Biosynthetic Phase — Revision Notes

⚡ 30-Second Revision

Location: — Stroma of chloroplasts.
Energy Source: — ATP and NADPH from light reactions.
Main Cycle: — Calvin Cycle (C3 pathway).
Steps:

1. Carboxylation: $CO_2 + RuBP \xrightarrow{RuBisCO} 2 \times 3-PGA$ 2. Reduction: $3-PGA \xrightarrow{ATP, NADPH} G3P$ 3. Regeneration: $G3P \xrightarrow{ATP} RuBP$

Key Enzyme: — RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase).
Energy Cost for 1 Glucose: — 18 ATP, 12 NADPH.
First Stable Product (C3): — 3-PGA (3-carbon).
Photorespiration: — Wasteful process where RuBisCO binds $O_2$ instead of $CO_2$.

2-Minute Revision

The biosynthetic phase, also known as the Calvin cycle or light-independent reactions, is where plants convert atmospheric carbon dioxide into sugars. This crucial process occurs in the stroma of chloroplasts and relies entirely on the ATP (energy) and NADPH (reducing power) produced during the light-dependent reactions.

The cycle begins with carboxylation, where the enzyme RuBisCO fixes $CO_2$ by combining it with a 5-carbon sugar, RuBP, forming two molecules of 3-PGA. Next, in the reduction phase, 3-PGA is converted into glyceraldehyde-3-phosphate (G3P) using the energy from ATP and electrons from NADPH.

G3P is the direct precursor to glucose. Finally, most of the G3P molecules are used in the regeneration phase to reform RuBP, a process that also consumes ATP, ensuring the cycle can continue. To synthesize one molecule of glucose, the Calvin cycle needs 6 turns, consuming a total of 18 ATP and 12 NADPH.

Understanding the role of RuBisCO and the energy stoichiometry is vital for NEET.

5-Minute Revision

The biosynthetic phase, or Calvin cycle, is the heart of sugar production in plants, occurring in the chloroplast stroma. It's 'light-independent' but critically dependent on the ATP and NADPH generated by the light reactions. The cycle proceeds in three stages:

1
Carboxylation: — This is the entry point for $CO_2$. One molecule of $CO_2$ combines with Ribulose-1,5-bisphosphate (RuBP), a 5-carbon sugar, catalyzed by the enzyme RuBisCO. This forms an unstable 6-carbon intermediate that immediately splits into two molecules of 3-Phosphoglyceric acid (3-PGA), a 3-carbon compound. This makes it the C3 pathway.

1
Reduction: — The 3-PGA molecules are then converted into glyceraldehyde-3-phosphate (G3P). This is an energy-intensive step: each 3-PGA molecule is first phosphorylated by ATP, then reduced by NADPH. For every $CO_2$ fixed (yielding 2 PGA), 2 ATP and 2 NADPH are consumed in this stage. G3P is the actual sugar precursor.

1
Regeneration: — Most G3P molecules (10 out of 12 for every 6 $CO_2$ fixed) are used to regenerate RuBP, the initial $CO_2$ acceptor. This regeneration requires additional ATP (1 ATP per RuBP regenerated). This ensures the cycle's continuous operation.

Overall Stoichiometry: To produce one molecule of glucose ($C_6H_{12}O_6$), the Calvin cycle must turn 6 times, fixing 6 $CO_2$ molecules. This requires a total of 18 ATP (12 in reduction, 6 in regeneration) and 12 NADPH (all in reduction).

A key challenge for C3 plants is photorespiration, where RuBisCO binds $O_2$ instead of $CO_2$ under certain conditions, leading to a wasteful process. C4 plants have evolved a mechanism to concentrate $CO_2$ around RuBisCO in bundle sheath cells, minimizing photorespiration, though at a higher ATP cost (30 ATP per glucose).

Prelims Revision Notes

Biosynthetic Phase (Calvin Cycle) - NEET Revision Notes

1. Overview:

Also known as Light-Independent Reactions or Dark Reactions (misnomer).
Occurs in the stroma of chloroplasts.
Indirectly dependent on light (uses ATP and NADPH from light reactions).
Converts $CO_2$ into sugars (glucose).

2. Key Components:

$CO_2$ Acceptor: — Ribulose-1,5-bisphosphate (RuBP), a 5-carbon sugar.
Key Enzyme: — RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase).
Energy Carriers: — ATP and NADPH (from light reactions).

3. Stages of Calvin Cycle:

a. Carboxylation: * $CO_2$ combines with RuBP. * Catalyzed by RuBisCO. * Forms an unstable 6-carbon intermediate, which immediately splits into two molecules of 3-Phosphoglyceric acid (3-PGA).

* 3-PGA is the first stable 3-carbon product (hence C3 pathway). b. Reduction: * 3-PGA is converted to Glyceraldehyde-3-phosphate (G3P) (also called PGAL or triose phosphate). * Requires ATP for phosphorylation and NADPH for reduction.

* For every 2 molecules of 3-PGA, 2 ATP and 2 NADPH are consumed. * G3P is the direct precursor for glucose and other carbohydrates. c. Regeneration: * Most G3P molecules are used to regenerate RuBP.

* Requires ATP for phosphorylation steps. * Ensures continuous operation of the cycle.

4. Stoichiometry for 1 Glucose Molecule ($C_6H_{12}O_6$):

$CO_2$ fixed: — 6 molecules (since glucose is 6-carbon, and 1 $CO_2$ per cycle turn).
ATP consumed: — 18 ATP (12 in reduction, 6 in regeneration).
NADPH consumed: — 12 NADPH (all in reduction).

5. Photorespiration (Wasteful Process):

Occurs when RuBisCO binds $O_2$ instead of $CO_2$ (oxygenase activity).
Favored by high $O_2$, low $CO_2$, and high temperatures.
Consumes ATP and NADPH, releases $CO_2$, but produces no sugar.
Reduces photosynthetic efficiency in C3 plants.

6. C4 Pathway (Adaptation):

Found in plants adapted to hot, dry climates (e.g., maize, sugarcane).
Kranz Anatomy: — Specialized leaf anatomy with bundle sheath cells.
Initial $CO_2$ Fixation: — In mesophyll cells, $CO_2$ combines with PEP (Phosphoenolpyruvate) catalyzed by PEP Carboxylase (PEPcase), forming a 4-carbon compound (e.g., OAA).
Calvin Cycle Location: — Occurs in bundle sheath cells.
Advantage: — Concentrates $CO_2$ around RuBisCO, minimizing photorespiration.
Energy Cost: — Higher ATP requirement (30 ATP per glucose) compared to C3 plants (18 ATP).

Vyyuha Quick Recall

Calvin Really Reduces RuBP's Carbon.

Calvin: Calvin Cycle
Really: Regeneration (of RuBP)
Reduces: Reduction (of PGA to G3P)
RuBP's: RuBP (initial acceptor)
Carbon: Carboxylation (of RuBP by $CO_2$)