In Photosynthesis you learned the inputs and outputs: carbon dioxide, water and light in, sugar and oxygen out. Now we look inside the chloroplast to see how the light's energy ends up in sugar, using the ATP you just met.
Key idea
$$6\text{CO}_2 + 6\text{H}_2\text{O} + \text{light} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$$ The light-dependent reactions (in the thylakoid membranes) use light to split water, releasing O₂ and making ATP and NADPH. The Calvin cycle (in the stroma) uses that ATP and NADPH to build sugar from CO₂.
| Stage | Where | In | Out |
|---|---|---|---|
| Light reactions | Thylakoid membranes | Light, H₂O, ADP, NADP⁺ | O₂, ATP, NADPH |
| Calvin cycle | Stroma | CO₂, ATP, NADPH | Sugar (G3P), ADP, NADP⁺ |
Why it works
Chlorophyll absorbs mostly red and blue light and reflects green, which is why leaves look green. Absorbed light excites electrons in chlorophyll. Those electrons pass down a chain of proteins in the thylakoid membrane, and their energy is used to make ATP and to load NADPH, a carrier of high-energy electrons. The electrons have to be replaced, and they come from water: splitting H₂O gives electrons, H⁺ and oxygen gas. In the Calvin cycle, the enzyme rubisco attaches CO₂ to a five-carbon sugar, and ATP and NADPH turn the products into the three-carbon sugar G3P. It takes six turns of the cycle, fixing six CO₂, to make one glucose.
Worked example: Count the molecules
A leaf fixes 18 molecules of CO₂. How many glucose molecules can it make, and how many O₂ molecules does it release?
- Each glucose needs 6 CO₂: $18 \div 6 = 3$ glucose.
- The equation releases one O₂ for each CO₂ fixed: 18 O₂.
Answer: $3$ glucose and $18$ O₂
Worked example: Trace the oxygen
In 1941, Samuel Ruben, Martin Kamen and their colleagues gave algae water made with a heavy isotope of oxygen, oxygen-18. Where did the oxygen-18 show up?
- The light reactions split water and release its oxygen as O₂.
- So the heavy oxygen appeared in the oxygen gas the algae gave off.
- The oxygen atoms of CO₂ end up in the sugar instead.
Answer: In the O₂ gas given off
Common mistake
Thinking the oxygen released comes from carbon dioxide. It comes from splitting water in the light-dependent reactions. The carbon and oxygen atoms of CO₂ end up in the sugar. Check with the isotope experiment: label the water, and the O₂ given off carries the label.