The Cell (Latin cella, meaning “small room”) is the smallest basic structural, functional, and biological unit of all living organisms. They are often called the ‘building blocks of life’. Cell biology is the study of cells. In 1665, Robert Hooke discovered them and named them after cells inhabited by Christian Monks in monasteries. In 1839, Matthias Jakob Schleiden and Theodor Schwann developed Cell Theory; it states that every organism is composed of one or many cells, and that each cell comes from a pre-existing cell. Cells emerged on Earth at least 3.5 billion years ago. The number of cells in any organism varies from species to species.
Cells contain organelles, a tiny structure of a cell that performs a specific function. A cell consists of cytoplasm, encompassed in a cell membrane, containing many bio-molecules such as proteins and nucleic acid. Other vital organelles are nucleus, ribosomes, vacuoles, cell wall, endoplasmic reticulum, flagella, and many more. Each organelle has a specific function and is vital to the existence of the cell, and by extension, of the organism.
The two major cells types are Prokaryote and Eukaryote.
Prokaryotes are unicellular organisms that do not have a membrane-bound nucleus, i.e., the DNA floats loose in the cytoplasm. Other organelles like mitochondria, ribosomes, and others, are also not membrane-bound. Prokaryotes are categorized as Archaea and Bacteria. Prokaryotes reproduce without fusion of gametes. Proteins, DNA, metabolites, and other intra-cellular water-soluble components are located together in the cytoplasm enclosed by the cell membrane, as opposed to separate cellular compartment. However, bacteria have protein-based bacterial micro-compartment that act as primitive organelles enclosed within protein shells. Certain prokaryotes, like Cyanobacteria, may form large colonies. Others, like Myxobacteria, have multi-cellular stages in their life cycles.
Prokaryotes have a more primitive cytoskeleton. Apart from homologous of actin and tubulin, flagellin, the helically arranged structure of the flagellum, is one of the significant cytoskeleton protein in the bacteria because it provides the structural blueprint of chemotaxis, the basic cell physiological response of bacteria. Some prokaryotes contain intra-cellular structures that can be thought of as primitive organelles. Some species of prokaryotes contain carbohydrate-enclosed micro-compartments, each having distinct physiological roles. Prokaryotes are often between 1 µm and 10 µm, but have been seen to vary in size from 0.2 µm to 750 µm.
Eukaryotes are organisms with cells that have a membrane-bound nucleus. They contain other membrane-bound organelles like mitochondria, Golgi apparatus, and chloroplasts, in some cells of plants and algae. Eukaryotes can be multi-cellular and include organisms that have many cell types, which together form different kinds of tissue; animals and plants are the most familiar eukaryotes. Eukaryotes reproduce asexually through mitosis and sexually through meiosis and gamete fusion. Through mitosis, one cell divides to produce two cells that are genetically identical. Through meiosis, cell division follows DNA replication and produces four haploid daughter cells that act as sex cells or gametes, each with one set of chromosomes making them a unique mix of the corresponding pair of parent chromosomes that result from genetic recombination during meiosis.
Eukaryotes form a minority of all living organisms. Nonetheless, because of their mostly large size, their collective worldwide biomass equals that of the prokaryotes. Eukaryotic cells form three types, namely, animal cells, plant cells, and fungal cells. Each of them have a lot of different internal membrane-bound organelles and a cytoskeleton made up of micro-tubules, micro-filaments, intermediate filaments, that helps define the cells shape and organization. Eukaryote DNA divides into many vertical bundles called chromosomes that get separated during nuclear division using micro-tubular spindles. Cyanelles in glaucophytes, haptonema in haptophytes, ejectosomes in cryptomonads, and others, are unique organelles found in some groups of eukaryotes; similarly, pseudopodia and other such structures are found in various eukaryote groups in different forms.