A fundamental distinction in biology is the one between the prokaryotes (eubacteria and archaea) and the eukaryotes (animals, plants, fungi and protists). Equally fundamental is the distinction between the eubacteria and the archaea. In fact, analyses of ribosomal RNA sequences and other molecular data suggest a tripartite view of organisms, with eukaryotes, eubacteria and archaea as the three main groups.

A plasma membrane and ribosomes are the only two structural features common to all three groups. All other organelles are found only in eukaryotic cells, where they play indispensable roles in the compartmentalisation of function. Prokaryotic cells are relatively small and structurally less complex, lacking most of the internal membrane systems and organelles of eukaryotic cells, which have at least four major structural features: a plasma membrane that defines the boundaries of the cell and retains its contents; a nucleus that houses most of the cell&’s DNA; a variety of organelles; and the cytosol with its cytoskeleton of tubules and filaments. In addition, plant cells almost always have a rigid cell wall and animal cells are usually surrounded by an extracellular matrix of collagen and proteoglycans.

The nucleus is surrounded by a double membrane called a nuclear envelope and the chromosomes within the nucleus contain most of the DNA of the cell, complexed with proteins.

Mitochondria play an important role in the oxidation of food molecules to release energy, which is used to make ATP. Chloroplasts trap solar energy and use it to “fix” carbon from carbon dioxide into organic form and convert it to sugar. Mitochondria and chloroplasts are surrounded by a double membrane and have an extensive system of internal membranes in which most of the components involved in ATP generation are embedded.

The endoplasmic reticulum is an extensive network of membranes that are either rough (studded with ribosomes) or smooth. Rough ER is responsible for the synthesis of secretory and membrane proteins, whereas smooth ER is involved in lipid synthesis and drug detoxification. Proteins synthesised on the rough ER are further processed and packaged in the Golgi complex and are then transported to the surface of file cell by secretory vesicles.

Lysosomes contain hydrolytic enzymes and are involved in cellular digestion. These were the first organelles to be discovered on the basis of their function rather man their morphology. Peroxisomes are often about the same size as lysosomes but function in the generation and degradation of hydrogen peroxide. Animal peroxisomes play an important role in the catabolism of long-chain fatty acids. In plants, specialised peroxisomes are involved in the conversion of stored fat into carbohydrate during seed germination and in the process of photorespiration.

Ribosomes serve as sites of protein synthesis in both prokaryotic and eukaryotic cells and also in mitochondria and chloroplasts. The striking similarities between mitochondrial and chloroplast ribosomes and those of bacteria and cyanobacteria, respectively, lend strong support to the endosymbiont theory that these organelles are of prokaryotic origin.

The cytoskeleton is an extensive network of microtubules, microfilaments and intermediate filaments that gives eukaryotic cells their distinctive shapes. The cytoskeleton is also important in cellular motility and contractility.

Viruses satisfy some, though not all, of the basic criteria of living things. They are important both as infectious agents that cause diseases in humans, animals and plants and as laboratory tools, particularly for geneticists. Viroids and prions are infectious agents that are even smaller (and less well understood) than viruses. Viroids are small RNA molecules, whereas prions are thought to be abnormal products of normal cellular genes.

The writer is Associate Professor, Head, Department of Botany, Ananda Mohan College, Kolkata, and also fellow, Botanical Society of Bengal, and can be contacted at tapanmaitra59@yahoo.co.in