Photosynthesis in Higher Plants Class Summary, Notes, MCQs, Important Questions

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Comprehensive notes on Photosynthesis in Higher Plants Class 12 Biology including summary, flowchart, keywords, short and long questions, MCQs, and exam tips for board exams.

Introduction of the Chapter

The chapter Photosynthesis in Higher Plants is one of the most important units in Class 12 Biology. It explains how green plants prepare their own food using sunlight, carbon dioxide, and water. This process forms the base of life on Earth because it produces oxygen and organic food for all living organisms.

In Photosynthesis in Higher Plants, students learn about pigments, light reaction, dark reaction, C3 and C4 pathways, photorespiration, and factors affecting photosynthesis. Understanding this chapter is essential for board exams and competitive exams like NEET.

Short Notes (Bullet Points)

Photosynthesis occurs in chloroplasts of green cells.
Overall equation:
6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O
Two main stages:
- Light Reaction (occurs in thylakoid membrane)
- Dark Reaction / Calvin Cycle (occurs in stroma)
Photosystems involved:
- Photosystem I (PS I)
- Photosystem II (PS II)
Photolysis of water releases oxygen.
C3 plants use Calvin cycle only.
C4 plants show Hatch and Slack pathway.
Photorespiration reduces efficiency of photosynthesis.
Factors affecting photosynthesis:
- Light
- Carbon dioxide
- Temperature

Detailed Summary of Photosynthesis in Higher Plants (1000–1200 Words)

The chapter Photosynthesis in Higher Plants explains the mechanism by which green plants synthesize carbohydrates. It takes place inside chloroplasts, which contain chlorophyll pigments.

What is Photosynthesis?

Photosynthesis is the process by which green plants convert light energy into chemical energy. The energy is stored in the form of glucose. Oxygen is released as a by-product.

Site of Photosynthesis

Photosynthesis occurs in chloroplasts. A chloroplast consists of:

Outer membrane
Inner membrane
Stroma
Thylakoids arranged in stacks called grana

The thylakoid membrane contains chlorophyll and other pigments.

Pigments Involved

Photosynthetic pigments include:

Chlorophyll a (primary pigment)
Chlorophyll b
Carotenoids
Xanthophylls

Chlorophyll a is the reaction center pigment.

Light Reaction

The light reaction occurs in the thylakoid membrane. It includes:

Absorption of light by photosystems.
Photolysis of water.
Formation of ATP (photophosphorylation).
Formation of NADPH.

There are two photosystems:

Photosystem II (PS II) – absorbs light at 680 nm.
Photosystem I (PS I) – absorbs light at 700 nm.

The movement of electrons through the electron transport chain results in ATP formation.

Dark Reaction (Calvin Cycle)

The dark reaction occurs in the stroma. It does not require light directly.

Steps of Calvin Cycle:

Carboxylation – CO₂ combines with RuBP.
Reduction – formation of glucose.
Regeneration – RuBP is regenerated.

The enzyme RuBisCO plays a major role.

C3 Pathway

Most plants follow C3 pathway.
First stable product is 3-phosphoglycerate (3-PGA).
Occurs in mesophyll cells.

C4 Pathway

C4 plants show special anatomy called Kranz anatomy.

Characteristics:

First stable product is oxaloacetic acid (OAA).
Occurs in mesophyll and bundle sheath cells.
More efficient under high temperature.

Examples: Maize, Sugarcane.

Photorespiration

Photorespiration occurs when RuBisCO reacts with oxygen instead of carbon dioxide. It reduces photosynthetic efficiency.

It occurs in:

Chloroplast
Peroxisome
Mitochondria

Factors Affecting Photosynthesis

Light intensity
Carbon dioxide concentration
Temperature
Water availability

Blackman’s Law of Limiting Factors states that the rate of photosynthesis is limited by the factor in minimum supply.

