NCERT Solutions for Class 11 Biology Chapter 13 Photosynthesis in Higher Plants

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What Are the NCERT Solutions for Class 11 Biology Chapter 13 Photosynthesis in Higher Plants?

NCERT Solutions for Class 11 Biology Chapter 13 Photosynthesis in Higher Plants explain the complete mechanism by which green plants trap sunlight and convert it into chemical energy stored in glucose. This chapter is one of the highest-weightage topics in Class 11 Biology for both CBSE board exams and NEET, since photosynthesis links directly to plant physiology, respiration, and ecosystem energy flow taught in later chapters.

The eSaral Class 11 Biology Chapter 13 solutions are structured topic-wise, so students can move from basic definitions (photosynthetic pigments, site of photosynthesis) to advanced NEET-level concepts (chemiosmotic hypothesis, C4 pathway, photorespiration) without gaps. Below is a complete, exam-ready breakdown of every sub-topic in the chapter.

What Do We Already Know About Photosynthesis?

Photosynthesis is the physico-chemical process by which green plants, algae, and some bacteria use light energy to synthesise organic compounds (primarily carbohydrates) from carbon dioxide and water, releasing oxygen as a by-product. The overall equation for photosynthesis is:

6CO₂ + 12H₂O + light energy → C₆H₁₂O₆ + 6O₂ + 6H₂O

Green plants are called autotrophs because they synthesise their own food. Photosynthesis is significant for two reasons that examiners repeatedly test:

• It is the primary source of all food on Earth, directly or indirectly sustaining almost all life forms.
• It is responsible for the release of oxygen into the atmosphere, which is essential for aerobic respiration in nearly all organisms.

Which Early Experiments Helped Scientists Discover Photosynthesis?

Several classical experiments, frequently asked as short-answer NEET and board questions, established our current understanding of photosynthesis:

Where Does Photosynthesis Take Place in a Plant Cell?

Photosynthesis occurs mainly in the mesophyll cells of leaves, which contain a large number of chloroplasts. Each chloroplast has:

• Outer and inner membranes enclosing the stroma
• Grana — stacks of membrane-bound sacs called thylakoids, where the light reaction takes place; grana are interconnected by stroma lamellae
• Stroma — the fluid matrix surrounding grana, where the dark reaction (Calvin cycle) takes place; it contains enzymes required for carbon fixation, including the most abundant enzyme on Earth, RuBisCO

How Many Pigments Are Involved in Photosynthesis?

Photosynthetic pigments are separated using chromatography and include:

• Chlorophyll a — the chief pigment directly involved in the light reaction (bright/blue-green)
• Chlorophyll b — an accessory pigment (yellow-green)
• Xanthophylls — accessory pigment (yellow)
• Carotenoids — accessory pigment (yellow to orange)

Accessory pigments absorb light at wavelengths chlorophyll a cannot, then transfer that energy to chlorophyll a — this is why they are also called antenna pigments. They also protect chlorophyll a from photo-oxidation.

A key NEET-level distinction tested from this section: the action spectrum (rate of photosynthesis at different wavelengths) closely resembles the absorption spectrum of chlorophyll a, confirming it is the primary pigment.

What Happens During the Light Reaction of Photosynthesis?

The light reaction takes place on the thylakoid membrane and includes:

• Light absorption by pigments organised into Photosystem I (P700) and Photosystem II (P680) — light-harvesting complexes (LHC) made of hundreds of pigment molecules feeding energy to a special reaction-centre chlorophyll a molecule.
• Splitting of water (photolysis) — occurs on the inner side of the thylakoid membrane, associated with PS II. Water is split into H⁺, electrons, and O₂. This step is the source of the oxygen released during photosynthesis.
• Electron transport — electrons move through the Z-scheme, passing from PS II to PS I via a chain of carriers, ultimately reducing NADP⁺ to NADPH.
• Photophosphorylation — synthesis of ATP using light energy.
  • Non-cyclic photophosphorylation — involves both PS I and PS II; produces ATP, NADPH, and O₂.
  • Cyclic photophosphorylation — involves only PS I when NADP⁺ is unavailable or far-red light is used; produces only ATP, no NADPH or O₂.

What Is the Chemiosmotic Hypothesis in Photosynthesis?

The chemiosmotic hypothesis explains how ATP is synthesised: protons (H⁺) accumulate inside the thylakoid lumen (from water splitting and electron transport), creating a proton gradient across the thylakoid membrane. These protons diffuse back into the stroma through ATP synthase, and this movement drives the synthesis of ATP from ADP and inorganic phosphate — the same fundamental principle used in mitochondrial ATP synthesis during respiration.

What Happens During the Dark Reaction (Calvin Cycle)?

Discovered by Melvin Calvin, the Calvin cycle occurs in the stroma and does not directly require light (though it depends on ATP and NADPH from the light reaction). It has three stages:

1. Carboxylation — fixation of CO₂ into a stable organic compound using the enzyme RuBisCO, which binds CO₂ to RuBP (ribulose bisphosphate), producing two molecules of 3-PGA (3-phosphoglyceric acid). This is the rate-limiting, most crucial step of the cycle.
2. Reduction — 3-PGA is reduced to G3P (glyceraldehyde-3-phosphate) using ATP and NADPH generated in the light reaction.
3. Regeneration — most G3P is used to regenerate RuBP so the cycle can continue; the rest is used to synthesise glucose.

Key numeric fact frequently tested in NEET: six turns of the Calvin cycle (fixing 6 CO₂ molecules) are required to synthesise one molecule of glucose, using 18 ATP and 12 NADPH.

