Chapter – 8 : Plant Physiology | Chapter Solution Class 9

(c) Photosynthesis

Explanation: During photosynthesis, green plants convert light energy into chemical energy, which is stored in glucose molecules. This process occurs in the chloroplasts and is essential for the survival of plants and oxygen production.

(a) Photosynthesis

Explanation: Photosynthesis is the process by which green plants absorb CO₂ from the atmosphere and release O₂ as a byproduct while synthesizing glucose.

(c) Photolysis of water

Explanation: In the light-dependent phase of photosynthesis, photolysis of water occurs, where light energy splits water molecules into hydrogen and oxygen.

(b) Entrapping of solar energy

Explanation: Chlorophyll absorbs light energy from the sun and initiates the photosynthetic process by exciting electrons.

(d) Photolysis of water

Explanation: The oxygen released during photosynthesis comes from water molecules (H₂O) that are split during photolysis.

(c) Photolysis

Explanation: Photolysis of water is a crucial step in photosynthesis, as it provides electrons for the light-dependent reactions and releases oxygen as a byproduct.

(a) Iron and Magnesium

Explanation: Magnesium is the central atom in the chlorophyll molecule, and iron is required for its synthesis.

(a) Glucose

Explanation: Glucose (C₆H₁₂O₆) is the primary and first visible product of photosynthesis, which can later be converted into starch, sucrose, or other organic compounds.

(d) None of the above

Explanation: The oxygen released during photosynthesis comes from water (H₂O), not CO₂.

(a) Endothermic process & (c) Anabolic process

Explanation: Photosynthesis is an anabolic process as it builds up glucose molecules, and it is endothermic because it absorbs solar energy.

(d) All the above

Explanation: Chlorophyll, CO₂, and water are essential for photosynthesis to take place.

(a) Glucose

Explanation: Glucose is a product of photosynthesis, not a requirement.

(a) Water

Explanation: The oxygen in the glucose molecule comes from CO₂, but the oxygen released as a byproduct comes from H₂O.

Red and Blue light

Explanation: Photosynthesis is most effective in red and blue wavelengths, as these are best absorbed by chlorophyll.

(b) Magnesium

Explanation: Magnesium (Mg²⁺) is the central atom in the chlorophyll molecule.

(a) C₅₅H₇₂O₅N₄Mg

(d) C₅₅H₇₂O₆N₄Mg

(c) Excitation of chlorophyll a molecule by the photonic energy

Explanation:

When sunlight strikes chlorophyll molecules, they get excited, leading to photolysis of water and ATP formation.

(a) H₂O

Explanation: Oxygen comes from the photolysis of water.

(c) Photosynthesis

Explanation: Photosynthesis is the process in which green plants synthesize carbohydrates (glucose) using carbon dioxide, water, and sunlight, releasing oxygen as a byproduct.

(c) Both of the above

Explanation: During the light reaction, ATP and NADPH (NADPH + H⁺) are formed, which are later used in the dark phase of photosynthesis to synthesize glucose.

(d) Nitrogen

Explanation: Chlorophyll, water, and CO₂ are essential for carbohydrate synthesis, but nitrogen is not directly involved in the process of photosynthesis.

(c) Chloroplastid

Explanation: Chloroplasts contain chlorophyll, which is responsible for capturing solar energy and converting it into chemical energy during photosynthesis.

(d) All chlorophyll-containing living cells

Explanation: Photosynthesis occurs in cells that contain chlorophyll, primarily in the leaves and sometimes in green stems. It does not take place in non-photosynthetic parts like roots.

(c) ATP

Explanation: ATP (Adenosine Triphosphate) is the energy currency of the cell, providing the necessary energy for various biochemical reactions, including the dark phase of photosynthesis.

(c) Phosphoglyceric acid

Explanation: The first stable product in the Calvin cycle (dark reaction) is 3-phosphoglyceric acid (PGA), which is formed after CO₂ fixation by RuBP (Ribulose Bisphosphate).

(b) Light phase of photosynthesis

Explanation: The Hill reaction refers to the light-dependent reaction in which water molecules are split (photolysis) to release oxygen, protons, and electrons, which help in ATP and NADPH formation.

