Photosynthesis: equation, light reactions, Calvin cycle

Q: How is photosynthesis related to cellular respiration?

They are close to opposites. Photosynthesis uses \mathrm{CO_2}, water and light energy to make glucose and \mathrm{O_2}. Cellular respiration uses glucose and \mathrm{O_2} to make \mathrm{CO_2}, water and ATP. Together they cycle carbon and oxygen through ecosystems.

Q: What limits how fast photosynthesis goes?

Light intensity, \mathrm{CO_2} concentration and temperature. Raising one speeds it up only until another factor runs short. Temperature matters because the Calvin cycle runs on enzymes, which slow down when cold and stop working when too hot.

Key ideas

\mathrm{6\,CO_2 + 6\,H_2O} \xrightarrow{\text{light}} \mathrm{C_6H_{12}O_6 + 6\,O_2}

$\mathrm{CO_2}$ (carbon dioxide) comes in from the air through small pores called stomata.
$\mathrm{H_2O}$ (water) comes up from the roots.
$\mathrm{C_6H_{12}O_6}$ (glucose) is the sugar made. The cycle's direct product is a 3-carbon sugar, G3P, which the plant builds into glucose, sucrose and starch.
$\mathrm{O_2}$ (oxygen) is released. It comes from the water that is split, not from the $\mathrm{CO_2}$ .

The two stages
	Light-dependent reactions	Calvin cycle (light-independent)
Where	Thylakoid membranes	Stroma
Needs	Light, water, $\mathrm{ADP}$ , $\mathrm{NADP^+}$	$\mathrm{CO_2}$ , ATP, NADPH
Makes	ATP, NADPH, $\mathrm{O_2}$	G3P (used to make glucose), ADP, $\mathrm{NADP^+}$
Key players	Chlorophyll, photosystems II and I, ATP synthase	The enzyme RuBisCO and RuBP

The Calvin cycle has three stages: fixation (RuBisCO attaches $\mathrm{CO_2}$ to RuBP), reduction (ATP and NADPH turn the product into G3P) and regeneration (most G3P is used to rebuild RuBP). One $\mathrm{CO_2}$ is fixed per turn.

Worked examples

Example 1: counting turns of the Calvin cycle

Problem How many turns of the Calvin cycle, and how many ATP and NADPH, does it take to make one glucose?

Each turn fixes one $\mathrm{CO_2}$ . Glucose has 6 carbons, so it takes 6 turns.
OpenStax counts per 3 turns (one exported G3P): 6 ATP and 6 NADPH in reduction, plus 3 ATP in regeneration. That is 9 ATP and 6 NADPH per G3P.
Glucose is two G3Ps' worth of carbon, so double it.
$2 \times 9 = 18\ \text{ATP} \qquad 2 \times 6 = 12\ \text{NADPH}$

Answer 6 turns, using 18 ATP and 12 NADPH

Example 2: grams of sugar from carbon dioxide

Problem A plant takes in 132 g of $\mathrm{CO_2}$ and uses all of it to make glucose. How many grams of glucose can it make?

Grams of $\mathrm{CO_2}$ to moles (molar mass 44.01 g/mol).
$\frac{132\ \text{g}}{44.01\ \text{g/mol}} = 2.999\ \text{mol CO}_2$
The equation shows 6 $\mathrm{CO_2}$ per glucose.
$2.999 \div 6 = 0.4999\ \text{mol glucose}$
Moles to grams (glucose is 180.16 g/mol).
$0.4999 \times 180.16 = 90.06\ \text{g}$

Answer 90.1 g of glucose

Example 3: where the oxygen comes from

Problem Scientists gave plants water made with a heavy oxygen isotope ( $^{18}\mathrm{O}$ ) and normal $\mathrm{CO_2}$ . Where did the heavy oxygen end up?

In the light-dependent reactions, photosystem II splits water to replace the electrons that chlorophyll loses.
Splitting water releases its oxygen atoms as $\mathrm{O_2}$ gas.
The oxygen atoms from $\mathrm{CO_2}$ end up in the sugar (and in new water), not in the gas released.

Answer In the $\mathrm{O_2}$ gas released, because the oxygen comes from split water

Common mistakes and how to fix them

Saying the oxygen comes from $\mathrm{CO_2}$ . It comes from water split in the light reactions.
Calling the Calvin cycle the dark reactions. It does not need light directly, but it runs in daylight because it needs ATP and NADPH from the light reactions. OpenStax notes that most scientists no longer use the term dark reactions.
Saying plants do photosynthesis instead of respiration. Plants do both. They make sugar in chloroplasts and break it down for ATP in mitochondria, day and night.
Mixing up thylakoid and stroma. Light reactions are in the thylakoid membranes. The Calvin cycle is in the stroma, the fluid around them.

Practice questions

Where do the light-dependent reactions take place?
1. Stroma
2. Thylakoid membranes
3. Mitochondria
4. Cytoplasm
Show answer
Answer: Thylakoid membranes
Chlorophyll and the photosystems are built into the thylakoid membranes, where light is absorbed.
Which two products of the light reactions does the Calvin cycle use?
1. Oxygen and glucose
2. ATP and NADPH
3. Water and carbon dioxide
4. ADP and NADP+
Show answer
Answer: ATP and NADPH
ATP supplies energy and NADPH supplies electrons to turn fixed carbon into G3P.
How many molecules of $\mathrm{CO_2}$ are needed to make 3 molecules of glucose?
Show answer
Answer: 18 molecules
Each glucose needs 6 $\mathrm{CO_2}$ , so $3 \times 6 = 18$ .
A leaf makes 0.50 mol of glucose. How many moles of $\mathrm{O_2}$ does it release?
Show answer
Answer: 3.0 mol
The equation gives 6 $\mathrm{O_2}$ per glucose: $0.50 \times 6 = 3.0$ mol.
Which enzyme fixes $\mathrm{CO_2}$ in the Calvin cycle?
1. ATP synthase
2. RuBisCO
3. DNA polymerase
4. Amylase
Show answer
Answer: RuBisCO
RuBisCO attaches $\mathrm{CO_2}$ to RuBP in the fixation stage.

Frequently asked questions

Why are plants green?

Chlorophyll absorbs mostly blue and red light and reflects green light. The reflected green light is what reaches your eyes. Other pigments, such as carotenoids, absorb other colors and show up in fall when chlorophyll breaks down.

How is photosynthesis related to cellular respiration?

They are close to opposites. Photosynthesis uses $\mathrm{CO_2}$ , water and light energy to make glucose and $\mathrm{O_2}$ . Cellular respiration uses glucose and $\mathrm{O_2}$ to make $\mathrm{CO_2}$ , water and ATP. Together they cycle carbon and oxygen through ecosystems.

What limits how fast photosynthesis goes?

Light intensity, $\mathrm{CO_2}$ concentration and temperature. Raising one speeds it up only until another factor runs short. Temperature matters because the Calvin cycle runs on enzymes, which slow down when cold and stop working when too hot.

Sources

OpenStax Biology 2e, 8.1 Overview of Photosynthesis, accessed October 1, 2026
OpenStax Biology 2e, 8.2 The Light-Dependent Reactions of Photosynthesis, accessed October 1, 2026
OpenStax Biology 2e, 8.3 Using Light Energy to Make Organic Molecules, accessed October 1, 2026

Key ideas

Worked examples

Common mistakes and how to fix them

Frequently asked questions

Sources

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