Photosynthesis Answer Key

 

SECTION A – (1 Mark Each)

1) Define Photosynthesis with balanced equation

1. Photosynthesis is the process by which green plants synthesize glucose from carbon dioxide and water using light energy.
2. Balanced Equation: 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O.

2) Difference between Chlorophyll-a and Chlorophyll-b

1. Chlorophyll-a contains a methyl (–CH₃) group.
2. Chlorophyll-b contains an aldehyde (–CHO) group.

3) Define Quantum Energy

1. Quantum energy is the minimum amount of energy required to excite an electron.
2. It is carried in the form of a photon of light.

4) Primary function of Accessory Pigments

1. They absorb light of different wavelengths.
2. They transfer absorbed energy to chlorophyll-a.

5) Name the ground substance of chloroplast

1. The ground substance is called stroma.
2. It contains enzymes required for the dark reaction.

6) Location of photosynthetic pigments

1. Photosynthetic pigments are present in the thylakoid membrane.
2. They are arranged in photosystems.

7) Chemical composition of Carotene

1. The chemical formula of carotene is C₄₀H₅₆.
2. It is an orange-red lipid soluble pigment.

8) Define Photolysis of water

1. Photolysis is the splitting of water molecules using light energy.
2. It produces oxygen, protons and electrons.

9) Site of Dark Reaction

1. The dark reaction occurs in the stroma of chloroplast.
2. It utilizes ATP and NADPH formed during light reaction.

10) First stable product of Calvin Cycle

1. The first stable product is 3-phosphoglyceric acid (3-PGA).
2. It is a three-carbon compound.

11) Define Photophosphorylation

1. Photophosphorylation is the formation of ATP in the presence of light.
2. It occurs in the thylakoid membrane during light reaction.

12) What are Scotoactive Stomata?

1. Scotoactive stomata open during night time.
2. They are found in CAM plants.

13) Reaction centre chlorophyll molecules

1. P680 is the reaction centre of Photosystem II.
2. P700 is the reaction centre of Photosystem I.

14) First 4-carbon product in Hatch-Slack pathway

1. Oxaloacetic acid (OAA) is the first stable product.
2. It is formed in mesophyll cells.

15) Two components of Chlorophyll

1. Porphyrin ring (light-absorbing head).
2. Phytol tail (membrane-anchoring tail).

16) Enzyme fixing CO₂ in C₄ plants

1. The enzyme is PEP carboxylase.
2. It fixes carbon dioxide in mesophyll cells.

17) First step of Calvin Cycle

1. The first step is carboxylation.
2. Carbon dioxide combines with RuBP in presence of RuBisCO.

18) Define Action Spectrum

1. Action spectrum shows the rate of photosynthesis at different wavelengths.
2. It indicates the most effective light for photosynthesis.

19) Percentage of chloroplast protein as RuBisCO

1. RuBisCO constitutes about 16% of chloroplast protein.
2. It is the most abundant enzyme in plants.

20) Example of CAM plant

1. Kalanchoe is an example of CAM plant.
2. It fixes carbon dioxide at night.

21) Optimum temperature for photosynthesis

1. The optimum temperature is about 25–30°C.
2. At this temperature the rate of photosynthesis is maximum.

22) Two important products of Light Reaction

1. ATP is produced to supply energy.
2. NADPH₂ is produced to provide reducing power.

23) Define Kranz Anatomy

1. Kranz anatomy is a special leaf structure found in C₄ plants.
2. Bundle sheath cells surround the vascular bundles.

24) State Blackman’s Law

1. The rate of photosynthesis is controlled by the slowest factor.
2. The limiting factor determines the overall rate of the process.

25) Carbon lost in Photorespiration

1. Approximately 25% of fixed carbon is lost.
2. This reduces photosynthetic efficiency.

PHOTOSYNTHESIS - COMPLETE ONE SHOT ANSWER KEY

SECTION A (1 Mark Each – 2 Points Each)

1) Photosynthesis

1. Synthesis of glucose from carbon dioxide and water using light energy.
2. 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O.

2) Chlorophyll-a vs Chlorophyll-b

1. Chlorophyll-a has –CH₃ group.
2. Chlorophyll-b has –CHO group.

3) Quantum Energy

1. Minimum energy required to excite an electron.
2. Carried by a photon.

4) Accessory Pigments

1. Absorb different wavelengths of light.
2. Transfer energy to chlorophyll-a.

5) Ground Substance

1. Stroma of chloroplast.
2. Contains enzymes of dark reaction.

6) Pigment Location

1. Present in thylakoid membrane.
2. Organized into photosystems.

7) Carotene Formula

1. C₄₀H₅₆.
2. Orange-red pigment.

8) Photolysis

1. Splitting of water using light.
2. Releases oxygen, protons and electrons.

9) Dark Reaction Site

1. Occurs in stroma.
2. Calvin cycle takes place here.

10) First Stable Product

1. 3-Phosphoglyceric acid.
2. Three-carbon compound.

SECTION B (2 Marks – 4 Points Each)

Importance of Photosynthesis

1. Produces food for all living organisms.
2. Converts solar energy into chemical energy.
3. Releases oxygen for respiration.
4. Maintains atmospheric balance.

