Photosynthesis in Higher Plants Class 11 Notes PDF

01
Introduction to Photosynthesis in Higher Plants Class 11

Mastering the concepts of photosynthesis in higher plants class 11 is essential for any NEET aspirant. This biological process is the ultimate source of food for all living organisms on Earth. It is a physico-chemical process by which plants use light energy to drive the synthesis of organic compounds. In simpler terms, photosynthesis is the transformation of solar energy into chemical energy stored in the bonds of sugar molecules. For NEET, understanding the molecular mechanisms within the chloroplast is the key to securing high marks in Plant Physiology.

OVERALL CHEMICAL EQUATION

6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O

SITE OF REACTION The chloroplast, specifically the thylakoid membranes for light reactions and the stroma for dark reactions.

IMPORTANCE Primary source of food on Earth and responsible for the release of Oxygen into the atmosphere.

02
Early Experiments: Paving the Way

Our current understanding of photosynthesis in higher plants class 11 is built upon centuries of scientific inquiry. NEET frequently asks matching-type questions based on these landmark experiments.

Scientist	Key Discovery / Experiment
Joseph Priestley	Discovered the role of air (oxygen) in the growth of green plants using a bell jar and mint plant.
Jan Ingenhousz	Showed that sunlight is essential for photosynthesis and that only green parts release oxygen.
Julius von Sachs	Provided evidence for the production of glucose and its storage as starch in plants.
T.W. Engelmann	Described the first action spectrum using Cladophora and aerobic bacteria.
Cornelius van Niel	Demonstrated that photosynthesis is a light-dependent reaction where hydrogen from an oxidizable compound reduces CO₂.

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03
Photosynthetic Pigments & Light Absorption

Pigments are substances that have the ability to absorb light at specific wavelengths. In photosynthesis in higher plants class 11, we study several pigments found in leaves: Chlorophyll a (bright/blue green), Chlorophyll b (yellow green), Xanthophylls (yellow), and Carotenoids (yellow to yellow-orange).

TIP

Chlorophyll a is the primary pigment. Other pigments like Chlorophyll b and carotenoids are called accessory pigments; they protect chlorophyll a from photo-oxidation and widen the range of light absorption.

04
The Light Reaction: Photochemical Phase

The light reaction occurs in the thylakoid membranes (grana). It involves light absorption, water splitting, oxygen release, and the formation of high-energy chemical intermediates: ATP and NADPH.

PHOTOLYSIS OF WATER

2H₂O → 4H⁺ + O₂ + 4e⁻

(This process provides electrons to PS II and releases O₂ as a byproduct.)

Photosystems and the Z-Scheme

Pigments are organized into two discrete Light Harvesting Complexes (LHC) called Photosystem I (PS I) and Photosystem II (PS II). PS II has an absorption peak at 680 nm, while PS I peaks at 700 nm. The movement of electrons from PS II up to an acceptor, down the ETC to PS I, and then up again to reduce NADP⁺ is called the Z-scheme.

05
Photophosphorylation: Cyclic vs Non-Cyclic

The process of synthesizing ATP from ADP and inorganic phosphate in the presence of light is called photophosphorylation. This is a critical comparison in photosynthesis in higher plants class 11 notes.

Feature	Non-Cyclic	Cyclic
Photosystems involved	PS II and PS I	Only PS I
Photolysis of Water	Occurs	Does not occur
Products	ATP, NADPH, and O₂	Only ATP
Location	Grana lamellae	Stroma lamellae

06
Dark Reaction: The Biosynthetic Phase

The dark reaction, or Calvin Cycle (C₃ Cycle), occurs in the stroma of the chloroplast. It uses the ATP and NADPH produced during the light reaction to fix CO₂ into sugars. It consists of three main phases:

CARBOXYLATION Fixation of CO₂ into a stable organic intermediate. RuBP + CO₂ → 2 molecules of 3-PGA, catalyzed by **RuBisCO**.

REDUCTION Uses 2 ATP and 2 NADPH per CO₂ fixed to form glucose.

REGENERATION Regeneration of the CO₂ acceptor RuBP is crucial for the cycle to continue. Requires 1 ATP.

ENERGY BUDGET FOR 1 GLUCOSE

6 CO₂ + 18 ATP + 12 NADPH → 1 Glucose

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07
The C₄ Pathway & Kranz Anatomy

Plants adapted to dry tropical regions use the Hatch and Slack Pathway (C₄ Cycle). These plants have a special leaf anatomy called Kranz anatomy (bundle sheath cells arranged in a wreath-like manner). This pathway avoids the energy-wasting process of photorespiration.

First Stable Product: Oxaloacetic acid (OAA), a 4-carbon compound.
Primary Acceptor: Phosphoenolpyruvate (PEP) in mesophyll cells.
Enzyme: PEPcase (in mesophyll) and RuBisCO (in bundle sheath).

