Photosynthesis – Complete Study Material for Class 8 to 10

Photosynthesis is one of the most important biological processes studied in Class 8, Class 9 and Class 10 Science. Green plants prepare their own food using sunlight, carbon dioxide and water. This process takes place mainly in the leaves and releases oxygen into the atmosphere.

The word photosynthesis is derived from two Greek words: photo, meaning light, and synthesis, meaning putting together. Therefore, photosynthesis means the process of making food with the help of light energy.

What Is Photosynthesis?

Photosynthesis is the process by which green plants prepare glucose from carbon dioxide and water in the presence of sunlight and chlorophyll. Oxygen is released as a by-product.

The general word equation is:

Carbon dioxide + Water → Glucose + Oxygen
In the presence of sunlight and chlorophyll

The glucose produced may be used immediately for energy or converted into starch for storage.

Where Does Photosynthesis Take Place?

Photosynthesis takes place mainly in the green leaves of plants. Leaves are called the food factories of plants because they contain chlorophyll and have a large surface area for absorbing sunlight.

The process occurs inside tiny cell structures called chloroplasts. Chloroplasts contain a green pigment called chlorophyll.

Role of Different Parts of a Plant

• Roots: Absorb water and minerals from the soil.
• Stem: Transports water to the leaves and carries prepared food to different parts.
• Leaves: Prepare food through photosynthesis.
• Stomata: Allow the exchange of gases.
• Chloroplasts: Contain chlorophyll and serve as the main site of photosynthesis.

Raw Materials Required for Photosynthesis

Four important conditions or materials are required:

1. Sunlight

Sunlight provides the energy needed for photosynthesis. Chlorophyll absorbs light energy, especially light in the red and blue regions of the visible spectrum.

2. Chlorophyll

Chlorophyll is the green pigment present in chloroplasts. It traps solar energy and makes it available for preparing food.

3. Carbon Dioxide

Plants obtain carbon dioxide from the atmosphere through tiny pores called stomata, which are generally found on the surface of leaves.

4. Water

Roots absorb water from the soil through root hairs. Water is transported to the leaves through special conducting tissues called xylem.

Role of Stomata in Photosynthesis

Stomata are tiny pores present mainly on the lower surface of leaves. Each stoma is surrounded by two guard cells.

Stomata perform the following functions:

• Allow carbon dioxide to enter the leaf.
• Allow oxygen to leave the leaf.
• Help in the process of transpiration.
• Open and close according to the water requirements of the plant.

When water is scarce, stomata may close to reduce water loss.

How Do Plants Obtain Carbon Dioxide?

Plants obtain carbon dioxide from the air. Carbon dioxide enters the leaf through the stomata. The concentration of carbon dioxide inside the leaf is usually lower than outside because it is continuously used during photosynthesis.

How Do Plants Obtain Water?

Roots absorb water from the soil through root hairs. The water moves upward through the xylem vessels and reaches the leaves. Minerals dissolved in water are also transported to different parts of the plant.

Steps of Photosynthesis

Photosynthesis can be explained in the following steps:

1. Roots absorb water and minerals from the soil.
2. Water is transported to the leaves through xylem vessels.
3. Carbon dioxide enters the leaves through stomata.
4. Chlorophyll traps energy from sunlight.
5. Light energy is used to convert carbon dioxide and water into glucose.
6. Oxygen is released through the stomata.
7. Excess glucose is converted into starch and stored in different parts of the plant.

What Happens to the Glucose Produced?

The glucose produced during photosynthesis has several possible uses:

• It is used for respiration and energy production.
• It is converted into starch and stored.
• It is used to form cellulose for cell walls.
• It helps in the formation of fats and oils.
• It is used to produce proteins when combined with nitrogen and other minerals.

Importance of Photosynthesis

Photosynthesis is essential for plants, animals and human beings.

• It provides food for green plants.
• It is the primary source of food for almost all living organisms.
• It releases oxygen needed for respiration.
• It maintains the balance of oxygen and carbon dioxide in the atmosphere.
• It provides raw materials for the formation of starch, fats, proteins and cellulose.
• It supports food chains and food webs.
• It helps maintain life on Earth.
• It reduces atmospheric carbon dioxide.
• It stores solar energy in the form of chemical energy.

Factors Affecting Photosynthesis

The rate of photosynthesis depends on several factors:

Light Intensity

Photosynthesis generally increases with an increase in light intensity up to a certain limit. Extremely strong light may damage plant tissues.

Carbon Dioxide Concentration

An increase in carbon dioxide concentration can increase the rate of photosynthesis up to an optimum level.

Temperature

Photosynthesis is controlled by enzymes. It occurs efficiently within a suitable temperature range. Very low or very high temperatures reduce the rate.