Grana vs Stroma

1. Grana are stacks of thylakoids.
2. Light reaction occurs in grana.
3. Stroma is fluid matrix of chloroplast.
4. Dark reaction occurs in stroma.

Chlorophyll Appearance

1. Absorbs red and blue light.
2. Reflects green light.
3. Shows red fluorescence in transmitted light.
4. Due to emission of absorbed energy.

SECTION C (3 Marks – 6 Points Each)

Absorption vs Action Spectrum

1. Absorption spectrum shows light absorbed by pigments.
2. Action spectrum shows rate of photosynthesis.
3. Absorption measured using instruments.
4. Action measured by oxygen evolution.
5. Both peak in red and blue region.
6. Spectra overlap each other.

Cyclic vs Non-cyclic Photophosphorylation

1. Cyclic involves only PS-I.
2. Non-cyclic involves PS-I and PS-II.
3. Cyclic produces only ATP.
4. Non-cyclic produces ATP and NADPH.
5. Cyclic does not release oxygen.
6. Non-cyclic releases oxygen.

SECTION D (5 Marks – 10 Points Each)

Calvin Cycle

1. Occurs in stroma.
2. First phase is carboxylation.
3. RuBP combines with CO₂.
4. 3-PGA is formed.
5. Reduction phase uses ATP.
6. NADPH is used for reduction.
7. 3-PGAL formed.
8. Glucose synthesized.
9. RuBP regenerated.
10. Cycle continues continuously.

Hatch-Slack Pathway

1. CO₂ fixed by PEP carboxylase.
2. Oxaloacetic acid formed.
3. Malate transported to bundle sheath.
4. CO₂ released in bundle sheath.
5. Calvin cycle operates there.
6. Pyruvate returns to mesophyll.
7. ATP consumed in regeneration.
8. Kranz anatomy present.
9. No photorespiration.
10. Highly efficient in tropical regions.

SECTION C (3 Marks Each – 6 Points Per Answer)

1) Distinguish between Absorption Spectrum and Action Spectrum

1. Absorption spectrum shows the amount of light absorbed by photosynthetic pigments.
2. Action spectrum shows the rate of photosynthesis at different wavelengths of light.
3. Absorption spectrum is obtained using a spectrophotometer.
4. Action spectrum is measured by observing oxygen evolution.
5. Absorption spectrum indicates pigment efficiency in absorbing light.
6. Action spectrum indicates effectiveness of light in driving photosynthesis.

2) Describe the structure and role of Reaction Centre

1. Reaction centre is present in the thylakoid membrane.
2. It contains special chlorophyll molecules P680 in PS-II and P700 in PS-I.
3. It is surrounded by antenna pigment molecules.
4. Antenna pigments transfer absorbed light energy to the reaction centre.
5. The reaction centre releases high-energy electrons.
6. These electrons initiate the electron transport chain.

3) What are Carotenoids? Mention their role

1. Carotenoids are lipid-soluble accessory pigments present in chloroplasts.
2. They include carotenes and xanthophylls.
3. They absorb light mainly in the blue-violet region.
4. They transfer absorbed energy to chlorophyll-a.
5. They protect chlorophyll from photo-oxidative damage.
6. They enhance the efficiency of photosynthesis.

4) Explain Hill Reaction

1. Hill isolated chloroplasts and exposed them to light.
2. He observed oxygen release even in absence of carbon dioxide.
3. This proved that oxygen evolved during photosynthesis comes from water.
4. Water undergoes photolysis into protons, electrons and oxygen.
5. The electrons reduce an artificial electron acceptor.
6. The reaction occurs only in presence of light.

5) Distinguish between Cyclic and Non-cyclic Photophosphorylation

1. Cyclic photophosphorylation involves only Photosystem-I.
2. Non-cyclic photophosphorylation involves both Photosystem-I and Photosystem-II.
3. Cyclic pathway produces only ATP.
4. Non-cyclic pathway produces ATP and NADPH.
5. Cyclic pathway does not release oxygen.
6. Non-cyclic pathway releases oxygen due to photolysis of water.

6) Explain the three phases of Calvin Cycle

1. The first phase is carboxylation where CO₂ combines with RuBP.
2. This reaction is catalyzed by the enzyme RuBisCO.
3. The second phase is reduction where 3-PGA is converted into 3-PGAL.
4. ATP and NADPH produced in light reaction are utilized.
5. The third phase is regeneration of RuBP.
6. This ensures continuation of the Calvin cycle.

7) Significance of Kranz Anatomy

1. Bundle sheath cells surround vascular bundles.
2. CO₂ concentration becomes high in bundle sheath cells.
3. RuBisCO functions mainly as carboxylase.
4. Photorespiration is prevented.
5. Photosynthetic efficiency increases.
6. Plants adapt well to high temperature conditions.

8) Explain Photosystem-I and Photosystem-II

1. Photosystem-II contains P680 reaction centre.
2. It splits water and releases oxygen.
3. Photosystem-I contains P700 reaction centre.
4. It reduces NADP⁺ to NADPH.
5. Both photosystems are present in thylakoid membrane.
6. They work together in non-cyclic photophosphorylation.

