The Fundamental Unit of Life

Cell is the fundamental structural and functional unit of life.

All living organisms are made up of cells. It is similar to a house; as a house is made of bricks all living organisms whether it is a plant or animals are made up by combining together many cells. The number of cells varies from organism to organism. According to estimates an adult human has about 100 trillion cells while an amoeba is a single celled organism. The human body has about 200 types of cells. Thus, it can be said that Cell is the basic structural unit of life.

All of the basic functions required for the life are performed at the cellular level. Thus it can be said that the Cell is the basic functional unit of life.

Combining together both statements, it can be said that CELL IS THE BASIC STRUCTURAL AND THE FUNCTIONAL UNIT OF LIFE.

Cell Theory

The cell Theory is a scientific theory in Biology that tells that the cell is the basic functional and fundamental unit of life. The credit for the Cell Theory goes to two scientists; The German Physician Theodor Schwann and the German Botanist Matthias Jakob Schleiden.

The three tenets of the Cell Theory are:

(1) All living organisms are composed of one or more cells.

(2) The cell is the basic unit of structure and organization in organisms.

(3) Cells arise from pre-existing cells.

Measurement of the size of Cells

Cells are very small in size and cannot be seen by the naked eye.

The size of a cell of an organism is measured in micrometers because of its small size.

Micro meter is denoted as "μm".

1 micro meter = 1 × 10^{– 6}

1 micro meter = 0.000001 meter

This means 1 micro meter = 1 one millionth of a metre

Or 1 micro meter = one thousandth of a millimetre

Or 1 micro meter = 0.001 mm

The Largest Cell

The egg of an ostrich is the largest cell. The length of cell of an ostrich is about 15 cm long and 13 cm wide.

The longest cell in a human is the neuron.

The smallest cell is Mycoplasma gallicepticum which is a bacterium that causes disease in chickens and some other birds.

Cell – Tissue – Organs

Many cells combined together form tissue and many tissues combined together form organs. Different organs in a living object perform different types of work as the eye is an organ that facilitates us to see the world, the ear is an organ that facilitates us to hear a sound, the heart is an organ that pumps blood to the whole body and by this way the oxygenated blood is transported to every part of the body, etc.

Many organs combined together and form an organ system, such as the digestive system, circulatory system, nervous system, skeletal system, and excretory system.

Cell → Tissue → Organs → Organ System

Discovery of Cell

Robert Hooke an English scientist discovered the cell in the year 1665. He examined the cork using his own designed microscope and found that the cork has a similar structure to a honeycomb which has little compartments. He called these boxes "cells".

Cork is a substance that comes from the bark of a tree and the word cell comes from Latin which means "a little room". The word "cell" is used to describe the fundamental unit of life in biology.

The Structure of an Onion Peel

By seeing an onion peel through a microscope, it can be observed that an onion peel is made up of many tiny compartment like structures. These structures resemble a honeycomb. These small structures are the cells of an onion peel. All structures look similar to each other.

Each cell of an onion peel has a dot like structure in the middle. These dot like structures is called nucleus.

Regardless of places, varieties, and sizes, similar structures are found in all the onion peels.

What are these structures?

The rectangular structures seen through the microscope in the onion peel are the "cells" of the onion. Many such cells combined together form onion peel, and finally many onion peels combined together make an onion.

Not only onion or onion peel rather all of the forms of life have similar structures, i.e. all forms of life are made up of cells.