WARN

C₄ plants are more efficient than C₃ plants in high temperatures and low CO₂ concentrations because they maintain a high CO₂ level near the RuBisCO enzyme, preventing oxygenation.

08
Photorespiration & Limiting Factors

Photorespiration occurs in C₃ plants when RuBisCO binds with O₂ instead of CO₂. This results in the loss of fixed carbon and energy, producing no ATP or sugar. It is a wasteful process unique to C₃ plants.

Blackman’s Law of Limiting Factors

Proposed in 1905, it states: “If a chemical process is affected by more than one factor, then its rate will be determined by the factor which is nearest to its minimal value.” In photosynthesis in higher plants class 11, CO₂ concentration is the major limiting factor in nature.

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Quick Revision Summary

Equation: 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O.
PS II: P680; PS I: P700. PS II is located on appressed parts of thylakoids.
Non-cyclic photophosphorylation: Produces ATP and NADPH.
Calvin Cycle: Carboxylation → Reduction → Regeneration.
RuBisCO: Most abundant enzyme; can act as carboxylase or oxygenase.
C₄ plants: Kranz anatomy; Maize, Sugarcane.
CAM plants: Stomata open at night; Scotoactive stomata (e.g., Pineapple, Cactus).
Photorespiration: RuBP + O₂ → Phosphoglycolate + PGA.
CO₂ compensation point: Lower for C₄ plants (0-10 ppm) than C₃ (25-100 ppm).
Light compensation point: Point where rate of photosynthesis equals rate of respiration.

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09
Frequently Asked Questions

Why is the action spectrum of photosynthesis not exactly the same as the absorption spectrum of chlorophyll a?

While chlorophyll a is the primary pigment, the action spectrum includes the contributions of accessory pigments like chlorophyll b, xanthophylls, and carotenoids. These pigments absorb light at wavelengths where chlorophyll a does not, transferring that energy to the reaction center.

Explain the significance of Kranz Anatomy in C₄ plants.

Kranz anatomy involves bundle sheath cells with large, agranal chloroplasts and mesophyll cells with normal chloroplasts. This arrangement spatially separates the initial CO₂ fixation (PEPcase) from the Calvin cycle (RuBisCO), ensuring high CO₂ levels around RuBisCO to prevent photorespiration.

Why is RuBisCO called a dual-nature enzyme?

RuBisCO stands for Ribulose Bisphosphate Carboxylase-Oxygenase. Its active site can bind to both CO₂ and O₂. When CO₂ concentration is high, it acts as a carboxylase; when O₂ is high, it acts as an oxygenase, leading to photorespiration.

What is the “Z-scheme” of electron transport?

The Z-scheme represents the pathway of electron flow during non-cyclic photophosphorylation. Electrons from water splitting move from PS II (redox potential) up to an acceptor, then down an ETC to PS I, and finally up again to NADP⁺, forming a “Z” shape when plotted by redox potential.

How do CAM plants conserve water?

CAM (Crassulacean Acid Metabolism) plants open their stomata at night to fix CO₂ as malic acid. During the day, stomata remain closed to prevent transpiration, and the stored malic acid is decarboxylated to release CO₂ for the Calvin cycle using light-reaction products.

What happens to the rate of photosynthesis if CO₂ concentration exceeds 0.05%?

Atmospheric CO₂ is about 0.03-0.04%. Increasing it up to 0.05% can enhance photosynthesis. However, beyond this concentration, the rate can become damaging over long periods as it leads to the closure of stomata or metabolic inhibition.

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Table of Contents — Biology Class 11

Biology — Class 11

01	The Living World	Go to page
02	Biological Classification	Go to page
03	Plant Kingdom	Go to page
04	Animal Kingdom	Go to page
05	Morphology of Flowering Plants	Go to page
06	Anatomy of Flowering Plants	Go to page
07	Structural Organisation in Animals	Go to page
08	Cell: The Unit of Life	Go to page
09	Biomolecules	Go to page
10	Cell Cycle and Cell Division	Go to page
11	Photosynthesis in Higher Plants	Go to page
12	Respiration in Plants	Go to page
13	Plant Growth and Development	Go to page
14	Breathing and Exchange of Gases	Go to page
15	Body Fluids and Circulation	Go to page
16	Excretory Products and their Elimination	Go to page
17	Locomotion and Movement	Go to page
18	Neural Control and Coordination	Go to page
19	Chemical Coordination and Integration	Go to page

Photosynthesis in Higher Plants Class 11 PDF: Complete NEET Guide

01
Introduction to Photosynthesis in Higher Plants Class 11

02
Early Experiments: Paving the Way

03
Photosynthetic Pigments & Light Absorption