What Is the C4 Pathway (Hatch and Slack Pathway)?

Plants such as maize and sugarcane, adapted to hot and dry environments, fix CO₂ using a different, more efficient route:

• CO₂ is first fixed in mesophyll cells by the enzyme PEPcase, combining with PEP (phosphoenolpyruvate) to form OAA (oxaloacetic acid) — a 4-carbon compound (hence "C4 pathway").
• OAA is converted to malate, which is transported to bundle sheath cells.
• In the bundle sheath cells, malate releases CO₂, which then enters the normal Calvin cycle (C3 pathway) via RuBisCO.

This two-step fixation is possible due to a special leaf anatomy called Kranz anatomy, where bundle sheath cells (containing chloroplasts) form a ring around the vascular bundle. By concentrating CO₂ around RuBisCO in the bundle sheath, C4 plants largely avoid photorespiration and remain efficient even in high light and high temperature.

What Is Photorespiration and Why Does It Lower Efficiency?

RuBisCO is not entirely specific to CO₂ — it can also bind O₂, especially when CO₂ concentration is low and O₂ concentration is high (as in hot, dry conditions with partially closed stomata). When RuBisCO binds O₂ instead of CO₂, the process is called photorespiration. Unlike normal respiration, photorespiration:

• Does not produce ATP or NADPH
• Does not fix carbon into sugar
• Releases previously fixed CO₂, effectively wasting energy and reducing photosynthetic efficiency

Because C4 plants concentrate CO₂ around RuBisCO in bundle sheath cells, photorespiration is largely suppressed in them — a key reason C4 plants outperform C3 plants in hot climates.

What Is the Difference Between C3 and C4 Plants (Table 13.1)?

Table 13.1 in Chapter 13 asks students to compare C3 and C4 plants. This is one of the most frequently asked NEET and board-exam questions from the chapter.

Want the full explanation with reasoning for each row, diagrams of Kranz anatomy, and NEET-level follow-up questions? Explore the detailed, expanded version of this comparison here: Table 13.1 Class 11 Biology — Full Explanation with Diagrams (Note: this expanded page is proposed and not yet live — create and publish it, then this link will resolve.)

What Factors Affect the Rate of Photosynthesis?

Photosynthesis is affected by both external (environmental) and internal (plant) factors. The concept most tested in NEET from this section is Blackman's Law of Limiting Factors (1905): when a chemical process is affected by more than one factor, its rate is limited by the factor that is nearest to its minimum value — that factor directly determines the rate, even if all other factors are abundant.

• Light intensity — rate increases with light intensity up to a point, then plateaus (light saturation); very high intensity can cause solarisation (pigment damage)
• Carbon dioxide concentration — a limiting factor in nature since atmospheric CO₂ is low (~0.03–0.04%); increasing CO₂ up to about 0.05% boosts photosynthesis rate significantly
• Temperature — dark reactions (enzyme-driven) are more temperature-sensitive than light reactions; optimum range is generally 25–35°C for most plants
• Water — affects photosynthesis mainly indirectly by causing stomatal closure and reducing CO₂ availability

How Does the Light Reaction Differ from the Dark Reaction?

What Are Some Important Conceptual Questions from Chapter 13? ( Quick Revision)

Why is the Calvin cycle also called the C3 pathway?

Because the first stable product of CO₂ fixation in this cycle, 3-PGA, is a 3-carbon compound.

What is the significance of the two photosystems working together?

Together, PS II and PS I enable non-cyclic photophosphorylation, which produces ATP, NADPH, and releases O₂ — the complete set of products required to run the Calvin cycle.

Why do C4 plants have a photosynthetic advantage in tropical climates?

Their CO₂-concentrating mechanism around RuBisCO in bundle sheath cells minimises photorespiration, allowing efficient photosynthesis even at high temperatures and low stomatal CO₂ availability.

What would happen to the Calvin cycle if NADPH supply stopped? The reduction step (3-PGA to G3P) would halt, stalling the entire cycle since RuBP regeneration and glucose synthesis both depend on this step.

Why Are These NCERT Solutions Important for NEET and Board Exams?

Photosynthesis in Higher Plants contributes directly to Unit 4 (Plant Physiology) of the NEET Biology syllabus and carries recurring weightage in CBSE board papers. Concepts like the C3–C4 comparison, the Z-scheme of electron transport, and the chemiosmotic hypothesis appear almost every year in NEET UG. Studying this chapter through the NCERT Solutions ensures students answer in the exact terminology examiners expect, rather than losing marks to imprecise phrasing.

How Should Students Use the Class 11 Biology Chapter 13 NCERT Solutions PDF?

1. Read the full concept breakdown above before attempting the exercise questions.
2. Solve each NCERT in-text and end-of-chapter question using the step-by-step solutions.
3. Revise Table 13.1 and the light-reaction diagram a day before your test — these are the most repeated NEET questions from this chapter.
4. Cross-check weak areas using the Respiration in Plants chapter, since both chapters are frequently asked together in Plant Physiology questions.

Which Other NCERT Solutions Should Class 11 Biology Students Check Next?

• NCERT Solutions for Class 11 Biology — All Chapters
• Class 11 Biology NCERT PDF Book — Full Chapter List

Preparing for NEET? Don't stop at Chapter 13. Get chapter-wise NCERT Solutions, topic tests, and Kota faculty video lectures for the complete Class 11 Biology syllabus with an eSaral NEET course. Explore NEET Biology Courses on eSaral →