CBSE Class 7 Science Photosynthesis in Plants Notes Set 08

What is photosynthesis?

Photosynthesis is the process by which green plants prepare food from carbon dioxide and water in the presence of sunlight and chlorophyll. The word comes from photo (light) and synthesis (to put together). It is the main way in which energy from the sun enters living systems, so almost all life on earth depends on it.

What a plant needs

1. Sunlight: it supplies the energy for the process.

2. Chlorophyll: the green pigment in leaves that absorbs sunlight. It is found in structures called chloroplasts.

3. Carbon dioxide: taken from the air.

4. Water: taken from the soil by the roots.

If any one of these is missing, photosynthesis stops or slows down.

The equation

In words: carbon dioxide + water → glucose + oxygen (in sunlight, with chlorophyll).

In symbols: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

How the raw materials reach the leaf

Leaves have tiny pores, mostly on the lower surface, called stomata. Each pore is guarded by two bean-shaped guard cells that open and close it. Carbon dioxide enters the leaf through the stomata and oxygen leaves through them. Water and dissolved minerals are absorbed by the roots and carried upward to the leaves through xylem, a set of pipe-like tubes running through the root, stem and branches.

What happens to the food

The glucose produced is used for energy or is changed into starch and stored. Starch is a carbohydrate. Leaves are called the food factories of the plant because most photosynthesis takes place in them. Green stems also photosynthesise to a small extent. In desert plants such as cactus, leaves are reduced to spines to save water, and the thick green stem carries out photosynthesis instead.

Testing a leaf for starch

Starch shows that photosynthesis has taken place. To test a leaf, boil it in water and then in alcohol to remove the green colour, wash it, and add a few drops of iodine solution. A blue-black colour shows starch. A leaf kept in the dark for some days, or one covered by black paper, does not turn blue-black, proving that light is needed.

Why photosynthesis is important

1. It makes food for the plant and, through food chains, for all animals.

2. It releases oxygen, which most organisms need for respiration.

3. It removes carbon dioxide from the air and helps balance the gases in the atmosphere.

Worked examples

Example 1. Why do leaves look green?
Leaves contain chlorophyll, a green pigment. It reflects green light to our eyes while absorbing the light needed for photosynthesis.

Example 2. Count the atoms to check that 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ is balanced.
Left side: carbon 6, hydrogen 12, oxygen 12 + 6 = 18. Right side: carbon 6, hydrogen 12, oxygen 6 + 12 = 18. Both sides are equal, so the equation is balanced.

Example 3. A potted plant is kept in a dark cupboard for three days and then a leaf is tested with iodine. What will you observe and why?
The leaf will not turn blue-black. Without light the plant could not photosynthesise, so no new starch was made, and the stored starch was used up.

Quick Check Questions

Answer in one or two lines.