Class 7 Science

Plant Nutrition: Photosynthesis

Learning Goals

The process of making of food by green plants in the presence of sunlight and chlorophyll is known as photosynthesis. Photosynthesis is the combination of two words- Photo + Synthesis. ‘Photo’ means light and ‘Synthesis’ means to make.

Process of Photosynthesis

Green plants make their food themselves. Green leaves make food from Carbon dioxide and water in the presence of sunlight and chlorophyll.

Uptake of Carbon Dioxide

Leaves have several tiny pore-like structures on the lower surface. These pores are called stomata. Leaves absorb carbon dioxide from air, through these pores.

Transport of Water

Water is transported to leaves through very thin pipelines from the roots. These pipelines are present throughout the plant, i.e. from roots to branches and leaves. These pipelines are known as Xylem. Xylem is a type of tissue. You will learn about tissues in your higher classes.

Role of Chlorophyll

Chlorophyll is a green pigment that is found in green leaves. Chlorophyll absorbs sunlight and gives energy. Chlotrophyll is inside chloroplast. Chloroplast is the site of photosynthesis.

Reaction during Photosynthesis

At the end of photosynthesis, carbohydrate and oxygen are formed. Carbohydrate is used as food and oxygen is emitted out to the atmosphere. This whole process of making food by plants is called photosynthesis.

The reaction that takes place in the process of photosynthesis can be written as:

6CO2 + 6H2O → C6H12O6 + 6O2

Carbohydrate is ultimately converted into starch and stored in leaves. From leaves, starch is transported to different parts of a plant. Starch is a type of carbohydrate.

Leaf

Leaves are known as the kitchen or food factory of plants because photosynthesis takes place in leaves. Leaves look green because of the presence of chlorophyll.

Besides leaves, photosynthesis also takes place in other green parts of the plant, such as in green stems. Chlorophyll is necessary for photosynthesis, hence photosynthesis takes place only in green plants.

The leaves of plants that grow in desert areas are modified in spine like structure or scales to reduce the loss of water in the course of transpiration. In such plants photosynthesis takes place in green stems. Stem is modified into thick spongy leaf-like structures in such plants.

Photosynthesis helps to maintain a balance between oxygen and carbon dioxide in the atmosphere as it absorbs carbon dioxide and releases oxygen.

Sunlight is necessary for photosynthesis. Thus sun is the ultimate source of energy for all living organism.

Our earth is the unique planet, where photosynthesis takes place. In the absence of photosynthesis life would not be possible on earth.

Necessary Factors for Photosynthesis:

Photosynthesis in Algae

Green patches in ponds or near the stagnant water can be seen easily. These green patches are living organism called algae. Algae are plants. Often algae grow near shallow waterlogged areas such as near tube-wells, taps, etc. One may slip over it. Algae look green because of presence of Chlorophyll. Algae prepare their own food by the process of photosynthesis.

Review Questions

Questions 1: What is photosynthesis?

Answer: The process of making food in green plants in the presence of sunlight is known as photosynthesis.

Questions 2: What are the essentials factors for the photosynthesis?

Answer: Carbon dioxide, water, chlorophyll and sunlight are essentials factors for photosynthesis to take place.

Question 3: What is chlorophyll?

Answer: Chlorophyll is the green pigment present in green leaves.

Question 4: Why do leaves look green?

Answer: Leaves look green because of the presence of chlorophyll, which is a green pigment.

Questions 5: What is the function of chlorophyll?

Answer: Chlorophyll absorbs the sunlight for photosynthesis.

Question 6: What are the final products made after photosynthesis?

Answer: Carbohydrate and oxygen are the final products after photosynthesis.

Questions 7: What are stomata?

Answer: The small pores present on the lower surface of leaf, are called stomata.

Questions 8: What is function of stomata?

Answer: Stomata absorb carbon dioxide from air for photosynthesis. Stomata facilitates exchange of gases and transpiration.

Question 9: What is the ultimate source of energy?

Answer: Sun is the ultimate source of energy.

Question 10: How water is transported to the leaves?

Answer: Water is transported to the leaves through pipe like structures from the roots of plant. These pipe-like structures are present from root to leaves through branches throughout.