Chapter 5 — The Fundamental Unit of Life
Welcome to HSLC Guru! This study guide is prepared for ASSEB Class 9 Science learners who want a clear, exam-ready understanding of Chapter 5 — The Fundamental Unit of Life. The cell is the basic structural and functional unit of every living organism. From the unicellular Amoeba to the trillions of cells inside a human body, life begins, functions and continues only because of cells. In this chapter we shall explore how cells were discovered, the cell theory, the structure of prokaryotic and eukaryotic cells, the various organelles and their functions, the basic transport processes such as diffusion, osmosis and plasmolysis, and finally how cells multiply through mitosis and meiosis.
This article presents a complete summary, NCERT/ASSEB-aligned textbook question answers, additional MCQs, fill in the blanks, true/false statements and a glossary table — everything you need to revise Chapter 5 in one place. Read the summary first, then attempt the questions on your own before checking the answers. Bookmark this page for revision before your half-yearly and final examinations.
Summary of the Chapter
Discovery of the Cell: The cell was discovered in 1665 by Robert Hooke, an English scientist, while examining a thin slice of cork (the dead bark of a tree) under his self-designed microscope. He noticed honeycomb-like compartments and named them “cells” (from the Latin cellula meaning “a little room”). A few years later, the Dutch scientist Anton van Leeuwenhoek (1674) observed living cells such as bacteria and protozoa in pond water using a more powerful microscope, becoming the first person to see free-living cells. Robert Brown (1831) discovered the nucleus, Purkinje (1839) coined the term “protoplasm” for the living substance of the cell, and Camillo Golgi (1898) discovered the Golgi apparatus.
Cell Theory: In 1838 the German botanist M. J. Schleiden proposed that all plants are composed of cells. In 1839 the German zoologist Theodor Schwann stated that all animals are made up of cells, and together they formulated the Cell Theory. Later in 1855, Rudolf Virchow extended the theory by adding that “all cells arise from pre-existing cells” (Omnis cellula e cellula). The modern cell theory states: (i) all living organisms are made of cells, (ii) the cell is the basic unit of life, and (iii) all cells come from pre-existing cells.
Types of Cells — Prokaryotic vs Eukaryotic: Prokaryotic cells (e.g., bacteria, blue-green algae, mycoplasma) are small (1–10 µm), lack a true membrane-bound nucleus and lack membrane-bound organelles. Their genetic material lies free in the cytoplasm in a region called the nucleoid. Eukaryotic cells (e.g., plant cells, animal cells, fungi, protozoa) are larger (5–100 µm), possess a true nucleus enclosed by a nuclear membrane, and contain well-developed membrane-bound organelles such as mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes and plastids. On the basis of number of cells, organisms are classified as unicellular (Amoeba, Paramecium, bacteria) or multicellular (plants, animals, fungi).
Cell Organelles and Their Functions: The plasma membrane is a thin, flexible, selectively permeable layer made of lipids and proteins that allows entry and exit of substances; it follows the fluid mosaic model proposed by Singer and Nicolson (1972). The cell wall is found only in plant cells, fungi and bacteria; in plants it is made of cellulose and provides shape and rigidity. The nucleus, bounded by a nuclear membrane with pores, contains chromatin (DNA + protein) and a nucleolus; it controls all cell activities and stores hereditary information. The cytoplasm is the jelly-like fluid between the plasma membrane and nucleus where the organelles float and most metabolic reactions occur.
