Photosynthesis and Respiration- Chapter 4 (Class 7 - Concise Biology Selina) ICSE

 

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Photosynthesis and Respiration- Chapter 4  (Class 7 - Concise Biology Selina) ICSE

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Photosynthesis and Respiration- Chapter 4

(Class 7 - Concise Biology Selina) ICSE

 

A Brief Notes :

Photosynthesis

Photosynthesis is a crucial biological process that occurs in the chloroplasts of plant cells, specifically within the chlorophyll-containing thylakoids. During this process, light energy is absorbed and used to convert carbon dioxide (CO₂) and water (H₂O) into glucose (C₆H₁₂O₆) and oxygen (O₂). Photosynthesis consists of two main stages: the light-dependent reactions and the Calvin cycle (light-independent reactions). In the light-dependent reactions, which take place in the thylakoid membrane, light energy is used to produce ATP and NADPH. These energy-rich molecules then fuel the Calvin cycle in the stroma, where CO₂ is fixed into glucose. Photosystem I and II play key roles in the electron transport chain, ultimately leading to the production of oxygen and the synthesis of glucose.

Respiration

Respiration is a metabolic process that occurs in the mitochondria of cells, where glucose (C₆H₁₂O₆) is broken down to release energy in the form of ATP. This process can be divided into three main stages: glycolysis, the Krebs cycle (citric acid cycle), and the electron transport chain. During glycolysis, glucose is converted into pyruvate, which then enters the mitochondria and is converted into acetyl-CoA, entering the Krebs cycle. Here, NADH and FADH₂ are produced, which carry electrons to the electron transport chain. In the final stage, oxidative phosphorylation occurs, where the energy from electrons is used to produce a large amount of ATP. Oxygen (O₂) is the final electron acceptor, combining with electrons and hydrogen ions to form water (H₂O). Respiration can occur aerobically, with oxygen, or anaerobically, through processes like fermentation, depending on oxygen availability.

Multiple Choice Questions

Question 1

Carbohydrates are stored by plants in the form of:

1. Starch
2. Proteins
3. Fats
4. Glucose

Answer

Starch

Reason — Glucose is formed in leaves very rapidly during the photosynthesis and it cannot be transported to other parts with the same rapidity. So, several molecules of glucose join together to form insoluble starch for temporary storage.

Question 2

Stomata are present on the surface of:

1. Leaves
2. Roots
3. Stem
4. Flower petals

Answer

Leaves

Reason — Stomata are present on the surface of leaves to facilitate exchange of gases between the leaf and the atmosphere.

Question 3

Which one of the following is the immediate end-product of photosynthesis?

1. Fructose
2. Glucose
3. Cellulose
4. Lactose

Answer

Glucose

Reason — Fructose, Cellulose and lactose are polymers of glucose.

Short Answer Questions

Question 1

Why do leaves generally look green?

Answer

Leaves generally look green because of the presence of chlorophyll, a pigment that imparts green colour to leaves.

Question 2

Which four of the following are needed for photosynthesis in a leaf?

1. Carbon dioxide
2. Oxygen
3. Nitrates
4. Water
5. Chlorophyll
6. Soil
7. Light

Answer

Following are the four things which are needed for photosynthesis in a leaf:

1. Carbon dioxide
2. Water
3. Chlorophyll
4. Light

Question 3

What is the source of energy for photosynthesis?

Answer

The source of energy for photosynthesis is Sunlight.

Question 4

Which gas is taken in and which one is given out by a green leaf in bright sunlight?

(i) Taken in: ............... (ii) Given out: ...............

Answer

(i) Taken in: Carbon dioxide (ii) Given out: Oxygen

Question 5

Suppose we compare the leaf with a factory, match the items in Column A with those in Column B.

Sl. No.	Column A	Column B
1.	Cells in the leaf	Raw materials
2.	Chloroplast	Power
3.	Sunlight	Machinery
4.	Oxygen and water	End product
5.	Carbon dioxide and water	By product
6.	Glucose	Work room

Answer

Sl. No.	Column A	Column B
1.	Cells in the leaf	Work room
2.	Chloroplast	Machinery
3.	Sunlight	Power
4.	Oxygen and water	By product
5.	Carbon dioxide and water	Raw materials
6.	Glucose	End product

Question 6

State whether the following statements are True or False:

(i) Green plants prepare their food by using two raw materials, oxygen and water.

