The Light-Independent Reactions

The light-independent reactions constitute a pathway known as the Calvin-Benson cycle or the C3 Pathway. The participants and their roles in the synthesis of carbohydrates are:

ATP, which provides energy.
NADPH, which provides hydrogen atoms and electrons.
Atmospheric air, which provides the carbon and oxygen from carbon dioxide.

The reactions take place in the stroma of chloroplasts and are not dependent on sunlight.

Carbon dioxide diffuses into a leaf, across the plasma membrane of a photosynthetic cell, and into the stroma of a chloroplast. An enzyme called ribulose carboxylase (Rubisco) joins carbon dioxide to a five carbon sugar called ribulose biphosphate (RuBP) to produce an unstable six carbon intermediate that splits to form two molecules of phosphoglycerate (PGA) which each have three carbon atoms.

Each PGA then receives a phosphate (Pi) from ATP plus H+ and electrons from NADPH to form phosphoglyceraldehyde (PGAL).

Most of the PGAL molecules continue in the cycle to fix more carbon dioxide, but two PGAL join to form a sugar-phosphate, which will be modified to sucrose, starch, and cellulose.

This animation (Audio - Important) describes the Calvin-Benson cycle.

REVIEW: The carbon dioxide acceptor in the Calvin-Benson cycle is

REVIEW: The joining of carbon dioxide to RuBP occurs in the

REVIEW: The Calvin-Benson cycle starts when _____ .

REVIEW: ATP phosphorylates _____ in the light-independent reactions.

REVIEW: Identify which of the following substances accumulates inside the thylakoid compartment of chloroplasts during the light-dependent reactions:
a. glucose
b. carotenoids
c. chlorophyll
d. fatty acids
e. hydrogen ions