The Bio Brief

• In 1665, Robert Hooke observed a thin slice of cork under a self-designed microscope.
• He saw many small box-like structures arranged like a honeycomb.
• He named these boxes cells.
• The word cell comes from the Latin word “cellula”, meaning “a little room.”
• This discovery was very important because it showed for the first time that living things are made of small units.

 Cells are still called “cells” today in biology.

What Are Living Organisms Made Up Of?

All living organisms are made up of cells.

 Onion Peel Experiment

Steps:

1. Take a small piece of onion bulb.
2. Peel off the thin epidermis (skin) from the inner side.
3. Place it in water to prevent drying.
4. Put the peel on a glass slide with a drop of water.
5. Add a drop of safranin stain.
6. Place a cover slip carefully (avoid air bubbles).
7. Observe under a microscope.

 What Do We Observe?

• Many small, similar structures are seen.
• All structures look same, even if onions are of different sizes.
• These structures together form the onion bulb.
• These basic units are called cells.

 Conclusion: Cells are the basic building units of plants, animals, and all living organisms.

 Types of Organisms Based on Cells

 Unicellular Organisms (Uni = One)

• Made up of only one cell.
• One cell performs all life activities.

Examples:

• Amoeba
• Paramecium
• Chlamydomonas
• Bacteria

 Multicellular Organisms (Multi = Many)

• Made up of many cells.
• Different cells perform different functions.

Examples:

• Humans
• Plants
• Animals
• Some fungi

Flow Chart: Types of Organisms

Living Organisms

        ↓

   Made of Cells

        ↓

 ┌───────────────┐

 │               │

Unicellular   Multicellular

 (One cell)   (Many cells)

Origin of Cells

• Every multicellular organism starts from a single cell.
• Cells divide to form new cells.
• All cells come from pre-existing cells.

 Important Scientists & Their Discoveries

Scientist	Year	Discovery
Robert Hooke	1665	Discovered cells in cork
Leeuwenhoek	1674	First observed living cells
Robert Brown	1831	Discovered nucleus
Purkinje	1839	Coined term “protoplasm”
Schleiden	1838	Plants are made of cells
Schwann	1839	Animals are made of cells
Virchow	1855	Cells arise from pre-existing cells

 Cell Theory

1. All living organisms are made of cells.
2. The cell is the basic unit of life.
3. New cells arise from pre-existing cells.

 Importance of Microscope

• Discovery of magnifying lenses helped explore the microscopic world.
• Electron microscope (1940) helped study detailed cell structure and organelles.