The endoplasmic reticulum (ER) is a network of membrane-bound tubules; rough ER bears ribosomes and helps in protein synthesis, while smooth ER lacks ribosomes and helps in lipid synthesis and detoxification. The Golgi apparatus, discovered by Camillo Golgi, consists of flat membranous sacs (cisternae) that modify, package and dispatch materials; it also forms lysosomes. Lysosomes are tiny membrane-bound sacs filled with digestive enzymes; they digest worn-out organelles and foreign particles, hence called the “suicidal bags” of the cell. Mitochondria are double-membrane structures (inner membrane folded into cristae) that produce ATP through cellular respiration and are therefore called the “powerhouse of the cell”; they have their own DNA and ribosomes. Plastids are found only in plant cells and are of three kinds — chloroplasts (green, contain chlorophyll, carry out photosynthesis), chromoplasts (yellow/orange/red pigments, give colour to flowers and fruits) and leucoplasts (colourless, store starch, oils and proteins). Vacuoles are fluid-filled sacs; in plant cells a single large central vacuole maintains turgor pressure, while animal cells have small temporary vacuoles. Ribosomes are tiny non-membranous granules of RNA and protein, found free in the cytoplasm or attached to rough ER, and are the sites of protein synthesis.
Cell Division — Mitosis vs Meiosis: The cell cycle consists of interphase (G1, S, G2) and the M phase (division). Mitosis occurs in somatic (body) cells, produces two daughter cells identical to the parent cell with the same chromosome number (diploid → diploid); it is responsible for growth, repair and replacement of cells. The phases are prophase, metaphase, anaphase and telophase. Meiosis occurs in reproductive cells (germ cells), produces four daughter cells with half the number of chromosomes (diploid → haploid); it is essential for the formation of gametes and brings about genetic variation. Meiosis has two successive divisions — meiosis I (reductional) and meiosis II (equational).
Diffusion, Osmosis and Plasmolysis: Diffusion is the movement of molecules of a substance from a region of higher concentration to a region of lower concentration; gases such as O₂ and CO₂ enter and leave cells by diffusion. Osmosis is a special type of diffusion in which water molecules move through a semi-permeable membrane from a region of higher water concentration (dilute/hypotonic) to a region of lower water concentration (concentrated/hypertonic). When a living plant cell is placed in a hypertonic solution, water moves out and the cytoplasm shrinks away from the cell wall — this phenomenon is called plasmolysis. The reverse process, in which a plasmolysed cell regains its turgidity in a hypotonic solution, is called deplasmolysis. These processes are vital for absorption of water by roots, opening and closing of stomata, and maintaining cell shape.
Textbook Questions and Answers
A. Very Short Answer Type Questions (1 Mark)
Q1. Who discovered the cell and in which year?
Answer: The cell was discovered by Robert Hooke in 1665 while examining a thin slice of cork under his microscope.
Q2. Who first observed living cells?
Answer: Anton van Leeuwenhoek first observed free-living cells (bacteria and protozoa) in pond water in 1674.
Q3. Who discovered the nucleus?
Answer: Robert Brown discovered the nucleus in 1831.
Q4. Why is the mitochondrion called the powerhouse of the cell?
Answer: Because it produces energy in the form of ATP through cellular respiration.
Q5. Why are lysosomes called suicidal bags of the cell?
Answer: Because they contain powerful digestive enzymes that can digest the cell’s own contents when it is damaged or dead.
Q6. Name the organelle responsible for protein synthesis.
Answer: Ribosomes are responsible for protein synthesis.
Q7. What is the chemical composition of plant cell wall?
Answer: The plant cell wall is mainly made of cellulose.
Q8. Define osmosis.
Answer: Osmosis is the movement of water molecules through a semi-permeable membrane from a region of higher water concentration to a region of lower water concentration.
Q9. Name the green plastid that performs photosynthesis.
Answer: Chloroplast.
Q10. Who proposed the cell theory?
Answer: M. J. Schleiden (1838) and Theodor Schwann (1839) proposed the cell theory; it was later modified by Rudolf Virchow (1855).
B. Short Answer Type Questions (2–3 Marks)
Q1. Differentiate between prokaryotic and eukaryotic cells (any three points).
Answer:
- Prokaryotic cells lack a true nucleus, while eukaryotic cells have a true membrane-bound nucleus.
- Prokaryotic cells lack membrane-bound organelles; eukaryotic cells have well-developed organelles like mitochondria, ER, Golgi apparatus.
