NCERT Class 9 Science Chapter 5 – The Fundamental Unit of Life

NCERT Class 9 Science Chapter 5 explains that the cell is the basic structural and functional unit of life. NCERT Class 9 Science Chapter 5 builds strong conceptual clarity about cell structure, cell organelles and differences between plant and animal cells, which are essential for advanced biology.

The chapter traces important historical discoveries such as Robert Hooke (1665) observing cork cells, Anton van Leeuwenhoek (1674) observing living cells, and the formulation of Cell Theory by Matthias Schleiden (1838) and Theodor Schwann (1839). Later, Rudolf Virchow (1855) stated that all cells arise from pre-existing cells.

NCERT Class 9 Science Chapter 5 explains major cell organelles such as nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, ribosomes, plastids and vacuoles. It also differentiates between prokaryotic and eukaryotic cells and highlights structural differences between plant and animal cells.

The chapter introduces biological measurements such as cell size in micrometres (µm) and explains that 1 micrometre = 10⁻⁶ metre.

For structured preparation of NCERT Class 9–12 for UPSC, BPSC and State PCS examinations, strengthen your basics with our complete NCERT Book Notes PDF for Class 9-12, available inside the NCERT foundation course level-2.

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1. Introduction

• All living organisms are made up of basic structural and functional units called cells.
• The word cell was first used by Robert Hooke (1665) while observing cork under a microscope in London.
• Hooke observed box-like structures and named them cells meaning “small rooms”.
• Anton van Leeuwenhoek (1674) first observed living cells in pond water using a simple microscope.
• Matthias Schleiden (1838) stated that all plants are made of cells.
• Theodor Schwann (1839) stated that all animals are made of cells.
• Rudolf Virchow (1855) proposed that new cells arise from pre-existing cells, stated as “Omnis cellula e cellula”.
• Cell is therefore called the fundamental unit of life.

2. Cell theory

• Proposed jointly by Schleiden and Schwann (1838–1839).
• All living organisms are composed of cells.
• Cell is the basic structural and functional unit of life.
• Virchow (1855) added that all cells arise from pre-existing cells.
• Cell theory applies to both unicellular and multicellular organisms.

3. Discovery and development of microscope

• Robert Hooke (1665) observed dead cork cells using a compound microscope.
• Leeuwenhoek (1674) improved lenses and observed bacteria and protozoa.
• Compound microscope uses two lenses – objective and eyepiece.
• Electron microscope has resolving power of about 0.2 nanometre, much greater than light microscope.
• Cell organelles like mitochondria and ribosomes are clearly visible under electron microscope.

4. Diversity in cells

• Organisms may be unicellular like Amoeba and Paramecium.
• Multicellular organisms like humans and plants contain millions of cells.
• Cells vary in shape, size and function.
• Smallest cell known is Mycoplasma (0.1 µm).
• Largest cell in human body is nerve cell, and largest cell in general is ostrich egg.
• Size of most cells ranges from 0.1 µm to 100 µm.

5. Cell structure

• Every cell consists of plasma membrane, cytoplasm and nucleus.
• Cell organelles are suspended in cytoplasm.
• Structure differs slightly in plant and animal cells.
• Diagram in textbook shows labelled structures of both types.

6. Plasma membrane

• Also called cell membrane.
• Composed of lipids and proteins.
• Selectively permeable membrane.
• Controls movement of substances in and out of cell.
• Allows diffusion and osmosis.

7. Diffusion and osmosis

• Diffusion is movement of molecules from region of higher concentration to lower concentration.
• Osmosis is diffusion of water through selectively permeable membrane.
• Hypotonic solution causes cell to swell.
• Hypertonic solution causes cell to shrink (plasmolysis in plants).
• Isotonic solution maintains cell size.

8. Cell wall

• Present in plant cells only.
• Made of cellulose.
• Provides rigidity and protection.
• Freely permeable.
• Prevents bursting of cell in hypotonic solution.

9. Nucleus

• Dense, spherical organelle present in most cells.
• Surrounded by nuclear membrane.
• Contains chromosomes, composed of DNA and proteins.
• DNA carries hereditary information.
• Nucleolus present inside nucleus.
• Prokaryotic cells lack true nucleus.

10. Prokaryotic and eukaryotic cells

• Prokaryotes lack membrane-bound nucleus.
• Example: Bacteria, Blue-green algae (Cyanobacteria).
• Eukaryotes have well-defined nucleus.
• Example: Plants, animals, fungi and protozoa.
• Prokaryotic cells are generally smaller in size.

