Three events in a leaf

The idea

Photosynthesis involves absorption of light energy by chlorophyll, conversion of light energy to chemical energy with splitting of water into hydrogen and oxygen, and reduction of carbon dioxide to carbohydrates; these steps need not follow immediately.

Inside a green leaf

A green leaf makes food from carbon dioxide and water, using sunlight.

How many separate events do you think make up photosynthesis?

• One
• Three
• Ten

Follow photosynthesis

Press Next step to see the events one by one.

Events, not one jump

Photosynthesis is not one jump. It has three events: chlorophyll absorbs light energy; that light energy is converted to chemical energy while water is split into hydrogen and oxygen; and carbon dioxide is reduced to carbohydrates. The events need not follow each other at once, as the desert plants showed.

Photosynthesis involves absorption of light energy by chlorophyll, conversion of light energy to chemical energy with splitting of water into hydrogen and oxygen, and reduction of carbon dioxide to carbohydrates; these steps need not follow immediately.

Notes

Photosynthesis: chlorophyll absorbs light energy; light energy becomes chemical energy as water splits into hydrogen and oxygen; carbon dioxide is reduced to carbohydrates. The steps need not follow immediately.

Check yourself

When water molecules are split during photosynthesis, what are formed?

Which job does chlorophyll do in photosynthesis?

Desert plants take up carbon dioxide at night. When is the intermediate they prepare acted on by the energy absorbed by chlorophyll?

How many events of photosynthesis are listed in this lesson?

Answer: 3

The events are (i) absorption of light energy, (ii) conversion to chemical energy with splitting of water, (iii) reduction of carbon dioxide to carbohydrates.

• Carbohydrates and carbon dioxide. Carbohydrates are made when carbon dioxide is reduced. Carbon dioxide is a raw material, not a product of splitting water.
• Hydrogen and oxygen — correct. Yes!
• Chlorophyll and light. Chlorophyll absorbs light energy. It is not made by splitting water.
• It reduces carbon dioxide to carbohydrates. That is event (iii), a separate event in which carbon dioxide is reduced to carbohydrates.
• It takes up water from the soil. In land plants the roots take up water from the soil.
• It absorbs light energy — correct. Yes!
• During the day — correct. Yes!
• At night, at the same moment. At night the plant takes up carbon dioxide and prepares the intermediate. The energy absorbed by chlorophyll acts on it during the day.
• It is not acted on at all. The intermediate is acted on by the energy absorbed by chlorophyll during the day.