In 1655, the English scientist Robert Hooke made an observation that would change basic biological theory and research forever. While examining a dried section of cork tree with a crude light microscope, he observed small chambers and named them cells.
Over the next 175 years, research led to the formation of the cell theory, first proposed by the German botanist Matthias Jacob Schleiden and the German physiologist Theodore Schwann in 1838 and formalized by the German researcher Rudolf Virchow in 1858. In its modern form, this theorem has four basic parts:
1. The cell is the basic structural and functional unit of life; all organisms are composed of cells.
2. All cells are produced by the division of preexisting cells (in other words, through reproduction). Each cell contains genetic material that is passed down during this process.
3. All basic chemical and physiological functions - for example, repair, growth, movement, immunity, communication, and digestion - are carried out inside of cells.
4. The activities of cells depends on the activities of subcellular structures within the cell (these subcellular structures include organelles, the plasma membrane, and, if present, the nucleus)
There are millions of living organisms. They are of different shapes and sizes. Their organs also vary in shape, size and number of cells.
Number of Cells
Human body has trillions of cells which vary in shapes and sizes. Different groups of cells perform a variety of functions. Organisms made of more than one cell are called multicellular organisms. An organism with billions of cells begins life as a single cell which is the fertilized egg. The fertilised egg cell multiplies and the number of cells increase as development proceeds.
The single-celled organisms are called unicellular organisms. A single celled organism performs all the necessary functions that multicellular organisms perform.
A single-celled organism, like amoeba, captures and digests food, respires, excretes, grows and reproduces. Similar functions in multicellular organisms are carried out by groups of specialised cells forming different tissues. Tissues, in turn, form organs.
Shape of Cells
Generally, cells are round, spherical or elongated. Some cells are long and pointed at both ends. They exhibit spindle shape . Cells sometimes are quite long. Some are branched like the nerve cell or a neuron The nerve cell receives and transfers messages, thereby helping to control and coordinate the working of different parts of the body.
Components of the cell are enclosed in a membrane. This membrane provides shape to the cells of plants and animals. Cell wall is an additional covering over the cell membrane in plant cells. It gives shape and rigidity to these cells.
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