Introduction

A cell is the smallest part of an organism's body that can carry out all the basic functions of life, like metabolism, growth, and reproduction. It is the structural and functional unit of all known living organisms. It is the simplest unit of a living being called the "building block of life." Cells may be compared to bricks. Bricks are assembled to make a building. Similarly, cells are assembled to make the body of every organism.

Discovery of the cell 

Cells were discovered by Robert Hooke in 1665. He observed a thin slice of cork under the microscope that he had discovered and saw that it was made up of many small box-like structures. These boxes appeared like honeycomb. The boxes were separated from each other by a partition. He called these boxes cells. In 1674, Anton von Leeuwenhoek, a Dutch scientist, discovered cells moving freely and could also see some minute components within the cells with the help of his improvised microscope.

Cell theory

The concept that all living organisms, whether plants or animals, are made up of cells were elaborated as cell theory by German botanist Matthias Schleiden and British Zoologist Theodor Schwann. They put forward the cell theory of life. The main points of this theory are as follows:

All living organisms are composed of cells and products of cells. 

All cells arise from pre-existing cells.

Variations of cells 

There are millions of living organisms. They all have different shapes and sizes and consist of different cells; living cells show variations in size, shape, and number, leading to a wide variety in nature.

Number of cells

Different organisms have a different number of cells. For example, the human body has trillions of cells with different shapes and sizes. Based on the number of cells present in organisms, these are classified as unicellular and multicellular. Organisms, such as Amiche, Peremroum, Euglena, and bacteria, are single cells. These are examples of unicellular organisms. Although made up of a single cell, they perform all activities necessary for life. They grow, move, digest food, give out wastes and reproduce. All these activities Pants and animals can be seen with the naked eye are made up of many cells. They are multicellular are carried out by single-cell organisms. All multicellular organisms begin life as a single cell, which is the fertilized egg. The fertilized cell multiplies, and the number of cells increases as the organism develops.

Shapes of cells

The shape of the cell varies from organism to organism. In most cases, the shape is round. However, in some, it is irregular, as in Amha and leucocytes, whereas in bacteria, the shape is rod-like, spiral d comma-shaped. Some cells are long and thin, for example, muscle cells and nerve cells of multicellular organisms. The shape of the cells varies considerably in multicellular organisms; they may be flat, columnar elliptical, spindle-shaped, cuboidal, polygonal, and so on. Their shape is mainly determined by their function and position. The cell's shape may be mostly fixed or frequently changing, as in Amha and leukocytes (white blood cells).

A has no definite shape, unlike other organisms. It keeps on changing its shape. The projections of varying lengths protruding out of its body are called pseudopodia (pseudo: false; podia: feet). These projections appear and disappear as Amorha moves or feed The shape of the leucocyte remains spherical during circulation, but in other conditions (destroying germs), it becomes irregular. Cells sometimes are quite long. Some are branched like the nerve cell or neuron. The nerve cell receives and transfers messages, thereby helping to control and coordinate different parts of the body. Ciliated cells from the lining of the nose have hair-like projections called cilia. The sticky mucus the lashing movement of cilia keep the dust and the germs away.