13. PHOTOSYNTHESIS

DARK REACTION (BIOSYNTHETIC PHASE) - USE OF ATP & NADPH

Products of light reaction are ATP, NADPH and O2.Dark reaction is the use of ATP and NADPH to drive the processes for the synthesis of food (sugars).

This phase does not directly depend on the light but is dependent on the products of the light reaction.

It can be verified as follows: Immediately after light becomes unavailable, the biosynthetic process continues for some time, and then stops. If light is available, the synthesis starts again.

CO2 combines with H2O to form (CH2O)n or sugars.

CO2 assimilation during photosynthesis is 2 types:

C3 pathway: In this, first stable product of CO2 fixation is a C3 acid (3-phosphoglyceric acid - PGA). Melvin Calvin discovered this using 14C in algal photosynthesis.
C4 pathway: In this, first stable product is oxaloacetic acid (OAA), a 4-carbon (C4) organic acid.

C₃ PATHWAY (CALVIN CYCLE)

It occurs in all photosynthetic plants (C3 or C4 pathways).

It has 3 stages: Carboxylation, Reduction & Regeneration.

1. Carboxylation of RuBP

RuBP (ribulose bisphosphate - a 5-carbon ketose sugar) is the primary CO2 acceptor.

It is the most crucial step. CO2 is fixed by RuBP to two 3-PGA in presence of the enzyme RuBP carboxylase.

Since this enzyme also has an oxygenation activity it is called RuBP carboxylase-oxygenase (RuBisCO).

RuBisCO is the most abundant enzyme in the world.

2. Reduction

It is a series of reactions leading to the glucose formation.

Here, 2 ATP molecules for phosphorylation and two NADPH for reduction per CO2 molecule are used.

Fixation of 6 CO2 molecules and 6 turns of the cycle are needed to remove one glucose molecule from the pathway.

3. Regeneration of RuBP

It is crucial for continuation of the cycle.

It requires one ATP for phosphorylation to form RuBP.

Hence for every CO2 molecule, 3 ATP molecules and 2 NADPH are required.

It is probably to meet this difference in number of ATP and NADPH used in the dark reaction that the cyclic phosphorylation takes place.

To make 1 glucose molecule, 6 turns of the cycle are needed.

What does go in and come out of the Calvin cycle?	In	Out
	6 CO₂	1 glucose
	18 ATP	18 ADP
	12 NADPH	12 NADP

C₄ PATHWAY (HATCH & SLACK PATHWAY)

It is present in plants adapted to dry tropical regions.

They also use C3 pathway as main biosynthetic pathway.

The large cells around the vascular bundles of the C4 plants are called bundle sheath cells. Such anatomy is called ‘Kranz’ anatomy (‘Kranz’ = ‘wreath’).

The bundle sheath cells may form several layers around the vascular bundles.

They have large number of chloroplasts, thick walls impervious to gas exchange and no intercellular spaces.

Steps of Hatch and Slack Pathway

Primary CO2 acceptor is phosphoenol pyruvate (PEP) - a 3-carbon molecule seen in mesophyll cells. The enzyme for this fixation is PEP carboxylase (PEPcase).

The mesophyll cells lack RuBisCO enzyme.

The C4 acid OAA is formed in the mesophyll cells.

It then forms other 4-carbon acids like malic acid or aspartic acid. They are transported to bundle sheath cells.

In the bundle sheath cells, C4 acids are broken down to release CO2 and a C3 molecule.

The C3 molecule is transported back to mesophyll where it is converted to PEP again.

The released CO2 enters the C3 pathway.

Bundle sheath cells are rich in RuBisCO, but lack PEPcase. Thus C3 pathway is common to C3 & C4 plants.

C4 plants are special because:

They have a special type of leaf anatomy (Kranz).
They tolerate higher temperatures.
They show a response to high light intensities.
They lack photorespiration.
They have greater productivity of biomass.

PHOTORESPIRATION

In Calvin pathway, RuBP combines with CO2.

Active site of RuBisCO can bind to CO2 & O2 – so the name.

RuBisCO has a greater affinity for CO2 than for O2. This binding is competitive. Relative concentration of O2 and CO2 determines which one will bind to the enzyme.

In C3 plants, some O2 bind to RuBisCO. Hence CO2 fixation is decreased. Here RuBP binds with O2 to form one molecule of phosphoglycerate and phosphoglycolate. This pathway is called photorespiration.

In this, there is no synthesis of sugars, ATP and NADPH. Hence photorespiration is a wasteful process. Rather it causes the release of CO2 by using ATP.

In C4 plants, photorespiration does not occur because they can increase CO2 concentration at the enzyme site. This takes place when C4 acid from the mesophyll is broken down in the bundle cells to release CO2. This minimises the oxygenase activity of RuBisCO.

Due to the lack of photorespiration, productivity and yields are better in C4 plants. Also, these plants show tolerance to higher temperatures.

Differences between C₃ and C₄ plants

C₃ plants	C₄ plants
1. Photosynthesis occurs in mesophyll cells.	In mesophyll and bundle sheath cells.
2. Kranz anatomy is absent.	Present.
3. RuBP is the primary CO₂ acceptor.	PEP is the primary CO₂ acceptor.
4. 3-PGA, a 3-C compound is the first stable product.	OAA, a 4-C compound is the first stable product.
5. Chloroplasts are of only one type (granal).	Dimorphic (granal in mesophyll and agranal in bundle sheath).
6. Photorespiratory loss is high.	Photorespiration is absent or negligible.
7. High CO₂ compensation point (25-100 ml. CO₂ l^-1).	Low CO₂ compensation point (0-10 ml. CO₂ l^-1).
8. Optimum temperature for photosynthesis is about 25^oC.	About 35^oC - 45^oC.
9. Photosynthetically less efficient and productivity low.	Photosynthetically more efficient and productivity high.
10. E.g. Rice, wheat, bean, potato.	E.g. Maize, sugarcane, amaranth, sorghum.

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Photosynthesis - Notes | Class 11 | Part 4: Dark Reaction

13. PHOTOSYNTHESIS