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Dark Reaction and the Cycles Involved for NEET

Last updated date: 17th Apr 2024
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Dark Reaction of Photosynthesis

To carry out their various biological processes, all living beings require energy. Green plants are remarkable in this process. They synthesise food from simple molecules like carbon dioxide and water in the presence of sunlight. Photosynthesis is an oxidation-reduction process in which CO2 is reduced to generate starch and sugars while water is oxidised to liberate O2.

Photosynthesis is accomplished in two phases, according to scientists: light phase or photochemical processes, light-dependent reactions, or Hill's reactions.

Solar energy is received and transformed into chemical energy, which is stored in ATP and NADPH + H+ during this step. The dark phase is also known as chemical dark reactions, light-independent processes, the Blackman reaction, or the biosynthetic phase.

What is a Dark Reaction?

Dark reaction, also known as the photosynthetic carbon reduction (PCR) cycle or carbon fixation, is a method through which all photosynthetic eukaryotic organisms eventually assimilate CO2 into glucose. The dark reaction is sensitive to temperature changes but not to light; thus, its name, yet, it is dependent on the products of photosynthesis' light reaction, namely NADP, 2H+, and ATP.

Dark Reaction Equation

The equation depicting the overall process of dark reaction is as follows:

$6CO_{2}+12 NADPH+12H_{2}O+18ATP \rightarrow C_{6} H_{12} O_{6}+12 NADP^{+}+18ADP+18P_{i_6}$

Site of Dark Reaction

The dark reaction happens in the exterior of the thylakoids which is the stromal part of a plant cell. Carbon dioxide fixation occurs in the stroma of chloroplasts, which contains enzymes required for CO2 fixation and sugar production.

Different Kinds of Cycles Involved in the Dark Reaction 

Calvin Cycle

In this approach, Calvin and Benson found the carbon route. The Calvin cycle is known as C3-cycle because the first stable molecule is 3C-PGA (phosphoglyceric acid). Calvin investigated Chlorella and Scenedesmus species for the process. These are small unicellular algae that are simple to keep in the lab. 

He employed chromatography and a radioactive tracer (C14) to detect C3-cycle reactions during his experiment. RuBisCO (Ribulose Bisphosphate Carboxylase Oxygenase) is the major enzyme in the C3-cycle, which is found in the stroma and contributes to the production of around 16 per cent of chloroplast protein.

Ribulose -1,5- diphosphate is a CO2-acceptor in the C3 cycle (RuBp). RuBisCO catalyses this carboxylation process. Atriplex hastata and Atriplex patula are C3-plants that grow in temperate climates. For one glucose molecule to be formed, the Calvin cycle must be turned six times. In this cycle, 18 ATP and 12 NADPH2 are utilised to make one mole of hexose sugar (glucose). The dark reaction of photosynthesis steps is further categorised into three distinct stages: carboxylation, glycolytic reversal, and regeneration of RuBP.

1. Carboxylation

The process of carboxylation is the most integral step in the Calvin cycle. CO2 binds to RuBP in the essential step of carbon fixation, yielding 2 to 3 carbon molecules of phosphoglycerate. The enzyme that catalyses this reaction is ribulose bisphosphate carboxylase/oxygenase, which is found in the chloroplast stroma and is quite big with four subunits. This enzyme is extremely slow, digesting only about three RuBP molecules per second (a typical enzyme process of about 1000 substrate molecules per second). RuBisCO accounts for more than half of the protein in a typical leaf. It is estimated to be the world's most prevalent protein.

$6 RuBp+6 CO_{2}\left(HCO_{3}^{-}\right) \underset{\text { Carboxy Dismutase }}{\stackrel{\text { RubiSco }}{\rightarrow}} 6~C~\text{unstable compound}$ 

$\rightarrow ~12~~\text{mol.}~~3 - \text{phosphoglyceric acid}~(P G A)$

2. Glycolytic Reversal

The Calvin cycle has progressed to the second stage. Carbon fixation produces 3-PGA molecules, which are then transformed into simple sugar molecules, such as glucose. This stage gets its energy from ATP and NADPH produced during photosynthesis' light-dependent processes. As a consequence, the Calvin cycle is used by plants to convert solar energy into long-term storage molecules such as sugars. Energy is delivered to the sugars from ATP and NADPH. Because electrons are transferred to 3-PGA molecules to generate glyceraldehyde-3- phosphate, this procedure is called reduction.

$12~\text{mol. of}~3-PGA+12~ATP \stackrel{\text { Triokinase }}{\longrightarrow}$

$12~\text{mol. of}~1,~3 -\text{bisphosphoglyceric acid}$ $(1,3~\text{BiPGA})\underset{12 N A D P H_{2} \rightarrow 12 \mathrm{NADP}^{+}}{\stackrel{\text { Dehydrogenase }}{\longrightarrow}}$

$12~\text{mol. of}~3-\text{phosphoglyceraldehyde}~(3-PGAL)+12H_{3} PO_{4}$

Two of these 12 molecules are utilised to produce sugar, starch, and other carbohydrates, while the other ten is recycled to regenerate six molecules of Ribulose-5-phosphate through a series of complex processes. One molecule of PGAL is transformed to its isomer 3-hydroxy acetone phosphate out of every two.

