Cell — The Building Block of Life (RBSE Class 9 · Science)
A drop of pond water looks empty to your eye, yet under a microscope it teems with whole living worlds. The reason you cannot see them is not that they are absent — it is that your eye cannot resolve anything smaller than about 0.1 mm. This chapter is the story of what lies below that limit: the cell, the smallest thing that is alive.
RBSE note (2026-27). Class 9 follows the new NCF (Curiosity) Science textbook. This chapter, Cell: The Building Block of Life, merges what older books split into "The Fundamental Unit of Life" and parts of "Tissues". BSER (Ajmer) sets the paper; the content is the NCERT/NCF book.
1. How do we study cells?
The human eye can just separate two points about 0.1 mm apart — its resolution. Most cells are far smaller, so we need a microscope, which does two jobs: magnification (makes the image bigger) and resolution (separates fine detail). A light (compound) microscope resolves down to ~0.2 µm; an electron microscope reaches ~0.2 nm and reveals organelles.
Units to remember: 1 mm = 1000 µm; 1 µm = 1000 nm. A typical animal cell is 10–100 µm across.
2. Discovery of the cell and the cell theory
- Robert Hooke (1665) examined a thin slice of cork under his microscope and saw tiny boxes he called "cells" (Latin cella, a small room). He was actually seeing dead cell walls.
- Anton van Leeuwenhoek first saw living, moving cells ("animalcules") in pond water.
- Robert Brown (1831) discovered the nucleus.
These observations grew into the Cell Theory (Schleiden, Schwann, later Virchow):
- All living organisms are made of one or more cells.
- The cell is the basic structural and functional unit of life.
- All cells arise from pre-existing cells (Omnis cellula e cellula — Virchow).
3. Unicellular and multicellular — and the levels of organisation
- Unicellular organisms (bacteria, Amoeba, Paramecium, yeast) carry out all life functions within a single cell.
- Multicellular organisms (plants, animals) share the work among many cells.
In a multicellular body the work is shared in a hierarchy:
A group of similar cells doing the same job forms a tissue; tissues build organs (e.g. nasal cavity → trachea → lungs form the respiratory system). Even so, the cell remains the fundamental unit of structure and function.
4. Prokaryotic vs eukaryotic cells
| Feature | Prokaryotic | Eukaryotic |
|---|---|---|
| Nucleus | No true nucleus (no membrane around DNA — a nucleoid) | True, membrane-bound nucleus |
| Size | Small (1–10 µm) | Larger (10–100 µm) |
| Membrane-bound organelles | Absent | Present (mitochondria, ER, etc.) |
| Examples | Bacteria, cyanobacteria | Plant, animal, fungal, protist cells |
5. The plasma (cell) membrane — the living boundary
Every cell is bounded by a plasma membrane made of lipids and proteins. It is selectively permeable: it decides what enters and leaves. Two passive processes do this without using energy:
Diffusion
Movement of particles from a region of higher concentration to lower concentration. It supplies gases — CO₂ and O₂ cross the membrane by diffusion.
Osmosis
Diffusion of water across a selectively permeable membrane, from a solution of higher water concentration (dilute) to lower (concentrated). Put a cell in:
- a hypotonic solution (more dilute outside) → water enters → the cell swells;
- an isotonic solution → no net movement;
- a hypertonic solution (more concentrated outside) → water leaves → the cell shrinks.
In a plant cell placed in a strong sugar/salt solution, the cytoplasm and membrane pull away from the cell wall — this is plasmolysis, and it proves the cell was alive.
6. The cell wall
Plant cells (and bacteria, fungi) have an extra cell wall outside the membrane, made mainly of cellulose in plants. It is rigid and freely permeable, gives the cell shape and strength, and lets plant cells survive hypotonic surroundings without bursting. Animal cells have no cell wall.
7. The nucleus
The nucleus is the control centre. It has a double nuclear membrane with pores, a nucleolus, and chromatin — threads of DNA and protein that condense into chromosomes during division. The DNA carries the instructions (genes) for building and running the cell and is passed to daughter cells.
8. Cytoplasm and the organelles
The jelly-like cytoplasm fills the cell; suspended in it are the organelles, each a tiny specialised machine:
- Endoplasmic reticulum (ER): a membrane network. Rough ER (with ribosomes) makes proteins; smooth ER makes lipids and detoxifies.
- Ribosomes: the sites of protein synthesis.
- Golgi apparatus: modifies, packages and dispatches materials (the "post office").
- Mitochondria: the powerhouse — release energy as ATP in respiration. Have their own DNA.
- Plastids (plant cells only): chloroplasts (green, contain chlorophyll, do photosynthesis), and chromoplasts/leucoplasts (colour/storage).
- Lysosomes: "suicide bags" full of digestive enzymes; clear worn-out parts and waste.
- Vacuoles: storage sacs. Plant cells have one large central vacuole giving turgidity; animal cells have small ones.
Plant vs animal cell, in one line: plant cells have a cell wall, plastids and a big central vacuole; animal cells do not (but both have a nucleus, membrane, cytoplasm, mitochondria, ER, Golgi and ribosomes).
9. How cells multiply
New cells arise only from existing cells by cell division:
- Mitosis: one cell → two identical daughter cells with the same chromosome number. Used for growth and repair.
- Meiosis: forms gametes (sex cells) with half the chromosome number, so the number is restored at fertilisation. Also creates variation.
10. Quick recap
- The cell is the basic unit of life; resolution (~0.1 mm for the eye) is why we need microscopes.
- Cell theory: all life is cellular; cells come from cells.
- Prokaryotic (no true nucleus) vs eukaryotic (true nucleus, organelles).
- The plasma membrane is selectively permeable — diffusion (gases) and osmosis (water); plasmolysis shows a cell is alive.
- Know every organelle's job, and the plant-vs-animal differences (cell wall, plastids, central vacuole).
