Photosynthesis: the light-dependent reactions and the Calvin cycle, and how it mirrors respiration

Photosynthesis takes carbon dioxide and water and, with light, makes glucose and oxygen: 6 CO₂ + 6 H₂O + light → C₆H₁₂O₆ + 6 O₂. It runs in two stages in the chloroplast. The light-dependent reactions, in the thylakoid membranes, split water (that is where the oxygen comes from) and use the light's energy to load two carriers, ATP and NADPH. The Calvin cycle, in the stroma, spends those carriers to fix CO₂ into sugar, three CO₂ for each three-carbon sugar it releases. Below, the two stages with every count, the Calvin cycle traced turn by turn, the two things students swap, and the mirror with cellular respiration.

The photosynthesis equation, the organelle, and the two stages

The equation is the reverse of respiration's, and reading it that way is the first thing to carry:

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

It happens in the chloroplast, which has two compartments that matter here. The thylakoids are flattened membrane sacs, stacked into grana, each enclosing a space called the thylakoid lumen. The stroma is the fluid around the thylakoids. Where does photosynthesis take place? In both, one stage each.

Stage	Where	In	Out
Light-dependent reactions	Thylakoid membranes	Light, H₂O, ADP, NADP⁺	O₂, ATP, NADPH
Calvin cycle	Stroma	CO₂, ATP, NADPH	G3P (sugar), ADP, NADP⁺

The two stages are joined by four molecules and nothing else. The light reactions send ATP and NADPH into the stroma; the Calvin cycle spends them and sends ADP and NADP⁺ back to be reloaded. If you can name those four shuttles, you have the connection between the stages, and most of the diagram.

Stage 1, the light dependent reactions: water split, carriers loaded

The light reactions run in the thylakoid membrane, which holds two photosystems and an electron transport chain between them. The pigments are chlorophyll a and chlorophyll b, which absorb red and blue light and reflect green, with accessory pigments widening the range; that is one sentence, and the course wants no more than that.

Photosystem II absorbs light first. (The numbering is historical: II was discovered second but acts first.) The energy knocks electrons out of its reaction-centre chlorophyll, and the gap is filled by electrons pulled from water. Water is split:

2 H₂O → O₂ + 4 H⁺ + 4 e⁻

Count it: two water molecules give one O₂, four protons and four electrons. The oxygen a plant releases is water's oxygen. The carbon dioxide's carbon goes into the sugar.

The electrons pass down an electron transport chain in the membrane, and the energy they release pumps H⁺ from the stroma into the thylakoid lumen. The lumen becomes acidic and positive relative to the stroma. Protons flow back out through ATP synthase, and the flow makes ATP from ADP in the stroma. This is chemiosmosis, the same mechanism the mitochondrion uses; here it is called photophosphorylation because light is the energy source.

Photosystem I then absorbs light and re-energises the electrons, and an enzyme (NADP⁺ reductase) loads them onto NADP⁺ to make NADPH, also in the stroma.

The products of the light-dependent reactions, then: O₂, which leaves; ATP and NADPH, which stay in the stroma for the next stage.

Stage 2, the Calvin cycle traced: three turns, one G3P

The Calvin cycle runs in the stroma and has three phases: fixation, reduction and regeneration. One turn fixes one CO₂, but a turn does not release a whole sugar, so the smallest round that balances is three turns. Trace three turns and count the carbons at every step.

Fixation. 3 CO₂ (1 C each, 3 C) are each joined to a 5-carbon molecule, RuBP (3 RuBP, 15 C), by the enzyme rubisco. Each 6-carbon product splits at once into two 3-carbon molecules of 3-PGA. So: 3 RuBP + 3 CO₂ → 6 3-PGA. Carbon check: 15 + 3 = 18 C in, 6 × 3 = 18 C out.

Reduction. Each 3-PGA is phosphorylated by one ATP and reduced by one NADPH to make one G3P. Six of them: 6 ATP and 6 NADPH spent, 6 G3P made (18 C). The reduction is where NADPH's electrons go into the sugar.

Regeneration. 1 G3P leaves the cycle (3 C, the product). The other 5 G3P (15 C) are rearranged into 3 RuBP (15 C) using 3 more ATP. Carbon check: 15 C in, 15 C out, and the cycle is back where it started.

The ledger per G3P released: 3 CO₂ fixed, 9 ATP spent (6 + 3), 6 NADPH spent, 3 RuBP regenerated. Per glucose, which needs two G3P: 6 CO₂, 18 ATP, 12 NADPH, six turns. Those are the numbers a test asks for, and every one of them is three turns' worth multiplied by one or two.

One wording the course marks: the Calvin cycle's product is G3P, from which glucose is made. Glucose is built from two G3P by other enzymes outside the cycle, so "the Calvin cycle makes glucose" is a half-mark sentence and "the Calvin cycle makes G3P, from which glucose is made" is the full one. Rubisco, by the way, is an enzyme, and a slow one: the plant makes a great deal of it to compensate.