$1~\text{mol. of PGAL}+1\text{mol. of DHAP}~(9 C) \underset{\text { Aldolase }}{\longrightarrow}$

$1~\text{mol. of Fructose}~1,~6~Bisphosphate~(18 C) \stackrel{\text { Isomerose }}{\longrightarrow}$ 

$C_{6}H_{12} O_{6}( Glucose ) \rightarrow ~Sucrose / Starch$

3. Regeneration of Ribulose 1, 5 Bisphosphate

Some G-3-P molecules are recycled to replenish the RuBP acceptor, while others are used to produce glucose. Regeneration necessitates the use of ATP and entails a complex network of processes.

$2 \text { mol.Fructose }(12 C)-6-P+2 \text { mol.PGAL }(6 C) \stackrel{\text { Transketolase }}{\rightarrow} 2 \text { mol.Erythrose }-4-P(8 C)$

$+2 \text { mol.Xylulose }-5-P(10)$

$2 \text { mol.Erythrose }-4-P(8 C)+2 \text { mol.DHAP }(6 C) \stackrel{\text { Aldolase }}{\rightarrow} 2 \text { mol.Sedoheptulose } 1,7-B i P(14 C)$  

$2 \text { mol.Sedoheptulose }-P(14 C)+2 \text { mol.PGAL }(6 C) \stackrel{\text { Transketolase }}{\rightarrow} 2 \text { mol.Xylulose }-5-P$

$+2 \text { molRibose }-5-P(10 C) $

$2 \times 2 \text { mol.Xylulose } \stackrel{\text { Epimerase }}{\rightarrow} 4 \text { mol.Ribulose }-5-P(20 C)$

$2 \text { molRibose }-5 P \stackrel{\text { Isomerase }}{\rightarrow} 2 \text { mol.Ribulose }-5 P$ 

$6 \text { mol.Ribulose }-5 p+6 A T P \stackrel{\text { Kinase }}{\rightarrow} 6 \text { mol.Ribulose }-1,5-B i P\left(\mathrm{CO}_{2} \text { acceptor }\right)+6 A D P$

C4 cycle/ Hatch and Slack Pathway

  • The 4C-compound OAA (OxaloAcetic Acid) is generated during the dark reaction in sugarcane leaves, according to Kortschak and Hartt. 

  • Hatch and Slack (1967) investigated dark reactions in sugarcane and maize leaves in-depth and identified a mechanism.

  • The first stable product of this reaction is OAA, which is a 4C molecule, a dicarboxylic substance; as a result, the Hatch and Slack pathway are referred to as the C4 cycle or dicarboxylic acid cycle (DCA). 

  • The C4-cycle is found in 1500 species belonging to 19 angiosperm groups; however, the majority of the plants are monocots belonging to the Gramineae and Cyperaceae families (sugarcane, maize, sorghum, oat, Chloris, sedges, bajra, Panicum, Alloteropsis, etc.). 

  • Euphorbia spp., Amaranthus, Chenopodium, Boerhavia, Atriplex rosea, Portulaca, and Tribulus are C4-cycle dicots.

  • The C3 cycle occurs in the bundle sheath cells of the C4 plant, while the C4 cycle occurs in the mesophyll cells.

  • Hatch and Slack route operation necessitates the photosynthetic cells, namely mesophyll cells, and bundle sheath cells.

  • C4 plants are more efficient photosynthetically because there is no Warburg effect or photorespiration in C4 plants, and less O2 is available at the RuBisCo (BS cells) site (mesophyll cells pump more CO2 for the C3 cycle).

  • C4 plants thrive in tropical environments, where they have evolved to high temperatures, limited water supply, and intense sunshine. As a result, they have a higher level of development and adaptation than C3 plants. They do not affect photorespiration.


This article gives a brief explanation of the dark reaction of photosynthesis and its occurrence in the chloroplast cell. It also elucidates the various steps involved in dark reactions in the process of photosynthesis. Different pathways like C4 and C3 cycles are involved in dark reactions. Carbohydrates are created from carbon dioxide utilising the energy stored in ATP and NADPH produced in light-dependent processes at this stage.

Competitive Exams after 12th Science

FAQs on Dark Reaction and the Cycles Involved for NEET

1. What is the difference between the C3 cycle and C4 cycle?

There are a number of differences in C3 and C4 cycles. The carbon dioxide fixation occurs just once in the C3 cycle. Carbon dioxide fixing occurs twice in the C4 cycle (first in mesophyll cells and second in bundle sheath cells). C3 gets inhibited when the concentration of oxygen is increased, there is no such effect on the cycle.  C3 cycle requires 18 ATP molecules to synthesise one molecule of glucose while C4 cycle takes 30 molecules of ATP to synthesise one molecule of glucose.

2. What is the difference between light and dark reactions?

The light reaction is the first stage of photosynthesis, in which light energy is trapped to make ATP and NADPH, whereas the dark reaction is the second stage, in which energy from ATP and NADPH is used to produce glucose. Chlorophyll is involved in the light reaction, the reaction occurs in the thylakoid membrane of the chloroplast. No pigments are involved in dark reactions, the reaction occurs in the stroma of the chloroplast. Photolysis occurs in light reactions but not in dark reactions.