(ii) The chlorophyll enables the plants to use light energy.

(iii) The free oxygen in the atmospheric air is the result of photosynthesis.

(iv) Photosynthesis occurs only in chlorophyll-containing parts of the plant.

Answer

(i) False
Corrected Statement — Green plants prepare their food by using two raw materials; Carbon dioxide and water.

(ii) True

(iii) True

(iv) True

Question 7

Differentiate between aerobic and anaerobic respiration. Write the overall chemical equations of the two kinds of respiration in plants.

(i) Aerobic: ...............

(ii) Anaerobic: ...............

Answer

Sl. No.	Aerobic Respiration	Anaerobic Respiration
1.	It utilizes oxygen.	It does not utilize oxygen.
2.	Food molecules (glucose) completely break down.	Food molecules (glucose) partially break down.
3.	Carbon dioxide and water are formed.	Ethanol along with carbon dioxide (in plants) and lactic acid (in animals) are formed.
4.	More energy is released (38 ATP molecules).	Less energy is released (2 ATP molecules).

Chemical equations of the two kinds of respiration in plants are:

(i) Aerobic:

Glucose + Oxygen →Enzymes Enzymes​ Carbon dioxide + water + Energy (38 ATP)

(ii) Anaerobic:

Glucose →Enzymes Enzymes​ Ethanol + Carbon dioxide + Energy (2 ATP)
                   (ATP- Adenosine Triphosphate)

Question 8

Explain how photosynthesis is different from respiration.

Answer

The differences between Photosynthesis and Respiration are explained below:

Sl. No.	Photosynthesis	Respiration
1.	Food is synthesized.	Food is broken down.
2.	Oxygen is released as a by-product.	Carbon dioxide is released as a by-product.
3.	Occurs in plant cells containing chlorophyll.	Occurs in all cells of both plants and animals.
4.	Occurs only during the day.	Occurs both during the day and the night.

Question 9

Do the plants respire all day and night or only during the day? Give reasons.

Answer

Plants respire all day and night as plants require a constant supply of energy for all the different processes that go on inside the cells of the plant body. Energy is required for the growth of the plant, for root cells to penetrate the soil and absorb water and minerals and so many other such activities. Thus, Respiration occurs continuously in plants, during both day and night.

Question 10

What happens to the energy liberated during respiration?

Answer

The energy liberated during respiration is utilized for all the different processes that go on inside the cells of the plant body. Energy is required for the growth of the plant, for root cells to penetrate the soil and absorb water and minerals.

Long Answer Questions

Question 1

In order to carry out photosynthesis, what are the substances that a plant must take in? Also mention their sources.

Answer

In order to carry out photosynthesis, the substances that a plant must take in are Carbon dioxide and Water. Photosynthesis also require the presence of Chlorophyll and Sunlight.

Following are their sources:

• Carbon dioxide — Atmosphere
• Water — Soil
• Sunlight — Sun
• Chlorophyll — contained in the leaves.

Question 2

What is the role of chlorophyll in photosynthesis?

Answer

Chlorophyll is found in mesophyll cells of leaf, inside the plastids called chloroplast. Chlorophyll is very important as it absorbs the sunlight which provides energy for the process of photosynthesis. This energy is used to split water molecules (H₂O) into hydrogen ions (H⁺) and hydroxyl ions (OH^-). Oxygen from the hydroxyl ion is released into the air and hydrogen ions combine with carbon dioxide to form glucose.

Question 3

Do plants need oxygen? If so, what is its source?

Answer

Plants need oxygen for respiration. The source of oxygen is atmosphere.

