Table of Contents
Overview: What “Light-Independent” Really Means
The light-independent reactions of photosynthesis, often called the “dark reactions” or Calvin cycle, are the series of biochemical steps in which carbon dioxide $(\mathrm{CO_2})$ is converted into organic molecules (first a simple 3‑carbon sugar, then other carbohydrates and biomolecules).
Important clarifications:
- “Light-independent” does not mean that they only occur in darkness.
- It means the reactions do not require light directly the way the light-dependent reactions do.
- They depend on the products of the light-dependent reactions (ATP and NADPH) as their energy and reducing power.
These reactions mainly take place in the stroma of chloroplasts, using:
- $CO_2$ from the atmosphere (or water, in aquatic organisms),
- ATP and NADPH from the light reactions,
- and specific enzymes and intermediates.
The central outcome:
- Fixation of inorganic carbon ($CO_2$) into organic molecules that can be used to build sugars, starch, lipids, proteins, etc.
The Calvin Cycle: Three Main Phases
The Calvin cycle can be divided into three conceptual phases:
- Carboxylation (CO₂ fixation)
- Reduction
- Regeneration of the CO₂ acceptor (RuBP)
The entire cycle uses a 5‑carbon sugar, ribulose-1,5-bisphosphate (RuBP), as the starting molecule that accepts $CO_2$.
1. Carboxylation: Fixing CO₂ onto RuBP
The Key Enzyme: Rubisco
The central enzyme of this phase is:
- Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco)
Key properties (without going into its full structure–function, which belongs elsewhere):
- Catalyzes the reaction between RuBP and $CO_2$.
- Is extremely abundant in plants and algae.
- Is relatively slow and somewhat error-prone: it can also react with $O_2$ (this will be relevant in photorespiration, treated in another chapter).
The Carboxylation Reaction
For each $CO_2$ molecule entering the cycle:
- RuBP (5 carbons) + $CO_2$ (1 carbon) → unstable 6‑carbon intermediate
- This 6‑carbon compound immediately splits into two molecules of 3‑phosphoglycerate (3‑PGA or 3‑phosphoglyceric acid), each with 3 carbons.
So, per $CO_2$ fixed:
- 1 RuBP (5C) + 1 $CO_2$ (1C) → 2 × 3‑PGA (3C each)
If you track it on a larger scale (more realistic biochemically):
- 3 RuBP (3 × 5C = 15C) + 3 $CO_2$ (3 × 1C = 3C) → 6 × 3‑PGA (6 × 3C = 18C)
2. Reduction: From 3‑PGA to G3P (A More Reduced Sugar)
The 3‑PGA molecules are still relatively oxidized and not yet “sugar-like.” To turn them into a higher-energy 3‑carbon sugar, the cycle uses ATP and NADPH from the light reactions.
Two-Step Conversion
For each 3‑PGA molecule:
- Phosphorylation by ATP
- 3‑PGA + ATP → 1,3-bisphosphoglycerate (1,3‑BPG) + ADP
- Reduction by NADPH
- 1,3‑BPG + NADPH + $H^+$ → glyceraldehyde-3-phosphate (G3P) + $P_i$ + NADP⁺
G3P (also called triose phosphate) is a 3‑carbon sugar phosphate. It is the key output of the Calvin cycle:
- Some G3P leaves the cycle to be used in:
- synthesis of glucose and other sugars,
- starch synthesis in the chloroplast,
- sucrose synthesis in the cytosol (transport sugar in many plants),
- or as starting material for amino acids, fatty acids, etc.
- The rest of the G3P is recycled to regenerate RuBP.
Energy and Reducing Power Used in Reduction
To follow the stoichiometry, consider 3 $CO_2$ molecules entering the cycle (a common way to describe it):
- 3 $CO_2$ + 3 RuBP → 6 3‑PGA (from carboxylation)
- 6 3‑PGA → 6 1,3‑BPG → 6 G3P
This reduction phase uses:
- 6 ATP
- 6 NADPH
Of the 6 G3P molecules:
- 1 G3P can be considered a net gain (used for biosynthesis),
- 5 G3P are used in the next phase to regenerate RuBP.
3. Regeneration of RuBP: Closing the Cycle
To keep fixing $CO_2$, the cycle must regenerate its starting compound, RuBP.
Rearranging Carbons
From the 6 G3P produced:
- 1 G3P (3C) exits the cycle → used to build sugars and other compounds.
- 5 G3P (5 × 3C = 15C) remain in the cycle.
These 15 carbons are rearranged, through a series of enzyme-catalyzed steps, into 3 molecules of RuBP (3 × 5C = 15C).
