How a Leaf Builds Food Out of Air and Sunlight
Learn the four things photosynthesis needs, what stomata actually do, how the iodine test proves a leaf made starch, and why plants are called producers.
What does a plant need to make its own food?
Photosynthesis is the process by which green plants make their own food. It needs four things: carbon dioxide from the air, water from the soil, sunlight for energy, and chlorophyll, the green pigment in leaves that captures that energy.
This page covers everything in the CBSE Class 7 Science chapter on photosynthesis: the raw materials and conditions, the leaf as the site of the process, the iodine test that proves it happened, and why plants sit at the base of every food chain.
This page covers everything in the CBSE Class 7 Science chapter on photosynthesis: the raw materials and conditions, the leaf as the site of the process, the iodine test that proves it happened, and why plants sit at the base of every food chain.
Formula
What is the word equation for photosynthesis?
The process can be written as:
carbon dioxide + water → (in the presence of sunlight and chlorophyll) → glucose + oxygen
The glucose is the food the plant has made. Some is used immediately for energy, and the rest is stored — often converted into starch, which is what the iodine test later detects.
The oxygen is released into the air as a by-product, and it is the oxygen that animals, including us, breathe.
Notice that sunlight and chlorophyll are written above the arrow, not as reactants. They are conditions required for the reaction, not raw materials consumed by it — chlorophyll is still there afterwards.
Compare it with respiration from the animals chapter and you will see the two are near opposites: respiration uses glucose and oxygen to release energy, while photosynthesis uses energy to build glucose and release oxygen.
carbon dioxide + water → (in the presence of sunlight and chlorophyll) → glucose + oxygen
The glucose is the food the plant has made. Some is used immediately for energy, and the rest is stored — often converted into starch, which is what the iodine test later detects.
The oxygen is released into the air as a by-product, and it is the oxygen that animals, including us, breathe.
Notice that sunlight and chlorophyll are written above the arrow, not as reactants. They are conditions required for the reaction, not raw materials consumed by it — chlorophyll is still there afterwards.
Compare it with respiration from the animals chapter and you will see the two are near opposites: respiration uses glucose and oxygen to release energy, while photosynthesis uses energy to build glucose and release oxygen.
What do the stomata and guard cells do?
The leaf is the main site of photosynthesis, because it is broad and flat — giving a large surface to catch sunlight — and it is packed with chlorophyll.
On the leaf's surface, mostly underneath, are thousands of tiny pores called stomata (singular: stoma). Each stoma is bordered by two guard cells that change shape to open or close the pore.
Gases move through these pores: carbon dioxide enters for photosynthesis, and oxygen leaves. Water vapour also escapes through them.
The guard cells give the plant control. When water is plentiful they swell and the pore opens; when water is scarce they close it, saving water even though that also slows photosynthesis.
For example, on a very hot afternoon many plants close their stomata and effectively pause food-making rather than lose water they cannot spare.
That is the trade-off worth understanding: the same opening that lets food-making gas in also lets precious water out.
On the leaf's surface, mostly underneath, are thousands of tiny pores called stomata (singular: stoma). Each stoma is bordered by two guard cells that change shape to open or close the pore.
Gases move through these pores: carbon dioxide enters for photosynthesis, and oxygen leaves. Water vapour also escapes through them.
The guard cells give the plant control. When water is plentiful they swell and the pore opens; when water is scarce they close it, saving water even though that also slows photosynthesis.
For example, on a very hot afternoon many plants close their stomata and effectively pause food-making rather than lose water they cannot spare.
That is the trade-off worth understanding: the same opening that lets food-making gas in also lets precious water out.
How does the iodine test prove a leaf made starch?
Iodine solution turns blue-black in the presence of starch, and since a leaf stores its glucose as starch, this becomes a test for photosynthesis.
The experiment runs in steps:
1. Destarch the plant by keeping it in the dark for two or three days, so any starch already present is used up.
2. Cover part of a leaf with black paper, or use a leaf that is partly non-green, then leave the plant in sunlight for a few hours.
3. Pluck the leaf and boil it in water to soften it.
4. Boil it in alcohol to remove the green chlorophyll, so the colour change will be visible.
5. Wash it, spread it flat and add iodine solution.
The result: the parts that were exposed to sunlight and contained chlorophyll turn blue-black, showing starch. The covered parts, and the non-green parts, stay brownish — no starch.
That comparison is the whole point. Destarching first is what makes the result meaningful, because otherwise old starch would show up everywhere and prove nothing.
