What is the structural and functional unit of life ?

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Correct — B, Cell. The phrase in the stem is the standard formulation of the second tenet of cell theory, and the two words in it are doing different work. STRUCTURAL means that every living body is built of cells and of nothing smaller that is itself alive — a bacterium is one cell, a human being is some tens of trillions, and there is nothing in between. FUNCTIONAL means that the cell is the smallest thing that carries out the whole set of activities we call life: it takes in nutrients, releases energy from them, synthesises its own proteins, responds to stimuli, excretes wastes, grows and divides. A fragment of a cell can do none of that, and a virus, which is not made of cells, cannot do it without hijacking one. Cell theory itself was assembled in the nineteenth century: Matthias Schleiden proposed in 1838 that plants are made of cells, Theodor Schwann extended the claim to animals in 1839, and Rudolf Virchow completed it in 1855 with omnis cellula e cellula — every cell arises from a pre-existing cell — which ruled out spontaneous generation. The observation that started it all was earlier: Robert Hooke, examining a thin slice of cork under his microscope in 1665, saw rows of empty compartments and called them cells, after the small rooms of a monastery, and published the drawing in Micrographia. The other three options are all levels of biological organisation, but every one of them is BUILT FROM cells rather than being the unit itself.

Why the other options are wrong — (a) An organ — the heart, a leaf, a kidney — is an assembly of several different tissues working together on one job, and so stands two levels above the cell. It is a functional unit of the body in ordinary speech, which is what makes the option plausible, but it is not the smallest such unit and it is not the structural building block.  (c) The organism is the complete individual and therefore the largest level in the hierarchy, not the unit from which the hierarchy is built. The confusion is worth naming: in a unicellular organism such as an amoeba the cell and the organism happen to coincide, but that is a special case, not a definition.  (d) The most dangerous distractor, because a tissue sits immediately above the cell and shares its language of structure and function — it is a group of similar cells performing a common function. But it is an aggregate of cells, so it cannot be the unit of which living things are made. Note also that single-celled organisms have no tissues at all, which alone disqualifies tissue as a universal unit of life.

Concept: Living matter is organised in a strict hierarchy: cell, then tissue, then organ, then organ system, then organism. The cell is the base of that ladder, and cell theory states three things about it — that all living organisms are composed of one or more cells, that the cell is the basic unit of structure and function in living things, and that all cells arise from pre-existing cells. The range of what counts as a cell is vast. Mycoplasma, at about a tenth of a micrometre, is the smallest known cell; the ostrich egg is the largest single cell; nerve cells are the longest, running up to a metre in a tall human. Cells divide into two great classes: PROKARYOTIC cells, as in bacteria, which have no nucleus bounded by a membrane and no membrane-bound organelles; and EUKARYOTIC cells, as in protists, fungi, plants and animals, which have a true nucleus and a set of organelles that partition the work — chloroplasts for photosynthesis, mitochondria for respiration, ribosomes for protein synthesis, the plasma membrane for controlling what enters and leaves. Plant cells differ from animal cells in having a cellulose cell wall, a large central vacuole and plastids.

State PCS 2024 Q90 — Science & Technology PYQ with Answer

State PCS 2024 Q90 — Science & Technology PYQ with Answer