Photosynthesis explained simply, the equation and the leaf

This guide is for you if you need a clear picture of photosynthesis for school. In CBSE you first learn it in Class 7 (Nutrition in Plants) and then in more depth in Class 10 (Life Processes). It is also a core topic in ICSE biology, Key Stage 3, GCSE and IGCSE biology. Everything below stays at school level, so you can use it for revision.

The idea in plain words

Photosynthesis is how green plants make their own food. They take in two simple substances, carbon dioxide from the air and water from the soil, and use light energy to turn them into glucose (a sugar). Oxygen is released as a by-product.

The light is absorbed by chlorophyll, a green pigment found in chloroplasts.

The word equation

carbon dioxide + water → glucose + oxygen

(light energy and chlorophyll are written above or below the arrow, because they are needed but are not reactants)

The balanced symbol equation

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

You can check it is balanced:

Element	Left	Right
C	6	6
H	12	12
O	12 + 6 = 18	6 + 12 = 18

Where it happens: inside the leaf

Part	Job in photosynthesis
Chloroplasts	Contain chlorophyll, which absorbs light. Most are in the palisade cells near the top of the leaf.
Stomata	Tiny pores, mostly on the underside of the leaf. Carbon dioxide diffuses in and oxygen diffuses out.
Guard cells	Open and close the stomata.
Xylem (in veins)	Carries water up from the roots to the leaf.
Phloem (in veins)	Carries sugars made in the leaf to other parts of the plant.
Broad, thin leaf	Large surface to catch light, short distance for gases to diffuse.

The CBSE Class 10 textbook describes three events during photosynthesis:

1. Light energy is absorbed by chlorophyll.
2. Light energy is converted to chemical energy, and water is split into hydrogen and oxygen.
3. Carbon dioxide is reduced to carbohydrates.

What the plant does with glucose

• Uses it in respiration to release energy
• Stores it as starch (insoluble, so it does not affect water movement in cells)
• Makes cellulose for cell walls
• Combines it with nitrates from the soil to make proteins
• Makes fats and oils, often stored in seeds
• Turns it into sucrose to transport in the phloem

Limiting factors

The rate of photosynthesis depends on three main factors. Whichever is in shortest supply holds the rate back. That one is called the limiting factor.

• Light intensity: more light, faster rate, up to a point
• Carbon dioxide concentration: more CO₂, faster rate, up to a point
• Temperature: photosynthesis is controlled by enzymes, so it speeds up with warmth, but too hot and the enzymes are damaged and the rate falls

Worked questions

Question 1: the starch test

A student tests a leaf for starch. Describe the steps and explain each one.

1. Boil the leaf in water for a minute. This kills the cells and softens the leaf so iodine can get in.
2. Place it in warm ethanol (in a water bath, never over a flame, because ethanol catches fire easily). This removes the chlorophyll, so the colour change is easy to see.
3. Rinse in water to soften the leaf, which becomes brittle in ethanol.
4. Add iodine solution. Where starch is present, iodine turns from orange-brown to blue-black.

A blue-black leaf shows that photosynthesis has happened, because the starch was made from glucose.

Question 2: a variegated leaf

A plant with leaves that are green in the middle and white at the edges is kept in light, then tested for starch. Only the green part turns blue-black. What does this show?

The white parts have no chlorophyll. They could not photosynthesise, so they made no starch. This shows that chlorophyll is needed for photosynthesis.

Why was the plant kept in the dark for about two days first? To use up the starch already in the leaves (this is called destarching). Otherwise you could not tell whether the starch was new.

Question 3: reading a rate graph

A graph shows that as light intensity increases, the rate of photosynthesis increases and then levels off. Explain the shape.

• At low light, light is the limiting factor. More light gives a faster rate, so the line rises.
• Where the line levels off, adding more light makes no difference. Something else is now limiting the rate, most likely carbon dioxide concentration or temperature.

Common mistakes and how to avoid them

“Plants get their food from the soil.” They get water and mineral ions from the soil, but they make their own food (glucose) by photosynthesis.

“Plants photosynthesise instead of respiring.” Plants do both. Respiration happens all the time in every living cell.

“The oxygen comes from carbon dioxide.” It comes from the splitting of water.

Mixing up stomata and chloroplasts. Stomata are pores that let gases in and out. Chloroplasts are the structures inside cells where photosynthesis happens.

Saying glucose is stored as glucose. It is stored mainly as starch.

Saying “temperature kills enzymes”. Enzymes are not alive. Say they are denatured at high temperatures.

Try these

1. Name the gas released during photosynthesis.
2. In which part of a plant cell does photosynthesis take place?
3. Why is a leaf placed in warm ethanol during the starch test?
4. Why is a plant kept in the dark for two days before a starch test?
5. In bright light, increasing light further does not increase the rate of photosynthesis. Name two factors that could be limiting it.

Answers

1. Oxygen.
2. The chloroplasts.
3. To remove the chlorophyll, so the iodine colour change can be seen clearly.
4. To destarch it, so any starch found later must have been made during the experiment.
5. Carbon dioxide concentration and temperature.

Still unsure?

If photosynthesis still feels like a list of facts rather than one connected idea, a Qlive teacher can work through it with you one-to-one, starting with a free assessment that finds the exact gap. You can read more on our biology page, and our guide to plant and animal cells explains chloroplasts and the other parts of the cell.