(b) Photophosphorylation

Explanation: ATP is synthesized from ADP and inorganic phosphate in the presence of light, a process called photophosphorylation, which occurs during the light phase of photosynthesis.

(c) Phosphoglyceraldehyde is synthesized

Explanation: During the dark phase (Calvin cycle), CO₂ is fixed, and phosphoglyceraldehyde (PGAld) is synthesized using ATP and NADPH from the light reaction.

(b) Solarization

Explanation: Solarization refers to the inhibition of photosynthesis due to excessive light intensity, which leads to chlorophyll degradation and reduced efficiency.

(b) Phosphoglyceric acid

Explanation: The first product formed in the Calvin cycle is 3-phosphoglyceric acid (PGA), which is further converted into other carbohydrates.

(c) Chlorophyll

Explanation: Chlorophyll is the main photosynthetic pigment that absorbs light and drives the process of photosynthesis.

(a) Photophosphorylation

Explanation: During photosynthesis, ATP is formed from ADP and inorganic phosphate (Pi) in the presence of light, which is termed photophosphorylation.

(c) Photon

Explanation: Photons are quantum particles of light energy that excite chlorophyll molecules and drive the photosynthetic process.

(b) Photosynthesis

Explanation: Photosynthesis is the process through which green plants produce carbohydrates (glucose) using CO₂, water, and light energy.

(d) Photosynthesis

Explanation: Photosynthesis plays a crucial role in maintaining atmospheric balance by absorbing CO₂ and releasing O₂.

(a) Anabolic

Explanation: Photosynthesis is an anabolic process because it involves the formation of complex organic molecules (glucose) from simpler substances (CO₂ and H₂O).

(b) CO₂

Explanation: The oxygen present in the glucose molecule originates from CO₂, while the oxygen released as a byproduct comes from H₂O.

(c) Utilizes assimilatory power formed in light reaction

Explanation: The dark reaction (Calvin cycle) does not require direct light but uses ATP and NADPH produced in the light reaction to fix CO₂ and form glucose.

(c) RuBP

Explanation: RuBP (Ribulose Bisphosphate) is the first acceptor of CO₂ in the Calvin cycle, forming an unstable 6-carbon compound that quickly splits into two molecules of PGA.

(c) Excitation of an electron of chlorophyll a by a photon of light

Explanation: The first step in photosynthesis is the absorption of light energy by chlorophyll molecules, leading to the excitation of electrons. These excited electrons initiate the light-dependent reactions.

(d) All of the above

Explanation: Photosynthesis is a vital biological process because it converts solar energy into chemical energy (organic matter), releases oxygen into the atmosphere, and connects the physical world (light energy) with the biological world (life processes).

(b) Red light

Explanation: Photosynthesis is most efficient in red and blue wavelengths of light because chlorophyll absorbs these colors best. Green light is least effective because it is mostly reflected by plants.

(c) Excitation of chlorophyll molecules

Explanation: The first step in photosynthesis is the absorption of sunlight by chlorophyll molecules, which excites electrons and starts the light-dependent reactions.

(d) All of the above

Explanation: Photosynthesis is essential for oxygen production, energy conversion, and the maintenance of the balance between the physical and biological world. It is the foundation of the food chain.

(b) RuBP

Explanation: Ribulose Bisphosphate (RuBP) is the primary acceptor of CO₂ in the Calvin cycle (dark reaction). It reacts with CO₂ to form an unstable 6-carbon compound, which then splits into two molecules of PGA (Phosphoglyceric acid).

chlorophyll

Explanation: Chlorophyll is the green pigment found in plants that absorbs light energy required for photosynthesis. It plays a crucial role in capturing solar energy and converting it into chemical energy.

