Every Meal You Eat Began Inside a Leaf
Learn the equation for photosynthesis and its raw materials, how stomata and chloroplasts make the leaf the site, the experiments that prove sunlight and chlorophyll are needed, and why it matters.
Where does the food in your meal actually come from?
From a leaf. Whether your plate holds rice, dal, vegetables, milk or mutton, every bit of that energy was captured from sunlight by a green plant somewhere along the chain.
Animals cannot make food; they can only pass it on. This page covers everything in the ICSE Class 7 Biology chapter's first part: the equation for photosynthesis, the leaf as its site, the experiments proving what it needs, and its significance.
Animals cannot make food; they can only pass it on. This page covers everything in the ICSE Class 7 Biology chapter's first part: the equation for photosynthesis, the leaf as its site, the experiments proving what it needs, and its significance.
What is photosynthesis and what is its equation?
Photosynthesis is the process by which green plants make their own food — glucose — from carbon dioxide and water, in the presence of sunlight and chlorophyll, releasing oxygen.
The word equation:
carbon dioxide plus water, in the presence of sunlight and chlorophyll, gives glucose plus oxygen.
The chemical equation:
The parts to name separately:
- Raw materials — carbon dioxide, taken from the air through the stomata, and water, absorbed from the soil by the roots.
- Products — glucose, which is stored as starch, and oxygen, which is released into the air.
- Source of energy — sunlight, trapped by chlorophyll.
Because plants make their own food they are called autotrophs; animals, which depend on ready-made food, are heterotrophs.
The detail that earns marks is the position of sunlight and chlorophyll. They are written above the arrow, not as reactants, because sunlight supplies energy and chlorophyll is not consumed — it acts rather like a catalyst, still present after the reaction.
The word equation:
carbon dioxide plus water, in the presence of sunlight and chlorophyll, gives glucose plus oxygen.
The chemical equation:
The parts to name separately:
- Raw materials — carbon dioxide, taken from the air through the stomata, and water, absorbed from the soil by the roots.
- Products — glucose, which is stored as starch, and oxygen, which is released into the air.
- Source of energy — sunlight, trapped by chlorophyll.
Because plants make their own food they are called autotrophs; animals, which depend on ready-made food, are heterotrophs.
The detail that earns marks is the position of sunlight and chlorophyll. They are written above the arrow, not as reactants, because sunlight supplies energy and chlorophyll is not consumed — it acts rather like a catalyst, still present after the reaction.
How is the leaf built to carry out photosynthesis?
The leaf is the main site, and every part of its structure serves the process.
- Broad, flat blade — offers the largest possible surface for catching sunlight.
- Stomata — tiny pores, mostly on the lower surface, each guarded by two kidney-shaped guard cells. They let carbon dioxide in and oxygen and water vapour out.
- Mesophyll cells — the leaf's middle tissue, packed with chloroplasts. The upper palisade layer does most of the photosynthesis; the looser spongy layer below has air spaces so gases can circulate.
- Chloroplasts — green bodies inside the mesophyll cells containing the pigment chlorophyll, which traps sunlight. This is where photosynthesis actually happens.
- Veins — carry water and minerals in through the xylem and take the food away through the phloem.
Four conditions are necessary:
- Sunlight
- Chlorophyll
- Carbon dioxide
- Water
A money plant kept away from a window grows pale and leggy because one of the four is short in supply.
The placing of the stomata is worth explaining rather than just stating. They are mostly on the lower surface, away from direct sun, which reduces the water lost as vapour while still letting carbon dioxide in.
- Broad, flat blade — offers the largest possible surface for catching sunlight.
- Stomata — tiny pores, mostly on the lower surface, each guarded by two kidney-shaped guard cells. They let carbon dioxide in and oxygen and water vapour out.
- Mesophyll cells — the leaf's middle tissue, packed with chloroplasts. The upper palisade layer does most of the photosynthesis; the looser spongy layer below has air spaces so gases can circulate.
- Chloroplasts — green bodies inside the mesophyll cells containing the pigment chlorophyll, which traps sunlight. This is where photosynthesis actually happens.
- Veins — carry water and minerals in through the xylem and take the food away through the phloem.
Four conditions are necessary:
- Sunlight
- Chlorophyll
- Carbon dioxide
- Water
A money plant kept away from a window grows pale and leggy because one of the four is short in supply.
