Cell: Introduction, Discovery, Size and Cell Theory MCQ

Quick Facts

Introduction to Cell Structure and Function

CELL: Introduction, Discovery, Size and Cell Theory

Introduction:

A cell is the unit of a living thing.
A living thing can be made up of one cell like a bacterium of many cells like a human. Irrespective of the number of cells, living things are referred to as organisms.
Hence, the cell is the basic unit of an organism.
A cell can be defined as the basic structural and functional unit of life.
It is called the structural unit because it forms the body/structure of the organisms.
It is called the functional unit because each cell is capable of performing the essential metabolic functions.

Discovery of Cell:

In 1665, Robert Hooke first discovered the cell using his self-designed microscope. He observed the dead cells of cork of plants. He described his observation as empty cells and termed them as ‘cells’.
In 1674, the documentation of live cells was made by Anton van Leeuwenhoek, who observed bacterial cells and algal cells.

Cell Theory:

In 1838, the German botanist Matthias Schleiden observed that all plants are made up of cells.
In 1839, the German zoologist Theodor Schwann declared that all animals are composed of cells.
These observations laid the foundation of the Cell Theory.
Later, the German physician Rudolf Virchow stated Omnis cellula e cellula, that is, new cells arise from pre-existing cells.

Based on these observations, the cell theory can be stated as:

As technology advanced, the insight into the cells became clearer and the cell theory got modified as follows:

All known living things are made up of cells.
The cell is the structural and functional unit of all living things.
All cells come from pre-existing cells by division.
Cells contain hereditary information which is passed from cell to cell during cell division.
All cells are basically the same in chemical composition.
All energy flow (metabolism & biochemistry) of life occurs within cells.

Exceptions to Cell Theory:
There are following three exceptions to the postulates of cell theory:

Viruses are acellular organisms. They are neither considered living nor non-living because they are metabolically dead when not in the host’s cell and live as soon as they enter the host cell. Also, they cannot reproduce on their own.
Semi-autonomous organelles in eukaryotic cells, that is, mitochondria and plastids have their own genetic material. They can divide on their own while being in the cell.

Cell Size and Shape:

Cells are microscopic. Mycoplasma (Smallest cells) measure only 0.3 μm in length.
Bacterial cells have a size of 3 to 5 μm.
Human red blood cell is 7.0 μm in diameter.
Egg of the ostrich is the largest isolated single cell.
The shape of the cell depends on the function it performs.