Cell: The Building Block of Life Class 9 Notes, Diagrams and Questions

What Cell: The Building Block of Life is about

Weightage: This chapter is part of Unit I, World of Living, which carries 27 of the 80 theory marks in the school annual exam, shared with Tissues in Action, Reproduction: How Life Continues and Patterns in Life: Diversity and Classification.Source: CBSE Science (086) syllabus, Class IX 2026-27, course structure, Unit I World of Living

Key concepts

Seeing cells: resolution and microscopes

NCERT section 2.1 How to Study Cells?

The limit of resolution of the human eye is about 0.1 mm: two dots closer than that (viewed from about 25 cm) look like one. Most cells are smaller, so we need lenses.

• Robert Hooke (1665) looked at thin cork with his own microscope (about 200 to 300 times magnification) and named the box-like compartments cells.
• A school light microscope uses an objective lens and an eyepiece; total magnification = objective × eyepiece (e.g. 40X × 10X = 400X). It uses visible light.
• An electron microscope uses electrons instead of light and can show organelles such as ribosomes and the details of membranes.

Typical sizes: most plant and animal cells are 10 to 100 μm; most bacteria 1 to 10 μm; viruses are smaller still.

Worked example

A student uses a 10X eyepiece with a 40X objective. A cell's image appears 20 mm across. What is the magnification, and the cell's real size in micrometres?

Try it first, then show the solutionSolution

1. Magnification $=10\times 40=400$.
2. Real size $=\frac{20\text{ mm}}{400}=0.05$ mm.
3. $0.05$ mm $=50$ μm (since 1 mm = 1000 μm).

Answer: 400X; the cell is about 50 μm across, too small for the unaided eye (limit 0.1 mm = 100 μm).

Cell membrane: selectively permeable, fluid-mosaic

NCERT section 2.2 Structure of a Cell

The cell membrane (plasma membrane) surrounds every cell, protects its contents and controls what enters and leaves. It is selectively permeable: some substances pass, others are blocked.

Structure (fluid-mosaic model): a lipid bilayer, a double sheet of fat molecules whose water-loving heads face out and water-avoiding tails face in, with proteins set into it. It is only about 7 to 10 nm thick.

• 'Fluid' because the molecules can move sideways, flip and rotate.
• 'Mosaic' because the molecules are arranged like tiles.
• Membrane proteins act like gatekeepers that help substances cross.

Worked example

Why is the cell membrane described as 'fluid' and as a 'mosaic'? What would happen if it let everything through?

Try it first, then show the solutionSolution

1. Fluid: its lipid and protein molecules move sideways, flip and rotate.
2. Mosaic: the molecules are arranged like tiles in a pattern, with proteins scattered in the lipid bilayer.
3. If it let everything through, useful substances would leak out and harmful ones would enter; the cell could not keep its own conditions.

Answer: Its molecules can move (fluid) and are arranged like tiles (mosaic); selective permeability keeps the cell's contents under control.

Diffusion and osmosis

NCERT section 2.2 Structure of a Cell

Diffusion is the net movement of particles from a region of higher concentration to lower concentration; no membrane is needed for it.

Osmosis is the special case where water spreads through a selectively permeable membrane, from a dilute solution (more water, less solute) to a concentrated one (less water, more solute).

Compare the solution outside the cell with the cell's contents:

• Hypotonic outside (more dilute): water enters, the cell swells.
• Isotonic (equal): no net movement.
• Hypertonic outside (more concentrated): water leaves, the cell shrinks.

NCERT's potato activity: a piece in plain water swells and gains mass; a piece in 20% salt or sugar solution shrinks and loses mass. Roots absorb water from the soil by osmosis.

Worked example

Two potato pieces of 50 g each are placed in plain water (A) and 20% salt solution (B). After an hour A weighs 54 g and B weighs 45 g. Find the percentage change in each and explain.

