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INTRODUCTION

A cell is defined as the smallest, basic unit of life that is responsible for all of life’s processes. They  are the basic, fundamental unit of life. Cells are the structural, functional, and biological units of all living beings. A cell can replicate itself independently. Hence, they are known as the building blocks of life. They  range in size from 0.0001 mm to nearly 150 mm across

                                      The cell structure

All organisms are made up of cells. They may be made up of a single cell (unicellular), or many cells (multicellular).  Mycoplasmas are the smallest known cells. Cells are the building blocks of all living beings.

 Cells are the lowest level of organisation in every life form. From organism to organism, the count of cells may vary. Humans have higher number of cells compared to that of bacteria.

Cells provide structure to the body and convert the nutrients taken from the food into energy.

The study of cells from its basic structure to the functions of every cell organelle is called Cell Biology.

Cells comprise several cell organelles that perform specialized functions to carry out life processes. Every organelle has a specific structure. The hereditary material of the organisms is also present in the cells. Each cell contains a fluid called the cytoplasm, which is enclosed by a membrane. Also present in the cytoplasm are several biomolecules like proteins, nucleic acids and lipids. Moreover, cellular structures called cell organelles are suspended in the cytoplasm.

 Cells are complex and their components perform various functions in an organism. They are of different shapes and size. Our body is made up of cells of different shapes and size

Robert Hooke was the first Biologist who discovered cells in 1665. He observed a piece of bottle cork under a compound microscope and noticed minuscule structures that reminded him of small rooms. Consequently, he named these “rooms” as cells. However, his compound microscope had limited magnification, and hence, he could not see any details in the structure. Owing to this limitation, Hooke concluded that these were non-living entities.

Later Anton Van Leeuwenhoek observed cells under another compound microscope with higher magnification. This time, he had noted that the cells exhibited some form of movement (motility). As a result, Leeuwenhoek concluded that these microscopic entities were “alive.” Eventually, after a host of other observations, these entities were named as animalcules.

In 1883, Robert Brown, a Scottish botanist, provided the very first insights into the cell structure. He was able to describe the nucleus present in the cells of orchids.

The discovery of cells is one of the remarkable advancements in the field of science. It helps us know that all the organisms are made up of cells, and these cells help in carrying out various life processes. The structure and functions of cells helped us to understand life in a better way.

Cell Theory

Cell Theory was proposed by the German scientists,  Theodor Schwann, Matthias Schleiden, and Rudolf Virchow. The cell theory states that:

1. All living species on Earth are composed of cells.
2. A cell is the basic unit of life.
3. All cells arise from pre-existing cells.

A modern version of the cell theory was eventually formulated, and it contains the following postulates:

Characteristics of Cells

Following are the various essential characteristics of cells:

1. Cells provide structure and support to the body of an organism.
2. The cell interior is organised into different individual organelles surrounded by a separate membrane.
3. The nucleus (major organelle) holds genetic information necessary for reproduction and cell growth.
4. Every cell has one nucleus and membrane-bound organelles in the cytoplasm.
5. Mitochondria, a double membrane-bound organelle is mainly responsible for the energy transactions vital for the survival of the cell.
6. Lysosomes digest unwanted materials in the cell.
7. Endoplasmic reticulum plays a significant role in the internal organisation of the cell by synthesizing selective molecules and processing, directing and sorting them to their appropriate locations.

Types of Cells

Cells are similar to factories with different labourers and departments that work towards a common objective. Various types of cells perform different functions. Based on cellular structure, there are two types of cells:

1. Prokaryotes
2. Eukaryotes

Prokaryotic Cells

1. Prokaryotic cells have no nucleus. Instead, some prokaryotes such as bacteria have a region within the cell where the genetic material is freely suspended. This region is called the nucleoid.
2. They all are single-celled microorganisms. Examples include archaea, bacteria, and cyanobacteria.
3. The cell size ranges from 0.1 to 0.5 µm in diameter.
4. The hereditary material can either be DNA or RNA.
5. Prokaryotes generally reproduce by binary fission, a form of asexual reproduction. They are also known to use conjugation – which is often seen as the prokaryotic equivalent to sexual reproduction (however, it is NOT sexual reproduction).

Eukaryotic Cells

1. Eukaryotic cells are characterized by a true nucleus.
2. The size of the cells ranges between 10–100 µm in diameter.
3. This broad category involves plants, fungi, protozoans, and animals.
4. The plasma membrane is responsible for monitoring the transport of nutrients and electrolytes in and out of the cells. It is also responsible for cell to cell communication.
5. They reproduce sexually as well as asexually.
6. There are some contrasting features between plant and animal cells. For eg., the plant cellcontains chloroplast, central vacuoles, and other plastids, whereas the animal cells do not