Water Availability

Water is a raw material for photosynthesis. Lack of water causes stomata to close, reducing the entry of carbon dioxide.

Chlorophyll Content

Leaves with sufficient chlorophyll can absorb more light energy and carry out photosynthesis effectively.

Photosynthesis and Respiration

Photosynthesis and respiration are different but related processes.

Experiment to Show That Sunlight Is Necessary for Photosynthesis

Materials Required

• A healthy potted plant
• Black paper or aluminium foil
• Clips
• Iodine solution
• Alcohol
• Beaker
• Water bath

Procedure

1. Keep the potted plant in darkness for 24–48 hours to remove stored starch.
2. Cover a portion of one leaf with black paper on both sides.
3. Keep the plant in sunlight for a few hours.
4. Pluck the leaf and boil it in water.
5. Heat the leaf in alcohol using a water bath to remove chlorophyll.
6. Wash the leaf and add iodine solution.

Observation

The exposed part of the leaf turns blue-black, while the covered part remains yellowish-brown.

Conclusion

The exposed part received sunlight and formed starch. Therefore, sunlight is necessary for photosynthesis.

Experiment to Show That Chlorophyll Is Necessary

A variegated leaf contains green and non-green areas.

1. Keep the plant in sunlight for a few hours.
2. Remove a leaf and test it for starch using iodine.
3. Only the green parts turn blue-black.

Conclusion

Only the green parts containing chlorophyll prepared starch. Therefore, chlorophyll is necessary for photosynthesis.

Experiment to Show That Carbon Dioxide Is Necessary

1. Take two similar destarched plants.
2. Place one plant in a jar containing potassium hydroxide, which absorbs carbon dioxide.
3. Place the other plant in a jar without potassium hydroxide.
4. Keep both plants in sunlight.
5. Test their leaves for starch.

The leaf from the jar containing carbon dioxide turns blue-black, while the other leaf does not.

Conclusion

Carbon dioxide is necessary for photosynthesis.

20 Solved MCQs on Photosynthesis

1. What is the main function of photosynthesis?

A. Production of carbon dioxide
B. Preparation of food
C. Absorption of minerals
D. Removal of water

Answer: B. Preparation of food
Explanation: Green plants prepare glucose through photosynthesis.

2. Which pigment is essential for photosynthesis?

A. Haemoglobin
B. Melanin
C. Chlorophyll
D. Keratin

Answer: C. Chlorophyll
Explanation: Chlorophyll absorbs sunlight required for photosynthesis.

3. Which gas is used by plants during photosynthesis?

A. Oxygen
B. Nitrogen
C. Carbon dioxide
D. Hydrogen

Answer: C. Carbon dioxide
Explanation: Plants use carbon dioxide to prepare glucose.

4. Which gas is released during photosynthesis?

A. Oxygen
B. Carbon dioxide
C. Nitrogen
D. Methane

Answer: A. Oxygen
Explanation: Oxygen is released as a by-product.

5. The food produced during photosynthesis is initially stored as:

A. Protein
B. Starch
C. Fat
D. Cellulose

Answer: B. Starch
Explanation: Excess glucose is converted into starch for storage.

6. The tiny pores present on leaves are called:

A. Lenticels
B. Stomata
C. Veins
D. Root hairs

Answer: B. Stomata
Explanation: Stomata allow gaseous exchange.

7. Photosynthesis mainly takes place in:

A. Roots
B. Flowers
C. Green leaves
D. Fruits

Answer: C. Green leaves
Explanation: Green leaves contain chlorophyll and chloroplasts.

8. Which cell organelle contains chlorophyll?

A. Nucleus
B. Mitochondrion
C. Chloroplast
D. Ribosome

Answer: C. Chloroplast
Explanation: Chloroplasts are the main sites of photosynthesis.

9. Water is transported from roots to leaves through:

A. Phloem
B. Xylem
C. Stomata
D. Epidermis

Answer: B. Xylem
Explanation: Xylem carries water and minerals upward.

10. Food is transported from leaves through:

A. Xylem
B. Phloem
C. Stomata
D. Root hairs

Answer: B. Phloem
Explanation: Phloem transports prepared food to different parts.

11. Which of the following is not required for photosynthesis?

A. Sunlight
B. Chlorophyll
C. Carbon dioxide
D. Oxygen

Answer: D. Oxygen
Explanation: Oxygen is released during photosynthesis.

12. The product of photosynthesis is:

A. Glucose
B. Urea
C. Carbon dioxide
D. Nitrogen

Answer: A. Glucose
Explanation: Glucose is the food prepared by plants.

13. Iodine turns starch:

A. Red
B. Green
C. Blue-black
D. White

Answer: C. Blue-black
Explanation: Iodine is used to test the presence of starch.