Thus, Photosynthesis in Higher Plants provides complete knowledge of how plants prepare food and maintain ecological balance.

Flowchart / Mind Map (Text-Based)

Photosynthesis in Higher Plants
│
├── Site → Chloroplast
│ ├── Grana → Light Reaction
│ └── Stroma → Calvin Cycle
│
├── Light Reaction
│ ├── PS II
│ ├── Photolysis
│ ├── ATP Formation
│ └── NADPH Formation
│
├── Dark Reaction
│ ├── Carboxylation
│ ├── Reduction
│ └── Regeneration
│
├── C3 Pathway
├── C4 Pathway
└── Photorespiration

Important Keywords with Meanings

Photosynthesis – Food synthesis using sunlight.
Chloroplast – Organelle where photosynthesis occurs.
Photolysis – Splitting of water by light.
RuBisCO – Enzyme that fixes CO₂.
Calvin Cycle – Dark reaction pathway.
Kranz Anatomy – Special leaf structure in C4 plants.
Photophosphorylation – ATP formation using light.
Photorespiration – Oxygenation reaction by RuBisCO.

Important Questions & Answers

10 Very Short Answer Questions

Define photosynthesis.
Photosynthesis is the process of converting light energy into chemical energy.
Where does light reaction occur?
In thylakoid membrane.
Name primary pigment.
Chlorophyll a.
What is RuBisCO?
Enzyme that fixes CO₂.
First product of C3 cycle?
3-PGA.
Define photolysis.
Splitting of water using light.
What is Kranz anatomy?
Special structure in C4 leaves.
Define photorespiration.
Oxygenation of RuBP.
State Blackman’s law.
Rate depends on limiting factor.
Name two C4 plants.
Maize, Sugarcane.

9 short Answer Questions

Describe light reaction in detail.
Light reaction occurs in thylakoids. It includes excitation of electrons, photolysis, ATP and NADPH formation.
Explain Calvin cycle.
It consists of carboxylation, reduction, regeneration steps.
Differentiate C3 and C4 plants.
C3: First product 3-PGA.
C4: First product OAA.
Explain photorespiration mechanism.
Occurs in chloroplast, peroxisome, mitochondria.
Describe structure of chloroplast.
Double membrane, stroma, grana.
Explain factors affecting photosynthesis.
Light, CO₂, temperature.
Explain non-cyclic photophosphorylation.
Produces ATP and NADPH.
Explain cyclic photophosphorylation.
Produces only ATP.
Write chemical equation of photosynthesis.
6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O

30 one words with Answers

Photosynthesis occurs in – Chloroplast
Primary pigment – Chlorophyll a
PS II absorbs – 680 nm
PS I absorbs – 700 nm
Oxygen released from – Water
Enzyme in Calvin cycle – RuBisCO
First product C4 – OAA
C3 plants produce – 3-PGA
ATP formed in – Light reaction
Dark reaction occurs in – Stroma
Law of limiting factor – Blackman
Kranz anatomy seen in – C4 plants
Photorespiration decreases – Efficiency
NADPH formed in – Light reaction
Photolysis releases – O₂
Bundle sheath cells present in – C4 plants
Electron transport chain located in – Thylakoid
Pigment group – Carotenoids
Non-cyclic flow produces – ATP & NADPH
Cyclic flow produces – ATP

Exam Tips / Value-Based Questions

Why are C4 plants efficient in tropical regions?
They minimize photorespiration.
Why is photosynthesis important for life?
Produces oxygen and food.
Why does water stress reduce photosynthesis?
Stomata close, reducing CO₂.
How can farmers increase crop yield?
By maintaining optimum light and CO₂.
Why is RuBisCO called dual enzyme?
It acts as carboxylase and oxygenase.

Important Questions from Photosynthesis in Higher Plants

Part A: 10 Short Answer Questions with Answers

1. Define photosynthesis.

Answer:
Photosynthesis is the process by which green plants synthesize organic food (glucose) from carbon dioxide and water using sunlight in the presence of chlorophyll, releasing oxygen as a by-product.

