizobium lives in the root nodules of leguminous plants — gram, pulses, beans, peas — and converts atmospheric nitrogen into a form the plant can use.
• The relationship is symbiotic: Rhizobium cannot make its own food and gets it from the plant, while the plant gets usable nitrogen from the bacterium. Both benefit, which is exactly the definition of symbiosis.
• Growing a crop repeatedly removes nutrients from the soil. Farmers restore them by adding manure and fertilisers, or by sowing a leguminous crop, which enriches the soil with nitrogen at no cost.

Worked Examples

• Iodine turns a boiled leaf blue-black — Blue-black with iodine means starch is present, so that part of the leaf photosynthesised. This is the standard starch test, and it is the evidence that food was actually made.
• A variegated leaf goes blue-black only in the green patches — The white patches have no chlorophyll, so no starch forms there. This single experiment proves chlorophyll is necessary for photosynthesis.
• Cuscuta twining over a shrub — Cuscuta has no chlorophyll and makes no food. It is a parasite; the shrub carrying it is the host and is deprived of nutrients.
• Mushrooms appearing on a rotting log after rain — Saprotrophic nutrition. The fungus secretes digestive juices onto the dead wood, digests it outside its body and absorbs the dissolved food.
• A farmer sows gram after harvesting wheat — Gram is leguminous, so Rhizobium in its root nodules fixes atmospheric nitrogen and enriches the soil, reducing the need for nitrogen fertiliser.

Common Mistakes Students Make

• Writing that plants take in oxygen and give out carbon dioxide, or the reverse, as an absolute. In photosynthesis the plant takes in carbon dioxide and releases oxygen; plants also respire, taking in oxygen, all the time.
• Putting sunlight and chlorophyll on the left of the equation as reactants. They are conditions and belong on the arrow — neither is consumed.
• Calling Cuscuta a saprotroph. It feeds on a living host, so it is a parasite. Saprotrophs feed on dead and decaying matter.
• Saying the pitcher plant cannot photosynthesise. It is green and does photosynthesise; it traps insects for nitrogen, not for carbohydrate.
• Claiming plants absorb nitrogen directly from the air. They cannot use atmospheric nitrogen; they take it from the soil, and Rhizobium is what converts it into a usable form.
• Writing that the host benefits from the parasite. In parasitic nutrition only the parasite gains — the host is harmed. Mutual benefit is symbiosis, which is a different mode.

Where This Shows Up in Real Life

Crop rotation and why pulses are sown between cereals

Cereals such as wheat and rice are heavy feeders on soil nitrogen. Sowing a leguminous crop such as gram, pea or moong between cereal crops lets Rhizobium in the root nodules fix atmospheric nitrogen into the soil, restoring fertility without buying fertiliser. This is why traditional Indian farming alternates cereals with pulses, and it is the practical pay-off of the symbiosis described in this chapter.

Why deforestation is an oxygen problem as well as a tree problem

Every green leaf is releasing oxygen as a by-product of photosynthesis and absorbing carbon dioxide as a raw material. Removing large areas of forest removes that exchange at scale — less carbon dioxide taken out of the air and less oxygen put back. The chapter's process is the mechanism behind an environmental argument students meet repeatedly in later classes.

Insectivorous plants and marshy soil

Pitcher plants grow in soils that are waterlogged and poor in nitrogen, so the usual route of absorbing nitrogen compounds from the soil is unreliable. Trapping insects is a workaround for one missing nutrient. This explains why insectivorous plants are found in particular habitats rather than everywhere.

Storing food in parts that never made it

Carrots, radishes, potatoes and sweet potatoes are storage organs swollen with starch, yet none of them photosynthesised. The food was made in the leaves and transported down. This is why a plant can be dug up for food even when the edible part grew underground in the dark.

Board-Style Questions with Model Answers

Why are leaves called the food factories of plants? (2 marks)

Leaves contain chlorophyll, which traps energy from sunlight, and they have stomata through which carbon dioxide enters. Water absorbed by the roots reaches them through vessels. Because all three requirements for photosynthesis come together in the leaf, food is manufactured there and then transported to the rest of the plant, so the leaf functions as the plant's factory.

Write the word equation for photosynthesis and state where the conditions are written. (3 marks)

Carbon dioxide plus water, in the presence of sunlight and chlorophyll, gives carbohydrate plus oxygen. Sunlight and chlorophyll are written above and below the arrow, not as reactants, because they are conditions required for the reaction and are not used up in it. Oxygen is released as a by-product, and the carbohydrate first formed is glucose, which the plant stores as starch.

Distinguish between parasitic and saprotrophic nutrition, giving one example of each. (3 marks)

In parasitic nutrition the organism obtains ready-made food from another living organism, which is harmed in the process; Cuscuta growing on a host shrub is an example. In saprotrophic nutrition the organism secretes digestive juices onto dead and decaying matter, digests it outside its body and then absorbs the dissolved food; a mushroom growing on a rotting log is an example. The key difference is that a parasite feeds on a living host while a saprotroph feeds on dead matter.

How is the relationship between Rhizobium and a leguminous plant beneficial to both? (3 marks)

Rhizobium lives in the root nodules of leguminous plants and converts atmospheric nitrogen into a form the plant can absorb and use to make proteins. Rhizobium cannot make its own food, and it obtains food from the leguminous plant. Since each organism supplies something the other cannot obtain for itself, the association benefits both and is described as symbiotic.

