Chapter 5 · NCERT Class 9 Science

The Fundamental Unit of Life

Every living thing — from a single bacterium to a blue whale — is built from cells. Zoom inside one and you find tiny machines, each doing a job. Tap each part to see what it does.

🔬 3 topics⏱ ~25 min📝 12-question quiz

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Inside a cell

A cell is a busy little factory. Tap each part to see what it is and the job it does for the cell.

Explore · Parts of a celltap a part

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The three big ideas

The cell is the basic structural and functional unit of all living things. Robert Hooke first saw cells in a slice of cork (1665).
Some organisms are a single cell (unicellular, e.g. Amoeba); others are made of many cells (multicellular).
Prokaryotic cells have no true (membrane-bound) nucleus — e.g. bacteria.
Eukaryotic cells have a true, membrane-bound nucleus — plant and animal cells.

Cell membrane — the outer boundary; selectively permeable, it controls what enters and leaves.
Nucleus — the control centre; holds the DNA / chromosomes that direct the cell.
Cytoplasm — the jelly-like fluid where organelles float and reactions happen.
Mitochondria — the "powerhouse"; release energy from food (respiration).

Worked example. Which structure decides what enters or leaves a cell?

The cell membrane. It is selectively permeable — it allows some substances through while blocking others, so it controls movement in and out of the cell.

Both plant and animal cells have a cell membrane, nucleus, cytoplasm and mitochondria.
Plant cells also have a cell wall (cellulose), chloroplasts (for photosynthesis) and a large central vacuole.

Common mistake: thinking every cell has a cell wall. The cell wall is found only in plant cells (and bacteria/fungi). Animal cells have just a cell membrane — no wall.

Where you'll meet it

Cells at work

How medicines reach a cell

A drug only works if it can cross the selectively permeable cell membrane to reach the inside. Pharmacologists design molecules small enough and soluble enough to slip through — the membrane decides what gets in.

Why plants stand upright

When a plant cell takes in water by osmosis it swells and presses against its rigid cell wall (turgor pressure). This turgor plus the cell wall keeps stems and leaves firm; lose the water and the plant wilts.