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Dark Reaction of Photosynthesis

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What is Photosynthesis: An Introduction

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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

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

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

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.


Light Reaction

Dark Reaction

These reactions depend on sunlight.

This reaction does not depend on sunlight.

Here oxygen is liberated.

Here carbon dioxide is fixed.

Here ATP and NADPH are produced.

Here glucose is produced.

Here chlorophyll is involved.

Here chlorophyll does not involve.

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

  1. Minimum photosynthesis takes place in which light?

  2. What Major limiting factors for photosynthesis in C3 plants are?

  3. Write and explain dark reaction equations.

  4. 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.

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FAQs on Dark Reaction of Photosynthesis

1. Define accessory pigments and their significance.

Accessory pigments are xanthophyll, chlorophyll b, and carotenoids, also known as photosynthetic pigments. They help in accumulating solar energy and passing them to chlorophyll-a. They are not directly concerned with the emission of excited electrons. While doing so, this pigment absorbs maximum radiation in regions – blue and green. As a result, chlorophyll-a is the main pigment, and all other pigments, such as carotenoid, chlorophyll b, and xanthophyll, are accessory pigments.

2. What is the relationship between photosynthesis and respiration?

They are related. In both, plants gain energy from solar radiation, while in respiration, glucose molecules break down to get energy in the form of ATP molecules. Both processes are dependent on each other. Respiration uses the end product of photosynthesis(glucose in the form of food). To produce ATP for energy. In the process, water and carbon dioxide are released, which photosynthesis uses to produce more sugars such as glucose.

3. Why does the rate of photosynthesis decrease at higher temperatures?

The process of photosynthesis is an enzyme-specific process. All the enzymes act at an optimum temperature. If the temperature increases beyond 35 °C, enzymes denature, affecting the photosynthesis rate. Thus higher temperatures lead to a decrease in the rate of photosynthesis; the temperature should be optimum.

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