- Prokaryotic cells are small (1–10 µm) e.g., bacteria; eukaryotic cells are larger (5–100 µm) e.g., plant and animal cells.
Q2. What is plasmolysis? Give an example.
Answer: Plasmolysis is the shrinkage of the cytoplasm of a living plant cell away from its cell wall when placed in a hypertonic (highly concentrated) solution. The cell loses water by osmosis. Example: when raisins or grapes are placed in concentrated sugar/salt solution, they shrink — this is due to plasmolysis.
Q3. Write three differences between a plant cell and an animal cell.
Answer:
- Plant cells have a rigid cellulose cell wall, while animal cells lack a cell wall.
- Plant cells contain plastids (chloroplasts); animal cells lack plastids.
- Plant cells have one large central vacuole; animal cells have small, scattered vacuoles or none at all.
Q4. What are the functions of the plasma membrane?
Answer: The plasma membrane (i) gives shape to the cell and encloses its contents, (ii) is selectively permeable — it controls the entry and exit of substances such as ions, water and nutrients, and (iii) helps in cell-to-cell communication and the transport of materials by diffusion, osmosis and active transport.
Q5. Distinguish between diffusion and osmosis.
Answer:
- Diffusion is the movement of any molecule (solid, liquid or gas) from higher to lower concentration; osmosis is specifically the movement of water (solvent) molecules.
- Diffusion does not require a semi-permeable membrane; osmosis requires a semi-permeable membrane.
- Example of diffusion — exchange of O₂ and CO₂ in lungs; example of osmosis — absorption of water by root hairs.
Q6. Describe the three types of plastids briefly.
Answer: (i) Chloroplasts — green plastids containing chlorophyll, perform photosynthesis. (ii) Chromoplasts — coloured plastids (yellow, orange, red) which give colour to flowers and fruits. (iii) Leucoplasts — colourless plastids that store food materials such as starch (amyloplasts), oils (elaioplasts) and proteins (aleuroplasts).
C. Long Answer Type Questions (5–6 Marks)
Q1. Describe the structure and functions of the nucleus.
Answer: The nucleus is the largest and most important organelle of a eukaryotic cell, often called the “control centre” or “brain” of the cell. Structure: It is a spherical or oval body enclosed by a double-layered nuclear membrane (nuclear envelope) which has many small pores called nuclear pores. The nucleus contains a dense fluid called nucleoplasm in which a deeply stained, thread-like material called chromatin is suspended. Chromatin is composed of DNA and proteins, and during cell division it condenses into rod-shaped structures called chromosomes. Inside the nucleus is a small, dense, spherical body called the nucleolus, which is the site of ribosome synthesis. Functions: (i) it controls all the metabolic activities of the cell, (ii) it carries the genetic/hereditary information from one generation to another through DNA, (iii) it directs protein synthesis through mRNA, and (iv) it plays a key role in cell division and growth.
Q2. Explain the structure of mitochondria and justify why they are called the powerhouse of the cell.
Answer: Mitochondria are tiny rod-shaped or spherical organelles found in the cytoplasm of all eukaryotic cells (except mature RBCs). Structure: Each mitochondrion is bounded by a double membrane. The outer membrane is smooth and permeable to small molecules, while the inner membrane is highly folded into finger-like projections called cristae which increase the surface area for chemical reactions. The space inside the inner membrane is filled with a fluid called the matrix, which contains enzymes, ribosomes and a circular DNA. Function: Mitochondria are the sites of cellular respiration, in which glucose and oxygen react to release energy. This energy is stored in the form of ATP (adenosine triphosphate) molecules, which act as the energy currency of the cell. Since mitochondria generate almost all the energy required for the cell’s activities, they are rightly called the “powerhouse of the cell.” Their possession of their own DNA and ribosomes also indicates that they are semi-autonomous organelles.
Q3. Differentiate between mitosis and meiosis.
Answer:
- Site: Mitosis takes place in somatic (body) cells; meiosis takes place in reproductive (germ) cells.
