Microscopes allow us to study cells. The compound microscope uses two lenses and provides a more magnified view than a simple microscope. Key discoveries included Hooke observing cells and van Leeuwenhoek using microscopes to study them. The cell theory proposed that all living things are made of cells, cells are the basic functional unit of life, and cells come from pre-existing cells. Cells can be unicellular like bacteria or multicellular like plants and animals. They vary in size, shape, and internal structures like the cell membrane, cytoplasm, and nucleus.

1. Chapter – 2
The Cell
INTRODUCTION
What is Cell?
Cell is the basic building blocks of living organisms.
Cells are also called as basic unit of life.
MICROSCOPES
Microscopes are the most important instruments that have helped us to discover and study cells. A
microscope is an instrument that is used to view small objects by making them appear larger. Different types of
lenses are used in microscopes to make the objects appear larger.
Two Dutch spectacle, Zacharias Janssen and his father, Hans Janssen, are believed to be the inventors of the
microscope.
Types of Microscopes
The simple microscope uses only one lens and works like a magnifying glass.
The compound microscope uses two lenses and give a more magnified image than the simple
microscopes does.
Compound microscopes are used in school laboratories.
A third kind of microscope is also used; it is called the Electron microscope. It does not use any lens. It
is used in laboratories and can be used to study extremely small objects.
Hence, the two type of microscopes are Compound microscope and Electron microscope.
THE DISCOVERY OF THE CELL
The discovery of cell took a long time to be believed since most scientists did not have the tools to
study life like we do today.
Robert Hooke: He was the first person to observe and name cells.
Antonie Van Leeuwenhoek: He made and used hundreds of simple microscope.

2. CELL THEORY
Theodor Schwann, Matthias Schleiden and Rudolf Virchow proposed the cell theory of life, which says the
following:
All living things are made of cells.
A cell is the basic structure and functional unit of living things.
All cells come from pre-existing cells.
UNICELLULAR AND MULTICELLULAR ORGANISMS
Unicellular organisms – these organisms are made of only cells. It is thought that the first organisms that found
on Earth were unicellular.
They can function by themselves.
Examples: Amoeba, bacteria and so on.
Multicellular organisms – these organisms are made of more than one cell. They were originally formed
billions of years ago.
Examples: Plants and Animals
CELL DIVERSITY
Cell Size
Most cells are very small and only can be seen with a microscope.
Some cells are very long, for example: Some nerve cells in humans which maybe 1m long.
Ostrich eggs are one of the largest known cells. It is around 15cm long.

3. Cell Shape
Many bacteria are spherical or round.
Red Blood Cells are circular and flatted so that they can move easily through blood vessels.
The single-celled organisms Chlamydomonas is oval in shape.
Muscle cells are cylindrical with either tapering or non-tapering ends.
Many types of cells in plants are rectangular in shape.
Some cells are amoeboid (with no fixed shape). This shape is named after the unicellular organism Amoeba,
which keeps changing its shape as it moves
PARTS OF CELLS
Cell Membrane
Cell membrane (membrane: a thin layer that acts as a boundary in a cell) is a thin, delicate structure found in
all cells. Cell membrane is also called the plasma membrane.
Functions:
It protects the cell and keeps the contents of the cell contained within the cell.
It allows only some substances to either enter the cell or leave the cell. For this reason, the cell membrane is
called semi-permeable or selectively permeable.
Cytoplasm
The cytoplasm is a viscous (viscous: thick (used to describe a fluid)) liquid found within the cell membrane. It
is the place where all major life functions such as respiration and digestion take place.
Small structures from cytoplasm is called organelles, larges organelles is called the nucleus.
Nucleus
The nucleus (plural: nuclei) is a large spherical structure that is usually fund in center of a cell.
(PLEASE READ THE FUNCTIONS)
The main parts of nucleus are as follows:
1. Nuclear membrane
2. Nucleoplasm
3. Chromosomes