Photosynthesis in Higher Plants – Class 11 Biology Chapter 11 Notes & NCERT PDF

Introduction

Photosynthesis is the physico-chemical process by which green plants use light energy to drive the synthesis of organic compounds. It is the basis of almost all life on Earth: it is the primary source of all food, and it is responsible for the release of oxygen into the atmosphere. This chapter looks at the structure of the photosynthetic machinery and the reactions that convert light energy into chemical energy.

Key Concepts

11.1 What Do We Know?

Simple school experiments already show that chlorophyll, light and CO₂ are required. A variegated leaf (or a partly covered leaf) exposed to light shows starch only in the green, exposed parts.

11.2 Early Experiments

11.3 Where Does Photosynthesis Take Place?

In the chloroplasts of mesophyll cells (and other green parts). Inside the chloroplast:

11.4 How Many Types of Pigments?

Paper chromatography of leaf pigments shows four: chlorophyll a (bright/blue green), chlorophyll b (yellow green), xanthophylls (yellow) and carotenoids (yellow to yellow-orange). Chlorophyll a is the chief pigment; the others are accessory pigments that absorb other wavelengths and protect chlorophyll a from photo-oxidation. Maximum absorption and maximum photosynthesis both occur in the blue and red regions.

11.5 What is Light Reaction?

Light absorption, water splitting, oxygen release and formation of ATP and NADPH. Pigments are organised into light harvesting complexes (LHC) in Photosystem I and Photosystem II (named in order of discovery, not function). Each photosystem has an antenna of hundreds of pigment molecules and a single reaction centre chlorophyll a:

11.6 The Electron Transport

11.7 Where are the ATP and NADPH Used? (Calvin Cycle)

Melvin Calvin used radioactive ¹⁴C in algae: the first stable product was 3-phosphoglyceric acid (PGA), a 3-carbon compound — the C₃ pathway. The CO₂ acceptor is the 5-carbon ribulose bisphosphate (RuBP); the enzyme is RuBisCO (RuBP carboxylase-oxygenase). Three stages:

So each CO₂ needs 3 ATP + 2 NADPH; one glucose needs 6 turns: In — 6 CO₂, 18 ATP, 12 NADPH; Out — 1 glucose, 18 ADP, 12 NADP⁺. Cyclic photophosphorylation probably meets the extra ATP demand.

11.8 The C₄ Pathway (Hatch and Slack Pathway)

Found in plants of dry tropical regions (e.g., maize, sorghum). Features: Kranz anatomy ("wreath") — large bundle sheath cells with many chloroplasts, thick walls impervious to gases and no intercellular spaces; tolerance of high temperature and light; no photorespiration; higher productivity.

11.9 Photorespiration

RuBisCO can bind both CO₂ and O₂ (competitively). In C₃ plants some O₂ binds: RuBP forms one phosphoglycerate and one phosphoglycolate (2C). No sugar, ATP or NADPH is made; CO₂ is released with use of ATP — a wasteful process. C₄ plants avoid it by concentrating CO₂ at the enzyme site in bundle sheath cells.

11.10 Factors Affecting Photosynthesis

Internal: number, size, age, orientation of leaves; mesophyll cells and chloroplasts; internal CO₂; chlorophyll. External: light, temperature, CO₂, water.

Diagrams to Know (describe and label)

Key-Facts Sheet

Exam Focus

The chapter's exercises ask you to: tell C₃ from C₄ plants (internally, by Kranz anatomy — not reliably by external look); explain why C₄ plants are highly productive though only a few cells run the Calvin cycle; explain why RuBisCO carboxylates more in C₄ plants; discuss what would happen without chlorophyll a; explain why leaves kept in the dark turn yellow (chlorophyll breaks down, more stable carotenoids remain); interpret the light-intensity graph (Figure 11.10); and compare C₃ vs C₄ pathways and cyclic vs non-cyclic photophosphorylation.

Key Terms

Common Mistakes