The placing of the stomata is worth explaining rather than just stating. They are mostly on the lower surface, away from direct sun, which reduces the water lost as vapour while still letting carbon dioxide in.
How do you prove sunlight and chlorophyll are needed?
Each experiment removes one factor and then tests for starch, since starch present means photosynthesis has occurred.
The starch test itself: boil the leaf in water to kill it, then boil it in alcohol to remove the green colour, wash it, and add iodine solution. A blue-black colour means starch is present; a brown colour means it is absent.
Every experiment begins by destarching the plant — keeping it in complete darkness for two to three days so that all stored starch is used up. Without this step the test proves nothing, because leftover starch would show up anyway.
To prove sunlight is essential. Cover part of a destarched leaf with black paper or a cork on both sides, and leave the plant in sunlight for a few hours. Test the whole leaf with iodine. The exposed part turns blue-black; the covered part stays brown. So sunlight is essential.
To prove chlorophyll is essential. Use a variegated leaf, such as croton or money plant, which has green and non-green patches. Destarch the plant, expose it to sunlight, then test with iodine. Only the green parts turn blue-black. So chlorophyll is essential.
To prove carbon dioxide is essential. Place a destarched potted plant under a bell jar containing potassium hydroxide, which absorbs carbon dioxide, and keep a similar plant under a jar without it. After some hours in sunlight, only the leaf from the second plant gives a blue-black result.
These are the experiments to draw and label in a practical exam.
The variegated-leaf design is the cleverest of the three, and it is worth saying why: the green and non-green parts are on the same leaf, so light, water, carbon dioxide and temperature are identical for both. The only difference left is chlorophyll.
The starch test itself: boil the leaf in water to kill it, then boil it in alcohol to remove the green colour, wash it, and add iodine solution. A blue-black colour means starch is present; a brown colour means it is absent.
Every experiment begins by destarching the plant — keeping it in complete darkness for two to three days so that all stored starch is used up. Without this step the test proves nothing, because leftover starch would show up anyway.
To prove sunlight is essential. Cover part of a destarched leaf with black paper or a cork on both sides, and leave the plant in sunlight for a few hours. Test the whole leaf with iodine. The exposed part turns blue-black; the covered part stays brown. So sunlight is essential.
To prove chlorophyll is essential. Use a variegated leaf, such as croton or money plant, which has green and non-green patches. Destarch the plant, expose it to sunlight, then test with iodine. Only the green parts turn blue-black. So chlorophyll is essential.
To prove carbon dioxide is essential. Place a destarched potted plant under a bell jar containing potassium hydroxide, which absorbs carbon dioxide, and keep a similar plant under a jar without it. After some hours in sunlight, only the leaf from the second plant gives a blue-black result.
These are the experiments to draw and label in a practical exam.
The variegated-leaf design is the cleverest of the three, and it is worth saying why: the green and non-green parts are on the same leaf, so light, water, carbon dioxide and temperature are identical for both. The only difference left is chlorophyll.
Why does photosynthesis matter beyond the plant itself?
Because it is the source of both the food and the oxygen the rest of life depends on.
It maintains the gas balance of the atmosphere. Respiration, combustion and decay constantly remove oxygen and add carbon dioxide. Photosynthesis reverses both, keeping the air near 21 percent oxygen and 0.03 percent carbon dioxide.
It is the origin of all food. In any food chain, the green plant is the producer at the start. Grass makes food, a goat eats the grass, a human drinks the goat's milk — and the energy in that milk came from sunlight captured by grass.
It stores energy for later. The starch in rice and potatoes, the oil in mustard seeds, and even the coal and petroleum formed from ancient plants are all stores of energy first trapped by photosynthesis.
It supplies raw material for growth. The glucose made becomes cellulose for cell walls, and with nitrogen from the soil becomes the proteins a plant needs to grow.
That is why planting trees is both an oxygen measure and a food measure at once.
The consequence worth stating plainly is the dependence. If photosynthesis stopped, the oxygen in the air would fall as respiration and burning continued, and every food chain would collapse from its first link upward — because no animal, anywhere, can make food from sunlight.
It maintains the gas balance of the atmosphere. Respiration, combustion and decay constantly remove oxygen and add carbon dioxide. Photosynthesis reverses both, keeping the air near 21 percent oxygen and 0.03 percent carbon dioxide.
