Cell: The Unit of Life ICSE Class 9: Cell Theory, Organelles and Plant versus Animal Cells

What Cell: The Unit of Life is about

Weightage: CISCE publishes no chapter weightage for ICSE Biology. Cell: The Unit of Life is topic 1, Basic Biology (i), of the Class IX part of the 2028 syllabus; the Class IX paper (80 marks, 2 hours) is set and marked by the school.Source: CISCE ICSE Biology (Science Paper 3) syllabus, examination year 2028 (Class IX)

Key concepts

The cell, protoplasm and cell theory

Cell: the basic structural and functional unit of life. Robert Hooke first saw and named 'cells' in a thin slice of cork (dead cell walls); Leeuwenhoek first saw living cells.

Protoplasm: the whole living substance of a cell, everything inside the cell membrane: cytoplasm + nucleus. It is a jelly-like, colourless mixture of water, proteins, fats, carbohydrates and salts, and is called the 'physical basis of life'.

• Cytoplasm: the protoplasm outside the nucleus, in which the organelles lie.
• Nucleoplasm: the protoplasm inside the nucleus.

Cell theory (Schleiden and Schwann, extended by Virchow):
1. All living organisms are made of one or more cells.
2. The cell is the basic unit of structure and function of living things.
3. All cells arise from pre-existing cells (Virchow: 'omnis cellula e cellula').

Worked example

Is a virus an exception to cell theory? Give a reason.

Try it first, then show the solutionSolution

1. A virus has no cell membrane, cytoplasm or organelles.
2. It cannot carry out life activities on its own; it multiplies only inside a living host cell.

Answer: Yes: viruses are not made of cells, so they are not covered by cell theory.

Cell boundaries: cell wall and cell membrane

Cell wall (plant cells, also fungi and bacteria; absent in animal cells):

• The outermost, non-living, rigid layer, made of cellulose in plants.
• Freely permeable: lets water and dissolved substances pass.
• Gives shape, support and protection, and prevents the cell bursting when it takes in water.

Cell membrane (plasma membrane) (all cells):

• A thin, living, flexible layer of lipids and proteins around the cytoplasm (inside the wall in plants).
• Selectively (semi-) permeable: controls what enters and leaves the cell.
• Holds the cell contents together.

Worked example

Why can a plant cell placed in water swell but not burst, while a red blood cell may burst?

Try it first, then show the solutionSolution

1. Both take in water by osmosis.
2. The rigid cellulose wall of the plant cell resists further swelling.
3. The red blood cell has only a thin membrane, which can rupture.

Answer: The cell wall protects the plant cell.

Nucleus, nucleolus and the organelles that make and pack proteins

Nucleus: a large, usually round body bounded by a double nuclear membrane with pores. It contains chromatin (DNA, which forms chromosomes when the cell divides) and one or more nucleoli.

• Controls all activities of the cell (the 'control centre').
• Carries hereditary information (genes on DNA) and takes part in cell division.

Nucleolus: a dense, round body inside the nucleus; it makes ribosomes (ribosomal RNA).

Ribosomes: tiny granules, free in cytoplasm or attached to ER; sites of protein synthesis (the 'protein factories'). Not bounded by a membrane.

Endoplasmic reticulum (ER): a network of membrane-lined channels through the cytoplasm.

• Rough ER (ribosomes attached): makes and transports proteins.
• Smooth ER (no ribosomes): makes lipids (fats); helps detoxify drugs and poisons.
• Both form a transport system and give internal support.

Golgi bodies (Golgi apparatus): stacks of flattened sacs with vesicles. They modify, package and secrete proteins and other products (enzymes, hormones, mucus); help form lysosomes; in plants, help make the new cell wall (cell plate).

Worked example

A gland cell makes and releases large amounts of a digestive enzyme (a protein). Which three organelles will be especially well developed? Give the role of each.

Try it first, then show the solutionSolution

1. Ribosomes: make the protein.
2. Rough ER: carries the protein through the cell.
3. Golgi bodies: package and secrete it.

Answer: Ribosomes, rough ER, Golgi bodies.