Photosynthesis

Explanation: An anabolic process refers to a biochemical reaction in which simpler substances are built into more complex molecules. Photosynthesis is anabolic because it synthesizes glucose from carbon dioxide and water.

chloroplast

Explanation: Chloroplasts are the organelles in plant cells where photosynthesis occurs. They contain chlorophyll and other pigments that help in capturing light energy.

magnesium

Explanation: Magnesium is the central atom in the chlorophyll molecule. It plays a key role in absorbing light energy and transferring electrons in the photosynthetic process.

phosphorylation

Explanation: Phosphorylation is the process by which an inorganic phosphate (Pi) is added to ADP to form ATP. This process occurs during photosynthesis and cellular respiration, providing energy for various biological activities.

water (H₂O)

Explanation: The oxygen released during photosynthesis comes from the photolysis (splitting) of water molecules in the light reaction, not from carbon dioxide.

photons

Explanation: Photons are tiny packets of light energy that travel from the sun and are absorbed by chlorophyll to excite electrons, initiating the light-dependent reactions of photosynthesis.

phosphoglyceric acid (PGA)

Explanation: The first stable compound formed in the Calvin cycle (dark reaction) is PGA, which is later converted into glucose and other organic molecules.

photosynthesis

Explanation: Photosynthesis removes CO₂ from the air and releases O₂, maintaining the atmospheric balance necessary for life on Earth.

light reaction, dark reaction

Explanation: The light reaction occurs in the thylakoids of chloroplasts and involves the capture of light energy, ATP synthesis, and water splitting. The dark reaction (Calvin cycle) occurs in the stroma and uses ATP and NADPH to fix carbon dioxide into glucose.

anabolic

Explanation: Anabolic processes involve the building of complex molecules from simpler ones. In photosynthesis, carbon dioxide and water are converted into glucose, making it an anabolic process.

Light

Explanation: Light provides the energy required for the process of photosynthesis, where CO₂ and water are converted into glucose and oxygen.

light reaction

Explanation: Photolysis, or the splitting of water molecules, occurs in the light reaction. It provides electrons for the electron transport chain and releases oxygen as a byproduct.

spectrophotometer

Explanation: A spectrophotometer is a scientific instrument that measures the amount of light absorbed by pigments like chlorophyll at different wavelengths, helping scientists understand the efficiency of photosynthesis.

photorespiration

Explanation: When oxygen concentration is high, it competes with CO₂ for the enzyme RuBisCO, leading to a process called photorespiration, which reduces photosynthetic efficiency by consuming energy without producing glucose.

solarization

Explanation: Excessive light intensity can damage chlorophyll molecules, reducing their ability to absorb light and leading to a decline in photosynthesis. This process is known as solarization.

True

Explanation: The light reaction takes place in the grana (thylakoid membranes) of chloroplasts, where light energy is captured by chlorophyll and converted into ATP and NADPH.

True

Explanation: The dark reaction (Calvin cycle) takes place in the stroma of chloroplasts, where ATP and NADPH from the light reaction are used to fix carbon dioxide into glucose.

False

Explanation: Photosynthesis is an anabolic process because it involves the synthesis of complex organic molecules (glucose) from simpler substances (CO₂ and H₂O).

True

Explanation: Photophosphorylation is the process of ATP formation using light energy during the light reaction of photosynthesis.

True

Explanation: Photons are packets of light energy that excite electrons in chlorophyll molecules, initiating the process of photosynthesis.

True

Explanation: Photophosphorylation occurs in the light reaction when light energy is used to generate ATP from ADP and inorganic phosphate.

False

Explanation: Photosynthesis occurs in two main phases—light reaction and dark reaction. There is no third phase.

True

Explanation: Carotenoids are accessory pigments in photosynthesis that help absorb light energy and protect chlorophyll from damage.

False

Explanation: Oxygen released during photosynthesis comes from water (H₂O), not from CO₂. This process occurs in the light reaction during photolysis.

True

Explanation: ATP and NADPH are the energy-rich molecules produced during the light-dependent reaction and used in the dark reaction for glucose synthesis.

False

Explanation: The Calvin cycle (dark reaction) occurs in the absence of light, using ATP and NADPH produced in the light reaction to fix CO₂ into glucose.

False

Explanation: CO₂ is absorbed during the dark reaction (Calvin cycle), not during the light reaction.

False

Explanation: Magnesium (Mg) is the central metal ion in chlorophyll, not iron.

True

Explanation: Magnesium (Mg) is the essential metal in the chlorophyll molecule, playing a role in light absorption and electron transport.

