By | September 29, 2018

Question 1 Define the term nutrition?

Question 2 Name any two modes of nutrition?

Question 3 Name the pores through which leaves exchange gases?

Question 4 What is photosynthesis?

Question 5 Why do organisms need food?

Question 6 Why green plants are called autotrophs?

Question 7 Name the green pigment present in the leaves of plants?

Question 8 “Human being cannot make their own food”?Explain?

Question 9 Why animals are called as heterotrophs?

Question 10 Explain the process of synthesis of food in green plants?

Question 11 State the conditions necessary for the process of photosynthesis to take place?

Question 12 What is the role of chlorophyll in green plants?

Question 13 Explain how the plants obtain Carbon Dioxide for making food by photosynthesis?

Question 14 Explain how , water and minerals are transported to the leaves of a plant to be used in food making by photosynthesis?

Question 15 What is the role of sunlight in photosynthesis?

Question 16 What is the importance of photosynthesis in life?

Question 17 Name foods made by plants which are important part of our diet?

Question 18 Why are algae green?

Question 19 Why leaves of certain plants are coloured?


The organisms need to take food

1) To obtain energy
2) To obtain materials for growth and
3) To obtain materials for the repair of damaged parts of the body.

The process of taking food by an organism as well as the utilisation of this food by the organism is called Nutrition.

Plants can make their own food but animals can not make food themselves. Human beings and animals depend on plants for their food, directly or indirectly.

Modes of Nutrition

The methods of obtaining food are called modes of nutrition. On the basis of their modes of nutrition, all the organisms can be divided into two main groups:

(1) Autotrophs
(2) Heterotrophs

Autotrophs: Autotrophic Mode of Nutrition

Those organisms which can make food themselves from simple substances (like carbon dioxide and water) by the process of photosynthesis, are called autotrophs (and their mode of nutrition is called autotrophic).

All the green plants are autotrophs because green plants can make their own food from simple substances like carbon dioxide and water present in their surroundings by the process of photosynthesis.

Autotrophs contain a green pigment called chlorophyll which helps them make food by absorbing energy from sunlight. The green plants produce food for non green plants as well as for animals.

Our body cannot make food from carbon dioxide and water present around us by the process of photosynthesis (like plants do) because our body does not have the green pigment called

chlorophyll. The green pigment chlorophyll is necessary to absorb energy from sunlight required for making food by photosynthesis.

Heterotrophs: Heterotrophic Mode of Nutrition

Those organisms which can not make food themselves by the process of photosynthesis and take food from green plants or animals, are called heterotrophs (and their mode of nutrition is called heterotrophic). All the non green plants and animals (including human beings) are heterotrophs. The non green plants called fungi are heterotrophs.


The plants use the energy in sunlight to prepare food in the presence of a green colour matter called “chlorophyll” present in the leaves of a green plant.

The process by which green plants make their own food (like glucose) from carbon dioxide and water by using sunlight energy (in the presence of chlorophyll) is called photosynthesis.

Carbon dioxide  + Water  ————–> Glucose  + Oxygen

The process of photosynthesis takes place in the leaves of a plant. Oxygen gas is produced during photosynthesis which is utilised by all the living organisms for their survival.

(1)The process of photosynthesis first produces a simple carbohydrate called ‘glucose’ as food.

(2)The glucose carbohydrate then gets converted into a complex carbohydrate called Starch.

(3)Starch gets stored as food in the various parts of plant including leaves.

(4)Some of the glucose is also converted into other types of plant foods such as fats and oils, proteins as well as vitamins.

(5)The synthesis of food (or making of food) occurs in the leaves of a plant, so leaves are the food factories of a plant. The leaves of a plant can synthesise food because they contain a green pigment chlorophyll (which is necessary for making food). Other parts of a plant usually cannot synthesise food because they do not contain chlorophyll.

Conditions necessary for Photosynthesis

The presence of carbon dioxide, water, chlorophyll and sunlight is necessary for the process of photosynthesis to take place.

(1) How the plants obtain Carbon Dioxide for photosynthesis – The plants take carbon dioxide gas from air through the tiny pores (called stomata) present on the surface of leaves.Each pore is surrounded by a pair of guard cells. The opening and closing of stomatal pores in the leaves is controlled by the guard cells. The carbon dioxide gas present in air enters the leaves of a plant through the stomatal pores present on their surface and utilised in photosynthesis. The oxygen gas produced in the leaves during photosynthesis goes out into air through the same stomatal pores. The stomatal pores of leaves during photosynthesis goes out into air through the same stomatal pores. The stomatal pores of leaves open only when carbon dioxide is to be taken in or oxygen is to be released otherwise they remain closed.

(2) How the plants obtain water for photosynthesis – Water present in the soil is absorbed by the roots of a plant and then transported to the leaves through the vessels which run like inter-connected pipes throughout the roots, stem, branches and leaves. The tiny, pipe-like vessels which transport water from the roots of a plant to its leaves are called xylem.

