Cell: Introduction, Discovery, Size and Cell Theory - Practice Questions & MCQ

Introduction to Cell Structure and Function

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:

• All living organisms are made up of one or more cells.
• Cells are the basic unit of structure and function of organisms.
• Cells come only from preexisting cells because cells are self-reproducing.

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.