The Calvin Cycle

A growing plant can gain much more mass than its soil loses. Water contributes substantially to its fresh mass, while atmospheric carbon dioxide supplies the carbon built into its organic molecules. In photosynthetic cells, carbon enters the Calvin cycle through a reaction in the chloroplast stroma. The cycle must regenerate its carbon acceptor to keep incorporating carbon dioxide.

Watch the process

Photosynthesis

The light reactions supply ATP and NADPH to the stroma. ATP supports energy-requiring steps, while NADPH supplies reducing electrons. If the light reactions are disrupted, trace which Calvin-cycle reaction loses its input before predicting how an intermediate’s concentration changes.

Three Phases, One Loop

The Calvin cycle in the stroma uses the ATP and NADPH the light reactions made to reduce carbon dioxide into sugar. It has three phases. Carbon fixation attaches \(\text{CO}_2\) to the five-carbon acceptor RuBP, short for ribulose bisphosphate, in a step catalyzed by rubisco, the enzyme that performs that attachment and the most abundant protein on Earth. The product is an unstable six-carbon molecule that immediately splits into two molecules of the three-carbon 3-PGA, short for 3-phosphoglycerate, which is an acid rather than a sugar. Reduction spends ATP and NADPH to convert 3-PGA into G3P, short for glyceraldehyde 3-phosphate, a three-carbon sugar. Regeneration spends more ATP to rebuild RuBP from most of the G3P so the cycle can continue.

Keep the three intermediates separate by their roles. RuBP is the acceptor the cycle keeps rebuilding, 3-PGA is the acid made the instant carbon is fixed, and G3P is the sugar that appears only after ATP and NADPH have been spent. To predict a concentration change, identify which reaction produces the intermediate and which consumes it.

The accounting is worth one pass. Three turns fix three \(\text{CO}_2\) and make six G3P, but five are consumed regenerating RuBP, so one leaves; six turns are needed to export the two G3P a cell combines into one glucose, at a cost of 18 ATP and 12 NADPH.

Calling the Calvin cycle the “dark reactions” is misleading. Light need not strike its enzymes, but sustained carbon fixation in a typical photosynthetic leaf depends on light-supported ATP and NADPH supply and regulation of its enzymes. The dependency runs both ways: block the light reactions and fixation stops; block fixation and the light reactions slow because \(\text{NADP}^+\) and ADP stop coming back.

The Full Cost of One Glucose

Using three turns of the cycle to fix three \(\text{CO}_2\), work out how many turns, ATP, and NADPH are required to export enough G3P for one glucose, and show where each number comes from.

Step 1: count carbons entering per turn. One turn fixes one \(\text{CO}_2\), adding one carbon to the pool. Three turns add three carbons, which is exactly one G3P worth, since G3P has three carbons.

Step 2: count the G3P made and the G3P kept. Three turns produce six G3P, but five of those six are consumed rebuilding the three RuBP molecules the cycle started with. Check that with carbons: \[6 \times 3 = 18\ \text{carbons made}, \qquad 3 \times 5 = 15\ \text{carbons needed for RuBP},\] leaving 3 carbons, one G3P, to leave the cycle.

Step 3: double it for glucose. Glucose has six carbons, so two exported G3P are needed, which takes six turns.

Step 4: total the inputs. Each turn spends 3 ATP, two in the reduction phase and one in regeneration, and 2 NADPH, both in the reduction phase. For six turns: \[6 \times 3 = 18\ \text{ATP}, \qquad 6 \times 2 = 12\ \text{NADPH} .\]

Step 5: sanity-check the ratio. The cycle consumes ATP and NADPH in a 3 to 2 ratio. Linear electron flow can supply less ATP relative to NADPH than the cycle requires, which is the shortfall cyclic electron flow makes up. The arithmetic and the mechanism agree, and an answer that connects them is a stronger answer than one that reports 18 and 12 alone.

Answer

Six turns, 18 ATP, and 12 NADPH per glucose, because three turns yield only one exportable G3P and glucose needs two.

Predicting the Effect of Withdrawing \(\text{CO}_2\)

A well-lit algal culture is fixing carbon at a steady rate. Carbon dioxide is abruptly removed from the medium while light continues. Consider the initial Calvin-cycle response and neglect photorespiration and alternative sources of 3-PGA in this model. Predict what happens to the concentrations of RuBP and 3-PGA in the stroma, and support the prediction.

Claim. RuBP accumulates and 3-PGA falls.

Evidence. RuBP is consumed only in the fixation step, which requires \(\text{CO}_2\). 3-PGA is produced only by that same step and is consumed by the reduction step, which requires ATP and NADPH but not \(\text{CO}_2\).

