Subjects and lessons to explore.

Law

Concepts of Biology · Photosynthesis

The Calvin Cycle

8 min read
10 key termsSourcesReport an error
The 9 ATP + 6 NADPH accounting per G3P and the description of rubisco as the most abundant protein are commonly taught textbook reference concepts; verify specific values against current primary sources before formal citation.
Want it in plain words first? Jump to Eli explains — the same idea, no jargon.
On this page 9 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Worked example
  6. Key takeaway
  7. Check yourself
  8. Study tools
  9. Sources & references

In 30 seconds

The is the second act of photosynthesis: the sugar-building stage. It takes place in the stroma — the fluid surrounding the thylakoids — and uses the ATP and NADPH produced by the light-dependent reactions to reduce carbon dioxide into a three-carbon sugar. Because it does not require light directly, it is also called the light-independent reactions (historically the "dark reactions" — a misleading name, since most plants run it during the day, powered by ATP and NADPH made minutes earlier).

The cycle has three phases, repeated over and over: (capturing CO₂), reduction (using ATP and NADPH to build a sugar), and regeneration (rebuilding the starting molecule so the cycle can turn again). The starting molecule is a five-carbon sugar called ribulose-1,5-bisphosphate (). The enzyme that captures CO₂ — — is famously slow and famously abundant, often described as the most abundant protein on Earth (a claim worth verifying against current sources). Because it is slow, plants must make enormous amounts of it, which is why it dominates leaf protein.

For every three turns, one molecule of the three-carbon product glyceraldehyde-3-phosphate () is exported to build glucose and other sugars; the other five G3P are recycled to regenerate three RuBP. In other words, the cycle is a sugar factory whose "profit" of one G3P is taken out only after the machinery is fully restocked.

Why this matters

  • The food-making step: Everything the light reactions accomplished is only useful once the Calvin cycle builds sugar. This is the step that actually removes CO₂ from the air and produces the organic carbon that feeds plants, herbivores, and everything above them.
  • The carbon cycle: Carbon fixation by the Calvin cycle (and similar pathways in other organisms) is the main route by which inorganic CO₂ becomes organic carbon — the foundation of the global carbon cycle and a key term in climate conversations.
  • Rubisco's limitations shape agriculture: Rubisco is slow and sometimes captures O₂ instead of CO₂ (), which wastes energy. This is why C₄ and CAM plants evolved alternative carbon-fixing strategies — and why crop scientists study rubisco so intensely.
  • Connecting the two stages: The Calvin cycle is where the ATP:NADPH balance matters; the plant must run cyclic electron flow when the cycle needs more ATP. Understanding this link explains why the light reactions and the Calvin cycle are inseparable.
  • Exams: Expect to name the three phases in order, count the turns needed to produce a G3P (or a glucose), identify the enzyme that fixes carbon, and state the energy cost in ATP and NADPH.

The college version

Core Concepts

Phase 1: Carbon fixation

The cycle begins when CO₂ diffuses into the stroma and is attached to RuBP (a five-carbon molecule) by the enzyme rubisco. The six-carbon intermediate is unstable and immediately splits into two molecules of 3-phosphoglycerate (), each with three carbons. Nothing is reduced yet — the carbon has simply been "fixed," converted from inorganic CO₂ into an organic molecule. This phase consumes no ATP or NADPH; it is the capture step.

Phase 2: Reduction

Now the captured carbon is upgraded. Each 3-PGA is phosphorylated using ATP, then reduced using NADPH (the electrons from the light reactions) to become glyceraldehyde-3-phosphate (G3P), a three-carbon sugar. For every three CO₂ fixed, six G3P are produced — but only one is the cycle's "profit"; the other five are recycled. The exported G3P is the raw material for glucose, starch, cellulose, and everything else the plant builds.

Phase 3: Regeneration of RuBP

The five remaining G3P (15 carbons total) are rearranged through a series of reactions — spending more ATP — to rebuild three molecules of RuBP (5 carbons each). This restocking is what makes the cycle a cycle: without regenerated RuBP, the next CO₂ has nothing to bind to, and carbon fixation grinds to a halt.

The energy budget of the cycle

Counting per G3P exported (commonly taught textbook accounting): three turns fix three CO₂, consume nine ATP and six NADPH, and produce one net G3P. Six turns yield two G3P, which combine into one glucose. The ATP and NADPH spent here are exactly what the light-dependent reactions supply — which is why the two stages are locked together. Note that ATP is consumed even in the regeneration phase, making the ATP cost roughly 50% higher than the NADPH cost — why plants sometimes run cyclic electron flow to top up ATP.

Rubisco's flaw and the C₄/CAM workarounds

Rubisco is not selective: when O₂ is high and CO₂ low, it attaches O₂ to RuBP instead of CO₂, starting a wasteful process called photorespiration that consumes ATP and NADPH without making sugar. Hot, dry conditions make this worse because plants close their stomata to save water, letting CO₂ drop and O₂ build up inside the leaf. C₄ plants (like corn and sugarcane) fix CO₂ in one cell type and run the Calvin cycle in another, concentrating CO₂ around rubisco. CAM plants (like cacti and pineapple) fix CO₂ at night, when stomata can open without losing much water. Both are rubisco workarounds — the same enzyme, different plumbing.