The two things students swap

"Light-independent" does not mean "at night." The Calvin cycle needs ATP and NADPH, and the light reactions are what make them, at that moment. So in a leaf the Calvin cycle runs in daylight, right beside the light reactions. The old name "dark reactions" is the cause of this error, and it meant only that the cycle's own steps do not absorb light.

Where the oxygen comes from. Water, split in the light reactions. Not CO₂. The test asks "what is the source of the O₂ released in photosynthesis?" and the answer is H₂O. The carbon of CO₂ goes into the sugar, along with some of its oxygen; the rest of CO₂'s oxygen ends up in water. None of the released O₂ was ever in CO₂.

A third, close behind: where the carriers are made and used. ATP and NADPH are made in the thylakoid membrane (released into the stroma) and used in the stroma by the Calvin cycle. Never the reverse.

Cellular respiration vs photosynthesis: the mirror

Set the two side by side and the mirror is exact enough to learn one from the other.

	Photosynthesis	Cellular respiration
Organelle	Chloroplast	Mitochondrion
Equation	6 CO₂ + 6 H₂O + light → C₆H₁₂O₆ + 6 O₂	C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + ATP
Electron carrier	NADPH	NADH (and FADH₂)
Where the chain pumps H⁺	Into the thylakoid lumen	Into the intermembrane space
Energy in	Light	Glucose
What the ATP is for	Building sugar in the Calvin cycle	The cell's work
Who does it	Plants, algae, some bacteria	Nearly every eukaryotic cell, plants included, and many bacteria

The one sentence students need: plants do both. Respiration runs in every plant cell all the time; photosynthesis runs in the green cells when there is light. A plant in the dark is respiring, using the sugar it made in the day. Cellular respiration, traced the same way, is the other half of this page, and the chain idea in each is the same: electrons move down a chain, protons are pumped, and the proton flow through ATP synthase makes the ATP.

Check: trace it cold, then one sibling

Draw the chloroplast as two boxes, thylakoid and stroma. Write the four shuttles between them: ATP and NADPH out of the thylakoid, ADP and NADP⁺ back. In the first box write 2 H₂O → O₂ + 4 H⁺ + 4 e⁻ and the two photosystems in order. In the second write 3 CO₂ + 3 RuBP → 6 3-PGA → 6 G3P → 1 G3P out + 3 RuBP, with the 9 ATP and 6 NADPH beside it. Then open the page and mark it, line by line, with the honest-marking rules. Karpicke and Blunt (2011) found that drawing from memory and checking produced more learning than re-studying the material, even when the re-study was careful concept mapping from the book; a concept map of the two stages is worth drawing, but only after the attempt. How to study biology sets the order for a whole unit: the vocabulary first, then the mechanism.

Then a sibling, cold: a leaf fixes 12 CO₂ through the Calvin cycle. How many G3P leave the cycle, how much ATP and NADPH is spent, and how many glucose can be built?

Answer. Three CO₂ per G3P, so 12 ÷ 3 = 4 G3P released. ATP: 4 × 9 = 36. NADPH: 4 × 6 = 24. Two G3P per glucose, so 4 ÷ 2 = 2 glucose. Check against the per-glucose ledger: 2 × 6 = 12 CO₂, 2 × 18 = 36 ATP, 2 × 12 = 24 NADPH. It agrees.

If you had the 4 G3P but not the 36, the regeneration's 3 ATP is the piece you dropped; if you wrote 4 glucose, the two-G3P rule is tonight's card.

Where this sits in the course

These are ideas 12 and 13 of the 27 in the SBI4U course map: The light-dependent reactions and The Calvin cycle, the end of strand C. Before them comes idea 11, Anaerobic respiration and fermentation, and before that the respiration ideas this page mirrors. After them idea 14, The structure of DNA, opens strand D, which the DNA-to-protein page traces.

Photosynthesis on one screen

Stage	Where	In	Out
Light-dependent reactions	Thylakoid membranes	Light, H₂O, ADP, NADP⁺	O₂, ATP, NADPH
Calvin cycle	Stroma	CO₂, ATP, NADPH	G3P, ADP, NADP⁺

• Per G3P: 3 CO₂ fixed, 9 ATP and 6 NADPH spent, 3 RuBP regenerated; carbon check 15 + 3 = 18 = 15 + 3.
• Per glucose: two G3P, so 6 CO₂, 18 ATP, 12 NADPH, six turns.
• Water split: 2 H₂O → O₂ + 4 H⁺ + 4 e⁻; the O₂ released is water's.
• The mirror in six words each: chloroplast and mitochondrion; NADPH and NADH; lumen and intermembrane space; light in and glucose in; sugar built and work done; green cells and nearly every cell.
• The two swaps: light-independent is not "at night"; the oxygen comes from water, not CO₂.