24 Calvin cycle & PPP

Definition: The light reactions of photosynthesis require visible light as an energy source.
Products: The light reactions produce:
- Reducing power in the form of NADPH
- ATP
- Oxygen (O2)
Chemical Equation:
2 ext{ H}2 ext{O} + 2 ext{ NADP}^+ + 3 ext{ ADP} + 3 ext{ Pi} + ext{ light} ightarrow 2 ext{ NADPH} + 2 ext{ H}^+ + 3 ext{ ATP} + ext{ O}2

Function: The carbon reactions, also called dark reactions or light-independent reactions, utilize ATP and NADPH to fix carbon dioxide (CO2) into carbohydrates.
Chemical Equation:
3 ext{ CO}2 + 9 ext{ ATP} + 6 ext{ NADPH} + 6 ext{ H}^+ ightarrow ext{ C}3 ext{H}6 ext{O}3- ext{phosphate} + 9 ext{ ADP} + 8 ext{ Pi} + 6 ext{ NADP}^+ + 3 ext{ H}_2 ext{O}
Location of Reactions: Both NADPH and ATP are produced in the stroma of chloroplasts, which is where they are used in the Calvin Cycle.

Phases of the Calvin Cycle: The Calvin cycle consists of three main phases:
1. Carboxylation: Conversion of CO2 to 3-phosphoglycerate (3PG)
2. Reduction: Transformation of 3PG to glyceraldehyde-3-phosphate (GAP)
3. Regeneration of Starting Material: Conversion of GAP back to ribulose 1,5-bisphosphate (RuBP)
Historical Context: Originally termed "dark reactions" because they do not require light. The Calvin cycle centers around building sugar from CO2 and is a reductive process, necessitating electrons provided by NADPH.
Significance: Melvin Calvin was awarded the Nobel Prize in 1961 for elucidating this cycle.

Enzyme Involved: RuBisCO (Ribulose Bisphosphate Carboxylase/Oxygenase)
- Function: Attaches CO2 to ribulose 1,5-bisphosphate (RuBP) converting the gas into carbohydrate.
- Characteristics: Considered "slow" with a turnover rate of approximately 3 reactions per second, and can mistakenly attach O2 instead of CO2.
- Most abundant protein on Earth, making up about 40 million tons or 50% of leaf protein.
- It is termed the "cornerstone of autotrophy" because it catalyzes the fixation of CO2 into organic molecules.

Process: Involves the reduction of 3-phosphoglycerate (3PG) to glyceraldehyde-3-phosphate (GAP).
Chemical Mechanism: The reduction process represents the reverse of two reactions in glycolysis:

Cleavage: Cleavage of the acyl-phosphate bond drives reduction.
Redox Pair: NADPH/NADP+ are the redox pairs utilized in this reaction, in contrast to glycolysis which uses NADH/NAD+.
- Cycle Turn: For every 3 turns of the Calvin Cycle using 3 CO2s, one GAP is siphoned off for net carbohydrate synthesis.

Purpose: Regenerates the CO2 acceptor, RuBP.
Requirements: Conversion of five 3C GAPs into three 5C RuBPs is essential to maintain cycle continuity.
In Reactions: Some reactions exchange carbons while others involve hydrolysis or phosphorylation, supported by specific enzymes:
- Aldolase: Catalyzes the combination of three-carbon aldose and ketose into a six-carbon carbohydrate.
- Transketolase: Transfers two carbon units, crucial for moving carbons around during the regeneration phase.
- Bisphosphatases: Catalyze the hydrolysis of phosphates in some of the reactions.
- Kinase: Adds phosphates to important substrates.
- Isomerases and Epimerases: Facilitate structural rearrangements within molecules.

Key Regulatory Steps: Rubisco and kinase/phosphatases are the points of regulation within the Calvin cycle. The cycle is activated by light, ensuring coordination between light and dark reactions.
Reversibility: All regulated enzymatic reactions are irreversible, while other reactions can be reversible.

During the cycle, a total of nine ATPs and six NADPHs are utilized for each three turns.
One GAP is used for net carbohydrate synthesis while five are directed towards regenerating RuBP.

Final Products of Calvin Cycle: 2 GAPs from two turns are used to synthesize hexoses, which may then be further processed into starch or sucrose.