Photosynthesis in Higher Plants
Photosynthesis
Photosynthesis is how a green plant uses light to turn carbon dioxide and water into sugar, releasing oxygen. It is the process that feeds almost all life.
It happens in the chloroplast in two stages. The light reactions capture sunlight and use it to make energy carriers (ATP and NADPH), splitting water and releasing oxygen. The dark reactions (the Calvin cycle) then use that energy to build sugar from carbon dioxide.
Light is needed only for the first stage. The second stage can run without light as long as ATP and NADPH are available, which is why it is called light-independent, not 'happens at night'.
Where students slip up
The 'dark reactions' do not need darkness, they need the products of the light reactions. Students read 'dark' and think it runs at night. It runs whenever ATP and NADPH are supplied, usually in daylight. It is light-independent, not night-time.
Hold onto this
Light makes the energy; the Calvin cycle spends it to build sugar.
Why it helps in the exam: the two stages, where each happens, and the light-independent trap are asked repeatedly.
Where this connects
Photosynthesis stores energy in sugar; respiration burns that sugar back for energy. They are near mirror images, and the oxygen one releases is the oxygen the other consumes.
The reverse process: respiration →Quick check
The oxygen released during photosynthesis comes from:
- ACarbon dioxide
- BWater
- CSugar
- DThe chloroplast membrane