Mitochondria, plastids, lysosomes, centrosome and vacuoles

Mitochondria: rod-shaped or round bodies with a double membrane; the inner membrane is folded into cristae. They are the sites of aerobic respiration, releasing energy from food as ATP: the 'powerhouse of the cell'. Very active cells (muscle, nerve) have many.

Plastids (plant cells only):

• Chloroplasts: green, contain chlorophyll; site of photosynthesis (the 'kitchen of the cell').
• Chromoplasts: coloured (yellow, orange, red); give colour to flowers and fruits, attracting pollinators and seed dispersers.
• Leucoplasts: colourless; store food such as starch (in potato), oil and proteins.

Lysosomes: small sacs with a single membrane containing digestive enzymes. They digest food particles, worn-out organelles and invading germs. If the cell is damaged they burst and digest the cell itself: 'suicide bags'. Mainly in animal cells.

Centrosome: a region near the nucleus containing two centrioles (in animal cells). It forms the spindle during cell division. Absent in most plant cells.

Vacuoles: fluid-filled sacs bounded by a membrane (tonoplast).

• Plant cells: usually one large central vacuole filled with cell sap; keeps the cell turgid (firm), stores water, salts, sugars and wastes.
• Animal cells: small, temporary vacuoles (food or contractile vacuoles), or none.

Worked example

Why are lysosomes called suicide bags?

Try it first, then show the solutionSolution

1. They hold powerful digestive enzymes.
2. If the cell is damaged or old, the lysosome membrane bursts.
3. The enzymes digest the cell's own contents.

Answer: They can destroy their own cell.

Plant cell versus animal cell

Feature	Plant cell	Animal cell
Cell wall	present (cellulose)	absent
Plastids	present (chloroplasts, chromoplasts, leucoplasts)	absent
Vacuole	one large, central, permanent, with cell sap	small, temporary, many or none
Centrosome	absent (in most)	present
Shape	fixed, usually rectangular	irregular or round, can change
Nucleus position	pushed to one side by the vacuole	usually central
Lysosomes	rare	common
Stored food	starch	glycogen

Both have a cell membrane, cytoplasm, nucleus, mitochondria, ER, ribosomes and Golgi bodies.

Worked example

A cell has a cell wall and a large central vacuole but no chloroplasts. Is it a plant cell? Where might it come from?

Try it first, then show the solutionSolution

1. Cell wall and large vacuole are plant features.
2. Cells not exposed to light (e.g. in roots or potato tuber) lack chloroplasts.

Answer: Yes, a plant cell, e.g. from a root or a potato tuber.

Prokaryotic versus eukaryotic cells

Feature	Prokaryotic cell	Eukaryotic cell
Nucleus	no true nucleus; DNA lies in the cytoplasm (nucleoid), no nuclear membrane	true nucleus with a nuclear membrane
Nucleolus	absent	present
Membrane-bound organelles (mitochondria, ER, Golgi, plastids, lysosomes)	absent	present
Ribosomes	smaller	larger
Size	small (about 1-10 µm)	larger (about 10-100 µm)
Chromosome	single, circular DNA	several, linear, with proteins
Examples	bacteria, blue-green algae (cyanobacteria)	plants, animals, fungi, protozoa

'Pro' means before, 'eu' means true, and 'karyon' means nucleus.

Worked example

A cell has ribosomes and a cell wall but no nuclear membrane and no mitochondria. Classify it and give an example.

Try it first, then show the solutionSolution

1. No nuclear membrane and no membrane-bound organelles: prokaryotic.
2. Ribosomes and a wall are found in bacteria.

Answer: Prokaryotic, e.g. a bacterium.

Cell: The Unit of Life in real life

Each concept, met in an everyday situation. Read the situation, answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Topics in this chapter

• Cell Organelles and Their Functions
• Plant and Animal Cells, Prokaryotic and Eukaryotic Cells Compared

Cell: The Unit of Life: key facts

Protoplasm

$\text{protoplasm}=\text{cytoplasm}+\text{nucleus}$

Living part of the cell.