Try it first, then show the solutionSolution

1. A: $\frac{54-50}{50}\times 100=+8\%$.
2. B: $\frac{45-50}{50}\times 100=-10\%$.
3. Plain water is hypotonic to the potato cells, so water enters by osmosis.
4. Salt solution is hypertonic, so water leaves the cells; the membrane does not let the salt in.

Answer: A gains 8% (water enters); B loses 10% (water leaves) by osmosis.

Cell wall and plasmolysis

NCERT section 2.2 Structure of a Cell

Plants, fungi and bacteria have a cell wall outside the cell membrane. In plants it is made mainly of cellulose (many glucose units joined), which is roughage in our diet.

• It is rigid, so it gives shape, support and protection, helping plants stand upright against wind and rain.
• It is permeable (not selectively permeable): water and dissolved minerals pass through it freely.

When a *Rhoeo* (cradle lily) or onion peel is put in concentrated sugar solution, water leaves by osmosis. The cell wall keeps its shape but the contents shrink away from the wall (this is called plasmolysis). Cheek cells, with no wall, simply shrink. Animal cells' lack of a wall lets them change shape, which suits movement.

Worked example

Rhoeo leaf peel cells and human cheek cells are both kept in 20% sugar solution for half an hour. Describe and explain what you would see under the microscope.

Try it first, then show the solutionSolution

1. Both kinds of cell lose water by osmosis, as the solution is hypertonic.
2. Rhoeo cells: the rigid cell wall stays in place, but the membrane and contents pull away from it, leaving a gap.
3. Cheek cells: there is no cell wall, so the whole cell shrinks and may crinkle.

Answer: Rhoeo cells keep their outline while their contents shrink from the wall; cheek cells shrink as a whole.

Prokaryotic and eukaryotic cells

NCERT section 2.3 The Cell Interior — A Coordinated Working System

Feature	Prokaryotic (e.g. bacteria)	Eukaryotic (plants, animals, fungi)
Nucleus	no true nucleus; DNA in a nucleoid	well-defined nucleus with a membrane
Membrane-bound organelles	absent	present
Typical size	1 to 10 μm	10 to 100 μm
Organisms	usually unicellular	unicellular or multicellular

'Pro' means primitive and 'karyon' means nucleus; 'eu' means true. Eukaryotic cells also have a cytoskeleton of fine fibres for shape, movement and internal transport. Cells may contain inclusions such as stored starch or crystals.

Viruses, viroids and prions are acellular infectious agents, not cells.

Worked example

A cell has a cell wall, ribosomes and DNA in a nucleoid, but no mitochondria. Is it prokaryotic or eukaryotic? Give two reasons, and name a likely organism.

Try it first, then show the solutionSolution

1. Its DNA is in a nucleoid, not a membrane-bound nucleus.
2. It has no membrane-bound organelles such as mitochondria.
3. Both are features of prokaryotes; ribosomes and a cell wall are found in bacteria too.

Answer: Prokaryotic, for example a bacterium.

The nucleus, ribosomes, ER and Golgi apparatus

NCERT section 2.3 The Cell Interior — A Coordinated Working System

• Nucleus (house of coded instructions): double nuclear membrane with pores; a nucleolus that makes ribosome subunits; chromatin (DNA with proteins) that coils into chromosomes when the cell is about to divide. Functional segments of DNA are genes. Mature human red blood cells have no nucleus (more room for haemoglobin; they live about 120 days).
• Ribosomes: free or on the ER; sites of protein synthesis.
• Endoplasmic reticulum (ER): a network continuous with the nuclear envelope. Rough ER has ribosomes and makes and secretes proteins; smooth ER has none and makes and stores fats and hormones.
• Golgi apparatus (post office): stacks of flat sacs that modify, sort and package proteins and lipids into vesicles. Named after Camillo Golgi (seen in 1898).

Worked example

Pancreatic cells secrete large amounts of digestive enzymes (proteins). Which two organelles would you expect to be especially well developed in them, and why?