9) Explain ATP synthesis by Chemiosmosis

1. Photolysis of water releases protons inside thylakoid lumen.
2. Electron transport chain pumps protons across membrane.
3. A proton gradient is established across thylakoid membrane.
4. Protons move back through ATP synthase enzyme.
5. The energy of proton flow synthesizes ATP.
6. This process is called chemiosmosis.

10) Adaptations of Xerophytic Plants for Photosynthesis

1. They exhibit CAM pathway.
2. Stomata open during night to reduce water loss.
3. CO₂ is fixed into malic acid at night.
4. Malic acid is stored in vacuoles.
5. During day CO₂ is released for Calvin cycle.
6. These adaptations help survival in dry environments.

11) Why does RuBisCO prefer carboxylation in C₄ plants?

1. C₄ plants maintain high CO₂ concentration in bundle sheath cells.
2. Oxygen concentration remains comparatively low.
3. PEP carboxylase initially fixes CO₂ in mesophyll cells.
4. Photorespiration is minimized.
5. RuBisCO mainly acts as carboxylase.
6. Photosynthetic efficiency increases.

12) Interdependence of Light and Dark Reactions

1. Light reaction produces ATP.
2. Light reaction produces NADPH.
3. Dark reaction utilizes ATP.
4. Dark reaction utilizes NADPH.
5. Dark reaction regenerates ADP and NADP⁺.
6. Both reactions are interlinked and mutually dependent.

SECTION D (5 Marks – 10 Points Each)

1) Ultrastructure of Chloroplast

1. Chloroplast is a double membrane bound organelle.
2. Outer and inner membranes enclose an intermembrane space.
3. The internal fluid matrix is called stroma.
4. Grana are stacks of thylakoids arranged like coins.
5. Thylakoid membranes contain photosynthetic pigments.
6. Stroma lamellae connect different grana.
7. Chloroplast contains its own DNA.
8. 70S ribosomes are present inside stroma.
9. Light reaction occurs in thylakoid membrane.
10. Dark reaction occurs in stroma.

2) Hatch-Slack (C₄) Pathway

1. Carbon dioxide is fixed by PEP carboxylase in mesophyll cells.
2. Oxaloacetic acid is formed as first product.
3. Oxaloacetic acid is converted into malic acid.
4. Malic acid moves into bundle sheath cells.
5. Carbon dioxide is released in bundle sheath cells.
6. Calvin cycle operates in bundle sheath cells.
7. Pyruvate returns to mesophyll cells.
8. ATP is used to regenerate PEP.
9. Kranz anatomy is present in C₄ plants.
10. Photorespiration is absent, increasing efficiency.

3) Photorespiration (PCO Cycle)

1. Occurs when oxygen concentration is high.
2. RuBisCO acts as oxygenase enzyme.
3. RuBP forms PGA and phosphoglycolate.
4. Glycolate is transported to peroxisome.
5. Glycine is formed in mitochondria.
6. Carbon dioxide is released during conversion.
7. About 25% of fixed carbon is lost.
8. Occurs in chloroplast, peroxisome and mitochondria.
9. No ATP is produced in this process.
10. It reduces overall photosynthetic efficiency.

4) Calvin Cycle

1. Occurs in stroma of chloroplast.
2. First phase is carboxylation.
3. RuBP combines with carbon dioxide.
4. 3-Phosphoglyceric acid is formed.
5. Reduction phase uses ATP.
6. NADPH is used to reduce PGA.
7. 3-Phosphoglyceraldehyde is formed.
8. Two molecules combine to form glucose.
9. Remaining molecules regenerate RuBP.
10. Cycle continues repeatedly to produce carbohydrates.

5) External Factors Affecting Photosynthesis

1. Light intensity directly influences the rate.
2. Red and blue light are most effective.
3. Carbon dioxide concentration increases rate up to optimum level.
4. About 1% CO₂ gives maximum efficiency.
5. Optimum temperature is around 25–30°C.
6. Very high temperature reduces enzyme activity.
7. Water is essential for photolysis.
8. Water maintains cell turgidity.
9. Blackman’s law of limiting factor applies.
10. The slowest factor limits the overall rate.

6) CAM Pathway

1. Found in xerophytic plants.
2. Stomata open during night.
3. Carbon dioxide is fixed into oxaloacetic acid at night.
4. Malic acid is stored in vacuole.
5. During day, carbon dioxide is released.
6. Calvin cycle operates in daytime.
7. Water loss is minimized.
8. No Kranz anatomy is present.
9. Example: Kalanchoe.
10. Helps plants survive in dry conditions.

7) Non-Cyclic Photophosphorylation

1. Involves both Photosystem II and Photosystem I.
2. Water undergoes photolysis.
3. Oxygen is released as by-product.
4. Electrons pass through electron transport chain.
5. Proton gradient is formed in thylakoid lumen.
6. ATP is synthesized by chemiosmosis.
7. Photosystem I absorbs light energy.
8. NADP⁺ is reduced to NADPH.
9. Electrons do not return to Photosystem II.
10. Main pathway for ATP and NADPH production.