Protein pathway

$\text{ribosome (make)}\to \text{rough ER (transport)}\to \text{Golgi (package)}\to \text{secretion}$

Secretory cells.

Plant-only parts

$\text{cell wall}+\text{plastids}+\text{large central vacuole}$

Centrosome absent.

Prokaryote

$\text{no nuclear membrane}+\text{no membrane-bound organelles}$

Bacteria, cyanobacteria.

Cell: The Unit 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 Unit of Life

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

Tricks to remember Cell: The Unit of Life

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

• Plant extras: 'Walls, Plastids, Pools'

  Plant cells have Walls, Plastids and a big Pool (vacuole); animal cells have a Centrosome instead.

  Use it when: Plant versus animal cell.

• Nicknames

  Mitochondria = powerhouse; Lysosome = suicide bag; Ribosome = protein factory; Chloroplast = kitchen; Nucleus = control room; Golgi = post office.

  Use it when: Quick function recall.

• Plastid colours: 'Chloro Green, Chromo Colour, Leuco Lockers'

  Chloroplasts green, chromoplasts coloured, leucoplasts store food.

  Use it when: Types of plastids.

• Pro = before

  Prokaryotes came before a true nucleus evolved; eukaryotes have a 'true' nucleus.

  Use it when: Prokaryote versus eukaryote.

Cell: The Unit 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 B style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 5-mark question split into 1 + 1 + 2 + 1 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 Unit of Life

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

• ✗Mistake: Calling the cell wall living or selectively permeable

  The roles of cell wall and cell membrane are swapped.

  Wrong working

  The cell wall controls what enters and leaves the cell.

  Right working

  The cell wall is non-living and freely permeable; the cell membrane is selectively permeable.

  ✓Do this: Wall supports; membrane selects.

  Usually costs: 1 mark.

• ✗Mistake: Saying plant cells have no vacuole difference

  Students write 'vacuole present in plant, absent in animal'.

  Wrong working

  Animal cells have no vacuoles.

  Right working

  Plant cells: one large central vacuole; animal cells: small, temporary vacuoles (or none).

  ✓Do this: Compare size and number, not just presence.

  Usually costs: 1 mark.

• ✗Mistake: One-sided differences

  Only one cell type is described in a 'difference' answer.

  Wrong working

  Plant cell has a cell wall.

  Right working

  Cell wall present in plant cell; absent in animal cell.

  ✓Do this: Each point must mention both cells.

  Usually costs: 1/2-1 mark per point.

• ✗Mistake: Protoplasm and cytoplasm confused

  The terms sound alike.

  Wrong working

  Protoplasm is the fluid outside the nucleus.

  Right working

  Cytoplasm is outside the nucleus; protoplasm = cytoplasm + nucleus.

  ✓Do this: Proto = whole; cyto = cell fluid outside nucleus.

  Usually costs: 1 mark.

• ✗Mistake: Unlabelled or freehand-sketched diagrams

  Labels drawn without ruled lines or crossing each other cannot be read.

  Wrong working

  Labels written inside the cell with arrows crossing.

  Right working

  Ruled label lines ending on the part, labels on one side, all in pencil.

  ✓Do this: Follow the drawing checklist.

  Usually costs: 1-2 marks.

Practice questions on Cell: The Unit 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 cell organelle known as the 'suicide bag' is:
   (a) ribosome
   (b) lysosome
   (c) Golgi body
   (d) vacuole

   Hint

   Digestive enzymes.

   Answer

   (b)

2. Q2 · 1 mark · MCQExam level

   Which is absent in a prokaryotic cell?
   (a) ribosome
   (b) cell membrane
   (c) nuclear membrane
   (d) DNA

   Hint

   True nucleus.

   Answer

   (c)

3. Q3 · 3 marks · Very short answerExam level

   Name the plastid that gives colour to (a) green leaves, (b) ripe tomatoes. (c) Name the plastid in potato tubers.

   Hint

   Chloro, chromo, leuco.

   Answer

   (a) Chloroplast (b) Chromoplast (c) Leucoplast

4. Q4 · 2 marks · Short answerHarder exam level

   Why is the nucleus called the control centre of the cell? Give two reasons.