2. Where does the light reaction occur?

Answer:
The light reaction occurs in the thylakoid membranes (grana) of the chloroplast.

3. Name the primary pigment involved in photosynthesis.

Answer:
Chlorophyll a is the primary photosynthetic pigment.

4. What is photolysis of water?

Answer:
Photolysis is the splitting of water molecules into hydrogen ions, electrons, and oxygen in the presence of light during the light reaction.

5. What is the role of RuBisCO?

Answer:
RuBisCO (Ribulose-1,5-bisphosphate carboxylase oxygenase) catalyzes the fixation of carbon dioxide in the Calvin cycle.

6. What is the first stable product of the C3 cycle?

Answer:
The first stable product of the C3 cycle is 3-phosphoglycerate (3-PGA), a three-carbon compound.

7. What is Kranz anatomy?

Answer:
Kranz anatomy is a special leaf structure found in C4 plants where bundle sheath cells surround vascular bundles in a ring-like arrangement.

8. Define photorespiration.

Answer:
Photorespiration is a process in which RuBisCO reacts with oxygen instead of carbon dioxide, leading to reduced photosynthetic efficiency.

9. State Blackman’s Law of Limiting Factors.

Answer:
Blackman’s Law states that when a process is influenced by several factors, the rate is limited by the factor that is in the shortest supply.

10. Name two C4 plants.

Answer:
Maize and Sugarcane are examples of C4 plants.

Part B: 10 Long Answer Questions with Answers

1. Describe the structure of chloroplast.

Answer:
Chloroplast is a double-membrane bound organelle found in green plant cells. It consists of:

Outer membrane
Inner membrane
Stroma (fluid matrix)
Thylakoids arranged in stacks called grana

The thylakoid membranes contain chlorophyll pigments and are the site of the light reaction. The stroma contains enzymes required for the Calvin cycle (dark reaction). The structural organization ensures efficient photosynthesis.

2. Explain the light reaction of photosynthesis.

Answer:
The light reaction occurs in the thylakoid membrane and involves:

Absorption of light by Photosystem II (PS II).
Photolysis of water releasing oxygen.
Transfer of electrons through electron transport chain.
Formation of ATP by photophosphorylation.
Excitation of Photosystem I (PS I).
Reduction of NADP⁺ to NADPH.

The light reaction produces ATP and NADPH, which are used in the Calvin cycle.

3. Describe the Calvin cycle in detail.

Answer:
The Calvin cycle occurs in the stroma and consists of three main steps:

Carboxylation – CO₂ combines with RuBP in the presence of RuBisCO to form 3-PGA.
Reduction – 3-PGA is reduced to glyceraldehyde-3-phosphate (G3P) using ATP and NADPH.
Regeneration – RuBP is regenerated to continue the cycle.

For the synthesis of one glucose molecule, six turns of the Calvin cycle are required.

4. Differentiate between C3 and C4 plants.

Feature	C3 Plants	C4 Plants
First product	3-PGA	OAA
Leaf anatomy	Normal	Kranz anatomy
Photorespiration	High	Very low
Efficiency	Moderate	High in tropical climate

C4 plants are more efficient under high temperature and low CO₂ conditions.

5. Explain photorespiration and its significance.

Answer:
Photorespiration occurs when RuBisCO acts as an oxygenase and combines RuBP with oxygen. It results in formation of phosphoglycolate and reduces photosynthetic efficiency.

It occurs in chloroplast, peroxisome, and mitochondria.
It is considered wasteful because it consumes energy without producing glucose.

6. Explain cyclic and non-cyclic photophosphorylation.

Answer:

Cyclic Photophosphorylation:

Involves only Photosystem I.
Produces only ATP.
No NADPH or oxygen formed.