- Number of divisions: Mitosis involves a single division; meiosis involves two successive divisions (Meiosis I and II).
- Daughter cells: Mitosis produces two daughter cells; meiosis produces four daughter cells.
- Chromosome number: In mitosis, daughter cells have the same chromosome number as the parent cell (diploid → diploid); in meiosis, the chromosome number is halved (diploid → haploid).
- Genetic identity: Mitotic daughter cells are genetically identical to the parent; meiotic daughter cells differ genetically due to crossing over and random assortment.
- Function: Mitosis is responsible for growth, repair and asexual reproduction; meiosis is responsible for the formation of gametes and brings about genetic variation in sexually reproducing organisms.
Q4. Describe the structure of a typical plant cell with the help of a labelled outline.
Answer: A typical plant cell is bounded by a rigid cell wall made of cellulose, which provides mechanical strength and a definite shape. Inside the cell wall lies the thin, selectively permeable plasma membrane. The cytoplasm is the jelly-like fluid that fills the cell and houses the organelles. A large, well-defined nucleus with a nuclear membrane, nucleoplasm, chromatin and nucleolus controls cell activities. The endoplasmic reticulum (rough and smooth) forms a network of channels for transport and synthesis of proteins and lipids. The Golgi apparatus packages and dispatches materials. Mitochondria generate ATP. Plastids — chloroplasts, chromoplasts and leucoplasts — perform photosynthesis, impart colour, and store food respectively. A single large central vacuole maintains turgor pressure and stores cell sap. Ribosomes synthesise proteins, and lysosomes (when present) digest waste materials. Together, these organelles allow the plant cell to carry out all the essential life processes.
Q5. Explain diffusion and osmosis with suitable examples and state their importance in living organisms.
Answer: Diffusion is the spontaneous movement of molecules (solid, liquid or gas) from a region of higher concentration to a region of lower concentration until they are uniformly distributed. Example — when a bottle of perfume is opened in one corner of a room, the smell soon spreads throughout the room because perfume molecules diffuse through the air. In living organisms, gaseous exchange (entry of O₂ and exit of CO₂ during respiration) takes place through diffusion across the cell membrane. Osmosis is a special kind of diffusion in which only water/solvent molecules move through a semi-permeable membrane from a region of higher water concentration (dilute solution) to a region of lower water concentration (concentrated solution). Example — when raisins are soaked in water they swell up because water enters them by osmosis. Importance: (i) absorption of water by root hairs from the soil takes place by osmosis, (ii) opening and closing of stomata is controlled by osmotic changes in guard cells, (iii) movement of water from cell to cell in plants and (iv) maintenance of turgidity and shape of cells all depend on diffusion and osmosis.
Additional Multiple Choice Questions (MCQ)
Q1. Who discovered the cell?
(a) Robert Brown (b) Robert Hooke (c) Schleiden (d) Schwann
Answer: (b) Robert Hooke
Q2. The basic structural and functional unit of life is —
(a) tissue (b) organ (c) cell (d) organelle
Answer: (c) cell
Q3. Which of the following is a prokaryote?
(a) Amoeba (b) Yeast (c) Bacterium (d) Paramecium
Answer: (c) Bacterium
Q4. Powerhouse of the cell is —
(a) Ribosome (b) Mitochondrion (c) Lysosome (d) Golgi body
Answer: (b) Mitochondrion
Q5. Which of the following is found only in plant cells?
(a) Mitochondria (b) Plastids (c) Ribosomes (d) Nucleus
Answer: (b) Plastids
Q6. The cell wall of a plant cell is mainly made of —
(a) Chitin (b) Cellulose (c) Lipid (d) Protein
Answer: (b) Cellulose
Q7. “Suicidal bags” of the cell are —
(a) Lysosomes (b) Vacuoles (c) Plastids (d) Centrosomes
Answer: (a) Lysosomes
Q8. The number of daughter cells produced after meiosis is —
(a) 2 (b) 4 (c) 6 (d) 8
Answer: (b) 4
Q9. Site of protein synthesis in the cell is —
(a) Nucleus (b) Mitochondria (c) Ribosome (d) Lysosome
Answer: (c) Ribosome
Q10. Movement of water through a semi-permeable membrane is called —
(a) Diffusion (b) Osmosis (c) Plasmolysis (d) Active transport
Answer: (b) Osmosis
Fill in the Blanks
Q1. The cell was discovered by ________ in the year ________.
