In the mid-1600s Robert Hooke, an English Microscopist examined a sharply cut wooden cork under a  microscope. He was curious about why the wooden stoppers were so good at sealing air in bottles. It was his observation that coined the term “Cell” in biology for he observed tiny chambers much like honeycomb in the wooden cork. Later in 1674, Anton Van Leeuwenhoek, a Dutchman whose hobby was to grind glasses and construct lenses and microscopes, observed the water from a lake with his microscope. Leeuwenhoek observed movement of microscopic particles which were referred as “animalcules”. He also observed various forms of bacterial cells and protozoans(Vorticella).

The concept and importance of cell grew wider in the 1800s. A German botanist in 1838, Matthias Schleiden concluded that, although the structures of tissues in plants differ, all of them were made of cells and had arisen from a single celled plant embryo. Theodor Schwann, German Zoologist later in 1839, in his comprehensive report on the cellular basis of animal life, proposed the two basic principles of cell theory. Until 1855, German pathologist Rudolf Virchow proposed a third one denying the pre-concept that cells could arise from non-cellular components.

The tenets of the modern cell theory are:

■ All organisms are composed of one or more cells.

■ The cell is the structural unit of life.

■ Cells can arise only by division from a preexisting cell.

Definitions of The Cell

A cell is a smallest autonomous living unit.

Merriam-Webster dictionary defines cell as - “a small usually microscopic mass of protoplasm bounded externally by a semipermeable membrane, usually including one or more nuclei and various other organelles with their products, capable alone or interacting with other cells of performing all the fundamental functions of life, and forming the smallest structural unit of living matter capable of functioning independently”.

The basic definition of cell goes as-  “The cell is the basic structural, functional, and biological unit of all known living organisms and capable of independent replication. They are the building blocks of life".

The broad categorization of cells into prokaryotic and eukaryotic is primarily based on the nucleus. Most of the Prokaryotic cells are single-celled organisms and do not contain a membrane bound nucleus. They have a nucleoid region instead. While in the eukaryotes, a double membrane bound true nucleus is present. Eukaryotes may be single celled or multicelled organisms with higher specialized cellular organization. Kingdom Eukarya includes plants, animals, fungi and certain types of algae.

Prokaryotic Cell

A Bacterial Cell

By: Ali Zafan, CreativeCommons

A prokaryotic cell is the characteristic cell type of Eubacteria(commonly called bacteria) and the Archaea(Ancient bacteria). These cells are much smaller in size than most of the eukaryotic cells. The average bacterial cell is ~1-5µm. The cell structure comprises an outer cell wall made of peptidoglycan(a complex of proteins and polysaccharides)on the adjacent outer side of plasma membrane(cell membrane), which provides rigid shape and protection to the cell. Some bacteria (for eg. E. Coli) also contain a outer membrane made of lipids and proteins. This outer membrane makes it non-stainable by the gram’s stain solution, hence categorized into Gram Negative bacteria. Prokaryotic cells typically lack a true nucleus but posses a circular DNA in the nucleoid region of the cytoplasm. Additional circular DNAs are found in the cytoplasm called the plasmids. Some of the bacterial cells posses an invagination of cell membrane into the cytoplasm, called mesosomes, which is associated with DNA synthesis and secretion of proteins. Most of the ribosomes (50S and 30S) in the cell are found the cytoplasm which can easily bind with mRNA, soon after it is synthesis by RNA polymerase; This eventually speeds up the transcription and translation processes in prokaryotes. Prokaryotic cells lack the the mitochondria and plastids, prominent structures in Eukaryotic cells. Cell movement occurs with the help of its flagella, made of flagellin protein. Prokaryotic cells usually contain a single chromosome and reproduce by binary fission.

Gram Positive Bacteria

Actinomyces, Bacillus, Clostridium, Corynebacterium, Enterococcus, Gardnerella, Lactobacillus, Listeria, Mycobacterium, Mycoplasma, Nocardia, Propionibacterium, Staphylococcus, Streptococcus, Streptomyces

Gram Negative Bacteria

Acetobacter, Bortadella, Borrelia, Campylobacter, Enterobacter, Escherichia, Helicobacter, Hemophilus, Klebsiella, Legionella, Neisseria, Pseudomonas, Proteus, Salmonella, Shigella, Treponema, Vibrio, Yersinia

Archaebacteria on the other hand differ from Eubacteria not only on the basis of its DNA sequence but also by the compounds in its cell wall and cell membrane. These are the bacteria that live in extreme environments i.e high PH (Thermoacidophiles), high salt concentrations(Halophiles),  Oxygen free environments(Methanobacteria) resembling to ancient conditions. Cell wall of these bacteria contain Polysaccharides (pseudopeptidoglycan), proteins and glycoproteins. Also, Archeal DNA and the translation and transcription processes are more similar to those of Eukaryotes.