The experiment runs in steps:
1. Destarch the plant by keeping it in the dark for two or three days, so any starch already present is used up.
2. Cover part of a leaf with black paper, or use a leaf that is partly non-green, then leave the plant in sunlight for a few hours.
3. Pluck the leaf and boil it in water to soften it.
4. Boil it in alcohol to remove the green chlorophyll, so the colour change will be visible.
5. Wash it, spread it flat and add iodine solution.
The result: the parts that were exposed to sunlight and contained chlorophyll turn blue-black, showing starch. The covered parts, and the non-green parts, stay brownish — no starch.
That comparison is the whole point. Destarching first is what makes the result meaningful, because otherwise old starch would show up everywhere and prove nothing.
Why are green plants called producers?
Because they produce their own food from simple raw materials, instead of eating other living things. Every other organism depends on them, directly or indirectly.
Animals that eat plants get that food directly; animals that eat other animals get it second-hand. Trace any food chain backwards and it ends at a green plant — which is why plants are the producers and everything else are consumers.
To grow, a plant needs sunlight for energy, air for carbon dioxide, water from the soil, and nutrients — minerals such as nitrogen, phosphorus and potassium absorbed from the soil through the roots.
For example, a farmer adding manure or fertiliser is supplying those soil nutrients; no amount of sunlight and water can replace them.
A point students often blur: minerals from the soil are not the plant's food. The food is the glucose the leaf makes; the minerals are additional materials needed to build the plant's body properly.
Animals that eat plants get that food directly; animals that eat other animals get it second-hand. Trace any food chain backwards and it ends at a green plant — which is why plants are the producers and everything else are consumers.
To grow, a plant needs sunlight for energy, air for carbon dioxide, water from the soil, and nutrients — minerals such as nitrogen, phosphorus and potassium absorbed from the soil through the roots.
For example, a farmer adding manure or fertiliser is supplying those soil nutrients; no amount of sunlight and water can replace them.
A point students often blur: minerals from the soil are not the plant's food. The food is the glucose the leaf makes; the minerals are additional materials needed to build the plant's body properly.
Exam tip
Exam tip: writing the conditions above the arrow
In the word equation, students list sunlight and chlorophyll alongside carbon dioxide and water as though all four were raw materials. They are not.
The raw materials are carbon dioxide and water — these are used up and become part of the products. Sunlight and chlorophyll are conditions — sunlight supplies energy and chlorophyll captures it, and the chlorophyll is unchanged at the end.
Write the equation with the conditions above the arrow, and if asked to "name the raw materials required for photosynthesis", give only carbon dioxide and water. Adding sunlight to that list is a common way to lose an easy mark.
The raw materials are carbon dioxide and water — these are used up and become part of the products. Sunlight and chlorophyll are conditions — sunlight supplies energy and chlorophyll captures it, and the chlorophyll is unchanged at the end.
Write the equation with the conditions above the arrow, and if asked to "name the raw materials required for photosynthesis", give only carbon dioxide and water. Adding sunlight to that list is a common way to lose an easy mark.
Did you know
Why must a leaf be boiled in alcohol before the iodine test?
A green leaf's chlorophyll would mask the colour change completely — blue-black against dark green is impossible to judge.
Boiling the leaf in alcohol dissolves the chlorophyll out, leaving the leaf pale and almost white. Only then does iodine's blue-black clearly show where starch is, and where it is not.
Boiling the leaf in alcohol dissolves the chlorophyll out, leaving the leaf pale and almost white. Only then does iodine's blue-black clearly show where starch is, and where it is not.
Key takeaways
Photosynthesis: quick revision
- Photosynthesis needs carbon dioxide and water as raw materials, with sunlight and chlorophyll as conditions, producing glucose and oxygen.
- The leaf is the main site: broad for catching light, full of chlorophyll, with stomata bordered by guard cells that let gases in and out.
- Opening stomata admits carbon dioxide but also loses water, so plants close them when water is scarce.
- The iodine test turns starch blue-black; destarch the plant first, then boil in water and alcohol to remove chlorophyll before testing.
- Plants are producers because they make their own food, and every food chain traces back to them; they also need soil minerals, which are not food.
You will remember all of this far better after answering five questions on it than after reading it twice.
- The leaf is the main site: broad for catching light, full of chlorophyll, with stomata bordered by guard cells that let gases in and out.
- Opening stomata admits carbon dioxide but also loses water, so plants close them when water is scarce.
- The iodine test turns starch blue-black; destarch the plant first, then boil in water and alcohol to remove chlorophyll before testing.
- Plants are producers because they make their own food, and every food chain traces back to them; they also need soil minerals, which are not food.
You will remember all of this far better after answering five questions on it than after reading it twice.