Answer: Robert Hooke; 1665.
Q2. The ________ is called the powerhouse of the cell.
Answer: Mitochondrion.
Q3. The green plastid containing chlorophyll is called ________.
Answer: Chloroplast.
Q4. The shrinkage of cytoplasm in a hypertonic solution is called ________.
Answer: Plasmolysis.
Q5. ________ is the type of cell division that produces gametes.
Answer: Meiosis.
True / False
Q1. Bacteria are eukaryotic organisms.
Answer: False — bacteria are prokaryotes.
Q2. Ribosomes are sites of protein synthesis.
Answer: True.
Q3. Animal cells contain a cell wall made of cellulose.
Answer: False — only plant cells (and fungi/bacteria) have a cell wall; animal cells do not.
Q4. Mitosis produces four daughter cells with half the chromosome number.
Answer: False — mitosis produces two identical daughter cells with the same chromosome number; meiosis produces four with half.
Q5. Osmosis requires a semi-permeable membrane.
Answer: True.
Glossary
| Term | Meaning |
|---|---|
| Cell | The basic structural and functional unit of all living organisms. |
| Cell Theory | Theory by Schleiden, Schwann and Virchow stating that all living things are made of cells and cells come from pre-existing cells. |
| Prokaryotic Cell | A cell without a true membrane-bound nucleus, e.g., bacteria. |
| Eukaryotic Cell | A cell with a true nucleus and membrane-bound organelles, e.g., plant and animal cells. |
| Plasma Membrane | A thin, selectively permeable lipid-protein layer enclosing the cell. |
| Cell Wall | A rigid outer layer of cellulose found in plant cells; provides shape and support. |
| Nucleus | The control centre of the cell containing DNA in chromatin form. |
| Cytoplasm | The jelly-like fluid between the plasma membrane and nucleus where organelles float. |
| Endoplasmic Reticulum | A network of membranes; rough ER bears ribosomes (protein synthesis), smooth ER makes lipids. |
| Golgi Apparatus | Stack of membranous sacs that modify, package and dispatch cell products. |
| Lysosome | Membrane-bound sac of digestive enzymes; “suicidal bag” of the cell. |
| Mitochondrion | Double-membrane organelle that produces ATP; “powerhouse of the cell”. |
| Plastid | Plant-cell organelle of three types — chloroplast, chromoplast, leucoplast. |
| Chloroplast | Green plastid with chlorophyll; carries out photosynthesis. |
| Chromoplast | Coloured plastid giving yellow/orange/red colour to flowers and fruits. |
| Leucoplast | Colourless plastid that stores food materials such as starch, oils and proteins. |
| Vacuole | Fluid-filled sac that stores cell sap and maintains turgor pressure. |
| Ribosome | Tiny non-membranous granule of RNA and protein; site of protein synthesis. |
| Mitosis | Cell division in body cells producing two identical diploid daughter cells. |
| Meiosis | Cell division in germ cells producing four haploid daughter cells (gametes). |
| Diffusion | Movement of molecules from higher to lower concentration. |
| Osmosis | Movement of water through a semi-permeable membrane from higher to lower water concentration. |
| Plasmolysis | Shrinkage of cytoplasm of a plant cell in a hypertonic solution due to loss of water. |
| ATP | Adenosine triphosphate — the energy currency of the cell. |
Quick Revision Points
Before sitting for the examination, run through the following one-line revision points to make sure all the key facts are at your fingertips.
- Cell discovered by Robert Hooke (1665) in cork; living cells first seen by Anton van Leeuwenhoek (1674).
