Cell

A cell is defined as the functional and structural unit of all living organisms. Robert Hooke discovered the first cell from the bark cell after the discovery of the electron microscope. He called it a comb, because the shape of a cell is similar to the honeycomb. Later it came to be known by the name of the cell.

Diversity of the cell

In the universe, all the species are made up of cells. The smallest organism, which is PPLO, is a cell, and the largest organism, which is a whale, is also made up of cells.

The cell is present in various sizes and shapes. They are as follows: –

• Red blood cells
• White blood cells
• Nerve cell
• Hepatocytes
• Osteocytes, etc.

The smallest cell in the human body is the sperm, and the largest cell, the ovum.

The group of cells is known as tissue; one of the best examples of tissue is the xylem. Xylem is complex tissue present in the plant for the conduction of water. 

Cell history

The first cell was discovered by Robert Hooke, but it was the dead cell. The first cell was discovered from the cork part of the plant, which is the dead part.

The first living cell was discovered by Anton van Leeuwenhoek. 

In 1838, Shewedon, a German botanist studied the plant cell, while in 1839, British biologist Theodor Schwann studied the first animal tissue and observed the animal cell.

Cell theory

The cell theory was given by Rudolf Virchow. He stated three points, as follows: –

• All living organisms are made up of cells.
• All the cells have the capability to divide.
• The cell is the structural and functional unit of a living species.

Exception: Virus can not follow the cell theory.

Basic criteria for defining the cell :

(i) Presence of genetic material

(ii)Presence of limiting plasma membrane.

(iii)Presence of a metabolic machinery.

Types of cells on the basis of their number

On the basis of their number, cells can be classified as: 

1. Unicellular: – Those organisms which are made up of only one cell are called unicellular. For example – amoeba, 
2. Multicellular: – Those species which are made up of more than one type of cell are called multicellular species. For example: – Plants and animals. 

Cell division

The formation of a new cell from the old cell is called cell division. There are two types of division; these are mitosis and meiosis.

Characteristics of Cancer Cell

The cell has a property to control its division rate, it is known as contact inhibition. Cancer cells lose this property and divide uncontrollably, leading to the formation of a clump of cells known as a tumour.  

There are two types of tumours – benign and malignant tumours.

Cancer cells have a property known as metastasis, in which the tumour can transfer to the other location through blood.

Metastasis is only present in the malignant tumour. The genes are present in the cell, called oncogenes, which are responsible for the cancer.

Cancer-causing genes are present in all cells, but in an inactive state. The thing which activates these genes is known as carcinogens.

Components of Cells

The cell is made up of three components: –

• Cell wall
• Cell membrane
• Cell organelles

Cell Wall

The cell wall is a rigid structure. It helps prevent the entry of pathogens or any antigen present in the surrounding. The cell wall also applies some pressure on the cell membrane, called wall pressure.

Cell Membrane

The outer flexible structure of the cell is called the cell membrane. It is present in all cells without exception. The cell membrane is made up of protein, lipid, and carbohydrates.

The cell membrane allows the solute and the solvent to pass through it, as the cell membrane is permeable to solutions. There are different types of cell membranes on the basis of permeability: –

1. Permeable –  It allows both solute and solvent
2. Impermeable – It allows none
3. Semipermeable – it allows only solute particles.

Cell membrane changes its shape accordingly to the size of the vessels and this property is known as diapedesis.

Cell Organelles

The cell organelles are divided into three categories depending on the presence of the cell membrane.

• Membraneless organelles 

The organelles that have no membranes are called membrane-less organelles. Ribosomes and Centrioles are the membrane-less organelles.

The ribosome is made up of more than two subunits and these are larger and smaller subunits. The major function of the ribosomes is to help the formation of the proteins by joining the amino acids by peptide bonds.

The centrioles are present in the poles of the cells and help in the formation of spindle protein which helps in cell division.

• Single membrane-bound organelles

The organelles which have only one membrane around it are called single membrane organelles. For example – ER, Golgi body, Vacuoles, lysosomes.

• ER stands for the endoplasmic reticulum. It involves the formation of protein and lipids.
• The Golgi body helps in the packing and modification of the protein and lipid, which is formed by the ER. The lysosome is also produced by the Golgi body.
• Lysosome helps in Phagocytosis and also helps in the digestion of worn out and infected cells.
• Vacuoles store the extra food in them. The vacuoles in plants are larger than the animals.

• Double membrane-bound organelles.

The organelles which have two membranes around it are called double-membrane organelles. For example –The nucleus, mitochondria, and chloroplast.

• The nucleus is the regulatory unit of the cell and it controls the functioning of the cell.
• Mitochondria is the powerhouse of the cell and helps in the production of the energy currency of the cell, which is ATP.
• Chloroplast is the kitchen house of the cell and helps the formation of food through the process of photosynthesis.
• Nucleus

• Robert Brown later discovered the nucleus.
• Nucleus is a double membrane bound dense protoplasmic body, which controls all cellular metabolism and encloses the genetic information of cells. 
• Nucleus is considered as the controller or director of a cell. Importance of nucleus in control of heredity, growth and metabolism was experimentally proved by Hammering.

Conclusion

All the functions of the human body are done and regulated by the cell and its organelles. A cell can be considered the basic component of any living organism. Unicellular organisms are capable of  independent existence and performing the essential functions of life. Anything less than a complete structure of a cell does not ensure independent living. Hence, cell is the

fundamental structural and functional unit of all living organisms. Anton Von Leeuwenhoek first saw and described a live cell. Robert Brown later discovered the nucleus. The invention of the microscope and its improvement leading to the electron microscope revealed all the

structural details of the cell.