The plants also need minerals to make foods other than carohydrates. The minerals dissolve in water present in the soil and get transported with it. Water and minerals present in the soil are absorbed by the roots of a plant and transported to its leaves through interconnected pipe like xylem vessels present throughout the roots, stem, branches and leaves of the plant.

(3) Role of chlorophyll in Photosynthesis – It is the presence of chlorophyll which makes the leaves look green. Chlorophyll can absorb the energy from sunlight. The sunlight energy absorbed by chlorophyll is used to combine carbon dioxide and water in the green leaves to produce food. Chlorophyll absorbs light energy from the sun and supplies this energy to the leaves to enable them to carry out photosynthesis for making food. Chlorophyll is present in every leaf of a plant in the form of hundreds of tiny structures called chloroplasts.

4) The role of Sunlight in photosynthesis  – The sunlight supplies energy for the food making process called photosynthesis. The sun’s energy is captured by plant leaves with the help of chlorophyll and converted into chemical energy of food. This chemical energy gets stored in the form of plant food. So, when plants utilise the food made by photosynthesis, they actually use the solar energy stored in it in the form of chemical energy. Since all the food on this earth is made by utilising solar energy, therefore, sun is the ultimate source of energy for all the living organisms.

Leaves of Various Colours

Most of the plants have green coloured leaves. Some of the plants, however, have leaves of other colours such as red, violet, brown etc.The leaves having colours other than green also have chlorophyll in them. Actually, the large amount of red, violet, brown or other pigments in such leaves masks the green colour of chlorophyll. So, photosynthesis also takes place in leaves having colour other than green.

Photosynthesis by Plant Parts Other Than Leaves

In some plants, photosynthesis also takes place in other parts of plants such as “green stems” and “green branches. The green stems and green branches can do photosynthesis because they contain chlorophyll.

For example: The desert plants such as cactus have tiny, spine-like leaves to reduce the loss of water by transpiration. These tiny, spine-like leaves of a cactus plant cannot do photosynthesis. The stem and branches of a cactus plant are green which contain chlorophyll . So, the green stem and green branches of a cactus plant carry out the process of photosynthesis to make food for the plant.

Photosynthesis by Algae

We see patches of slimy, green layer floating on the surface of a pond or lake, or even in the stagnant parts of a river . This green layer is formed by the growth of tiny green plant-like organisms called algae .

Algae are a large group of simple, plant-like organisms. Algae contain chlorophyll and produce food by photosynthesis just like plants. Algae differ from plants because they do not have proper roots, stems and leaves. The green colour of algae is due to the presence of chlorophyll in them.

Synthesis of Food other than simple carbohydrate

The simplest food synthesised by the plants by photosynthesis is, a simple carbohydrate called glucose.

The glucose carbohydrate is made up of three elements : carbon, hydrogen and oxygen. The plants use the simple carbohydrate glucose to make many other foods such as starch, oils (or fats), proteins and vitamins.

(1) Plants Make Starch as food

Some of the simple carbohydrate glucose made by the plants through photosynthesis is converted naturally into a complex carbohydrate called starch. The starch is a food which is stored in various parts of a plant such as roots, stems , leaves and seeds.

The seeds or grains of wheat and rice have a lot of starch in them. Potato and carrot plants store a lot of starch in their roots.

(2) Plants Make Oils (or Fats) as Food

Certain plants convert the simple carbohydrate glucose made during photosynthesis into oils and store them in their seeds. Such seeds are called oil-seeds and give us oil (or fats) for cooking food.

For example: the seeds of sunflower plant contain a lot of oil stored in them.

We can extract oil from sunflower seeds and use it as a food. The oils obtained from plant seeds are commonly known as vegetable oil.

(3) Plants make Proteins as Food

Plants combine some of the glucose carbohydrate made during photosynthesis with nitrate minerals(obtained from soil) to make amino acids which are then made into proteins.In this way, plants make proteins as food.

Proteins are nitrogenous substances which contain nitrogen element

Nitrogen element is present in abundance in air in the form of nitrogen gas. However, the plants cannot absorb nitrogen gas for their needs. The soil has certain bacteria which convert nitrogen gas of air into nitrogen compounds (like nitrates) and release them into soil. Nitrates are the water soluble nitrogen compounds which are absorbed by the plants from the soil along with water. The plants fulfill their requirement of nitrogen.  The plants also obtain nitrogen from the nitrogen fertilisers.

(4) Plants make Vitamins as Food

Vitamins are made by plants. Vitamins are contained in vegetables, fruits and cereals made by plants.

Importance of Photosynthesis

Photosynthesis is important for the existence of life on this earth. 

(1)Photosynthesis by plants provides food to animals (including human beings): The survival of animals (including human beings) depends on the food made by plants by photosynthesis. In the absence of photosynthesis, there would be no plants on this earth and hence no animals will survive.

(2) The process of photosynthesis by plants puts oxygen gas into the air: 

It is this oxygen gas which the animals (including human beings) use for breathing and respiration. In the absence of photosynthesis, there would be no oxygen in air and hence no animals could exist on this earth.