- Nucleus discovered by Robert Brown (1831); term “protoplasm” given by Purkinje (1839).
- Cell theory: Schleiden (1838) + Schwann (1839); modified by Virchow (1855) — “Omnis cellula e cellula”.
- Two main types of cells — prokaryotic (no true nucleus) and eukaryotic (true nucleus + organelles).
- Plasma membrane is selectively permeable; follows the fluid mosaic model (Singer & Nicolson, 1972).
- Plant cell wall is made of cellulose; bacterial cell wall is made of peptidoglycan; fungal cell wall is made of chitin.
- Nucleus = control centre of the cell; contains DNA in the form of chromatin.
- Mitochondrion = powerhouse of the cell; site of cellular respiration; produces ATP.
- Lysosome = suicidal bag; ribosome = site of protein synthesis; Golgi apparatus = packaging and dispatch unit.
- Plastids — only in plants — chloroplast (photosynthesis), chromoplast (colour), leucoplast (storage).
- Vacuole — large central in plants (turgor pressure); small and temporary in animals.
- Mitosis — body cells, 2 diploid daughters, growth and repair; meiosis — germ cells, 4 haploid daughters, gamete formation.
- Diffusion — high to low concentration; osmosis — water through semi-permeable membrane.
- Plasmolysis — shrinkage of cytoplasm in hypertonic solution; reversal is called deplasmolysis.
Important Scientists at a Glance
| Scientist | Year | Contribution |
|---|---|---|
| Robert Hooke | 1665 | Discovered the cell while observing a slice of cork. |
| Anton van Leeuwenhoek | 1674 | First observed living cells (bacteria, protozoa). |
| Robert Brown | 1831 | Discovered the nucleus. |
| J. E. Purkinje | 1839 | Coined the term “protoplasm”. |
| M. J. Schleiden | 1838 | Stated that all plants are made of cells. |
| Theodor Schwann | 1839 | Stated that all animals are made of cells; co-author of cell theory. |
| Rudolf Virchow | 1855 | “Omnis cellula e cellula” — all cells from pre-existing cells. |
| Camillo Golgi | 1898 | Discovered the Golgi apparatus. |
| Singer & Nicolson | 1972 | Proposed the fluid mosaic model of the plasma membrane. |
Differences Between Plant Cell and Animal Cell
| Feature | Plant Cell | Animal Cell |
|---|---|---|
| Cell wall | Present, made of cellulose | Absent |
| Shape | Generally fixed and rectangular | Generally irregular and rounded |
| Plastids | Present (chloroplast, chromoplast, leucoplast) | Absent |
| Vacuole | One large central vacuole | Many small or no vacuoles |
| Centrosome | Absent | Present, helps in cell division |
| Lysosomes | Rare | Common, well developed |
| Mode of nutrition | Autotrophic (photosynthesis) | Heterotrophic |
| Storage of food | As starch | As glycogen |
Exam Tips for Chapter 5
- Always memorise the year and the discoverer pair — Hooke (1665), Leeuwenhoek (1674), Brown (1831).
- Practise neat, well-labelled diagrams of a plant cell and an animal cell — they are commonly asked for 3-mark or 5-mark questions.
- Remember the “powerhouse” (mitochondrion), “suicidal bag” (lysosome), “control centre” (nucleus) and “packaging unit” (Golgi apparatus) — short identification questions are based on these phrases.
- Differences (prokaryotic vs eukaryotic, plant vs animal cell, mitosis vs meiosis, diffusion vs osmosis) are favourite questions — write them in tabular form for full marks.
- Define every key term in one or two sentences and add an example wherever possible.
That brings us to the end of Chapter 5 — The Fundamental Unit of Life. Revise the diagrams of plant and animal cells, learn the differences between mitosis and meiosis, and remember the major scientists associated with cell biology. For more ASSEB-aligned Class 9 Science notes, MCQs and previous-year solutions, keep visiting HSLC Guru.