1. The oxygen released by photosynthesis originates mainly from which reactant?
- Carbon dioxide
- Glucose already present
- Water
Check the answer and reasoning
Water
Photolysis splits water, supplying electrons and releasing oxygen.
Biology / A2 level
Photosynthesis: light reactions and carbon fixation
Photosynthetic pigments absorb light in thylakoid membranes.
Learning objectives
- Connect light absorption with ATP and reduced NADP formation.
- Explain carbon fixation and carbohydrate formation.
- Interpret limiting factors and action spectra.
The big idea
Photosynthetic pigments absorb light in thylakoid membranes. Excited electrons and electron-transfer pathways support ATP formation and reduction of NADP; photolysis supplies electrons and releases oxygen from water.
The Calvin cycle uses ATP and reduced NADP to convert fixed carbon into carbohydrate precursors and regenerate the carbon acceptor. Its reactions do not require light directly but depend on products and conditions linked to the light-dependent stage. Net gas exchange also includes respiration.
Worked example
A plant’s gross oxygen production is 12 units while respiration consumes 3 units in the same interval. Find net oxygen release.
Follow the solution
Net release is production minus consumption: 12−3=9 units in the stated simplified balance.
Investigate and explain
Using the worked example as supplied data, predict the effect of changing one condition. Draw a labelled model, record your reasoning and identify a variable that would need controlling. No human, animal or microbial experiment is required.
No exact PhET model is claimed for this topic. Use diagrams and supplied-data reasoning instead.
Check your understanding
2. A plant’s gross oxygen production is 12 units while respiration consumes 3 units in the same interval. Find net oxygen release.
- 9 units
- 15 units
- 4 units
Check the answer and reasoning
9 units
Net release is production minus consumption: 12−3=9 units in the stated simplified balance.
3. Must so-called light-independent reactions operate at full rate indefinitely in darkness?
- Yes
- No
- Only in green plants
Check the answer and reasoning
No
They need ATP and reduced NADP and are affected by changing conditions; the name does not imply independence from light-generated supplies.