Cell- The Unit of Life

Introduction

Have you ever wondered what distinguishes a living thing from a non-living thing? The obvious answer is that a living organism has life in it, while a non-living thing does not. A cell, the unit of life, is the basic functional and structural unit in all living organisms. In fact, the cellular organisation is the defining characteristic of living beings that distinguishes them from non-living beings. Independent existence is not possible without a complete cell structure. It was Robert Hooke who discovered the cell in 1665 by examining a piece of bottle cork using a compound microscope. Anton Von Leeuwenhoek was another prominent microbiologist who discovered the existence of single-celled organisms and bacteria by using his own microscopes. Robert Brown, another prominent botanist, discovered the nucleus in the cell. This discovery helped in putting the cell theory on a firm footing. Nature has cells of various shapes such as polygonal, columnar, cuboidal, thread-like and so on. There are also certain cells with irregular shapes. In the human body, the red blood cells are round and biconcave, white blood cells are amoeboids, and nerve cells are long with branches. The shape of the cell helps it perform a specific function. The smallest cell is mycoplasma. Its length varies from 0.3 to 0.8 µm. The largest isolated cell is the egg of an ostrich. Biologists have been able to analyse the structure of a cell in detail by means of an electron microscope. Thus, the usage of powerful electron microscopes has generated vast study material and research papers about cells. This, in turn, has led to the development of various medicines and vaccinations for the prevention and cure of diseases.

Types of Living Organisms

There are two types of organisms: unicellular and multicellular. Unicellular organisms such as paramecium, yeast, and bacteria have only one cell that carries out all the functions needed to sustain life. In such organisms, reproduction results in an increase in the number of cells. But in multicellular organisms such as plants, different kinds of cells form tissues. These various tissues have definite functions to perform. Reproduction in these organisms results in the production of offspring whose features are similar to that of their parents. The growth in multicellular organisms happens through cell division. While in plants, cell division occurs throughout their lifespan, it is limited only up to a certain age in animals.

Cell Theory

The cell theory was propounded by German botanist Matthias Schleiden and British zoologist Theodore Schwann in 1839. They presented the cell theory by studying different types of plant and animal cells, respectively. Rudolf Virchow (1855) later enhanced the cell theory, who put forward the facts related to the division of cells and the formation of new cells from pre-existing ones. His contributions to the original cell theory (given by Schleiden and Schwann) resulted in the development of modern cell theory on a firm footing. The contributions of these three scientists cemented the role of the cell as the unit of life. The following points are highlighted by the modern cell theory:

All living organisms are made up of cells that are the functional units of living things

A new cell is formed when there is a division of existing cells

During the cell division process, the hereditary information is passed from one cell to another

The flow of energy occurs within the cells

The chemical composition is the same in all the cells

Structure of a Cell

A cell consists of five components. Each component performs a specific function necessary to ensure life’s processes. These components are cell membrane, cell wall, cytoplasm, nucleus and cell organelles. The cell membrane is the outer covering that protects the cell. Cell wall, found only in plant cells, protects the other components of the cell. Inside the cell membrane, we find cytoplasm, which is a thick, clear, jelly-like substance. The hereditary material of the cell known as the DNA is present in the nucleus. The growth, maturity and death of the cell depend upon the DNA. Cell organelles perform specific functions such as determining the sex of an individual and protecting against foreign bodies. There are different types of cell organelles such as Golgi bodies, nucleolus, ribosomes, and mitochondria.

Types of Cells—Prokaryotes and Eukaryotes

With respect to the cellular structure, there are two types of cells: prokaryotic cells and eukaryotic cells. The characteristics of these two cell types are discussed below:-

Prokaryotic Cells

Living organisms with prokaryotic cells are mycoplasma, blue-green algae and bacteria. The diameter of these cells varies from 0.1 µm to 5 µm. Compared with eukaryotic cells, these cells are smaller and multiply quickly. They have wide differences regarding shape, size and functions. For instance, bacteria has four basic shapes—rod-like, spherical, comma-shaped and spiral. In all the organisms (except mycoplasma) with prokaryotic cells, a cell wall surrounds the cell membrane. These cells do not have a well-defined nucleus and are filled with a fluid matrix called cytoplasm. The genetic material is not enveloped by a nuclear membrane. In bacteria, along with genomic DNA, there is also small circular DNA that lies outside it, known as plasmoids. Due to plasmoids, the bacterium exhibits resistance to antibiotics. Motile bacterial cells have one or more flagella. The shape of a flagellum is in the form of a whip that contains a filament, basal body and hook. The cells of bacteria have a three-layered envelope that is chemically complex. The outermost layer is glycocalyx. The next layer is the cell wall, and the innermost layer is the plasma membrane. Together they form a single protective unit, but the function of each of these three layers is different. Glycocalyx may be either thick or thin in different bacteria. If it is a loose sheet, it is known as a slime layer. A thick and tough glycocalyx is known as a capsule. The cell wall provides solid support structurally, thereby preventing the collapse of the bacteria. The plasma membrane is the point of contact with the outside world.

Eukaryotic Cells

The living organisms containing eukaryotic cells are plants, animals, fungi, and protists. The diameter of these cells varies from 10 µm to 100 µm, which is about twenty times larger than prokaryotic cells. In contrast to prokaryotic cells, these cells have more than one chromosome. Another difference is that these cells have an organised nucleus surrounded by a nuclear membrane, unlike prokaryotic cells. Eukaryotic cells in plants differ from those found in animals. Plant cells have a cell wall, plastids, and large central vacuoles. Animal cells have centrioles that are not found in plant cells. When eukaryotic cells are studied under an electron microscope, we find a network of tiny tubular structures that are spread across the cytoplasm, known as the endoplasmic reticulum (ER). ER divides the intracellular space into two compartments known as luminal and extraluminal compartments. The ER with ribosomes attached to its outer surface is known as the rough endoplasmic reticulum (RER). When ribosomes are not present, it appears smooth and is called the smooth endoplasmic reticulum (SER).

Conclusion

The basic building block of life is the cell. In living organisms, these cells combine to form tissues. They are of varying sizes and shapes that perform different functions. A cell in which there is a membrane-bound nucleus is known as a eukaryotic cell, while other types of cells are prokaryotic cells. The cell theory in its modern version has presented many finer details about the characteristics of the cells. This has been possible due to the invention of the electron microscope.