Chapter 19 of 25

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Tamil Nadu (TNBSE)Class 9 Science★ 4-5 marks in assessments

Plant Physiology

Samacheer Kalvi (TN State Board) Class 9 Science — Biology Chapter 19, Plant Physiology. Plant physiology covers how plants function. Understanding tropisms, photosynthesis, transpiration, and gas exchange explains how plants respond to light/gravity, lose water, and produce oxygen for our atmosphere. Notes, worked examples, common mistakes, and quick revision.

⏱ 18 min readMedium difficulty✓ Free · NCERT-aligned

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By the end of this chapter you'll be able to…

1Describe plant tropic and nastic movements with examples
2Explain the mechanism of photosynthesis (light and dark reactions)
3Describe transpiration and its significance in plants
4Explain aerobic and anaerobic respiration in plants

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Why this chapter matters

Plant physiology covers how plants function. Understanding tropisms, photosynthesis, transpiration, and gas exchange explains how plants respond to light/gravity, lose water, and produce oxygen for our atmosphere.

Before you start — revise these

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Class 8 Science — Plant Kingdom

Basic plant structures.

Plant Physiology — Class 9 Science (Samacheer Kalvi)

TN State Board (Samacheer Kalvi) Class 9 Science, Biology — Chapter 19. Plant physiology covers how plants function. Understanding tropisms, photosynthesis, transpiration, and gas exchange explains how plants respond to light/gravity, lose water, and produce oxygen for our atmosphere.

1. About this chapter

This chapter covers plant movements (tropisms/nastic), photosynthesis, transpiration, and respiration.

2. Plant Movements

Tropic Movements: Directional growth responses.
- Phototropism: Response to light (stems).
- Geotropism: Response to gravity (roots).
- Hydrotropism: Response to water.
- Thigmotropism: Response to touch (tendrils).
Nastic Movements: Non-directional movements.
- Seismonastic: Touch response (closing of Mimosa pudica).
- Photonastic: Light response (opening/closing of flowers).

3. Photosynthesis

Process of synthesizing glucose from CO₂ and H₂O in the presence of sunlight and chlorophyll.
Hill Reaction (Light): Splitting of water (photolysis), ATP/NADPH produced in Grana.
Calvin Cycle (Dark): Reduction of CO₂ to glucose in Stroma.

4. Transpiration

Loss of water through stomata.
Significance: Transpiration pull helps in mineral/water absorption; cools leaf surfaces.

Key formulas & results

Everything you need to memorise, in one card. Screenshot this for revision.

Photosynthesis Equation

6 CO2 + 6 H2O -> C6H12O6 + 6 O2

Occurs in chloroplasts in presence of light and chlorophyll.

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Common mistakes & fixes

These are the exact errors that cost students marks in board exams. Read them once, save yourself the trouble.

WATCH OUT

✗ Confusing tropic movements and nastic movements.

✓ Tropic movements are directional and depend on the stimulus source (e.g. Phototropism, geotropism). Nastic movements are non-directional and independent of the stimulus source (e.g. Mimosa pudica closing).

WATCH OUT

✗ Assuming plants only respire at night.

✓ Plants respire (inhale oxygen, release CO₂) continuously day and night. Photosynthesis (which releases oxygen) occurs only during daylight.

NCERT exercises (with solutions)

Every NCERT exercise from this chapter — what it covers and how many questions to expect.

Multiple Choice Questions

Questions on tropisms, photosynthesis, and transpiration.

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Short Answer Questions

Phototropism, light reaction, transpiration pull, and nastic movement comparison.

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Practice problems

Try each one yourself before tapping "Show solution". Active recall > rereading.

Q1EASY· Concept

What is phototropism and geotropism?

▶ Show solution

1. Phototropism: movement of plant parts towards light (stems show positive phototropism). 2. Geotropism: growth of plant parts in response to gravity (roots show positive geotropism).

Q2MEDIUM· Concept

State the two phases of photosynthesis.

▶ Show solution

1. Light-dependent reaction (Hill reaction): occurs in thylakoid membranes, uses light to split water, releasing oxygen, ATP, and NADPH. 2. Light-independent reaction (Calvin cycle): occurs in stroma, uses ATP and NADPH to reduce carbon dioxide to glucose.

5-minute revision

The whole chapter, distilled. Read this the night before the exam.

•Tropic: directional (photo, geo, hydro, thigmo).
•Nastic: non-directional (seismo, photo).
•Photosynthesis: 6CO₂ + 6H₂O -> C₆H₁₂O₆ + 6O₂. Light and dark reactions.
•Transpiration cools the plant and creates suction pull.
•Respiration occurs day and night.

Tamil Nadu (TNBSE) marks blueprint

Where the marks come from in this chapter — so you can plan your prep.

Typical chapter weightage: 4-5 marks in assessments

Question type	Marks each	Typical count	What it tests
MCQ	1	1-2	Base concepts and definitions
Short Answer	2-3	1-2	Descriptive and application points

Prep strategy

Understand core definitions and solve standard textbook problems.
Review common mistakes to avoid losing easy marks.

Where this shows up in the real world

This chapter isn't just an exam topic — it lives in the world around you.

Agriculture

Irrigation schedules are designed based on plant transpiration rates under different seasonal climates.

Greenhouse Cultivation

Controlling light levels and CO₂ concentrations inside greenhouses boosts photosynthesis, increasing crop yields.

Exam strategy

Battle-tested tips from teachers and toppers for this chapter.

Write definitions precisely as defined in the textbook.
Draw neat, labeled diagrams for biology and physics chapters.

Going beyond the textbook

For olympiad aspirants and curious learners — topics that build on this chapter.

Read advanced reference materials to explore concepts beyond the school syllabus.

Where else this chapter is tested

CBSE board isn't the only one — other exams test this chapter too.

Class 9 Annual ExamsHigh

NTSE Stage 1Medium

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Questions students ask

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The loss of water in the form of water vapour from the aerial parts of a plant, primarily through stomata in leaves.

It causes water loss, but is necessary because it creates a suction pull (transpiration pull) to transport water and minerals from roots to leaves and cools the plant.

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