Non-Cyclic Photophosphorylation:

Involves both PS I and PS II.
Produces ATP and NADPH.
Oxygen is released.

Non-cyclic is the main pathway during normal photosynthesis.

7. Discuss the factors affecting photosynthesis.

Answer:

Light Intensity – Rate increases with light up to a certain limit.
Carbon Dioxide Concentration – Higher CO₂ increases rate.
Temperature – Affects enzyme activity.
Water – Deficiency reduces rate due to stomatal closure.

The rate is governed by Blackman’s Law of Limiting Factors.

8. Describe the Z-scheme of electron transport.

Answer:
The Z-scheme represents the flow of electrons from water to NADP⁺ during non-cyclic photophosphorylation. It involves:

PS II excitation
Electron transport chain
PS I excitation
NADPH formation

The energy level diagram resembles the letter “Z”.

9. Write the overall chemical equation of photosynthesis and explain it.

Answer:

6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O

Carbon dioxide and water combine using light energy to form glucose. Oxygen is released as a by-product.

10. Explain the significance of Photosynthesis in Higher Plants.

Answer:

Produces oxygen for respiration.
Forms base of food chain.
Maintains carbon dioxide balance.
Supports agriculture and food production.
Essential for ecological balance.

Photosynthesis is the foundation of life on Earth.

Conclusion

The chapter Photosynthesis in Higher Plants Class 12 Biology is one of the most significant and concept-based chapters in the NCERT syllabus. It explains the fundamental life process through which green plants prepare their own food by using sunlight, carbon dioxide, and water. This process not only supports plant life but also sustains all living organisms on Earth. A clear understanding of Photosynthesis in Higher Plants helps students build a strong foundation in plant physiology, ecology, and environmental science.

Photosynthesis is the primary source of energy for almost every ecosystem. The energy captured from sunlight is converted into chemical energy in the form of carbohydrates. These carbohydrates serve as food for plants and indirectly for animals and humans. Thus, Photosynthesis in Higher Plants forms the base of the food chain. Without this process, life on Earth would not be possible because oxygen would not be available for respiration.

The chapter explains that photosynthesis occurs inside chloroplasts, which are specialized organelles present in green plant cells. The structure of chloroplast, including grana and stroma, is directly related to the two major stages of photosynthesis: the light reaction and the dark reaction. Understanding the internal organization of chloroplast makes it easier to grasp how energy conversion takes place efficiently within plant cells.

The light reaction, which occurs in the thylakoid membranes, is responsible for trapping solar energy. In this stage, chlorophyll pigments absorb light and convert it into chemical energy in the form of ATP and NADPH. The splitting of water molecules, known as photolysis, releases oxygen as a by-product. This oxygen is essential for the survival of aerobic organisms. The production of ATP and NADPH during the light reaction provides the energy required for the next stage of photosynthesis.

The dark reaction, also known as the Calvin cycle, takes place in the stroma of chloroplasts. It does not directly require light but depends on the products of the light reaction. In this stage, carbon dioxide is fixed and converted into glucose through a series of enzyme-controlled steps. The enzyme RuBisCO plays a key role in carbon fixation. The Calvin cycle includes three main steps: carboxylation, reduction, and regeneration of RuBP. This part of Photosynthesis in Higher Plants is extremely important for exam preparation, as it involves biochemical pathways and energy transformations.

Another major concept covered in Photosynthesis in Higher Plants is the difference between C3 and C4 pathways. Most plants follow the C3 pathway, where the first stable product is a three-carbon compound called 3-phosphoglycerate. However, some plants growing in hot and dry conditions have developed a special adaptation known as the C4 pathway. In C4 plants, the first stable product is a four-carbon compound called oxaloacetic acid. These plants show a special structural feature called Kranz anatomy, which helps reduce photorespiration and increase efficiency.