Photosynthesis:

Respiration:

Chlorophyll Chloroplast Light energy Carbon dioxide (CO2) Water (H2O) Oxygen (O2) Glucose (C6H12O6) Thylakoid Stroma Light-dependent reactions Calvin cycle (Light-independent reactions) ATP (Adenosine triphosphate) NADPH Photosystem I and II Electron transport chain Photophosphorylation Chlorophyll Chloroplast Light energy Carbon dioxide (CO2) Water (H2O) Oxygen (O2) Glucose (C6H12O6) Thylakoid Stroma Light-dependent reactions Calvin cycle (Light-independent reactions)

Mitochondria Glycolysis Krebs cycle (Citric acid cycle) Electron transport chain ATP NADH FADH2 Oxidative phosphorylation Pyruvate Acetyl-CoA Carbon dioxide (CO2) Oxygen (O2) Glucose (C6H12O6) Anaerobic respiration Aerobic respiration Fermentation

FAQ on Photosynthesis and Respiration

1. What is photosynthesis?

Answer: Photosynthesis is the process by which green plants, algae, and some bacteria convert light energy into chemical energy. During this process, these organisms use sunlight, carbon dioxide (CO₂), and water (H₂O) to produce glucose (C₆H₁₂O₆) and oxygen (O₂).

2. Where does photosynthesis occur?

Answer; Photosynthesis occurs in the chloroplasts of plant cells, specifically within the thylakoid membranes. The pigment chlorophyll, which captures light energy, is located in these chloroplasts.

3. What are the main stages of photosynthesis?

Answer; Photosynthesis consists of two main stages:

• Light-dependent reactions: These occur in the thylakoid membranes and convert light energy into chemical energy in the form of ATP and NADPH.
• Calvin cycle (Light-independent reactions): This takes place in the stroma of the chloroplasts, where ATP and NADPH are used to fix carbon dioxide (CO₂) into glucose.

4. What is respiration?

Answer; Respiration is the process by which cells break down glucose (C₆H₁₂O₆) and other molecules to produce energy in the form of ATP. It can be either aerobic (requiring oxygen) or anaerobic (without oxygen).

5. Where does respiration occur?

Answer; Respiration occurs primarily in the mitochondria of cells. The initial stage, glycolysis, occurs in the cytoplasm, while the later stages (Krebs cycle and electron transport chain) occur in the mitochondria.

6. What are the main stages of respiration?

Answer; Respiration involves three main stages:

• Glycolysis: Occurs in the cytoplasm, where glucose is broken down into pyruvate.
• Krebs cycle (Citric Acid Cycle): Takes place in the mitochondria, where pyruvate is further broken down to produce NADH and FADH₂.
• Electron Transport Chain (ETC): Located in the inner mitochondrial membrane, where energy from NADH and FADH₂ is used to produce ATP through oxidative phosphorylation.

7. How are photosynthesis and respiration related?

Answer; Photosynthesis and respiration are complementary processes. Photosynthesis converts light energy into chemical energy stored in glucose, while respiration breaks down glucose to release energy in the form of ATP. The oxygen produced during photosynthesis is used in respiration, and the carbon dioxide produced during respiration is used in photosynthesis.

8. What is the role of ATP in photosynthesis and respiration?

Answer; ATP (Adenosine Triphosphate) is the primary energy carrier in cells. In photosynthesis, ATP is produced during the light-dependent reactions and used in the Calvin cycle to synthesize glucose. In respiration, ATP is produced through glycolysis, the Krebs cycle, and oxidative phosphorylation to provide energy for various cellular activities.

9. What is the difference between aerobic and anaerobic respiration?

Answer;

• Aerobic respiration requires oxygen and produces a large amount of ATP (up to 38 molecules per glucose molecule). It includes glycolysis, the Krebs cycle, and the electron transport chain.
• Anaerobic respiration occurs without oxygen and produces a smaller amount of ATP (2 molecules per glucose molecule) through glycolysis and fermentation.

10. Why is photosynthesis important for life on Earth?

Answer; Photosynthesis is crucial because it produces oxygen, which is essential for the survival of most living organisms, and glucose, which serves as a primary energy source for plants and, indirectly, for other organisms through the food chain. It also helps regulate atmospheric carbon dioxide levels.

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