11. Cytoplasm

• Jelly-like substance between nucleus and plasma membrane.
• Contains cell organelles.
• Site of most cellular activities.
• Supports metabolic reactions.

12. Mitochondria

• Called powerhouse of the cell.
• Double membrane structure.
• Inner membrane folded into cristae.
• Site of cellular respiration and ATP production.
• Have their own DNA and ribosomes.

13. Endoplasmic reticulum (ER)

• Network of membrane-bound tubules.
• Rough ER has ribosomes attached.
• Smooth ER lacks ribosomes.
• RER involved in protein synthesis.
• SER involved in lipid synthesis and detoxification.

14. Golgi apparatus

• Discovered by Camillo Golgi (1898).
• Composed of flattened sacs called cisternae.
• Modifies, packages and transports proteins.
• Forms lysosomes.
• Important in secretion.

15. Lysosomes

• Called suicide bags of the cell.
• Contain digestive enzymes.
• Break down waste materials and foreign bodies.
• Active in animal cells.

16. Ribosomes

• Site of protein synthesis.
• Present freely in cytoplasm or attached to RER.
• Composed of RNA and proteins.
• Not membrane-bound.

17. Vacuoles

• Storage sacs containing water, food or waste.
• Large central vacuole present in plant cells.
• Surrounded by membrane called tonoplast.
• Helps in maintaining turgor pressure.

18. Plastids

• Present only in plant cells.
• Three types: Chloroplast, Chromoplast, Leucoplast.
• Chloroplast contains chlorophyll and performs photosynthesis.
• Plastids have their own DNA.

19. Comparison between plant and animal cells

• Plant cells have cell wall, animal cells do not.
• Plant cells contain plastids, animal cells do not.
• Plant cells have large central vacuole.
• Animal cells have small or temporary vacuoles.
• Lysosomes prominent in animal cells.

20. Diagram explanation

• Plant cell diagram shows cell wall, plasma membrane, chloroplast, vacuole, nucleus, mitochondria, ER and Golgi body.
• Animal cell diagram lacks cell wall and chloroplast.
• Nucleus centrally placed in animal cell but peripheral in plant cell due to vacuole.
• Diagram highlights structural differences clearly.

21. Conclusion

• Cell is structural and functional unit of life.
• Discovery of microscope revolutionised cell biology.
• Organelles perform specialised functions.
• Differences exist between prokaryotic and eukaryotic cells.
• Plant and animal cells differ structurally.

22. Exam oriented facts

• Robert Hooke (1665) – Discovered cork cells.
• Leeuwenhoek (1674) – First observed living cells.
• Schleiden (1838) – Plants made of cells.
• Schwann (1839) – Animals made of cells.
• Virchow (1855) – Cells arise from pre-existing cells.
• Golgi (1898) – Discovered Golgi apparatus.
• Smallest cell – Mycoplasma (0.1 µm).
• Electron microscope resolution – 0.2 nm.
• Mitochondria and plastids contain their own DNA.
• Plasmolysis – Shrinking of cell membrane in hypertonic solution.

Understanding NCERT Class 9 Science Chapter 5 is essential for developing a strong foundation in cell biology.

This chapter forms the base for higher topics such as cell division, genetics, human physiology and biotechnology in senior secondary classes.

For school examinations, students must focus on cell theory, functions of organelles, differences between plant and animal cells, and prokaryotic vs eukaryotic cells.

For competitive examinations like NEET, NCERT Class 9 Science Chapter 5 provides fundamental clarity about cellular structure and biological organization.

Continue reading NCERT Class 9 Science Chapter 6 – Tissues to understand how cells group together to perform specific functions.

FAQs

Q1. What is NCERT Class 9 Science Chapter 5 about?
It explains the structure and function of cells, cell organelles, cell theory, and differences between plant and animal cells.

Q2. Who discovered the cell?
Robert Hooke discovered cells in 1665 while observing cork under a microscope.

Q3. What is cell theory?
Cell theory states that all living organisms are made of cells, the cell is the basic unit of life, and new cells arise from pre-existing cells.

Q4. What is the difference between prokaryotic and eukaryotic cells?
Prokaryotic cells lack a well-defined nucleus and membrane-bound organelles, while eukaryotic cells have a true nucleus and specialized organelles.

Q5. Why is NCERT Class 9 Science Chapter 5 important for competitive exams?
It builds the foundation for advanced topics in biology such as genetics, human physiology and biotechnology, which are important for NEET and other medical entrance exams.

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