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Mechanism of Chemical Reaction in Plants

Grade 8
Jun 7, 2023


Energy is the ability to do work. All living organisms use energy to survive, grow, respond to stimuli, reproduce, and carry out all biological processes. The energy that we need is obtained from food.

The food that we eat contains organic molecules that store energy in their chemical bonds. The chemical energy stored in food molecules is changed within the cells into various forms that are required to perform all the necessary life activities.

We need the energy to do everything we do in our daily lives




In every cell, various chemical reactions are involved to obtain energy from food. Every activity of an organism requires chemical reactions in one way or another. The total number of chemical reactions in a living organism is called metabolism. All chemical reactions required to carry out metabolic activities need enzymes.

Enzymes are basically proteins that help speed up a chemical reaction. Each enzyme catalyzes a specific chemical reaction.

Enzymes can be used again and again and are sensitive to temperature and PH. Enzymes work on substances called substrates and the part of the enzyme where the reaction takes place is called the active site.

Structure of Enzyme


Living organisms are divided into three groups based on how they obtain their food. Organisms that produce their own food, such as plants and some microorganisms, are called producers. Consumers are the organisms that are unable to produce their own food. Organisms that feed on dead and decaying matter are called decomposers.

Energy enters the biosphere as sunlight. The producers i.e. plants change this energy into chemical energy through the process of photosynthesis. This energy is then passed on to organisms that eat plants and further transferred between organisms when they feed on each other. After the consumers die, they are consumed by decomposers which break down the remains into simple materials. These are again used by the plant during photosynthesis.

Transfer of Energy in an Ecosystem.


Photosynthesis is a vital process by which producers such as plants convert light energy into chemical energy.

Green plants use light energy to make sugars, which are used as food. Plants make their own food by making use of carbon dioxide and water in the presence of sunlight and chlorophyll. Food is stored in the form of energy-rich molecules or carbohydrates.


This process is primarily seen in green plants and photosynthetic bacteria. The main reactants of photosynthesis are carbon dioxide and water whereas the products of photosynthesis are glucose and oxygen.

During photosynthesis, plants utilize electromagnetic radiation from the sun as a form of energy, and CO2 is obtained from the atmosphere. Plants absorb water with the help of roots from the soil to produce oxygen and glucose.

Since plants make their food, they are also called autotrophs.

Raw Materials for Photosynthesis:

Carbon Dioxide(Co2):

It is obtained from the atmosphere where it is present in 0/03-0.04%. Aquatic plants obtain CO2 dissolved in water.

At the time of respiration, animals and plants release CO2 which is then utilized by plants in addition to atmospheric CO2 to prepare their food.

Water (H2O):

Plants absorb water from the soil using root hairs. The xylem further helps the plant transport water up to the stem and leaves. Aquatic plants also absorb water on their surface.


Sun is the only major source of natural light energy. Sunlight is very important in photosynthesis as light energy is converted to potential chemical energy. Artificial light is also used in photosynthesis but the light should be intensive.

The rate of photosynthesis depends on the quality and quantity of light. In red-colored light, the rate of photosynthesis is maximum whereas in green-colored light there is no photosynthesis.


Plants contain substances called pigments that reflect and absorb light.  Plants that use photosynthesis are usually green due to the presence of a green pigment called chlorophyll.

Photosynthesis takes place inside chloroplasts. Chloroplast is a special structure in plant cells containing the green pigment called chlorophyll. Chlorophyll and other pigments are used in photosynthesis to trap light energy. Some of the captured light energy is stored in the chemical bonds that hold together the sugar molecules.

Reactions in Chloroplasts:

Chlorophyll is the primary pigment used in photosynthesis and is found inside the chloroplasts of plant cells. Chlorophyll mostly absorbs blue and red light and reflects green light.

The sunlight absorbed by chlorophyll and other pigments power the chemical reactions of photosynthesis. During photosynthesis, light energy, carbon dioxide, and water make sugars. Photosynthesis also produces oxygen that is released into the atmosphere as a by-product.

Reactions in Chloroplasts.

Process of Photosynthesis:

Photosynthesis is a step-by-step process that maintains the balance of oxygen and carbon dioxide in the atmosphere. However, there are two ways by which photosynthesis takes place in plant leaves. These include light-dependent reactions and light-independent reactions.

Light-dependent Reaction:

This reaction takes place in the stroma and can occur in the absence of light. It uses ATP and NADPH produced from the light reaction to break down carbon dioxide, and this process helps form three types of carbon sugars that form glucose.

Light-Dependent Reaction.

Light-independent Reaction::

This reaction takes place in the stroma and can occur in the absence of light. It uses ATP and NADPH produced from the light reaction to break down carbon dioxide. This process helps in forming three types of carbon sugars that form glucose.

Step-by-Step Procedure of Photosynthesis:

Leaves absorb photons from sunlight which excite chlorophyll and activate electrons. Initially, the absorbed water is split into oxygen and hydrogen ions. Then the excited electrons make ATP and NADPH through the electron chain. Meanwhile, oxygen is released into the atmosphere.

In the next step, the energy from the light reaction converts carbon dioxide into glucose. This entire process involves several stages, and these reactions start with 3-Ribulose biphosphate (RuBP) and end with the same molecule forming glucose.

The Rubisco enzyme is an essential component of the Calvin Cycle as it initiates all the reactions

Photosynthesis reaction.

Significance of Photosynthesis:

Photosynthesis provides the energy required for the growth and development of all living organisms, both producers and consumers. The food we eat grows because of photosynthesis and the cells of most organisms, even bacteria, use sugars made by photosynthesis.

Photosynthesis supplies Earth’s atmosphere with oxygen, which is required for cellular respiration. Photosynthesis reduces the amounts of carbon dioxide produced by organisms from cellular respiration by converting it into oxygen.

It is vital for the synthesis of many essential organic compounds.

Storage of Glucose:

During photosynthesis, plants make more glucose than what is required for their own metabolic processes. The excess glucose is converted and stored as starch or used to make other carbohydrates.

As glucose is water-soluble plants can transport the glucose easily. However, in order to store large amounts of glucose, plants have to convert it into compounds that are insoluble in water. As a result, the plant converts glucose into starch.

Plants make use of these carbohydrates as food for growth, maintenance, and reproduction.

Mechanism of Chemical Reaction in Plants


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