Lesson 1 of 6 / Photosynthesis and respiration
Capture light and make usable chemical energy
Where does the oxygen released by photosynthesis come from?
In this lesson: Locate photosynthesis and identify light-dependent inputs, outputs and energy conversion.
About 5 min
The key ideaIn chloroplasts, light-dependent reactions use light and water to supply ATP and reduced NADP; oxygen is released from water.
Explore the idea
What does light supply?
Water is split and oxygen is released. Light-driven energy transfer supports ATP formation from ADP and phosphate, and reduction of NADP.
Original models of required stage inputs and outputs. Shapes are not full chemical formulae; exact pathway ATP yields are not required here.
Explanation
Photosynthesis occurs in chloroplasts. Chlorophyll and other components associated with thylakoid membranes capture light energy. This energy is transferred into chemical forms that can support the synthesis of carbohydrate.
The light-dependent stage uses water, ADP and inorganic phosphate, and oxidised NADP. Light energy drives reactions that produce oxygen, ATP and reduced NADP, often written NADPH. Water supplies electrons and hydrogen; oxygen is released when water is split.
ATP provides a transferable energy supply, while reduced NADP provides reducing power for later carbon assimilation. These are not the same role: a molecule can carry high-energy electrons without being the final carbohydrate product.
The oxygen released by photosynthesis comes from water, not from carbon dioxide. Carbon dioxide supplies carbon for carbohydrate in the linked light-independent stage. H1 requires the stage inputs and outputs, not the names of electron-transfer complexes or detailed ATP-synthase mechanism.
Step by step
- 1
Locate light capture
The chloroplast thylakoid system contains the light-absorbing and energy-transfer machinery.
- 2
Track water
Water contributes electrons and hydrogen; oxygen is released.
- 3
Track chemical supplies
ATP and reduced NADP support the next stage rather than being the final stored food.
Worked example
Trace labelled oxygen
In a conceptual tracing experiment, the oxygen atoms of supplied water are labelled. Which gaseous photosynthesis product would acquire the label?
One way to explain it
The released oxygen gas, because photosynthetic oxygen originates from water splitting. Labelling water does not make carbon dioxide the source of that gas.
Why this answer works
- Trace the atoms through the relevant stage.
- Keep oxygen source separate from the carbon source for carbohydrate.
Is this true? "The oxygen released in photosynthesis is removed directly from carbon dioxide."
Released oxygen comes from water. Carbon dioxide is the carbon input to carbohydrate synthesis.