Lesson 1 of 6 / Photosynthesis and respiration
Capture light and make usable chemical energy
Where does the oxygen released by photosynthesis come from?
I can locate photosynthesis and identify light-dependent inputs, outputs and energy conversion.
I can locate photosynthesis in chloroplasts.Syllabus
Syllabus 8876, Core 3(a). Locate photosynthesis in chloroplasts.
I can identify initial reactants and final products of both photosynthetic stages.Syllabus
Syllabus 8876, Core 3(b). Identify initial reactants and final products of both photosynthetic stages.
I can explain light-to-chemical energy conversion and carbohydrate storage.Syllabus
Syllabus 8876, Core 3(c). Explain light-to-chemical energy conversion and carbohydrate storage.
About 5 min
Make a guess. You are not marked.
A plant gives off oxygen in light. Which molecule did that oxygen come from?
Show the answer
Water.
Light-dependent reactions split water, releasing oxygen. Carbon dioxide is used later to build carbohydrate.
The key idea
In 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.
Watch out for this
A student says: "The oxygen released in photosynthesis is removed directly from carbon dioxide." What is wrong with this?
Show the answer
Released oxygen comes from water. Carbon dioxide is the carbon input to carbohydrate synthesis.
Check your understanding
Which pair passes useful supplies from the light-dependent to the light-independent stage?
Show the answer
ATP and reduced NADP.
They provide energy and reducing power for carbon assimilation.
Exam-style questions
Check your understanding
Which is NOT a product of the light-dependent reactions?
Show the answer
Glucose
Carbohydrate is made in the light-independent reactions using ATP and reduced NADP.
Quick recall
Where does the oxygen released by photosynthesis come from?