True

Explanation: The oxygen released in photosynthesis comes from the splitting of water molecules (photolysis) during the light reaction.

True

Explanation: Ribulose Bisphosphate (RuBP) is the primary CO₂ acceptor in the Calvin cycle, forming an unstable 6-carbon compound that splits into two PGA molecules.

False

Explanation: The first stable product in the Calvin cycle is 3-phosphoglyceric acid (PGA), not phosphoglyceraldehyde (PGAld).

False

Explanation: The Hill reaction is a part of the light-dependent reaction, where water is split to release oxygen, electrons, and protons.

False

Explanation: The correct formula of chlorophyll b is C₅₅H₇₂O₆N₄Mg, not C₅₅H₇₂O₅N₄Mg.

False

Explanation: Photophosphorylation is an essential process in the light-dependent reaction, where ATP is synthesized using light energy.

The term photosynthesis is derived from two Greek words:

• “Photo” meaning light
• “Synthesis” meaning putting together

Photosynthesis

• Chlorophyll a
• Chlorophyll b

Carbon dioxide (CO₂)

Chloroplast

• Light reaction occurs in the thylakoid membranes (grana) of the chloroplast.
• Dark reaction (Calvin cycle) occurs in the stroma of the chloroplast.

Quantasome

Opuntia (Prickly Pear Cactus)

• RuBP: Ribulose-1,5-Bisphosphate
• PGA: Phosphoglyceric Acid

Dark reaction (Calvin cycle)

Photosynthesis is a process in which green plants, algae, and some bacteria synthesize organic compounds (glucose) using carbon dioxide, water, and sunlight in the presence of chlorophyll, releasing oxygen as a byproduct.

Chemical equation:

6CO₂ + 12H₂O → (Sunlight, Chlorophyll) → C₆H₁₂O₆ + 6O₂ + 6H₂O

Photosynthesis is called an anabolic process because it involves the synthesis of complex organic molecules (glucose) from simpler molecules (CO₂ and H₂O). It is a constructive process that results in an increase in the dry weight of the plant.

• Chlorophyll a: C₅₅H₇₂O₅N₄Mg
• Chlorophyll b: C₅₅H₇₂O₆N₄Mg

Photophosphorylation is the process of ATP formation from ADP and inorganic phosphate (Pi) using light energy during the light reaction of photosynthesis. It occurs in two forms:

• Cyclic photophosphorylation
• Non-cyclic photophosphorylation

The accessory pigments that help in photosynthesis are:

1. Carotenoids (Carotene and Xanthophyll)
2. Phycobilins (Phycocyanin and Phycoerythrin)

These pigments assist in capturing additional light energy and transferring it to chlorophyll for photosynthesis.

Photolysis of water is the splitting of water molecules into oxygen, protons (H⁺), and electrons under the influence of light during the light reaction of photosynthesis.

Reaction: 2H₂O → 4H⁺ + 4e⁻ + O₂

The chemical energies produced in the light reaction are:

1. ATP (Adenosine Triphosphate)
2. NADPH (Nicotinamide Adenine Dinucleotide Phosphate, reduced form)

• RuBP: Ribulose-1,5-Bisphosphate
• PGA: Phosphoglyceric Acid

The first stable product of photosynthesis is 3-Phosphoglyceric Acid (PGA), which is formed in the dark reaction (Calvin cycle) after the fixation of CO₂ by RuBP.

The Hill reaction is a light-dependent reaction where electrons are transferred from water to electron acceptors, resulting in the evolution of oxygen. This reaction proves that oxygen released during photosynthesis comes from water, not CO₂.

Photosynthesis absorbs CO₂ from the atmosphere and releases O₂, while respiration and combustion consume O₂ and release CO₂. This cycle helps maintain the CO₂ – O₂ balance in the environment.

Photosynthesis is an anabolic process because it synthesizes complex organic molecules (glucose) from simpler substances (CO₂ and H₂O).

The photosynthetic unit is known as Quantasome, which is present in the thylakoid membrane of chloroplasts. It consists of multiple chlorophyll molecules and is responsible for absorbing light energy.

Solarization is the inhibition of photosynthesis due to excessive light intensity, which leads to the destruction of chlorophyll and reduces the efficiency of photosynthesis.

