Photosynthesis

You’ll be able to

Write the overall equation of photosynthesis and identify its inputs and outputs
Describe how the light reactions convert light energy into ATP and NADPH
Explain how the Calvin cycle uses ATP and NADPH to fix CO₂ into sugar

Capturing sunlight to build sugar

Photosynthesis converts light energy into the chemical energy of sugar, feeding nearly every ecosystem on Earth. It takes place in the chloroplast, an organelle with two key regions: the thylakoids (stacked membrane sacs where the light reactions occur, holding the green pigment chlorophyll) and the stroma (the fluid surrounding the thylakoids, where the Calvin cycle runs). Overall, plants take in carbon dioxide and water and, powered by light, produce glucose and release oxygen.

Overall photosynthesis

6 CO₂ + 6 H₂O + light → C₆H₁₂O₆ + 6 O₂

Inputs: carbon dioxide, water, and light energy. Outputs: glucose and oxygen. This is the exact reverse of cellular respiration.

The light reactions: turning photons into fuel

In the thylakoid membranes, chlorophyll absorbs light and excites electrons. Water is split (photolysis) to replace those electrons, releasing O₂ as a byproduct — this is the source of the oxygen you breathe. Excited electrons pass down an electron transport chain, and the energy released pumps H⁺ into the thylakoid space, building a gradient that drives ATP synthase to make ATP. At the end of the chain, electrons and H⁺ reduce NADP⁺ to NADPH. So the light reactions require light and water and output ATP, NADPH, and O₂.

The Calvin cycle: building sugar from CO₂

The Calvin cycle runs in the stroma and does not directly need light — it is powered by the ATP and NADPH made in the light reactions. In carbon fixation, the enzyme RuBisCO attaches CO₂ from the air to a 5-carbon molecule (RuBP). The resulting molecules are then reduced using ATP and NADPH to form G3P, a 3-carbon sugar; some G3P leaves to build glucose, while the rest is used (with more ATP) to regenerate RuBP so the cycle can continue. Its inputs are CO₂, ATP, and NADPH; its output is sugar (G3P), plus ADP and NADP⁺ returned to the light reactions.

Worked example

A plant is placed in the dark. Within minutes the Calvin cycle slows and then stops, even though CO₂ is still available. Explain why the light-independent reactions halt in the dark.

1.The Calvin cycle does not use light directly, but it is powered by the ATP and NADPH produced by the light reactions.
2.In the dark, the light reactions cannot run, so no new ATP or NADPH is made and existing supplies are quickly used up.
3.Without ATP and NADPH to reduce fixed carbon, the Calvin cycle runs out of fuel and stops — even with plenty of CO₂ present.

Answer: The Calvin cycle depends on ATP and NADPH from the light reactions; in the dark those run out, so carbon fixation cannot proceed despite available CO₂.

Checkpoint

Where does the oxygen (O₂) released by photosynthesis come from?

Watch out

The "light-independent" Calvin cycle is not the same as "happens in the dark." It does not use photons directly, but it depends on the ATP and NADPH the light reactions supply — so it stops soon after the light does. Do not describe it as running normally at night.

Checkpoint

Which set correctly pairs the inputs and outputs of the light reactions?

Checkpoint

In which part of the chloroplast does carbon fixation by RuBisCO occur, and what powers it?

On the exam

Master the hand-off: the light reactions (thylakoid) turn light + water into ATP, NADPH, and O₂; the Calvin cycle (stroma) spends that ATP and NADPH to fix CO₂ into sugar, returning ADP and NADP⁺. Tracing which product feeds the next stage earns free-response points.

Answer the 3 checkpoints as you read.