The Calvin Cycle and Carbon Reactions

Overview of the Calvin Cycle

• Also referred to as the carbon reactions.

• Occurs outside of the thylakoid membranes.

• No electron transport chains involved in this part of photosynthesis.

Energy Sources

• The Calvin Cycle utilizes energy carried by two main molecules produced in the light reactions:

  • Adenosine Triphosphate (ATP)

  • Nicotinamide Adenine Dinucleotide Phosphate (NADPH)

• CO2 from the air is also a necessary input for the carbon reactions.

Primary Goal of the Calvin Cycle

• The core purpose of the carbon reactions is to transform carbon dioxide into glucose, represented chemically as C6H{12}O_6 (which contains six carbon atoms).

• The entire process involves a series of rearrangements of carbon atoms to eventually synthesize glucose.

Major Steps in the Calvin Cycle

Input Molecules

• The process starts with the intake of carbon dioxide, CO_2, which contains one carbon atom.

• This CO_2 combines with Ribulose bisphosphate (RuBP), a five-carbon molecule. Although the specific name isn't required, it's important to remember that RuBP is crucial for the cycle.

Formation of 6-Carbon Molecule

• The reaction between CO_2 and RuBP creates a temporary six-carbon molecule.

  • Chemical representation:

  • RuBP (5 carbons) + CO_2 (1 carbon) → temporary 6-carbon molecule

• This six-carbon compound is unstable and quickly breaks down into two molecules that each contain three carbons, termed 3-Phosphoglycerate (3-PGA).

  • Chemical representation:

  • 6-carbon → 2 x 3-PGA

Rearrangement of Carbons

1. From 3-PGA to 1,3-Bisphosphoglycerate

   • 3-PGA undergoes a rearrangement to become 1,3-Bisphosphoglycerate (1,3-BPG), an energy-requiring step.

   • Energy for this transformation is derived from ATP produced in light reactions.

   • No specific structural knowledge required, only the fact that an energy investment is needed.

2. From 1,3-BPG to Glyceraldehyde-3-phosphate (G3P)

   • The transformation from 1,3-BPG to G3P also requires energy, which is supplied by NADPH generated in the light reactions.

   • Chemical representation and molecular changes are not crucial to note, just that it is another rearrangement step.

   • G3P is nearly the end product from these reactions and is crucial for glucose formation.

3. Production of Glucose and Regeneration of RuBP

   • Some molecules of G3P are used to ultimately form glucose, while others are redirected to regenerate RuBP, enabling continuous cycles of carbon fixation.

   • This process requires additional ATP derived from the light reactions.

   • Key Points:

     • 6 turns of the Calvin Cycle are necessary to produce one molecule of glucose, primarily because RuBP needs to be replenished.

Energy Cycling

• Energy transformation involved in the cycle includes:

  • ATP dephosphorylation leading to the formation of ADP.

  • NADPH converting to NADP+ as energy is released.

• Both ADP and NADP+ are cycled back to the light reactions to be re-energized:

  • ADP → ATP (in light reactions)

  • NADP+ → NADPH (in light reactions)

Summary Connection

• Energy and carbon constantly cycle back through this process, thus emphasizing that the cycle is indeed repetitive.

• The focus is on the rearranging of carbons, with energy consistently provided from the light reactions to facilitate all transformations.