It is the origin of all food. In any food chain, the green plant is the producer at the start. Grass makes food, a goat eats the grass, a human drinks the goat's milk — and the energy in that milk came from sunlight captured by grass.
It stores energy for later. The starch in rice and potatoes, the oil in mustard seeds, and even the coal and petroleum formed from ancient plants are all stores of energy first trapped by photosynthesis.
It supplies raw material for growth. The glucose made becomes cellulose for cell walls, and with nitrogen from the soil becomes the proteins a plant needs to grow.
That is why planting trees is both an oxygen measure and a food measure at once.
The consequence worth stating plainly is the dependence. If photosynthesis stopped, the oxygen in the air would fall as respiration and burning continued, and every food chain would collapse from its first link upward — because no animal, anywhere, can make food from sunlight.
Exam tip
Exam tip: destarching before every experiment
Photosynthesis experiments lose marks in the same two places every time.
Always mention destarching — keeping the plant in the dark for two to three days — and say why: so that any starch found afterwards must have been made during the experiment.
Describe the starch test in full: boil in water, boil in alcohol to decolourise, wash, add iodine. Then give the result as blue-black for starch present, brown for absent.
Write sunlight and chlorophyll above the arrow in the equation, never as reactants, and give the balanced form when asked for the chemical equation.
Name all four conditions if the question asks for conditions — sunlight, chlorophyll, carbon dioxide and water.
And in the variegated-leaf answer, say that both parts share the same leaf, so chlorophyll is the only variable.
Always mention destarching — keeping the plant in the dark for two to three days — and say why: so that any starch found afterwards must have been made during the experiment.
Describe the starch test in full: boil in water, boil in alcohol to decolourise, wash, add iodine. Then give the result as blue-black for starch present, brown for absent.
Write sunlight and chlorophyll above the arrow in the equation, never as reactants, and give the balanced form when asked for the chemical equation.
Name all four conditions if the question asks for conditions — sunlight, chlorophyll, carbon dioxide and water.
And in the variegated-leaf answer, say that both parts share the same leaf, so chlorophyll is the only variable.
Did you know
Why is the oxygen you breathe a by-product rather than the point?
Because the plant is not making oxygen on purpose — it is making food, and the oxygen is what is left over.
Photosynthesis splits water molecules to get the hydrogen needed for building glucose. The oxygen atoms released have no further use inside the leaf, so they diffuse out through the stomata.
So every breath you take is, strictly speaking, a plant's waste product. That waste happens to be the gas every animal on Earth depends on — which is why the balance between photosynthesis and respiration matters so much.
Photosynthesis splits water molecules to get the hydrogen needed for building glucose. The oxygen atoms released have no further use inside the leaf, so they diffuse out through the stomata.
So every breath you take is, strictly speaking, a plant's waste product. That waste happens to be the gas every animal on Earth depends on — which is why the balance between photosynthesis and respiration matters so much.
Key takeaways
Photosynthesis: quick revision
- Photosynthesis: , with sunlight and chlorophyll above the arrow because neither is consumed.
- Raw materials are carbon dioxide from the air and water from the soil; products are glucose, stored as starch, and oxygen.
- Stomata on the lower surface admit carbon dioxide, mesophyll cells hold the chloroplasts where the process occurs, and veins bring water and carry food away.
- The four conditions are sunlight, chlorophyll, carbon dioxide and water.
- Destarch first, then test with iodine: blue-black means starch. A partly covered leaf proves sunlight, a variegated leaf proves chlorophyll, and potassium hydroxide under a bell jar proves carbon dioxide.
- Photosynthesis keeps the oxygen and carbon dioxide balance and is the first link in every food chain, since only plants can make food from sunlight.
You will remember all of this far better after answering five questions on it than after reading it twice.
- Raw materials are carbon dioxide from the air and water from the soil; products are glucose, stored as starch, and oxygen.
- Stomata on the lower surface admit carbon dioxide, mesophyll cells hold the chloroplasts where the process occurs, and veins bring water and carry food away.
- The four conditions are sunlight, chlorophyll, carbon dioxide and water.
- Destarch first, then test with iodine: blue-black means starch. A partly covered leaf proves sunlight, a variegated leaf proves chlorophyll, and potassium hydroxide under a bell jar proves carbon dioxide.
- Photosynthesis keeps the oxygen and carbon dioxide balance and is the first link in every food chain, since only plants can make food from sunlight.
You will remember all of this far better after answering five questions on it than after reading it twice.