Reasoning. Removing \(\text{CO}_2\) removes the substrate that converts RuBP into 3-PGA. RuBP therefore continues to be regenerated from existing G3P but is no longer consumed, so it builds up. 3-PGA is no longer produced, but the reduction step continues to draw on the existing pool because light is still supplying ATP and NADPH, so 3-PGA is drained. The prediction depends on identifying which step each intermediate enters and leaves, not on any memorized rule about light.

Conclusion

RuBP rises and 3-PGA falls, because the fixation step that consumes one and produces the other is the step that lost its substrate.

Distinctions to keep clear

The older name “dark reactions” can misleadingly suggest that the Calvin cycle requires darkness. The Calvin cycle does not require darkness, does not prefer darkness, and is strongly limited at night in a typical photosynthetic leaf. It is called light-independent only in the narrow sense that photons do not strike its enzymes. In a typical leaf, darkness limits the supply of ATP and NADPH and changes the regulation of several Calvin-cycle enzymes. The response depends on available pools and cellular conditions.

Carbon fixation and carbon reduction are different chemical changes. Fixation is the attachment of inorganic \(\text{CO}_2\) to an organic acceptor, and the rubisco-catalyzed step does not directly consume ATP. The energy goes into the reduction phase, where ATP and NADPH convert an acid into a sugar. When a stem asks where the ATP and NADPH are spent, ATP is used in reduction and regeneration, while NADPH supplies reducing electrons in the reduction phase. Rubisco fixation itself does not directly consume either.

The cycle needs both its carbon dioxide substrate and its RuBP acceptor. RuBP is not a catalyst; it is consumed by fixation and rebuilt by regeneration. Cut off the regeneration phase, by starving the cycle of ATP, and fixation stops even with plenty of carbon dioxide available.

Review: The Calvin Cycle: Turning Gas into Sugar

The Calvin cycle spends the ATP and NADPH from the light reactions to attach carbon dioxide to an organic acceptor and reduce it to sugar.

Fixation attaches \(\text{CO}_2\) to RuBP and makes 3-PGA, reduction converts 3-PGA to G3P, and regeneration rebuilds RuBP; six turns and 18 ATP and 12 NADPH per glucose.

The cycle is light-independent only in the sense that photons do not hit its enzymes; it halts in the dark because its inputs stop.

Calvin cycle accounting

Practice question 1

The ATP and NADPH consumed by the Calvin cycle are used chiefly to

1. attach carbon dioxide to RuBP during the fixation step

2. reduce 3-PGA to G3P and regenerate the RuBP acceptor

3. split water so that electrons are available for fixation

4. transport carbon dioxide across the chloroplast envelope

Practice question 2

Three turns of the Calvin cycle produce six molecules of G3P, yet only one leaves the cycle. The other five are used to

1. reduce \(\text{NADP}^+\)

2. regenerate RuBP

3. form carbon dioxide

4. build the thylakoid membrane

Practice question 3

A chloroplast is illuminated normally but an inhibitor blocks ATP synthase in the thylakoid membrane. Which downstream change in the stroma is expected as ATP becomes limiting?

1. RuBP regeneration slows, so carbon fixation falls even though carbon dioxide is abundant

2. Rubisco stops fixing carbon dioxide first, because fixation is the step that spends ATP

3. 3-PGA is converted directly to glucose without the reduction phase

4. Water splitting increases to compensate for the missing ATP

Practice answer key

1. B; 2. B; 3. A.

Practice answer explanations

1. Calvin cycle accounting, Question 1. Choice B is correct. Both ATP and NADPH are used in the reduction phase; ATP is also used to regenerate RuBP. The rubisco fixation reaction itself does not directly consume either. Choice A names the one step with no energy cost. Choice C describes photolysis, which happens in the thylakoid and supplies electrons to the light reactions. Choice D invents an active transport step for a gas that diffuses freely.

2. Calvin cycle accounting, Question 2. Choice B is correct. Five of every six G3P are used to regenerate the three RuBP acceptors consumed by three turns, which is why only one G3P is exported per three fixed carbons. Choice A names a reaction of the light reactions, not the stroma’s sugar pool. Choice C reverses the direction of the cycle, since carbon dioxide is the input. Choice D assigns a carbohydrate to membrane construction that lipids and proteins perform.

3. Calvin cycle accounting, Question 3. Choice A is correct. Blocking thylakoid ATP synthase cuts the stroma’s ATP supply, and without ATP the regeneration phase cannot rebuild RuBP, so fixation falls even with abundant carbon dioxide. Choice B misplaces the ATP cost, since fixation by rubisco spends no ATP and is therefore not the step that fails first. Choice C skips the reduction phase entirely and misstates the product. Choice D predicts compensation by a reaction that produces no ATP by itself.

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