Try it first, then show the solutionSolution

1. Proteins are made on ribosomes attached to the rough ER, so rough ER (with many ribosomes) should be extensive.
2. Proteins for secretion are packaged into vesicles by the Golgi apparatus, so it should be well developed too.

Answer: Rough endoplasmic reticulum (protein making) and Golgi apparatus (packaging for secretion).

Lysosomes, mitochondria, plastids and vacuoles

NCERT section 2.3 The Cell Interior — A Coordinated Working System

• Lysosomes (clean-up system): single-membrane sacs of enzymes that break down waste and worn-out parts; the products are reused.
• Mitochondria (powerhouses): two membranes, the inner one folded into cristae for more surface area; cellular respiration releases energy from glucose and stores it as ATP (the energy currency).
• Plastids (plants): chloroplasts contain chlorophyll in disc-shaped membranes inside the stroma and carry out photosynthesis; chromoplasts hold yellow, orange or red pigments in petals and fruits; leucoplasts are colourless and store starch, oils or proteins (e.g. in potato).
• Vacuoles: a large central vacuole in mature plant cells holds cell sap and keeps the cell firm; when it loses water, the plant wilts. Animal cells have small, temporary vacuoles.

Mitochondria and plastids have their own DNA and ribosomes, a sign they share an evolutionary history with bacteria.

Worked example

Why do potted plants droop when not watered for a few days, and recover after watering? Which organelle is involved?

Try it first, then show the solutionSolution

1. The large central vacuole stores water and presses the cell contents against the wall, keeping cells firm.
2. Without water, vacuoles lose water, cells become less firm and the plant wilts.
3. After watering, water enters by osmosis, vacuoles refill and the cells become firm again.

Answer: The vacuole; losing water makes cells limp (wilting) and refilling makes them firm.

Cell division: mitosis and meiosis

NCERT section 2.4 How do Normal Cells Grow and Divide?

New cells form only from existing cells. Cell division lets organisms grow, repair damage and reproduce; onion root tips show cells at different stages because they divide continuously.

	Mitosis	Meiosis
Where	body cells	reproductive organs (testes, ovaries; anthers and ovaries in plants)
Divisions	one	two in a row
Daughter cells	2	4
Chromosome number	same as parent	half the parent's
Genetic make-up	identical	varied
Purpose	growth, repair, asexual reproduction	gametes for sexual reproduction, variation

Fertilisation joins two gametes and restores the full chromosome number. Errors in mitosis can cause tumours; errors in meiosis can cause genetic disorders.

Worked example

A body cell of an organism has 20 chromosomes. How many chromosomes are in (i) each cell after mitosis (ii) each gamete after meiosis (iii) the cell formed at fertilisation? How many cells result from one cell in each division?

Try it first, then show the solutionSolution

1. (i) Mitosis keeps the number: 20 in each of 2 cells.
2. (ii) Meiosis halves it: 10 in each of 4 gametes.
3. (iii) Fertilisation joins two gametes: $10+10=20$.

Answer: (i) 20 (2 cells) (ii) 10 (4 cells) (iii) 20

Cell theory, life span and cancer

NCERT section 2.5 Cell Theory — The Unifying Principle of Biology

Cell theory, built from the work of Schleiden (1838, plants), Schwann (1839, animals) and Virchow (1855, new cells come from existing cells):
1. All living organisms are made of one or more cells.
2. The cell is the basic unit of structure and function.
3. All cells arise from pre-existing cells.

Cells have a definite life span. Many animal cells stop dividing when they touch neighbours (contact inhibition); cancer cells lose this control and keep dividing, forming tumours that may spread. Programmed cell death removes unwanted cells, for example between developing fingers. Plant cells can be totipotent (Haberlandt, 1902): one cell can grow into a whole plant, the basis of plant tissue culture. In cell culture, cells grow outside the body in a sterile nutrient medium, used to make vaccines and medicines.

