The Cell
1. Discovery of the Cell
All living organisms are made up of tiny building blocks called cells.
Robert Hooke (1665) was the first person to observe cells. He looked at a thin slice of cork (dead plant cells) under his self-made microscope and saw tiny box-like structures. He called them 'cells' (meaning 'small rooms' in Latin).
Anton van Leeuwenhoek later observed living cells in pond water using improved microscopes.
2. The Cell Theory
The cell theory states:
- All living organisms are made up of one or more cells.
- The cell is the basic structural and functional unit of life.
- All cells arise from pre-existing cells.
Types of organisms based on cell count:
| Type | Definition | Examples |
|---|---|---|
| Unicellular | Single-celled organism | Amoeba, Paramecium, bacteria, yeast |
| Multicellular | Many-celled organism | Humans, trees, dogs, insects |
Common Mistake: Thinking bacteria are too simple to have cells. Bacteria are unicellular — they are made of a single cell!
3. Basic Structure of a Cell
Every cell has three basic parts:
- Cell membrane: The outer covering that separates the cell from its surroundings.
- Cytoplasm: A jelly-like substance inside the cell where all activities happen.
- Nucleus: The control centre of the cell that contains genetic material (DNA).
Shape and Size of Cells
Cells come in various shapes and sizes:
| Cell Type | Shape | Size |
|---|---|---|
| Red blood cell | Round/disc-shaped | 7 micrometres |
| Nerve cell | Long with branches | Up to 1 metre |
| Muscle cell | Elongated | Microscopic to visible |
| Amoeba | Irregular | 0.2 mm |
| Egg cell | Round | Visible to naked eye |
Worked Example: Why are nerve cells long and branched?
Nerve cells must carry signals from one part of the body to another. Their long, branched shape helps them transmit messages quickly over long distances.
4. Plant Cell vs Animal Cell
| Feature | Plant Cell | Animal Cell |
|---|---|---|
| Cell wall | Present (made of cellulose) | Absent |
| Cell membrane | Present (inside cell wall) | Present (outermost) |
| Shape | Fixed, rectangular | Irregular, round |
| Chloroplast | Present (in green parts) | Absent |
| Vacuole | Large, central vacuole | Small, temporary vacuoles |
| Nucleus | On one side (pushed by vacuole) | Usually central |
| Storage | Stores starch | Stores glycogen |
Common Mistake: Saying animal cells have no vacuoles. Animal cells do have vacuoles, but they are much smaller and more numerous than the single large central vacuole in plant cells.
Exam Focus (3 marks): 'State three differences between plant and animal cells.'
(1) Plant cells have a cell wall; animal cells do not. (2) Plant cells have chloroplasts; animal cells do not. (3) Plant cells have a large central vacuole; animal cells have small vacuoles.
5. Cell Organelles (Basic Introduction)
| Organelle | Location | Function |
|---|---|---|
| Cell membrane | Outer boundary | Controls entry and exit of substances |
| Cell wall | Outside membrane (plants only) | Provides strength and support |
| Cytoplasm | Inside cell membrane | Site of cellular activities |
| Nucleus | Centre or side | Controls all cell activities; contains DNA |
| Mitochondria | In cytoplasm | Powerhouse of the cell; releases energy |
| Chloroplast | In plant cells | Contains chlorophyll; site of photosynthesis |
| Vacuole | In cytoplasm | Stores water, nutrients, and waste |
| Ribosomes | In cytoplasm | Make proteins |
| Endoplasmic reticulum | Connected to nucleus | Transport system within the cell |
Worked Example: Which organelle would be found in large numbers in a muscle cell?
Muscle cells need a lot of energy to contract, so they have many mitochondria (the powerhouse of the cell).
6. Levels of Organisation
Cell -> Tissue -> Organ -> Organ System -> Organism
Example: Muscle cell -> Muscle tissue -> Heart -> Circulatory system -> Human body
7. Self-Test
- Who discovered the cell? What did he observe?
- State the three points of the cell theory.
- Differentiate between unicellular and multicellular organisms with examples.
- List three differences between plant and animal cells.
- Name the organelle: (a) Controls all cell activities (b) Releases energy (c) Provides strength to plant cells.
- What is the function of the cell membrane?
- Arrange in order of organisation: Organ, Cell, Tissue, Organ system, Organism.
8. Answers to Self-Test
- Robert Hooke in 1665. He observed box-like structures in a thin slice of cork.
- (1) All living things are made of cells. (2) The cell is the basic unit of life. (3) All cells come from pre-existing cells.
- Unicellular: single cell (Amoeba, bacteria). Multicellular: many cells (human, tree).
- Plant cells have cell wall, chloroplasts, and large central vacuole; animal cells lack these.
- (a) Nucleus (b) Mitochondria (c) Cell wall.
- Controls the movement of substances into and out of the cell (selectively permeable).
ICSE Exam Tips for Class 6
'In ICSE Class 6 Biology exams, cell structure questions typically carry 3-4 marks. You may be asked to draw a labelled diagram of a plant or animal cell. Practice drawing clearly with a sharp pencil. Label at least 4 parts. The comparison between plant and animal cells is a guaranteed question — memorise the three key differences: cell wall, chloroplasts, and vacuole size.'
Fun Facts about Cells
The human body contains approximately 37 trillion cells. The largest cell in the human body is the female egg cell (ovum) — visible to the naked eye! The smallest cell is the male sperm cell. 'Cells were first observed by Robert Hooke in 1665 using a primitive microscope. The word "cell" comes from the Latin "cella" meaning a small room — because the cork cells he saw reminded him of the tiny rooms (cells) in a monastery.' 7. Cell -> Tissue -> Organ -> Organ system -> Organism.