14. Plants are called autotrophs because they:

A. Depend on animals for food
B. Prepare their own food
C. Cannot make food
D. Eat other plants

Answer: B. Prepare their own food
Explanation: Autotrophs make food from simple inorganic substances.

15. The energy required for photosynthesis comes from:

A. Soil
B. Water
C. Sunlight
D. Oxygen

Answer: C. Sunlight
Explanation: Sunlight supplies energy for converting raw materials into food.

16. Which substance absorbs carbon dioxide in an experiment?

A. Iodine
B. Potassium hydroxide
C. Alcohol
D. Limewater

Answer: B. Potassium hydroxide
Explanation: Potassium hydroxide absorbs carbon dioxide.

17. Photosynthesis helps maintain the balance of:

A. Oxygen and carbon dioxide
B. Nitrogen and hydrogen
C. Water and salt
D. Calcium and iron

Answer: A. Oxygen and carbon dioxide
Explanation: Plants use carbon dioxide and release oxygen.

18. The green colour of leaves is due to:

A. Starch
B. Chlorophyll
C. Glucose
D. Xylem

Answer: B. Chlorophyll
Explanation: Chlorophyll is a green pigment.

19. Which tissue transports prepared food?

A. Xylem
B. Phloem
C. Collenchyma
D. Sclerenchyma

Answer: B. Phloem
Explanation: Phloem carries food from leaves to other parts.

20. Photosynthesis converts solar energy into:

A. Sound energy
B. Chemical energy
C. Electrical energy
D. Mechanical energy

Answer: B. Chemical energy
Explanation: Solar energy is stored in glucose as chemical energy.

15 Solved Short-Answer Questions

1. Define photosynthesis.

Photosynthesis is the process by which green plants prepare glucose from carbon dioxide and water in the presence of sunlight and chlorophyll, releasing oxygen as a by-product.

2. Why are green plants called autotrophs?

Green plants are called autotrophs because they prepare their own food from carbon dioxide and water using sunlight and chlorophyll.

3. What is the role of chlorophyll?

Chlorophyll absorbs light energy from the Sun. This energy is used to prepare food during photosynthesis.

4. What are stomata?

Stomata are microscopic pores present on the surface of leaves. They help in the exchange of gases and transpiration.

5. Name the raw materials needed for photosynthesis.

The raw materials required are carbon dioxide and water. Sunlight and chlorophyll are essential conditions for the process.

6. What is the role of sunlight in photosynthesis?

Sunlight provides the energy required to convert carbon dioxide and water into glucose.

7. What happens to excess glucose in plants?

Excess glucose is converted into starch and stored in leaves, roots, stems, fruits and seeds.

8. Why are leaves called food factories?

Leaves are called food factories because they contain chlorophyll and prepare food through photosynthesis.

9. How does carbon dioxide enter a leaf?

Carbon dioxide enters a leaf through tiny pores called stomata.

10. How does water reach the leaves?

Water absorbed by the roots reaches the leaves through xylem vessels.

11. Name the green pigment present in plants.

The green pigment present in plants is called chlorophyll.

12. Why is photosynthesis important for animals?

Photosynthesis provides food directly or indirectly to animals and releases oxygen required for respiration.

13. What is the test for starch?

To test for starch, iodine solution is added to a leaf. If starch is present, the leaf turns blue-black.

14. What is the chemical equation for photosynthesis?

$6C{O}_2+6H_{2}O\stackrel{\text{Sunlight, Chlorophyll}}{\to }{C}_6{H}_{12}{O}_6+6O_2$

15. What is the difference between a product and a raw material in photosynthesis?

Carbon dioxide and water are raw materials because they are used in the process. Glucose and oxygen are products because they are formed during photosynthesis.

12 Solved Long-Answer Questions

1. Explain the process of photosynthesis.

Photosynthesis is the process by which green plants prepare food. Roots absorb water and minerals from the soil. Water is transported to the leaves through xylem vessels. Carbon dioxide enters the leaves through stomata. Chlorophyll absorbs sunlight and traps its energy. This energy is used to convert carbon dioxide and water into glucose. Oxygen is released into the atmosphere through the stomata. The excess glucose is converted into starch and stored.

2. Explain the importance of photosynthesis.

Photosynthesis is important because it is the main source of food and energy for living organisms. Green plants prepare their own food, and animals obtain food directly or indirectly from plants. The process releases oxygen, which is required for respiration. It also removes carbon dioxide from the atmosphere and helps maintain the balance of gases. Photosynthesis stores solar energy in the form of chemical energy in glucose. It also provides materials needed for forming starch, cellulose, fats and proteins.

3. Describe an experiment to show that sunlight is necessary for photosynthesis.

A healthy potted plant is kept in darkness for 24–48 hours to remove stored starch. A portion of one leaf is covered with black paper on both sides. The plant is then placed in sunlight for a few hours. The leaf is removed, boiled in water and heated in alcohol using a water bath to remove chlorophyll. After washing, iodine solution is added.