A variegated leaf with green and white patches is tested with iodine after being boiled in alcohol. Describe the result and explain what it proves. (5 marks)

Only the parts that were green turn blue-black with iodine; the white patches do not change colour. Blue-black with iodine indicates the presence of starch, so starch was made only where the leaf was green. The white patches lack chlorophyll, and the fact that no starch formed there while the rest of the leaf received the same sunlight, carbon dioxide and water shows that the missing factor is chlorophyll. The experiment therefore proves that chlorophyll is necessary for photosynthesis. Boiling in alcohol first removes the green pigment so that the colour change from iodine can be seen clearly.

Why do farmers add manure and fertilisers to fields, and how can a crop reduce this need? (3 marks)

Growing crops repeatedly removes nutrients from the soil, particularly nitrogen compounds, so the soil becomes deficient and yields fall. Farmers restore these nutrients by adding manure and fertilisers. The need can be reduced by growing leguminous crops such as gram, pea or beans, because Rhizobium bacteria in their root nodules fix atmospheric nitrogen into the soil and enrich it naturally.

Assertion–Reason Questions (CBSE Format)

For each question choose: (a) both A and R are true and R is the correct explanation of A; (b) both are true but R is not the correct explanation; (c) A is true but R is false; (d) A is false but R is true.

Quick Self-Check

Key Terms Glossary

Nutrition

The mode of taking in food by an organism and its utilisation by the body.

Nutrients

Components of food such as carbohydrates, proteins, fats, vitamins and minerals that are needed by the body.

Autotroph

An organism that makes its own food from simple inorganic substances; green plants and algae are autotrophs.

Heterotroph

An organism that depends on other organisms for food, either directly or indirectly.

Photosynthesis

The process by which green plants synthesise food from carbon dioxide and water in the presence of sunlight and chlorophyll.

Chlorophyll

The green pigment present in leaves that traps energy from sunlight for photosynthesis.

Stomata

Tiny pores on the surface of a leaf, each bounded by guard cells, through which gases are exchanged and water vapour escapes.

Parasite

An organism that obtains food from another living organism, the host, which is harmed in the process.

Host

The organism on which a parasite lives and from which it derives its food.

Saprotroph

An organism that secretes digestive juices onto dead and decaying matter, digests it outside the body and absorbs the dissolved food.

Symbiosis

An association in which two organisms live together and both derive benefit, as in lichens or Rhizobium and a legume.

Insectivorous plant

A green plant that traps and digests insects to obtain nitrogen, such as the pitcher plant.

Rhizobium

A bacterium living in the root nodules of leguminous plants that converts atmospheric nitrogen into a form the plant can use.

One-Minute Revision

• Autotroph makes its own food; heterotroph depends on others. Every food chain ends at an autotroph.
• Photosynthesis: carbon dioxide plus water, with sunlight and chlorophyll on the arrow, gives carbohydrate plus oxygen.
• Chlorophyll traps light; stomata let carbon dioxide in; roots bring water up. All three meet in the leaf.
• Glucose is made first; the plant stores the surplus as starch. Iodine turning blue-black is the test for it.
• Parasite feeds on a living host and harms it — Cuscuta. Saprotroph feeds on dead matter — mushroom.
• Insectivorous plants photosynthesise; they trap insects for nitrogen, not for food energy.
• Symbiosis means both partners gain — lichen, and Rhizobium with a legume.
• Plants cannot use nitrogen from the air. Rhizobium in root nodules converts it into a usable form, which is why legumes enrich soil.

Syllabus reference: NCERT Class 7 Science — Nutrition in Plants. The nitrogen-fixation idea is picked up again in Class 8 (Microorganisms) and Class 9 (Natural Resources).

Frequently Asked Questions

Why are plants called autotrophs?

Plants are autotrophs because they can produce their own food through photosynthesis using sunlight, carbon dioxide, and water, without depending on other organisms

What is the role of chlorophyll in photosynthesis?

Chlorophyll is the green pigment present in leaves that traps energy from sunlight. Without it the plant cannot use light energy to make food, which is why photosynthesis takes place only in the green parts of a plant

How do nitrogen-fixing bacteria help plants?

Nitrogen-fixing bacteria in root nodules convert atmospheric nitrogen into ammonia compounds that plants can absorb and use for protein synthesis

Do plants only take in carbon dioxide and only give out oxygen?

No. During photosynthesis a plant takes in carbon dioxide and releases oxygen, but plants also respire continuously, taking in oxygen and releasing carbon dioxide just as animals do. In bright sunlight photosynthesis is much faster than respiration, so the net movement is carbon dioxide in and oxygen out, which is where the common oversimplification comes from.

Why is a pitcher plant called insectivorous if it is green?

Being green means it has chlorophyll and does make its own carbohydrate by photosynthesis. It grows in soil that is deficient in nitrogen, and proteins cannot be built without nitrogen. Trapping and digesting insects supplies that one missing nutrient, so the plant is autotrophic for its energy needs and insectivorous only to obtain nitrogen.

What is the difference between a parasite and a symbiont?

Both live in close association with another organism, but the outcome differs. A parasite benefits while the host is harmed, as when Cuscuta draws nutrients from the shrub it twines around. In symbiosis both organisms benefit, as in a lichen where the alga supplies food and the fungus supplies shelter, water and minerals. The test is simply whether the partner gains or loses.

Why must the leaf be boiled in alcohol before the starch test?

Boiling the leaf in alcohol removes the green chlorophyll from it. If the chlorophyll were left in place, the leaf's own green colour would mask the blue-black colour that iodine produces with starch, and the result could not be read. Decolourising the leaf first makes the colour change clearly visible.

More Class 7 Science Revision Notes

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