Photorespiration is another important topic discussed in the chapter. It occurs when the enzyme RuBisCO reacts with oxygen instead of carbon dioxide. This process reduces the efficiency of photosynthesis because it leads to loss of energy and carbon. Photorespiration involves chloroplasts, peroxisomes, and mitochondria. Understanding photorespiration is crucial because it explains why C4 plants are more productive in tropical climates compared to C3 plants.

The chapter also highlights various factors affecting photosynthesis, such as light intensity, carbon dioxide concentration, temperature, and water availability. According to Blackman’s Law of Limiting Factors, the rate of photosynthesis is controlled by the factor that is in the least supply. This concept is highly relevant for board examinations and competitive exams. It also has practical applications in agriculture, where optimizing environmental conditions can increase crop yield.

From an ecological perspective, Photosynthesis in Higher Plants Class 12 Biology explains how plants maintain the balance of oxygen and carbon dioxide in the atmosphere. Through carbon fixation, plants help reduce atmospheric carbon dioxide levels, thereby playing a significant role in controlling global warming and climate change. Forests and green plants act as carbon sinks, making photosynthesis an essential natural process for environmental stability.

For students preparing for board exams, mastering this chapter is extremely important because it carries high weightage. Questions are frequently asked on light reaction, Calvin cycle, differences between C3 and C4 plants, photorespiration, and factors affecting photosynthesis. Diagrams of chloroplast structure, Z-scheme of electron transport, and Kranz anatomy are commonly asked in examinations. Therefore, regular practice of labelled diagrams and flowcharts can help in scoring full marks.

From a competitive exam point of view, especially for NEET aspirants, Photosynthesis in Higher Plants is a scoring topic. Conceptual clarity in photophosphorylation, electron transport chain, ATP synthesis, and carbon fixation pathways is essential. MCQs are often framed around wavelengths absorbed by photosystems, location of reactions, enzyme names, and differences between pathways. Hence, thorough revision of notes, keywords, and previous year questions is necessary.

In addition to academic importance, the study of Photosynthesis in Higher Plants helps students appreciate the importance of plants in daily life. Plants are primary producers that provide food, fuel, fiber, and oxygen. Agricultural productivity depends on efficient photosynthesis. By understanding this chapter, students can also learn how environmental stress conditions such as drought and high temperature affect crop yield.

The chapter also emphasizes energy transformation, which is a fundamental principle in biology. It shows how solar energy is converted into chemical energy and stored in glucose molecules. This stored energy is later released during respiration to perform life activities. Thus, photosynthesis and respiration are complementary processes that maintain energy balance in living systems.

In conclusion, Photosynthesis in Higher Plants Class 12 Biology summary, notes, and MCQs provide complete insight into one of the most vital biological processes. This chapter connects plant anatomy, biochemistry, ecology, and environmental science in a systematic manner. By understanding the mechanism of light reaction, Calvin cycle, C3 and C4 pathways, and photorespiration, students gain a comprehensive understanding of plant metabolism.

To score high marks, students should revise important keywords, practice diagrams, and solve multiple-choice questions regularly. Conceptual clarity is more important than rote learning in this chapter. Once the concepts are clear, answering both short and long questions becomes easy.

Overall, Photosynthesis in Higher Plants is not just a chapter for exams but a fundamental concept that explains the origin of food and oxygen on Earth. Mastering this chapter ensures strong preparation for board exams, competitive exams, and higher studies in biological sciences.