   Hint

   DNA.

   Answer

   It controls all cell activities through the DNA (genes) it contains, and it carries hereditary information and directs cell division.

5. Q5 · 2 marks · Short answerHarder exam level

   Distinguish between rough ER and smooth ER (two points).

   Hint

   Ribosomes.

   Answer

   Rough ER has ribosomes on its surface, smooth ER does not. Rough ER makes and transports proteins; smooth ER makes lipids and helps detoxify.

6. Q6 · 2 marks · Short answerHarder exam level

   A cell lacks mitochondria. Which activity of the cell will be most affected, and why?

   Hint

   Energy.

   Answer

   Release of energy by aerobic respiration: mitochondria are its site, so the cell will be short of ATP for its work.

7. Q7 · 3 marks · Short answerHarder exam level

   Why do plant cells need a cell wall but animal cells can manage without one?

   Hint

   Support; skeleton; osmosis.

   Answer

   Plants have no skeleton; the rigid walls support the plant and keep cells turgid without bursting when they absorb water. Animals have a skeleton for support, and body fluids keep cells from taking in too much water, so a flexible membrane allows movement and shape change.

8. Q8 · 3 marks · Very short answerExam level

   Name the structure: (a) the membrane around the vacuole, (b) the fluid inside a plant vacuole, (c) the folds of the inner membrane of a mitochondrion.

   Hint

   Tono-, sap, cristae.

   Answer

   (a) Tonoplast (b) Cell sap (c) Cristae

9. Q9 · 3 marks · Short answerHarder exam level

   Explain why mature red blood cells of mammals, which lack a nucleus and mitochondria, can still carry oxygen but cannot divide.

   Hint

   Haemoglobin; DNA.

   Answer

   They are packed with haemoglobin, which carries oxygen without needing a nucleus. Without a nucleus (DNA) they cannot divide or repair themselves, so they have a limited life and are replaced by new cells from bone marrow.

10. Q10 · 3 marks · Short answerHOTS / competency

    A student observes a cell with a cell wall, a nucleus, mitochondria but no chloroplasts and no large vacuole, and finds it stores glycogen, not starch. Is it a plant cell? Suggest what it could be, with reasons.

    Hint

    Neither plant nor animal.

    Answer

    Probably not a plant cell: plants store starch and usually have a large vacuole. A cell wall with glycogen storage suggests a fungal cell (e.g. yeast), which is eukaryotic (nucleus, mitochondria) but lacks plastids.

11. Q11 · 3 marks · Short answerHOTS / competency

    In a cell, a newly made enzyme is found first near the rough ER, then in the Golgi body, then in vesicles at the cell membrane. Explain the path and suggest the function of the cell.

    Hint

    Secretion.

    Answer

    Ribosomes on rough ER make the enzyme; ER carries it to the Golgi body, which modifies and packs it into vesicles; vesicles fuse with the membrane to release it. The cell is a secretory (gland) cell, e.g. of the pancreas or salivary gland.

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.Who stated that all cells arise from pre-existing cells?Show answer

   Rudolf Virchow.

2. 2.Powerhouse of the cell?Show answer

   Mitochondria.

3. 3.Which organelle makes ribosomes?Show answer

   Nucleolus.

4. 4.Plastid that stores starch?Show answer

   Leucoplast.

5. 5.Cell wall of plants is made of?Show answer

   Cellulose.

6. 6.Organelle absent in most plant cells but present in animal cells?Show answer

   Centrosome.

7. 7.Example of a prokaryote?Show answer

   Bacteria (or blue-green algae).

8. 8.Membrane of the vacuole?Show answer

   Tonoplast.

Quick revision checklist

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

• ☐Can you state the three points of cell theory?
• ☐Can you give the function of all twelve organelles and two boundaries?
• ☐Can you write four plant-animal differences on the four syllabus features?
• ☐Can you write three prokaryote-eukaryote differences with examples?
• ☐Can you draw and label a plant and an animal cell?

Cell: The Unit of Life: frequently asked questions