The Dark Reactions

The Dark Reactions are the series of reactions where Carbon dioxide is "fixed" to produce glucose. The reaction does not directly need light (hence the name), but it requires ATP and NADPH, both products of the light reaction. Other names for these reactions are:

Light Independent Reactions
The Calvin Cycle named after its discoverer, Melvin Calvin
C3 photosynthesis because the first molecule formed has 3 carbons.

The Dark reaction occurs in the stroma of the chloroplast and occurs in three phases.

Phase I: Carbon Fixation

Carbon Fixing is a term that describes the conversion of inorganic carbon (CO₂) into an organic carbon compound.

In this phase CO₂ combines with ribulose 1, 5-bisphosphate (Aka RuBP) to form an unstable 6-carbon molecule that splits into two 3-phosphoglycerate.

Since three CO₂ molecules enter the cycle, and combine with three RuBP, three 6-carbon molecules split producing six 3-phosphoglycerate.

This reaction is catalyzed by the enzyme Rubisco, the most common protein on earth!

Phase II: The Reduction Reactions

Each 3-phosphoglycerate (3PG) pick up a phosphate (phosphorylated), creating 1,3-bisphosphoglycerate (1,3-BPG)

Then NADPH reduces 1,3-BPG to glyceraldehyde 3-phosphate (G3P).

For every six 3PG, only ONE G3P is released from the Calvin Cycle.

G3P can produce glucose → amylose and amylopectin → stored as starch in chloroplasts or can be converted to sucrose for transport around the plant.

Plants can utilize glucose for cellular respiration (for energy), cellulose, to make amino acids, or to make lipids.

Phase III: RuBP Regeneration

The 5 remaining G3P continue in Calvin cycle to regenerate RuBP.

This phase requires water and 3 ATP.

Overall: 3 CO₂, 9 ATP, 6 NADPH, are needed to produce 1 G3P molecule and 3 molecules of RuBP (double all numbers to make 1 glucose)

Net Reaction:

6CO₂ + 12H₂O + Light E → C₆H₁₂O₆ + 6H₂O + 6O₂

Textbook Practice

p.226-228#8-10

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