What is a Cell?

The cell is defined as the basic structural and functional unit of life. The cell membrane bounds it. It is capable of independent existence. 

Evolution of Cell

About 3.5 billion years ago, the earliest cell arose in a mixture of an organic compound. This mixture of organic compounds is known as primordial soup. They were mainly chemoheterotrophs. The primitive heterotrophs derive energy and synthesize the precursor molecules from the environment, which leads them to become less dependent on outside sources of energy. A significant evolutionary event has occurred, which is the development of photosynthetic ability in organisms. Photosynthetic ability in organisms means the fixation of carbon dioxide into complex organic compounds. In this evolutionary process, one important landmark is the transition of small cells having a simple internal structure known as a prokaryotic cell into a complex eukaryotic cell. About 1.5 billion years ago, fossil records showed the evolution of the eukaryotic cell. The deduction of prokaryote to eukaryote is made based on fossil records, but organisms' biochemical and morphological comparison is also made. This study has given a reasonable sequence of events consistent with fossil evidence. Archaea and eukaryotes share common ancestry according to the universal phylogenetic tree. Bacteria and archaea share similar cellular construction; they are phylogenetically linked. They descend from the same ancestor, but later they diverge to become separate domains.

For the evolution of prokaryotes into eukaryotes, three major events must have occurred. First, more DNA is acquired by the cell to fold it more compactly in discrete complexes and to divide it equally in daughter cells. At the time of cell division, this compactly arranged discrete complex is known as a chromosome. Second, the increase in the size of the cell and the development of intracellular membrane-bound organelle. In eukaryotes, most of the cell's DNA is present in the nucleus. A double layer of the membrane encloses the nucleus. Third, the eukaryotic cell, which was incapable of doing photosynthesis, evolved into photosynthetic bacteria.

Based on internal architecture, the cell can be sub-divided into prokaryotic and eukaryotic cells. In a eukaryotic cell, a membrane-bound nucleus is present, whereas prokaryotic organisms lack a membrane-bound nucleus. In a eukaryotic cell, there is a more complex intracellular organization in comparison to a prokaryotic cell. Apart from the nucleus, in a eukaryotic cell, another membrane-bound organelle is present such as the endoplasmic reticulum, Golgi complex, mitochondria, vacuoles. The cytoplasm is the area of the cell lying between the plasma membrane and the organelle. Such membrane-bound organelle is not present in prokaryotic organisms. 

Eukaryotic Cell

The term eukaryote comes from the Greek word ''eu'' and ''karyon'', which means true nuclei. The complexity of eukaryotes is more than prokaryotes. Except for kingdom Monera, all other kingdom is a eukaryote. In eukaryotes, a cell wall is a present whose role is in protecting cell membrane or plasma membrane—the role of the plasma membrane in the regulation of the exit and entry of molecules into cells. Genetic information is stored in DNA which is present in the nucleus. The nucleolus is present in the nucleus, which has a major role in the synthesis of protein. Mitochondria is present in the eukaryotic cell, which is responsible for producing energy inside the cell. Example of eukaryotes is Protists, fungi, etc. Protists represent the diverse eukaryotic group. In eukaryotes, the cell wall is made up of chitin and cellulose.

Prokaryotic Cell

The term prokaryotes come from ''pro'' and ''karyon'', which means before nuclei. On earth, prokaryotes are one of the earliest groups of organisms. They arose 3.5 billion years ago. In the earth's ancient environment, prokaryotes survived, and they have also used chemical energy and energy from the sun. The size of a prokaryotic cell is less as compared to eukaryotic cells. The entire body of prokaryotic organisms is surrounded by an envelope that serves as a protective coat. Some scientist says that eukaryotes have arisen from prokaryotes. The envelope which surrounds the prokaryotes prevents phagocytosis. On the external surface of prokaryotes, an external appendage pilus is present, which helps attach to other surfaces. Mesosomes are present in the prokaryotic cell, which helps in cellular respiration. The plasmid is also present in prokaryotes which stores the genetic information. In the nucleoid prokaryotic chromosome is present. Nucleoid is circular. Flagella and pilus are present in prokaryotes which helps in locomotion. The cell wall of prokaryotes is made up of peptidoglycans. 

The Distinction between Prokaryotes and Eukaryotes

• It is mainly done based on their cell structure and molecular make-up. 
• In a prokaryotic cell, DNA complexed with histone is absent, whereas it is present in the eukaryotic cell.
• The internal structure of the prokaryotic cell is simple as compared to the eukaryotic cell. 
• The membrane-bound organelle is absent in prokaryotic cells, whereas it is present in the eukaryotic cell.
• In a prokaryotic cell, the number of chromosomes is generally one, whereas, in eukaryotes, it is more than one.
• In a prokaryotic cell, mitosis and meiosis are absent whereas it is present in eukaryotic cells.
• In a prokaryotic cell, sterol in the plasma membrane is absent, whereas it is a present eukaryotic cell.
• In a prokaryotic cell, the ribosome is 70S, whereas, in eukaryotes, it is 80S.
• In prokaryotic cell wall is made up of peptidoglycan, whereas the eukaryotic cell wall is made up of chitin and cellulose.

The classification of prokaryotes was done based on their phenotypic characteristics. The taxonomy of prokaryotes involves multiple morphological and biochemical characteristics. 

Bacterial Cell

The bacteria are microscopic, simple prokaryotic organisms lacking a nucleus. It is a single-celled organism. Most of the bacteria show a typical prokaryotic structure. 

Origin of Eukaryotic Semi-Autonomous Organelle

'Origin of Eukaryotic cell' is published by Lynn Margulis. According to her, a semi-autonomous eukaryotic organelle is mitochondria and chloroplasts. Various endosymbiotic theories are given as a result of details biochemical and molecular data. 

Virus

Viruses are non-cellular organisms. Either DNA or RNA is present in the virus. The genetic material of viruses is enclosed in a protein coat. Viruses are obligate intracellular parasites; they multiply only in living cells. The size of viruses is smaller than a prokaryotic cell. Virion is defined as a fully assembled infectious virus. Virion delivers its DNA or RNA to the host cells. The genome is expressed in the host cell and encodes a protein product. The host range of viral species is limited. The structure of the virus is diverse; it varies in size, shape, and chemical composition.

Context and Application

This topic is significant in entrance exams like Bachelor and Master of Science

• Bachelors of Science in botany and zoology
• Masters of Science in botany and zoology