Worked example

State the three points of the cell theory and say which scientist added the idea that cells come only from cells.

Try it first, then show the solutionSolution

1. All living organisms are made up of one or more cells.
2. The cell is the basic unit of structure and function.
3. All cells arise from pre-existing cells, added by Rudolf Virchow in 1855.

Answer: The three statements above; Virchow added 'cells come from pre-existing cells'.

Science in real life: Cell: The Building Block of Life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Topics in this chapter

• Cell Membrane, Diffusion and Osmosis
• Cell Organelles, Prokaryotic and Eukaryotic Cells

Cell: The Building Block of Life: formulas and key facts

Resolution of the eye

About 0.1 mm (at 25 cm); smaller details need a microscope.

Magnification

$Totalmagnification=objective\times eyepiece;realsize=imagesize÷magnification.$

Size units

$1mm=1000\mu m;1\mu m=1000nm.Cells10to100\mu m;bacteria1to10\mu m;membrane7to10nm.$

Osmosis

Water moves across a selectively permeable membrane from dilute (more water) to concentrated (less water).

Solutions and cells

Hypotonic outside: cell swells. Isotonic: no change. Hypertonic outside: cell shrinks (plant cells: plasmolysis).

Cell wall

Plants (cellulose), fungi and bacteria; rigid and freely permeable.

Organelle jobs

Nucleus	control and DNA
Ribosome	proteins
RER	proteins
SER	fats and hormones
Golgi	packaging
Lysosome	clean-up
Mitochondrion	ATP
Chloroplast	photosynthesis
Vacuole	storage and firmness

Organelles with their own DNA

Nucleus, mitochondria and chloroplasts (plastids).

Mitosis

1 cell → 2 identical cells, same chromosome number (growth, repair).

Meiosis

1 cell → 4 gametes, half the chromosome number (sexual reproduction, variation).

Cell theory

All organisms are made of cells; the cell is the unit of structure and function; cells come from pre-existing cells.

Key dates

$Hooke1665(cells),Golgi1898,Schleiden1838,Schwann1839,Virchow1855,Haberlandt1902(totipotency).$

Cell: The Building Block of Life: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks Cell: The Building Block of Life

The question patterns to expect, and the method that works for each.

Tricks to remember Cell: The Building Block of Life

Short memory aids. Say each one aloud once, then use it on a question the same day.

• The cell as a factory

  Nucleus = head office, ribosomes = workers, ER = assembly line, Golgi = packing and post office, lysosomes = cleaners, mitochondria = power plant, vacuole = store room.

  Use it when: Remembering organelle jobs.

• Water chases salt

  In osmosis, water moves towards the side with more dissolved stuff.

  Use it when: Predicting swelling and shrinking.

• Pro = no

  Prokaryotes: no true nucleus, no membrane-bound organelles.

  Use it when: Telling prokaryotes and eukaryotes apart.

• Mitosis two, meiosis four (and half)

  Mi-TWO-sis makes two identical cells; meiosis makes four with half the chromosomes.

  Use it when: Comparing the divisions.

• Three DNA homes

  DNA lives in the Nucleus, Mitochondria and Chloroplasts: 'Never Mind Chromosomes outside'.

  Use it when: MCQs on which organelles contain DNA.

Cell: The Building Block of Life: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section E style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 4-mark question split into 1 + 1 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Cell: The Building Block of Life

The slips examiners see most often, and what to write instead.

• ✗Mistake: Saying the cell wall is selectively permeable

  The wall and membrane are confused.

  Wrong working

  The cell wall controls which substances enter the cell.

  Right working

  The cell wall is freely permeable; the cell membrane is selectively permeable and controls entry.

  ✓Do this: Wall = support; membrane = control.

  Usually costs: 1 mark.

• ✗Mistake: Getting the direction of osmosis wrong

  Students think water moves towards more water.

  Wrong working

  In salt solution, water moves into the potato.

  Right working

  Salt solution is more concentrated, so water moves out of the potato cells.