The uncovered portion turns blue-black because it has formed starch. The covered portion does not turn blue-black because it did not receive sunlight. This proves that sunlight is necessary for photosynthesis.

4. Describe an experiment to show that chlorophyll is necessary for photosynthesis.

A variegated leaf containing green and non-green areas is selected. The plant is placed in sunlight for a few hours. The leaf is then tested for starch using iodine solution. The green portions turn blue-black, while the non-green portions remain unchanged.

The green areas contain chlorophyll and produce starch. The non-green areas do not contain chlorophyll and do not produce starch. This proves that chlorophyll is necessary for photosynthesis.

5. Describe an experiment to show that carbon dioxide is necessary for photosynthesis.

Two similar plants are first kept in darkness to remove stored starch. One plant is placed inside a bell jar containing potassium hydroxide, which absorbs carbon dioxide. The other plant is placed in a bell jar without potassium hydroxide. Both plants are kept in sunlight for a few hours. Their leaves are then tested for starch.

The leaf from the jar containing carbon dioxide turns blue-black. The leaf from the jar containing potassium hydroxide does not turn blue-black. This proves that carbon dioxide is necessary for photosynthesis.

6. Explain the role of stomata in photosynthesis.

Stomata are tiny pores present mainly on the lower surface of leaves. They allow carbon dioxide to enter the leaf for photosynthesis. Oxygen produced during photosynthesis passes out through these pores. Stomata also help in transpiration, which is the loss of water vapour from leaves. Guard cells control the opening and closing of stomata. Thus, stomata regulate gas exchange and water loss.

7. Explain how water and minerals reach the leaves.

Root hairs absorb water and minerals from the soil. The absorbed water enters the xylem vessels present in the roots. Xylem vessels transport water and minerals upward through the stem to the leaves. The movement is helped by root pressure, capillary action and transpiration pull. Water reaching the leaves becomes available for photosynthesis.

8. Explain the factors affecting the rate of photosynthesis.

The rate of photosynthesis is affected by light intensity, carbon dioxide concentration, temperature, water availability and chlorophyll content. An increase in light intensity increases photosynthesis up to a particular limit. More carbon dioxide can increase the rate up to an optimum concentration. Very high or very low temperatures reduce the rate because photosynthesis is controlled by enzymes. Lack of water causes stomata to close, reducing carbon dioxide entry. Plants with healthy green leaves generally photosynthesise more efficiently.

9. Differentiate between photosynthesis and respiration.

Photosynthesis is a food-making process, whereas respiration is a food-breaking process. Photosynthesis occurs mainly in green plants and requires sunlight and chlorophyll. Respiration occurs in all living cells and does not directly require sunlight. Photosynthesis uses carbon dioxide and releases oxygen. Respiration usually uses oxygen and releases carbon dioxide. Photosynthesis stores energy in glucose, while respiration releases energy from glucose.

10. Explain what happens to the glucose produced during photosynthesis.

The glucose produced during photosynthesis is used in several ways. Some glucose is immediately broken down during respiration to release energy. Extra glucose is converted into starch and stored in roots, stems, fruits and seeds. Glucose is also used to form cellulose, which makes plant cell walls. Plants use glucose to produce fats and oils. With the help of nitrogen and other minerals, glucose contributes to the formation of proteins.

11. Why is photosynthesis essential for life on Earth?

Photosynthesis is essential because it provides food and oxygen. Plants form the first level of most food chains, so nearly all animals depend on them directly or indirectly. Oxygen released during photosynthesis is used by living organisms for respiration. The process also absorbs atmospheric carbon dioxide and helps regulate the climate. It stores solar energy in chemical form and supports the growth and survival of plants, animals and human beings.

12. Explain why a plant kept in darkness cannot perform photosynthesis properly.

A plant kept in darkness does not receive light energy. Chlorophyll cannot trap solar energy, and the reactions required to prepare glucose cannot proceed effectively. As a result, the plant cannot produce enough food through photosynthesis. If darkness continues for a long time, the plant uses its stored starch for respiration. This may cause weak growth, yellowing of leaves and eventual death.

Important Revision Points

• Photosynthesis occurs mainly in green leaves.
• Chlorophyll absorbs sunlight.
• Carbon dioxide enters through stomata.
• Water is absorbed by roots.
• Xylem transports water.
• Phloem transports prepared food.
• Glucose is the main food product.
• Excess glucose is stored as starch.
• Oxygen is released during photosynthesis.
• Iodine turns starch blue-black.
• Sunlight, chlorophyll, carbon dioxide and water are essential for photosynthesis.
• Photosynthesis maintains the balance of oxygen and carbon dioxide.

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