Dark Reaction (Biosynthetic Phase) - Use of ATP & NADPH
-
Products of the light reaction are ATP, NADPH, and O₂. The dark reaction uses ATP and NADPH to drive the synthesis of food (sugars).
-
This phase does not directly depend on light but relies on light reaction products. When light becomes unavailable, biosynthesis continues briefly before stopping, resuming when light is available.
-
CO₂ combines with H₂O to form (CH₂O)â‚™ or sugars
-
CO₂ assimilation during photosynthesis occurs via two pathways:
- C3 pathway: The first stable product of CO₂ fixation is a C3 acid (3-phosphoglyceric acid - PGA). Discovered by Melvin Calvin using ¹⁴C in algal photosynthesis.
- C4 pathway: The first stable product is oxaloacetic acid (OAA), a 4-carbon organic acid.
C₃ Pathway (Calvin Cycle)
-
The Calvin cycle occurs in all photosynthetic plants (C3 or C4 pathways) and has three stages: Carboxylation, Reduction, and Regeneration.
1. Carboxylation of RuBP
-
RuBP (ribulose bisphosphate), a 5-carbon ketose sugar, is the primary CO₂ acceptor.
-
CO₂ is fixed by RuBP to form two 3-PGA molecules, catalyzed by the enzyme RuBP carboxylase.
-
This enzyme, also called RuBP carboxylase-oxygenase (RuBisCO), has oxygenation activity and is the most abundant enzyme globally.
2. Reduction
-
A series of reactions leads to glucose formation, using 2 ATP for phosphorylation and 2 NADPH for reduction per CO₂ molecule.
-
Fixation of 6 CO₂ molecules requires 6 turns of the cycle to produce one glucose molecule.
3. Regeneration of RuBP
-
Crucial for cycle continuation, it requires one ATP for phosphorylation to regenerate RuBP.
-
For each CO₂ molecule, 3 ATP and 2 NADPH are needed. Cyclic photophosphorylation may occur to balance ATP and NADPH usage.
-
Producing one glucose molecule requires 6 turns of the cycle.
| In | Out |
|---|---|
| 6 CO₂ | 1 glucose |
| 18 ATP | 18 ADP |
| 12 NADPH | 12 NADP |
C₄ Pathway (Hatch & Slack Pathway)
-
Present in plants adapted to dry tropical regions, which also use the C3 pathway as the main biosynthetic pathway.
-
Large cells around vascular bundles, called bundle sheath cells, form Kranz anatomy (‘wreath’), with multiple layers, many chloroplasts, thick walls impervious to gas exchange, and no intercellular spaces.
Steps of Hatch and Slack Pathway
-
The primary CO₂ acceptor is phosphoenol pyruvate (PEP), a 3-carbon molecule in mesophyll cells, fixed by the enzyme PEP carboxylase (PEPcase).
-
Mesophyll cells lack RuBisCO.
-
The C4 acid OAA forms in mesophyll cells, converting to malic acid or aspartic acid, which are transported to bundle sheath cells.
-
In bundle sheath cells, C4 acids break down to release CO₂ and a C3 molecule, which is transported back to mesophyll cells and converted to PEP.
-
Released CO₂ enters the C3 pathway. Bundle sheath cells are rich in RuBisCO but lack PEPcase.
-
C4 plants are special due to:
- Kranz anatomy.
- Tolerance to higher temperatures.
- Response to high light intensities.
- Lack of photorespiration.
- Greater biomass productivity.
Photorespiration
-
In the Calvin pathway, RuBP combines with CO₂.
-
RuBisCO can bind both CO₂ and O₂, with a higher affinity for CO₂. Binding is competitive, determined by O₂ and CO₂ concentrations.
-
In C3 plants, some O₂ binds to RuBisCO, reducing CO₂ fixation. RuBP binds O₂ to form one phosphoglycerate and one phosphoglycolate, a process called photorespiration.
-
Photorespiration produces no sugars, ATP, or NADPH, and is wasteful, releasing CO₂ using ATP.
-
C4 plants avoid photorespiration by increasing CO₂ concentration at the enzyme site when C4 acids break down in bundle sheath cells, minimizing RuBisCO’s oxygenase activity.
-
This lack of photorespiration enhances productivity and yields in C4 plants, along with tolerance to higher temperatures.
Differences between C₃ and C₄ Plants
| C₃ Plants | C₄ Plants |
|---|---|
| Photosynthesis occurs in mesophyll cells. | In mesophyll and bundle sheath cells. |
| Kranz anatomy is absent. | Present. |
| RuBP is the primary CO₂ acceptor. | PEP is the primary CO₂ acceptor. |
| 3-PGA, a 3-C compound, is the first stable product. | OAA, a 4-C compound, is the first stable product. |
| Chloroplasts are of one type (granal). | Dimorphic (granal in mesophyll, agranal in bundle sheath). |
| Photorespiratory loss is high. | Photorespiration is absent or negligible. |
| High CO₂ compensation point (25-100 μl CO₂ l⁻¹). | Low CO₂ compensation point (0-10 μl CO₂ l⁻¹). |
| Optimum temperature for photosynthesis is about 25°C. | About 35°C - 45°C. |
| Photosynthetically less efficient, low productivity. | Photosynthetically more efficient, high productivity. |
| E.g., rice, wheat, bean, potato. | E.g., maize, sugarcane, amaranth, sorghum. |