The functions of carotenoids are:

• It absorb light energy and transfer it to chlorophyll.
• It protect chlorophyll from oxidative damage due to excess light (acting as antioxidants).

The photosynthetic quotient (PQ) is the ratio of oxygen evolved to carbon dioxide absorbed during photosynthesis.

PQ = O₂ evolved / CO₂ absorbed

For normal photosynthesis, PQ = 1.

The end products of the light reaction are:

1. ATP (Energy storage molecule)
2. NADPH (Reducing power)
3. O₂ (Byproduct, released into the atmosphere)

The end product of the dark reaction is glucose (C₆H₁₂O₆), which is synthesized using ATP and NADPH from the light reaction.

• PGAld: Phosphoglyceraldehyde
• RuMP: Ribulose Monophosphate

The essential components required for photosynthesis are:

1. Light (Sunlight)
2. Chlorophyll
3. Water (H₂O)
4. Carbon dioxide (CO₂)

The role of chlorophyll pigments in photosynthesis are:

1. It absorbs light energy and initiate the light reaction.
2. It converts solar energy into chemical energy by exciting electrons.

Where: Photosynthesis takes place in the chloroplasts of green plant cells.

When: It occurs only in the presence of sunlight during the day.

A photon is a quantum particle of light energy that excites chlorophyll molecules to start the photosynthetic process.

Photolysis is the splitting of water molecules (H₂O) by light energy in the presence of chlorophyll during the light reaction. It produces oxygen, protons (H⁺), and electrons.

Reaction: 2H₂O → 4H⁺ + 4e⁻ + O₂

The significance of photosynthesis are:

1. Photosynthesis produces oxygen, essential for all aerobic organisms.
2. Photosynthesis converts solar energy into chemical energy, stored in glucose.
3. Photosynthesis maintains CO₂ – O₂ balance in the atmosphere.
4. Photosynthesis forms the base of the food chain, supporting all life on Earth.

The light reaction is the first phase of photosynthesis, taking place in the grana of the chloroplasts in the presence of sunlight. It is responsible for converting light energy into chemical energy, which is stored in the form of ATP and NADPH₂. These energy-rich molecules are later used in the dark phase of photosynthesis.

During the light reaction, chlorophyll absorbs light energy, leading to the excitation of electrons. These excited electrons pass through electron carriers, resulting in ATP formation through photophosphorylation. Another important event in the light reaction is the photolysis of water, where water molecules are split into oxygen, protons (H⁺), and electrons. The oxygen is released as a byproduct, while the hydrogen is used in the dark reaction.

The major steps of the light reaction include:

1. Activation of chlorophyll: Light energy excites chlorophyll molecules.
2. Photolysis of water: Water molecules split, releasing oxygen.
3. Electron transport: Excited electrons pass through carriers, producing ATP and NADPH₂.
4. Formation of NADPH₂: NADP⁺ accepts electrons and protons, forming NADPH₂.

The end products of the light reaction are ATP, NADPH₂, and O₂, which provide energy and reducing power for the next phase, the dark reaction.

Carbon assimilation refers to the process by which plants absorb carbon dioxide (CO₂) from the atmosphere and incorporate it into organic molecules like glucose during photosynthesis. This process takes place in the dark reaction, also known as the biosynthetic phase.

The dark reaction occurs in the stroma of the chloroplasts and does not require direct light. However, it relies on ATP and NADPH₂ produced during the light reaction. The dark reaction involves the fixation of CO₂ into organic compounds through the Calvin cycle.

The steps of the dark reaction include:

1. CO₂ Fixation: CO₂ enters the chloroplast and combines with Ribulose bisphosphate (RuBP) in the presence of RuBisCO enzyme, forming an unstable 6-carbon compound.
2. Formation of Phosphoglyceric Acid (PGA): The unstable 6-carbon compound breaks into two molecules of Phosphoglyceric acid (PGA).
3. Reduction Phase: Using ATP and NADPH₂, PGA is converted into Phosphoglyceraldehyde (PGAld).
4. Glucose Synthesis:Some PGAld molecules are used to synthesize glucose, while others regenerate RuBP for the next cycle.

