Fundamental functional units called cells are present deep inside our body, on the surface and in all parts. These fundamental units are busy keeping the body in working condition. Numerous activities takes place in the cell at all times, our nervous system handles the behavioral activity of the cells. This is an example the of the structural and functional composition of our body. Plant cells are similar to animal cells. The plant cells have cell wall which is completely absent in animal cell. There are a few fundamental characteristics that are particular to every cell. cellular modifications make the cell different from each other. In diameter most plant cells are approximately 0.002 inches, most bacteria are smaller at 10 to 50 nanometers long, they are impossible to be seen without magnification. The size of the cell is limited due to the inability of very large cells to provide nutrients and water and also remove cellular wastes efficiently.
Cell theory is a doctrine that states all organisms have organizations that are composed of similar units called cells. This theory was put forth in 1839 by Scheliden and Shwann. This theory remains till date the foundation of modern cell theory. The foundation of this theory are taken from the great frameworks of biology including Darwin's theory of evolution (1859), Mendel's laws of Inheritance (1865) and so on.
The cell theory states :
- All living things are made of cells and their products.
- New cells are created by division of old cells into two.
- The basic building units of life are cells.
There are three parts of cell theory as described below:
- All living things or organisms are composed of one or more cells.
- The cell is the basic unit of structure, function and organization in all organisms.
- Cells come from pre-existing living cells.
The development of magnifying lenses and light microscopes made the observation and description of living cells possible. Many scientists contributed to the formulation of the cell theory.
- In the late 1500's Hans and Zacharias Janssen, father and son, two Dutch lens grinders produced first compound microscope with 2 lenses.
- In 1665 Robert Hooke used light microscope to observe at thin slices of plant tissues - cork. He observed empty, monk's chambers. He called these tiny chambers as "cells".
- Antony von Leeuvenhoek invented the first microscope. He was a Dutch scientist and the first to see bacteria and protists. He made powerful magnifying microscopes out of magnifying glasses. In the year 1673 Anthony von Leeuvenhoek observed pond water organisms. He was the first person to see living microscopic organisms. He made careful sketches of the observed organisms and called them 'animalcules' meaning "little animals".
- Robert Brown in the year 1831 a botanist. He concluded from his studies that all plants had cells with a nucleus.
- Matthias Schleiden a German botanist studied plants. He viewed plant parts under a microscope. He discovered that plant parts are made of cells. In the year 1838 "all plants are made of cells."
- Theodore Schwann a German scientist studied animals. He observed that all animals he studied were made of cells. Hence, he concluded in the year 1839 that "all animals are made of cells."
- Rudolf Virchow another German physician studied reproduction in cells. He stated in the year 1855 that "where a cell exists, there must have been a pre-existing cell."
- Brown, Theodore and Schwann hence put forth the cell theory.
General accepted parts of modern cell theory are as follows:
- All living things are made up of one or more cells.
- All living cells come from per-existing cells, by division.
- The cell is the fundamental, structural and functional unit of all living organisms.
- The activity of an organism is the total activity of the independent cells of the organism.
- All energy flow of life occurs within the cell.
- The cells contain hereditary information and is passed from one to another during cell division.
- The chemical composition of cell are basically the same in organisms of similar species.