What Happens During the Dark Reaction? Steps, Enzymes, and Key Facts
Photosynthesis is a process by which green plants prepare their food by releasing oxygen. Green plants have chlorophyll pigments like chlorophyll-a or chlorophyll-b and carotenoids, which have photosystems to trap the sunlight. Do you know what the end products of dark reactions are? Do you know how many steps of dark reaction there are?
There are two main steps of photosynthesis - light reaction is the first step, and dark reaction is the second step of photosynthesis. To know more about dark or light reactions, continue reading this article.
What is Dark Reaction?
Dark reaction is called so because it is a light-independent process in which carbohydrate molecules are formed from carbon dioxide and water molecules. It is also known as the carbon-fixing reaction. The dark reaction occurs in the chloroplast's stroma utilising the light reaction's products.
Mechanism of Dark Reaction
It is a light-independent or biosynthetic phase of photosynthesis. It assimilates the carbon dioxide with the help of ATP and NADPH. The dark reactions occur through the Calvin cycle and CAM cycle.
Calvin Cycle
It occurs in all photosynthetic plants. The first stable product formed in this cycle is 3-phosphoglyceric acid or PGA.
The primary acceptor of carbon dioxide in this cycle is Ribulose bisphosphate (RuBP), and the fixation enzyme is RuBisCO. There are three steps of dark reaction, which occurs only through only Calvin Cycle. These steps are explained below:
CARBOXYLATION involves the fixation of carbon dioxide and forming two molecules of 3-PGA.
REDUCTION leads to the formation of glucose. In this step, two ATP and two NADPH molecules are used for each carbon dioxide for reduction.
REGENERATION for the cycle to keep going on, the synthesis of Ribulose 1,5- bisphosphate is regenerated, which acts as the most crucial carbon dioxide acceptor molecule. The dark reaction diagram (Calvin Cycle) is explained below.
Calvin Cycle
Dark Reaction Equation
6CO2 +18ATP+12NADPH + H+ → C6H12O6+ 6H2O+18ADP+18Pi+12NADP+
Hatch and Slack Pathway
This occurs in C4 plants, which have a special type of leaf anatomy called kranz anatomy. Kranz's anatomy helps the plant to photosynthesize more efficiently. It also helps in preventing photorespiration. Moreover, it also helps in twice the fixation of carbon dioxide.
They comprise two types of cells: mesophyll and bundle sheath cells.
The primary carbon dioxide acceptor is three carbon compounds called phosphoenol pyruvate or PEP catalysed by enzyme PEPcase.
In mesophyll cells, the four-carbon compound oxaloacetate acid OAA is formed and gets converted into malic or aspartic acid to enter the bundle-sheath cells.
Bundle Sheath cells have RuBisCO enzyme where regeneration of Phosphoenol pyruvate takes place to keep the cycle on without interruption.
Let's look at ATP consumption in the C4 cycle:
In the hatch and Slack pathway, 2 ATP per carbon dioxide molecule
In Calvin Cycle, 3 ATP per carbon dioxide molecule
In all, for the cycle, 5 ATP are required for each carbon dioxide molecule.
Hatch and Slack Pathway
CAM Cycle or Crassulacean Acid Metabolism
Some plants like cacti and pineapple are adapted to dry environments; they use the crassulacean acid metabolism (CAM) pathway to minimise photorespiration.
The separation of reactions in them is unlike the light-dependent reactions and the use of the Calvin cycle in CAM plants; these are separated by time.
At night, CAM plants open their stomata, allowing CO2 to diffuse into the leaves. This is fixed into oxaloacetate by PEP carboxylase and then converted to malate or another type of organic acid.
The organic acid is stored inside vacuoles for the next day's daytime. In the daylight, the CAM plants do not open their stomata. Still, they photosynthesize by transporting organic acid out of the vacuole and breaking it down to release the 3-carbon compound PEP, which enters the Calvin cycle.
CAM Pathway
What is the Light Reaction?
The light reaction generates ATP and provides NADPH to start the dark reaction. The light reaction is only possible in the presence of sunlight only. And it is the first step of photosynthesis. The light reaction occurs in thylakoids of chloroplasts.
Important Questions on Dark Reaction of Photosynthesis
1. What are the end products of dark reactions?
Ans: The end product of the dark reaction is glucose.
2. Write the difference between light reaction and dark reaction.
Ans:
3. How do aquatic plants get CO2 to initiate photosynthesis?
Ans: Aquatic plants are adapted to live in the limitation of carbon dioxide. It is the dissolved carbon dioxide in the water on which the aquatic plants depend for performing photosynthesis, unlike terrestrial plants, which get carbon dioxide through stomata.