Sample Question Paper

Photosynthesis in Higher Plants – Class 12 Biology

(Based on NCERT | Board Exam Pattern | With Answers)

Section A – MCQs (1 × 10 = 10 Marks)

1. The primary pigment involved in photosynthesis is:
a) Chlorophyll b
b) Carotene
c) Chlorophyll a
d) Xanthophyll
Answer: c) Chlorophyll a

2. Photolysis of water occurs in:
a) Stroma
b) Thylakoid lumen
c) Cytoplasm
d) Nucleus
Answer: b) Thylakoid lumen

3. The first stable product of C4 cycle is:
a) 3-PGA
b) OAA
c) RuBP
d) PEP
Answer: b) OAA

4. RuBisCO acts as:
a) Oxygen carrier
b) Carboxylase and Oxygenase
c) Pigment
d) Hormone
Answer: b) Carboxylase and Oxygenase

5. ATP synthesis during photosynthesis is called:
a) Oxidative phosphorylation
b) Substrate-level phosphorylation
c) Photophosphorylation
d) Glycolysis
Answer: c) Photophosphorylation

6. C4 plants show:
a) High photorespiration
b) No chloroplast
c) Kranz anatomy
d) No Calvin cycle
Answer: c) Kranz anatomy

7. Oxygen released during photosynthesis comes from:
a) CO₂
b) Glucose
c) Water
d) ATP
Answer: c) Water

8. Dark reaction occurs in:
a) Thylakoid
b) Grana
c) Stroma
d) Cytoplasm
Answer: c) Stroma

9. The law of limiting factors was given by:
a) Darwin
b) Blackman
c) Mendel
d) Pasteur
Answer: b) Blackman

10. Photosystem I absorbs light at:
a) 680 nm
b) 700 nm
c) 600 nm
d) 550 nm
Answer: b) 700 nm

Section B – Very Short Answer Questions (2 × 5 = 10 Marks)

11. Define photosynthesis.
Photosynthesis is the process by which green plants synthesize food using sunlight, CO₂, and water.

12. What is photolysis?
Splitting of water molecules in presence of light to release oxygen, protons, and electrons.

13. Name two C4 plants.
Maize and Sugarcane.

14. What is RuBP?
Ribulose-1,5-bisphosphate, a CO₂ acceptor in Calvin cycle.

15. What is photorespiration?
It is oxygenation of RuBP by RuBisCO, reducing efficiency of photosynthesis.

Section C – Short Answer Questions (3 × 6 = 18 Marks)

16. Explain cyclic photophosphorylation.
Cyclic photophosphorylation involves only Photosystem I. Electrons return to the same photosystem after passing through electron carriers. It produces only ATP and no NADPH or oxygen.

17. Write differences between C3 and C4 plants.

Feature	C3 Plants	C4 Plants
First product	3-PGA	OAA
Leaf structure	Normal	Kranz anatomy
Photorespiration	High	Low
Efficiency	Lower	Higher

18. Explain Blackman’s Law of Limiting Factors.
According to this law, when several factors affect a process, the rate is controlled by the factor present in minimum amount. In photosynthesis, if CO₂ is low, increasing light will not increase the rate.

19. What are photosystems? Name their types.
Photosystems are pigment systems that absorb light energy.
Types:

Photosystem II (P680)
Photosystem I (P700)

20. Write the overall chemical equation of photosynthesis.
6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O

21. State the significance of photosynthesis.

Produces oxygen
Provides food
Maintains CO₂ balance
Supports life on Earth

Section D – Long Answer Questions (5 × 4 = 20 Marks)

22. Describe the light reaction of photosynthesis.

The light reaction occurs in the thylakoid membrane and includes:

Absorption of light by PS II
Photolysis of water
Electron transport chain
ATP formation
Excitation of PS I
NADPH formation

It produces ATP, NADPH, and oxygen.

23. Explain the Calvin cycle in detail.

The Calvin cycle occurs in the stroma and has three stages:

Carboxylation – CO₂ combines with RuBP to form 3-PGA.
Reduction – ATP and NADPH convert 3-PGA into G3P.
Regeneration – RuBP is regenerated.

Six turns of cycle produce one glucose molecule.

24. Explain photorespiration and its pathway.

Photorespiration occurs when RuBisCO reacts with oxygen. It forms phosphoglycolate. The process involves chloroplast, peroxisome, and mitochondria. It consumes energy and reduces productivity.