The light reaction of photosynthesis occurs in the grana of the chloroplasts and is carried out by two pigment systems, known as Photosystem I (PSI) and Photosystem II (PSII). These photosystems are composed of chlorophyll molecules and accessory pigments, which work together to absorb and convert light energy into chemical energy.

In Photosystem II (PSII), light energy excites P680 chlorophyll molecules, which release high-energy electrons. These electrons are transferred to an electron acceptor and replaced by electrons from photolysis of water, which releases oxygen as a byproduct. The electrons then pass through the electron transport chain, generating ATP through photophosphorylation.

In Photosystem I (PSI), light energy excites P700 chlorophyll molecules, leading to the release of electrons that combine with NADP⁺ to form NADPH₂.

The products of the light reaction—ATP and NADPH₂—are used in the dark reaction to synthesize glucose.

Photosynthetic pigments are substances that absorb light energy and play a crucial role in photosynthesis.

The main pigments involved in photosynthesis are:

1. Chlorophyll a (C₅₅H₇₂O₅N₄Mg): This is the primary pigment responsible for capturing light energy and initiating photosynthesis. It absorbs blue and red light and reflects green light.
2. Chlorophyll b (C₅₅H₇₂O₆N₄Mg): This accessory pigment helps capture additional light energy and transfers it to chlorophyll a.
3. Carotenoids (Carotene and Xanthophylls): These yellow, orange, and red pigments assist in absorbing light and protect chlorophyll from damage.

Photosynthesis is a complex process that requires four essential components:

1. Light Energy: Sunlight is the primary source of energy for photosynthesis.
2. Chlorophyll: The green pigment present in chloroplasts absorbs light and initiates the photochemical reactions.
3. Water (H₂O): It is split into hydrogen and oxygen during the light reaction, providing electrons and protons.
4. Carbon Dioxide (CO₂): This gas is absorbed from the atmosphere and used to synthesize glucose in the dark reaction.

These components work together to convert solar energy into stored chemical energy in the form of glucose.

Photosynthesis is an essential life-sustaining process that has significant importance:

• Oxygen Production: It releases oxygen into the atmosphere, which is necessary for respiration.
• Food Source: It provides glucose, which serves as the primary energy source for all living organisms.
• Maintains CO₂ – O₂ Balance: Photosynthesis absorbs CO₂ and releases O₂, balancing atmospheric gases.
• Energy Storage: It converts sunlight into chemical energy, which is stored in glucose molecules and used by organisms.

Without photosynthesis, life on Earth would not be possible, as it forms the foundation of the food chain.

The conversion of light energy into chemical energy occurs during the light reaction of photosynthesis. When sunlight is absorbed by chlorophyll molecules, electrons gain energy and move to a higher energy state. These high-energy electrons pass through the electron transport chain, leading to the synthesis of ATP and NADPH₂.

Additionally, water molecules undergo photolysis, releasing oxygen, protons, and electrons. The protons contribute to ATP formation, while NADPH₂ carries hydrogen atoms for the dark reaction.

Thus, light energy is stored in ATP and NADPH₂, which are later used in the dark reaction to synthesize glucose.

The dark reaction, also known as the Calvin cycle, is a crucial phase of photosynthesis that occurs in the stroma of chloroplasts. It does not require light directly, but it relies on the products of the light reactions (ATP and NADPH). Here’s a brief outline of the process:

1. Carbon Fixation: CO2 from the atmosphere is captured by ribulose bisphosphate (RuBP) through the enzyme RuBisCO, forming a 6-carbon compound that quickly splits into two molecules of 3-phosphoglycerate (3-PGA).
2. Reduction Phase: ATP and NADPH generated in the light reactions are used to convert 3-PGA into glyceraldehyde-3-phosphate (G3P). This involves phosphorylation and reduction reactions.
3. Regeneration of RuBP: A portion of G3P is used to regenerate RuBP, enabling the cycle to continue. This step requires ATP and ensures that the cycle can process more CO2.
4. Production of Glucose: The remaining G3P molecules can be used to synthesize glucose and other carbohydrates, which serve as energy sources for the plant and other organisms.