Eukaryotic Cell

Eukaryotic cells are present in all the members of plant and animal kingdoms as well as protozoans including amoebae and fungi.  The size of the cell is much larger ~ 10–100 µm. Similar to the Prokaryotic cell, a plasma membrane encloses the cytoplasm. The cytoplasm contains an array of fibrous proteins called cytoskeleton; It provides strength and rigidity. An eukaryotic cell’s striking feature is its compartmentalization. Most of the internal cell organelles(except in the human RBC) in the eukaryotic cells, unlike in the prokaryotes are enclosed by a single phospholipid membrane; but the nucleus, mitochondrin and chloroplast are enclosed by two membranes.  The nucleus is the largest organelle in animal cell and contains the DNA Genome, RNA Synthetic Apparatus, and a Fibrous Matrix. The DNA comprises of multiple long linear molecules (chromosomes) which is present in the dense region of the nucleus termed as nucleolus. Each chromosome comprises a single DNA molecule associated with numerous histones and other proteins.  Some eukaryotic organelles such as mitochondria also contain some DNA.


Unlike Prokaryotic cells, Eukaryotic cells additionally contain large number of membraned sub-cellular constituents or organelles. They are endoplasmic recticulum and golgi complex, endosomes, lysosomes, perixosomes, vacuoles. The eukaryotic cells multiply both by mitosis(fission, budding) and meiosis cell divisions.  Movement of the eukaryotic cells occurs with motile cilia and flagella which are the extensions of the plasma membrane. They contain a bundle of microtubules that gives them shape and, together with motor proteins, allows them to beat rhythmically. Primary cilia play important roles in chemosensation, mechanosensation, and thermosensation.

Common features in all Cells:

Both the Prokaryotic and Eukaryotic cells, despite being anatomically diverse and different have some similarities.

■ DNA, both prokaryotic and eukaryotic cells have the genetic material contained in DNA, which maybe naked or within a membrane bound organelle.

■ Ribosomes, a minute particle consisting of RNA and associated proteins found in the cytoplasm of living cells. Ribosomes bind the mRNA and transfer RNA to synthesize polypeptides and proteins.

■ Plasma Membrane, is the outer membrane surrounding the cell, primarily composed of phospholipid bilayer and proteins and acting as a selective barrier for transport of materials to and from the cell

■Cytoplasm, is the gel like substance enclosed within the cell membrane. It contains the organelles, the nucleoid region and cytosol(organelle-free part of cytoplasm), water, dissolved ions, small molecules, and proteins.

 Additionally all the cells undergo the RNA and protein synthesis, the replication of its genome and divide. 

The common cycle of all Cells


An organism is a contiguous living system, for example an animal, plant, fungi, archaea, or bacteria. All these known types of organisms are capable of some degree of response to external stimulus, of reproduction, growth and development and homeostasis(a process by which the concentration of a substance or its temperature is regulated constantly).

An organism may consist of one or more cells; when the organism comprises only one cell, it is known as a unicellular organism; and when it has more than one, it is known as a multicellular organism. Most of the unicellular organisms found in nature are microscopic and are thus described as microorganisms.

Humans are multicellular and composed of many trillions of cells which are grouped into specialized tissues and organs.

Common features in all Organisms

■ In all the living organisms, energy is stored in the form of ATP(Adenosine Triphosphate). It is usually generated in the cytoplasm, during cellular respiration.

■ The genetic information of all the living beings is stored within the chemical structure of the DNA molecule. DNA molecule consists of two nucleotides coiled around each other to form a spiral helix and cross linked by the strings of chemicals known as bases along the nucleotide backbones.

■ The information is transcribed into the RNA.

■ A common triplet genetic code is universal, with some exceptions.

■ Translation begins when a ribosome docks on a start codon of an mRNA molecule in the cytoplasm i.e it involves the ribosomes.

■ All the organisms have shared metabolic pathways for energy generation and respiration(Glycolysis, Kreb’s Cycle, Oxidative Phosphorylation, Pentose Phosphate Pathway etc.)

■ There is similarity in the types of proteins found in various groups of organisms.


■ Alberts, Bruce; Johnson, Alexander; Lewis, Julian; Morgan, David; Raff, Martin; Roberts, Keith; Walter, Peter (2002). Molecular Biology of the Cell (4th ed.). Garland Science

■ Harvey Lodish, Arnold Berk, Chris A. Kaiser, Monty Krieger, Anthony Bretscher, Hidde Ploegh, Angelika Amon, Kelsey C. Martin(2016). Molecular Cell Biology (8th ed.). W. H. Freeman and Company

■ University of Helsinki, Basics on Molecular Biology, Lecture notes 1509 and 1709

■ Campbell Biology—Concepts and Connections. Pearson Education. 2009. p. 320.