The processes which perform various function of the body and are essential for maintenance of life are called life processes.  All the organisms do not have same mode of nutrition i.e. different organisms have different methods of obtaining and utilizing food. Based on the mode of nutrition all living organisms can be classified into two groups:

1.       Autotrophic

2.       Heterotrophic

1)      Autotrophic Nutrition:

By autotrophic nutrition we mean that 'auto'=self; 'trophic'= nutrition. The literal meaning of this is self nutrition. Autotrophic Nutrition is the process where the organism prepare their own food from the simple inorganic materials like CO2,H2O and mineral salts in the presence of sunlight.  All the green plants possess autotrophic mode of nutrition. The autotrophic organisms contains green pigment called as chlorophyll. It is capable of supplying sunlight and this trapped energy is utilized by the autotrophs to prepare their food, combining  CO2 and H2O by the process of photosynthesis which results in the formation of glucose.

Types of Autotrophic Nutrition:

Different types of autotrophic nutrition are there to synthesize organic molecule from simple inorganic substances. Energy is needed for the completion of this process. There are two types of autotrophic nutrition:

a)      Photoautotrophic Nutrition:  In this type of autotrophic nutrition sunlight is the source of energy which produces  CO2 and H2O by the process of photosynthesis. CO2 and H2O are taken in by the organisms which is used to synthesize glucose. This glucose will later be broken down during the process of respiration to release the energy needed for life. By far the greatest energy supply to support food chain and webs is obtained from photo-autotrophic nutrition. Most of the producers use this as nutritional method.

b)      Chemo-autotrophic Nutrition: This can also supply energy needs by some organisms. Simple inorganic substances are taken in and synthesized into organic molecule. Chemical energy is the source of energy for this process. Two examples of chemo-autotrophs are Nitrosomonas, Bacteria.

NH3 + O2-  Nitrite+ H2O +Energy

There are three factors which helps in autotrophic nutrition:

1)      Chlorophyll: It is the green pigment present in plants that facilitate photosynthesis. This green colour is generally present on the leaves of plants. Furthermore, stems and fruits may also possess green colour. In some organisms like algae the whole plant is green and participate in photosynthesis process.

2)      Carbon- dioxide: CO2 is present in a very small amount in the atmosphere, this CO2 is absorbed by terrestrial plants while aquatic ones make do with CO2 dissolved in water. CO2 is absorbed through the stomata present on the surface of leaves.

3)      Sunlight: All forms on life on earth depends on sunlight for energy. In autotrophic plants, light is used for splitting water molecule into hydrogen and oxygen. This process is known as photolysis.


2)      Heterotrophic Nutrition:

Term heterotrophic consist of two words 'hetero'= others; 'trophic'= nutrition. In heterotrophic nutrition organisms cannot make their own food. Such organisms dependent on plants for their food directly or indirectly. All the animals, fungi, some protists and bacteria have heterotrophic mode of nutrition.


Green plants which are autotrophic synthesize food through the process of photosynthesis. Photosynthesis is a process of by which green plants using chlorophyll, synthesize the simple sugar (glucose) from the simple raw material, CO2 and H2O using the energy of sunlight, O2 is released in this process. Photosynthesis converts the solar energy into chemical energy. The overall equation of photosynthesis is:

6CO2+ 12H2O - C6H12O6+6H2O+6O2

The sugar produced is stored in the form of starch in plants. These reserve food provide energy, when required by the organism. Since autotrophs are able to produce food, they are known as producers.

Requirements of photosynthesis:

Photosynthesis requires chlorophyll, carbon-dioxide, water, sunlight.

1.       Chlorophyll : Chlorophyll are green pigments found in all photosynthetic organisms and are responsible for their green colour. In plants chlorophyll is found in leaves, young stems and fruits. In lower plants like algae, the whole plant is green and takes part in photosynthesis.

2.       Carbon- dioxide: Air contains about 0.03% of CO2. Terrestrial plants use atmospheric CO2 in photosynthesis. Aquatic plants use the CO2 dissolved in water. Plants contain CO2 through pores called stomata present on the surface of leaves. The opening and closing of these pores are regulated by guard cells which surround them.

3.       Water: Water is an important raw material for photosynthesis. Plants absorb water from the soil through their root hairs. The water is then transported up to the leaves through the stem.

4.       Sunlight: Light energy is used in splitting water molecule into hydrogen and oxygen. The splitting of water in the presence of light is called photolysis.

Different parts of the plant plays a different role to complete this process. Such parts are discussed as follows:

1)      Leaves: It is considered as the food factories of plant. Leaves of the plant have special structure called chloroplast which possess chlorophyll. Chlorophyll is a green colour pigment that traps the energy from sunlight as sun is the biggest source of light for all the living organism on earth. Plants synthesize glucose with the help of oxygen and water in the presence of sunlight.

2)      Stomata: It is present in the lower epidermis of leaves which takes CO2 from the air for the completion of photosynthesis process.

3)      Roots: It absorbs minerals and water from the soil and transport to different parts of the plant.

Mechanism of Photosynthesis:

There are two main stages in the process of photosynthesis:

1.       Light Reaction or light dependent reaction:

They use light to produce organic energy molecule like ATP, NADPH. Mostly they are initiated by the green pigment, chlorophyll.

2.       Dark Reaction or light independent reaction:

They make use of these organic energy molecule that were produce previously. It is also known as Calvin Benson Cycle and it occurs in the stroma where NADPH provides electrons required to convert CO2 into carbohydrate and ATP which provides the energy.

 In this process light is trapped by chlorophyll pigment which is present in the chlorophyll of leaves. This absorbed energy results in the splitting of water molecule into hydrogen and oxygen. In this process light energy gets converted into chemical energy. CO2 and H2O are taken in by organisms and used to synthesize glucose which can be broken down later during the process of respiration to release energy needed for life. Finally CO2 gets reduced to carbohydrate.

This is a brief description of photosynthesis as a means of autotrophic nutrition.

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