25. Describe structure of chloroplast with functions.

Chloroplast has:

Double membrane
Stroma
Thylakoids forming grana

Light reaction occurs in thylakoids; dark reaction occurs in stroma.

26. Discuss factors affecting photosynthesis.

Light intensity
CO₂ concentration
Temperature
Water

Rate depends on limiting factor.

Case Study-Based Question (4 Marks)

27. A farmer grows maize in a tropical region. Explain why maize shows higher productivity than wheat under high temperature conditions.

Answer:
Maize is a C4 plant. It has Kranz anatomy and minimal photorespiration. Under high temperature, RuBisCO oxygenase activity increases in C3 plants like wheat, reducing productivity. Therefore, maize shows higher efficiency.

Internal Choice Question (Optional)

28. Explain Z-scheme of electron transport.
OR
Draw and explain Kranz anatomy.

(Z-scheme shows electron movement from water to NADP⁺ in zig-zag pattern.)

Total Marks: 80 (Full-Length Chapter Test Format)

Exam Tips for Scoring High Marks

Practice labelled diagrams of chloroplast and Calvin cycle.
Learn differences between C3 and C4 plants clearly.
Revise keywords like RuBisCO, photolysis, Kranz anatomy.
Write answers in points.
Practice MCQs for competitive exams.

Solutions – Sample Paper

Photosynthesis in Higher Plants Class 12 Biology

Below are the complete, step-wise solutions to the Sample Question Paper on Photosynthesis in Higher Plants Class 12 Biology. Answers are written in board-exam format for maximum scoring.

Section A – MCQs (1 Mark Each)

1. Chlorophyll a
Correct because it is the primary pigment that directly participates in light reaction.

2. Thylakoid lumen
Photolysis occurs on the inner side of thylakoid membrane where water splits.

3. OAA (Oxaloacetic Acid)
First stable product in C4 pathway.

4. Carboxylase and Oxygenase
RuBisCO has dual function—fixes CO₂ and also reacts with O₂.

5. Photophosphorylation
ATP synthesis in presence of light is called photophosphorylation.

6. Kranz anatomy
C4 plants show special arrangement of bundle sheath cells.

7. Water
Oxygen released comes from water, confirmed by isotopic experiments.

8. Stroma
Dark reaction (Calvin cycle) occurs in stroma.

9. Blackman
Blackman proposed the Law of Limiting Factors.

10. 700 nm
Photosystem I absorbs light at 700 nm (P700).

Section B – Very Short Answer Solutions (2 Marks Each)

11. Define photosynthesis.

Photosynthesis is the process by which green plants synthesize carbohydrates from CO₂ and water using sunlight in the presence of chlorophyll, releasing oxygen.

12. What is photolysis?

Photolysis is the light-induced splitting of water molecules into hydrogen ions, electrons, and oxygen during the light reaction.

13. Name two C4 plants.

Maize and Sugarcane.

14. What is RuBP?

Ribulose-1,5-bisphosphate (RuBP) is a five-carbon compound that acts as the primary CO₂ acceptor in the Calvin cycle.

15. What is photorespiration?

Photorespiration is the oxygenation of RuBP by RuBisCO leading to energy loss and reduced photosynthetic efficiency.

Section C – Short Answer Solutions (3 Marks Each)

16. Explain cyclic photophosphorylation.

Occurs only in Photosystem I.
Excited electrons return to PS I through electron carriers.
Produces ATP only.
No NADPH or oxygen produced.
Occurs when NADP⁺ is not available.

17. Differences between C3 and C4 Plants.

C3 plants:

First product: 3-PGA
High photorespiration
Found in moderate climates

C4 plants:

First product: OAA
Low photorespiration
Adapted to high temperature

18. Blackman’s Law of Limiting Factors.

The rate of photosynthesis is limited by the factor that is present in minimum quantity. For example, if CO₂ concentration is low, increasing light intensity will not increase photosynthesis rate.