Common Confusions

Do Not ConfuseWithDifference
"Dark reactions" = only at nightReactions that don't need light directlyMost plants run the Calvin cycle during the day using ATP/NADPH just made in the light.
The Calvin cycle produces glucose directlyIt produces G3PG3P is combined into glucose and other sugars in later steps, not produced as glucose by the cycle itself.
One turn of the cycle = one G3P exportedThree turns per G3P exportedFive of every six G3P produced are recycled to regenerate RuBP; only one of six is exported as profit.
The cycle runs on light energy directlyIt runs on ATP and NADPHLight energy was already converted to ATP/NADPH by the light reactions.
Rubisco only fixes CO₂Rubisco can also grab O₂O₂ uptake (photorespiration) wastes energy; C₄/CAM plants concentrate CO₂ to prevent it.
Regeneration phase costs nothingIt spends ATPThe ATP cost makes the total 9 ATP + 6 NADPH per G3P (commonly taught).
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

The Calvin cycle is the plant's sugar-making workshop. It takes carbon dioxide from the air and, using the batteries (ATP) and electron taxis (NADPH) charged up in the sunlight, snaps the carbon atoms onto a conveyor belt. After three trips around the belt, one piece of sugar is finished and sent out, while the belt parts are recycled so it can run again. The workshop never turns off as long as the batteries are charged.

Worked example

Follow the cycle through three complete turns in a corn leaf cell (a C₄ plant, so CO₂ is delivered at high concentration):

  1. Turn 1 — fixation. A CO₂ molecule is attached to RuBP by rubisco; the six-carbon intermediate splits into two 3-PGA. ATP and NADPH from the light reactions convert both 3-PGA into G3P. Two G3P now exist.
  2. Turn 2. Another CO₂ is fixed and reduced, adding two more G3P. The pool now holds four G3P. Still no profit taken — the plant is building inventory.
  3. Turn 3. A third CO₂ is fixed and reduced, making six G3P total. Now the plant takes its profit: one G3P is exported to the cytoplasm, where it will be combined into sucrose for transport or starch for storage.
  4. Regeneration. The remaining five G3P (15 carbons) are rearranged, spending three more ATP, to rebuild three RuBP (15 carbons). The starting line is restored; the next CO₂ can be captured.
  5. The bigger picture. Six turns produce two exported G3P, which the plant joins into one glucose (C₆H₁₂O₆) — the sugar written in the photosynthesis equation. The whole cycle ran on the ATP and NADPH charged up by the light reactions minutes earlier.

The lesson: the Calvin cycle is a machine that must be fully restocked every turn; only the surplus comes out as sugar.

Key takeaways

  • Location: stroma; inputs: CO₂, ATP, NADPH; outputs: G3P (sugar), ADP, NADP⁺.
  • Three phases in order: fixation → reduction → regeneration.
  • Rubisco fixes CO₂ onto RuBP (5C) → unstable 6C → two 3-PGA (3C each).
  • Reduction uses ATP + NADPH to convert 3-PGA → G3P.
  • For every 3 turns: 3 CO₂ in, 1 net G3P out, 5 G3P recycled into 3 RuBP (spending more ATP).
  • Energy cost per G3P (commonly taught): 9 ATP + 6 NADPH; 6 turns → 1 glucose.
  • Photorespiration: rubisco grabbing O₂ instead of CO₂ — the problem that C₄ and CAM pathways solve.
  • The cycle runs in the light, not "only at night"; it just doesn't need light directly.

Check yourself

6 review questions from the chapter. Try each one, then open the answer.

  1. Where does the Calvin cycle occur, and what are its three phases in order?

    Show answer

    In the stroma of the chloroplast. The three phases are carbon fixation, reduction, and regeneration of RuBP.

  2. What is the role of rubisco, and why is it so important?

    Show answer

    Rubisco attaches CO₂ to RuBP, splitting the product into two 3-PGA — the carbon fixation step that converts inorganic CO₂ into organic molecules. It is the entry point of carbon into the biosphere's food web.

  3. How many turns of the cycle are needed to export one G3P, and how many to make one glucose?

    Show answer

    Three turns export one net G3P; six turns produce two G3P, which combine into one glucose.

  4. What are the energy inputs per G3P produced (commonly taught values)?

    Show answer

    Nine ATP and six NADPH per net G3P (commonly taught textbook accounting); ATP is spent in both the reduction and regeneration phases.

  5. What is photorespiration, and how do C₄ and CAM plants avoid it?

    Show answer

    Photorespiration is rubisco attaching O₂ instead of CO₂ to RuBP, wasting ATP and NADPH. C₄ plants separate CO₂ capture and the Calvin cycle between cell types; CAM plants capture CO₂ at night — both raise CO₂ concentration around rubisco.

  6. Why is "dark reactions" a misleading name for the Calvin cycle?

    Show answer

    Because the cycle does not require darkness — it simply doesn't need light directly. It uses ATP and NADPH, so most plants run it during the day when those are being produced.

Keep learning

Ready to build on this? Continue to the next lesson.

Study tools & related lessonsKey vocabulary · Related

Key vocabulary

Calvin cycle
Light-independent sugar-building cycle in the stroma
Carbon fixation
Attaching inorganic CO₂ to an organic molecule
Rubisco
Enzyme that attaches CO₂ (or O₂) to RuBP
RuBP
Five-carbon molecule that accepts CO₂
3-PGA
Three-carbon molecule formed right after fixation
G3P
Three-carbon sugar produced by the reduction phase
Reduction (phase)
Using ATP and NADPH to convert 3-PGA to G3P
Regeneration (phase)
Rebuilding RuBP from recycled G3P
Photorespiration
Wasteful O₂ uptake by rubisco instead of CO₂
C₄ / CAM plants
Plants that concentrate CO₂ for rubisco

Sources & references

  1. openstax.org — Concepts Biology

This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.

Educational content only. It is not medical, legal or professional advice. Found an error? Tell us.