Practice Question
Minimum photosynthesis takes place in which light?
What Major limiting factors for photosynthesis in C3 plants are?
Write and explain dark reaction equations.
Explain the dark phase of photosynthesis.
Key Features
Plants create the oxygen and glucose needed for most organisms.
Chloroplasts are the site of photosynthesis in plants containing thylakoids, where light reactions occur.
Light reactions convert sunlight into ATP and NADPH. And the dark reactions, or Calvin Cycle, use ATP and NADPH to convert CO2 into sugar.
The light and dark reactions cooperate to convert light energy into chemical energy housed in glucose.
FAQs on Dark Reaction of Photosynthesis: Process and Importance
1. What is the dark reaction of photosynthesis, and where does it take place in the plant cell?
The dark reaction, also known as the biosynthetic or carbon-fixation phase, is the second stage of photosynthesis where carbon dioxide is converted into glucose. Unlike the light reaction, it does not directly require sunlight. This entire process occurs in the stroma, the fluid-filled space within the chloroplasts, using the energy (ATP and NADPH) produced during the light reaction. The primary pathway for this is the Calvin Cycle.
2. Why is the term 'dark reaction' considered a misnomer?
The term 'dark reaction' is misleading because it implies the process only happens in the dark, which is incorrect. A more accurate term is 'light-independent reaction'. It is called this because it does not use light as a direct reactant. However, it is critically dependent on the products of the light reaction (ATP and NADPH). Therefore, the dark reaction runs simultaneously with the light reaction and stops shortly after the light source is removed.
3. What are the three main stages of the Calvin Cycle?
The Calvin Cycle, which constitutes the dark reaction, is divided into three essential stages:
- Carboxylation: The fixation of atmospheric CO₂ into a stable organic intermediate. This is catalysed by the enzyme RuBisCO, which combines CO₂ with Ribulose-1,5-bisphosphate (RuBP).
- Reduction: A series of reactions where the fixed carbon is reduced to form triose phosphates (carbohydrates). This stage utilises the ATP and NADPH generated during the light-dependent reactions.
- Regeneration: The regeneration of the CO₂ acceptor molecule, RuBP, from the triose phosphates. This step also requires ATP and is crucial for the cycle to continue.
4. How are the light and dark reactions of photosynthesis interconnected?
The light and dark reactions are two distinct but inseparable phases of photosynthesis. The light reaction captures solar energy and converts it into chemical energy in the form of ATP and NADPH. The dark reaction then uses this chemical energy to fix atmospheric CO₂ into organic molecules like glucose. In turn, the dark reaction regenerates ADP and NADP⁺, which are sent back to the light reaction to be re-energised, forming a continuous and dependent cycle. You can learn more about the difference between the light reaction and dark reaction to understand their unique roles.
5. What is the main difference in how C3 and C4 plants carry out the dark reaction?
The primary difference lies in the initial carbon fixation step and cellular anatomy, which is an adaptation to minimise water loss and photorespiration.
- C3 Plants: The entire Calvin cycle, including the initial fixation of CO₂ by RuBisCO, occurs within the mesophyll cells. The first stable product is a 3-carbon compound.
- C4 Plants: These plants have a specialised 'Kranz' anatomy. They first fix CO₂ in mesophyll cells using a different enzyme (PEP carboxylase) to form a 4-carbon compound. This compound is then transported to adjacent bundle-sheath cells, where CO₂ is released and enters the standard Calvin cycle. This mechanism efficiently concentrates CO₂ around RuBisCO, reducing wasteful photorespiration.
6. What is the role of the enzyme RuBisCO in the dark reaction?
RuBisCO (Ribulose-1,5-bisphosphate carboxylase-oxygenase) is arguably the most critical enzyme in the biosphere. Its primary function during the dark reaction is carboxylation: it captures an atmospheric carbon dioxide molecule and attaches it to a five-carbon sugar called Ribulose-1,5-bisphosphate (RuBP). This is the first and rate-limiting step of the Calvin cycle, effectively converting inorganic carbon into an organic form that the plant can use to build sugars.
7. What would be the immediate consequence if the regeneration phase of the Calvin Cycle failed?
If the regeneration phase of the Calvin cycle failed, the plant would be unable to replenish its supply of Ribulose-1,5-bisphosphate (RuBP), the primary CO₂ acceptor molecule. Even with ample light, ATP, and NADPH, the cycle would grind to a halt because there would be no substrate to fix new CO₂ molecules. This would stop the synthesis of glucose and eventually lead to the accumulation of intermediates from the reduction phase, completely shutting down the plant's ability to produce food.