The detailed pathway involves several sugar phosphates with different chain lengths (4C, 5C, 7C), but at this level the key points are:
- Multiple enzymes (transketolase, aldolase, etc.) shuffle carbon skeletons.
- Additional ATP is used to phosphorylate some intermediates, finally regenerating ribulose-1,5-bisphosphate.
ATP Cost of Regeneration
For the “3 $CO_2$” cycle unit:
- Regeneration of RuBP from 5 G3P uses 3 additional ATP.
Now you can summarize the full cycle for 3 $CO_2$:
- Input:
- 3 $CO_2$
- 9 ATP (6 for reduction, 3 for regeneration)
- 6 NADPH (for reduction)
- Output:
- 1 net G3P (3C) for biosynthesis
- 9 ADP + 9 $P_i$
- 6 NADP⁺
- 3 RuBP regenerated (cycle ready to accept another 3 $CO_2$)
How Many CO₂ Molecules to Make One Glucose?
G3P is the direct product of the Calvin cycle; glucose is made by combining G3P molecules in subsequent metabolic steps.
- One G3P has 3 carbons.
- To build a 6‑carbon sugar like glucose, the plant needs two G3P (2 × 3C).
Since the cycle yields 1 net G3P per 3 $CO_2$, we need 6 $CO_2$ for the equivalent of one glucose:
- 6 $CO_2$ fixed → 2 net G3P → can be converted to 1 glucose (6C) or equivalent hexose.
Corresponding energy cost for fixing 6 $CO_2$ (to net 2 G3P):
- ATP: $2 \times 9 = 18$ ATP
- NADPH: $2 \times 6 = 12$ NADPH
This stoichiometry links the Calvin cycle to the light reactions, which must supply at least this much ATP and NADPH.
Chemical Summary of the Calvin Cycle
A simplified overall equation for fixing 6 $CO_2$ into one hexose equivalent via the Calvin cycle is:
$$
6 \, CO_2 + 12 \, NADPH + 18 \, ATP
\;\longrightarrow\;
C_6H_{12}O_6\text{ (or equivalent)} + 12 \, NADP^+ + 18 \, ADP + 18 \, P_i + 6 \, H_2O
$$
This equation:
- Emphasizes the use of ATP and NADPH,
- and the production of ADP, $P_i$, and NADP⁺, which return to the light reactions to be “recharged.”
Regulation and Dependence on the Light Reactions
Although called “light-independent,” the Calvin cycle is intimately tied to the light-dependent reactions.
Dependence on ATP and NADPH
- Without ATP and NADPH from the light reactions, the cycle cannot proceed.
- In darkness, ATP and NADPH supplies fall; the Calvin cycle slows and eventually stops.
Enzyme Activation Linked to Light
Several Calvin cycle enzymes are regulated in ways that make them more active in the light:
- Changes in stromal pH (more alkaline in light due to proton pumping into the thylakoid lumen).
- Changes in magnesium ion concentration.
- Redox regulation by small proteins (thioredoxin) that are reduced in the light and activate Calvin cycle enzymes.
Consequences:
- Dark reactions are functionally coupled to light reactions.
- This coupling prevents wasteful use of ATP and NADPH when light is not available.
Fates of G3P: Connection to Other Biosynthetic Pathways
G3P exported from the Calvin cycle can follow different routes:
- Starch synthesis in the chloroplast:
- Storage carbohydrate, accumulated especially during the day.
- Sucrose synthesis in the cytosol:
- Main transport sugar in many plants, moved via the phloem to non-photosynthetic tissues (roots, fruits, seeds).
- Conversion into other biomolecules, after further metabolic steps:
- Amino acids (and then proteins),
- Lipids,
- Nucleotides, etc.
Thus the Calvin cycle is not just “making sugar” but is the entry point for inorganic carbon into the entire network of plant metabolism.
Variants of Carbon Fixation (Brief Orientation Only)
The Calvin cycle described here is the core of C₃ photosynthesis, so-called because the first stable product of $CO_2$ fixation is a 3‑carbon compound (3‑PGA).
Other chapters deal in detail with:
- C₄ photosynthesis: $CO_2$ initially fixed into a 4‑carbon compound, with a spatial separation between initial fixation and the Calvin cycle.
- CAM photosynthesis: $CO_2$ fixation and the Calvin cycle are separated in time (night vs. day).
In all these variants, however, the Calvin cycle itself remains the fundamental pathway for converting $CO_2$ into carbohydrate; only the way $CO_2$ is delivered to the cycle is modified.