  ✓Do this: Water moves from dilute to concentrated across the membrane.

  Usually costs: The explanation marks.

• ✗Mistake: Drawing chloroplasts or a cell wall in an animal cell

  A single 'cell' diagram is memorised.

  Wrong working

  An animal cell drawn with a thick wall and green chloroplasts.

  Right working

  An animal cell has only a membrane, no chloroplasts and only small vacuoles.

  ✓Do this: Learn the plant and animal diagrams separately.

  Usually costs: Up to half the diagram marks.

• ✗Mistake: Saying meiosis makes two cells

  Mitosis and meiosis are mixed up.

  Wrong working

  Meiosis gives two identical cells.

  Right working

  Meiosis gives four cells, each with half the chromosomes and genetic variation.

  ✓Do this: Use the comparison table.

  Usually costs: 1 to 2 marks.

• ✗Mistake: Calling viruses cells

  They cause disease like bacteria.

  Wrong working

  Viruses are the smallest prokaryotic cells.

  Right working

  Viruses are acellular: genetic material in a protein coat, with no cell membrane or cytoplasm.

  ✓Do this: Remember viruses, viroids and prions are not cells.

  Usually costs: 1 mark.

NCERT exercise guide: Cell: The Building Block of Life

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

Practice questions on Cell: The Building Block of Life

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQExam level

   The site of protein synthesis in a cell is the
   (a) lysosome
   (b) ribosome
   (c) Golgi apparatus
   (d) vacuole

   Hint

   The 'protein factories'.

   Answer

   (b) ribosome

2. Q2 · 1 mark · MCQExam level

   Which of these is a prokaryotic cell?
   (a) Onion peel cell
   (b) Cheek cell
   (c) Bacterium
   (d) Yeast cell

   Hint

   No true nucleus.

   Answer

   (c) Bacterium (yeast is a eukaryote).

3. Q3 · 1 mark · Assertion-reasonExam level

   Assertion (A): Plant cells in plain water do not burst.
   Reason (R): The rigid cell wall resists the pressure of water entering the cell.
   (a) Both A and R are true and R is the correct explanation of A.
   (b) Both A and R are true but R is not the correct explanation of A.
   (c) A is true but R is false.
   (d) A is false but R is true.

   Hint

   What does the wall do?

   Answer

   (a)

4. Q4 · 2 marks · Very short answerExam level

   State two functions of the vacuole in a plant cell.

   Hint

   Storage and firmness.

   Answer

   Stores water, minerals, sugars and wastes in cell sap; keeps the cell firm by maintaining pressure (without it the plant wilts).

5. Q5 · 2 marks · Very short answerHarder exam level

   Why are lysosomes called the cell's clean-up system? What happens to the products of their work?

   Hint

   Enzymes in a sac.

   Answer

   They contain enzymes that break down waste, unwanted molecules and worn-out parts. The products go back into the cytoplasm and are reused.

6. Q6 · 3 marks · Short answerHarder exam level

   Complete the comparison of prokaryotic and eukaryotic cells for: nucleus, membrane-bound organelles, typical size and number of cells in the organism.

   Hint

   Use Table 2.2.

   Answer

   Prokaryotic: nucleoid only, no membrane-bound organelles, 1 to 10 μm, usually unicellular. Eukaryotic: membrane-bound nucleus, organelles present, 10 to 100 μm, unicellular or multicellular.

7. Q7 · 3 marks · Short answerHarder exam level

   A red blood cell placed in distilled water swells and bursts, but an onion peel cell placed in distilled water only swells a little and becomes firm. Explain both observations.

   Hint

   Which cell has a wall?

   Answer

   Distilled water is hypotonic to both cells, so water enters each by osmosis. The red blood cell has only a thin membrane, so it keeps swelling until it bursts. The onion cell has a rigid cell wall that resists the pressure of the incoming water, so the cell becomes firm instead of bursting.

