Q.Which pair of scientists is credited with independently proposing, in 1838-39, the founding postulates of the Cell Theory -- that all plants and all animals are composed of cells?

A cell is called the structural and functional unit of life because it is both the smallest self-contained structural building block of a body AND the smallest unit capable of carrying out life's processes (metabolism, growth, response, reproduction) on its own.

Structurally, every tissue, organ, and organism is built up from cells — there is no level of organisation smaller than a cell that is itself alive. Functionally, every life process you can name — respiration, protein synthesis, excitability, division — happens inside cells, using cell organelles. A unicellular organism like Amoeba performs all of life's functions within one single cell, which is direct proof that a cell alone is sufficient to sustain life; in multicellular organisms, these same functions are simply distributed among specialised cells working together, but the functions still originate at the cellular level.

The other options are incorrect because: not all cells have nuclei (mature RBCs and prokaryotic cells lack a true nucleus, yet they are still functional cells); visibility to the naked eye is not a defining property (most cells are microscopic); and cells certainly do not divide "only once" — cell division is a repeated, regulated process (the cell cycle).

According to cell theory, every new cell is formed by the division of an already-existing cell.

The cell theory was proposed by Schleiden and Schwann (1838-39), who stated that all plants and animals are composed of cells and that the cell is the basic unit of life. This was later expanded by Rudolf Virchow (1855), who added the crucial idea, summarised in the Latin phrase 'Omnis cellula-e cellula' ('every cell arises from a pre-existing cell'). This means new cells are never formed by spontaneous/abiotic synthesis or fermentation, and are not simply 'regenerated' from non-cellular material — they always arise through the division of cells that already exist.

The cell theory in its original form was proposed jointly by Schleiden and Schwann.

History of cell theory: Robert Hooke first observed and named 'cells' in cork tissue in 1665 (but these were dead cell walls, not living cells) using a self-designed microscope. Antony van Leeuwenhoek was the first to observe and describe a living cell (in 1674). Building on decades of microscopic observation, Matthias Schleiden (a botanist, 1838) and Theodor Schwann (a zoologist, 1839) together formulated the cell theory, stating that all plants and animals are composed of cells and that the cell is the basic unit of life. Rudolf Virchow later (1855) extended this with the crucial addition that all cells arise from pre-existing cells ('Omnis cellula-e cellula'), which corrected the earlier idea that cells arose de novo from non-cellular material.

Viruses lack cellular organisation and cannot multiply independently, which is why they are considered an exception to the Cell Theory.

The Cell Theory (Schleiden, Schwann, and later extended by Virchow) states that:

1. All living organisms are composed of cells.
2. The cell is the basic structural and functional unit of life.
3. All cells arise from pre-existing cells.

Fungi and Bacteria are both truly cellular — bacteria are prokaryotic cells and fungi are eukaryotic cells, so they follow the Cell Theory. Mycoplasma, though the smallest known cellular organism, still possesses a cell membrane and cytoplasm and is cellular. Viruses, in contrast, are non-cellular — they consist only of a nucleic acid core (DNA or RNA) surrounded by a protein coat (capsid), have no cytoplasm, no cell membrane of their own, and cannot carry out metabolism or reproduce unless they infect and take over the machinery of a living host cell. Because of this acellular, obligate-parasitic nature, viruses are treated as an exception to the Cell Theory.

New cells generate only from pre-existing cells, by cell division — this is a core part of the Cell Theory and rules out spontaneous generation.

One of the fundamental postulates of the Cell Theory (extended by Rudolf Virchow in 1855) is "Omnis cellula-e cellula" — every cell arises from a pre-existing cell, through the process of cell division (mitosis or meiosis). This directly disproved the earlier idea of spontaneous generation of cells from non-living matter.

Checking the options: (A) bacterial fermentation is a metabolic process, not a mode of cell origin; (B) "regeneration of old cells" is not how new cells arise — regeneration refers to replacement/repair of lost body parts using new cells produced by division, not old cells being converted; (D) abiotic (non-living) material cannot give rise to a cell — this idea (spontaneous generation) was scientifically disproved. Only pre-existing cells, by division, give rise to new cells.