It is the smallest fundamental unit that makes up a living organism. A cell can carry out necessary life activities, including replication. Replication of one cell gives rise to a new cell of a similar type. This is also called cell division. 

Every living organism is composed of cells. On the basis of the number of cells that make up the organisms, they can be of two types – Unicellular (if made of single-cell) and multicellular (if made of multiple cells). Every cell has an extremely complex structure and function.

The microscopic study of the cell structure reveals a cell membrane enclosing a mass of cytoplasm (the mass of fluid that occupies the majority of the cell) and a nucleus (it contains the hereditary material). The nucleus is the chief regulatory centre of the cell. There are various organelles within the cells, each recruited for a specialized function. All the cell organelles work together as a unit to discharge the complete activities of the cell. 

To date, the smallest known cell: structure and function are Mycoplasma. The first biologist who discovered cells was Robert Hooke.

Lets see how a cell provides structural support and stability to any living organism.

• The external membrane that separates a cell from the rest of the world. The plasma membrane is the outermost membrane for animal cells, whereas plant cells have an additional exterior most membrane known as the cell wall. 
• Cytoplasm (fluid in nature) that forms the mass of the cell
• Cell organelles (they are bounded individually by membranes)
• Nucleus (the primary regulatory center of the cell which encodes the genetic information of the organism).

There are a plethora of cell organelles within each cell. The chief organelles are:- 

• Mitochondria also are known as the powerhouse of the cell because it generates the energy (in the form of ATP) required to carry out the cellular functions
• Lysosomes perform the housekeeping function within the cell by digesting unwanted organelles, dead counterparts within the cell.
• Endoplasmic Reticulum is synthetic and concerns the synthesis, processing, and sorting of various biomolecules vital for the cell’s functioning. They can be of two types – smooth endoplasmic reticulum with no attached ribosomes on its surface and rough endoplasmic reticulum with ribosomes on the surface.
• There are various other organelles within the cell, such as Golgi Apparatus, Ribosomes, Centrosomes, Vacuoles, Cytoskeletal elements, Peroxisomes. Specific organelles like plastids and chloroplasts are found only in the plant cells.

Did you know that the discovery of cells is the single most crucial fundamental event in Biology that has revolutionized the life sciences concept? In 1665, Robert Hooke first noticed a bottle cork under a compound microscope and addressed each cork unit as cells because they resembled tiny rooms. However, he could not provide any further insight into the cells’ structural complexity and attributed cells as non-living entities. 

Anton Van Leeuwenhoek observed cells under compound microscopes and attributed them as living entities. He addressed these units as animalcules.

Later in the year 1893, Robert Brown proposed a detailed structure of cells and highlighted the nucleus presence in cells.

The first proposed cell theory was proposed by three eminent scientists – Theodor Schwann, Matthias Schleiden, and Rudolf Virchow. Following are the postulates of cell theory:- 

• A cell is the fundamental unit of life
• All living species are composed of cells
• All cells arise from pre-existing cells

Later on, the following postulates were added to the existing cell theory:- 

• Genetic or hereditary information is also passed from one cell to the other cells of the subsequent generation.
• Energy flow occurs within each cell.
• All cells exhibit similar chemical composition.

Cells are fundamental of two types:- 

Prokaryotic cells (primitive cells)

They have the following characteristics:

1. They lack a nucleus and exhibit a dense region of nuclear material aggregation termed a nucleoid.
2. The genetic material of prokaryotic cells can be either DNA or RNA
3. Single-celled microbes like bacteria, archaea, and blue-green algae (cyanobacteria) are the prokaryotes.
4. The diameter of prokaryotic cells lies in the range of 0.1 -0.5 micrometres.

Eukaryotic Cells (modern cells)

They have the following characteristics:- 

1. They have a true nucleus enclosed by a nuclear membrane
2. They follow both sexual and asexual modes of reproduction
3. The diameter of eukaryotic cells lie in the range of 10-100 micrometres
4. Fungi, Protozoa, Plants, and Animals have eukaryotic cells hence called eukaryotes

There are various types of cells depending on their location and structure like Epithelial cells, bone cells (osteoblasts), nephrons of kidney cells, Myocytes or muscle cells, cardiocytes or heart cells, etc.

Following are the essential functions of the cells

• To provide structural support to every living organism
• Produce energy
• Act as the central pillar of growth of an organism
• Form the basic unit of reproduction
• Help in the transportation of various substances needed to sustain life

1. What is the structure of a cell?
   A cell’s structure can be explained as a mass of cytoplasm enclosed by a cell membrane. The cytoplasm has several organelles embedded in it, each of which has a membranous covering of their own. The membrane-bound organelles form the endomembrane system of the cell. In the cytoplasm, a nucleus is present that acts as the brain of the cell.
2. What are the functions of a cell?
   The three primary functions of cell are:-
   • Provide structural support
   • Help in the growth of an organism
   • Divide and produce new cells 
3. What are the four-cell structures?
   Four major cell structures are:-
   • Plasma or Cell membrane
   • Cytoplasm
   • Nucleus
   • Cell organelles
4. Who discovered the cell?
   Robert Hooke discovered the cell in 1665.