8. Q8 · 2 marks · Very short answerHarder exam level

   Do white flowers contain pigment-carrying plastids? Explain which plastids they may have.

   Hint

   Colourless plastids.

   Answer

   White petals lack coloured pigments, so they do not have chromoplasts with coloured pigments; they may have colourless leucoplasts.

9. Q9 · 3 marks · Short answerHarder exam level

   Explain why errors in mitosis can lead to tumours and errors in meiosis to genetic disorders.

   Hint

   Which cells does each division make?

   Answer

   Mitosis controls the number of body cells; if control is lost, cells keep dividing and form tumours (or get abnormal chromosome numbers). Meiosis makes gametes; errors give gametes with wrong chromosome numbers, and the offspring may have genetic disorders or the pregnancy may be lost.

10. Q10 · 5 marks · Long answerHarder exam level

    Draw a labelled diagram of an animal cell. Name two organelles absent in it that a plant cell has, and one organelle that is larger in plant cells.

    Hint

    Wall, chloroplast, vacuole.

    Answer

    Diagram with membrane, cytoplasm, nucleus, nucleolus, mitochondria, ER, Golgi, ribosomes, lysosome. Absent: cell wall and chloroplasts (plastids). Larger in plants: the vacuole (one large central vacuole).

11. Q11 · 3 marks · Short answerHOTS / competency

    Scientists placed synthetic DNA copied from one bacterium into another bacterium whose own DNA had been removed. The cell then grew and divided following the new DNA. What does this show, and why is it not 'creating life from scratch'?

    Hint

    Which parts were new?

    Answer

    It shows DNA controls the structure and activities of a cell. Only the DNA was made in the lab; the membrane, cytoplasm and other parts came from an existing living cell.

12. Q12 · 3 marks · Short answerHOTS / competency

    Cancer cells keep dividing even when crowded. Explain what normal control they have lost and why this is dangerous.

    Hint

    Contact inhibition.

    Answer

    Normal animal cells stop dividing when they touch neighbours (contact inhibition) and die at the end of their life span. Cancer cells ignore these controls, so they pile up into tumours that can invade nearby tissue and spread to other parts of the body.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.What is the limit of resolution of the human eye?Show answer

   About 0.1 mm.

2. 2.Who first named cells, and from what?Show answer

   Robert Hooke, in 1665, from thin slices of cork.

3. 3.What does 'selectively permeable' mean?Show answer

   It lets some substances pass and blocks others.

4. 4.Define osmosis.Show answer

   Movement of water across a selectively permeable membrane from a dilute to a concentrated solution.

5. 5.Which organelle is the site of protein synthesis?Show answer

   The ribosome.

6. 6.What are cristae and why are they useful?Show answer

   Folds of the inner mitochondrial membrane; they increase the surface area for energy-releasing reactions.

7. 7.Name the three types of plastids.Show answer

   Chloroplasts (green), chromoplasts (coloured) and leucoplasts (colourless, storage).

8. 8.Which cell division produces gametes?Show answer

   Meiosis.

9. 9.What is contact inhibition?Show answer

   Many animal cells stop dividing when they touch neighbouring cells; cancer cells lose this control.

10. 10.Why do mature red blood cells lack a nucleus?Show answer

    It leaves more room for haemoglobin to carry oxygen.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you calculate total magnification and real size?
• ☐Can you explain the fluid-mosaic model in two lines?
• ☐Can you predict what happens to a cell in hypotonic, isotonic and hypertonic solutions?
• ☐Can you explain plasmolysis in Rhoeo cells?
• ☐Can you list five differences between prokaryotic and eukaryotic cells?
• ☐Can you name the job of each organelle in one phrase?
• ☐Can you draw and label plant and animal cells?
• ☐Can you compare mitosis and meiosis in a table?
• ☐Can you state the cell theory and its three scientists?
• ☐Can you explain why cancer cells form tumours?

Cell: The Building Block of Life: frequently asked questions