BPSC 70th CCE Prelims General Studies · Question 90 of 143

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.

• (a)Organ — 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)Organism — 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)Tissue — 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.

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.

The question is decided by reading the stem as a definition rather than as a general question about biology, and the useful move is to test each option against the word 'unit'. A unit is the thing others are made OF, not a thing made of others. Tissue is made of cells; organ is made of tissues; organism is made of organ systems. Only one option is not made of anything smaller that is alive, and that is the answer. A second test settles the same thing from the other direction: which of these four exists in every living thing without exception? Bacteria and amoebae have no tissues and no organs, so those two fail immediately as universal answers; the organism is not a unit at all; the cell is present in all of them. Keep the two adjacent terms distinct in your own words — a cell is the unit, a tissue is a group of similar cells doing a common job — because BPSC and UPSC both build items on the cell-tissue boundary. And keep 'cell' in biology clearly separate from its other exam senses: a photoelectric cell, a dry cell and a solar cell are devices, not living units, and BPSC has used the word in that sense too.

• Cell theory: all organisms are made of one or more cells; the cell is the basic unit of structure and function; and all cells arise from pre-existing cells
• It was built by Matthias Schleiden (plants, 1838), Theodor Schwann (animals, 1839) and Rudolf Virchow (1855), whose omnis cellula e cellula ruled out spontaneous generation
• Robert Hooke first saw and named cells in 1665, in a slice of cork, publishing the observation in Micrographia; Anton van Leeuwenhoek was the first to see living cells
• Levels of organisation: cell → tissue → organ → organ system → organism, with each level built from the one below it
• Size range: Mycoplasma is the smallest cell at about 0.1 micrometre, the ostrich egg is the largest single cell, and nerve cells are the longest
• Prokaryotic cells (bacteria) lack a membrane-bound nucleus and organelles; eukaryotic cells have both, with chloroplasts, mitochondria and ribosomes doing distinct jobs

• Choosing tissue because it sits next to the cell in the hierarchy; a tissue is a group of similar cells and does not exist in single-celled organisms at all
• Treating the organism as the 'functional unit' because it is what actually lives; the question asks for the unit things are built from
• Confusing the biological cell with a photoelectric, dry or solar cell — the same word, an entirely different subject

BPSC asks foundational biology in a single definitional line and relies on the candidate reading the stem precisely, since three of the four options are genuinely levels of biological organisation. UPSC takes the same chapter and asks it as organelle-to-function matching or as a single discriminating fact — which organelle makes protein, which one carries its own DNA — so the UPSC version needs the inside of the cell while the BPSC version needs the definition of it.

• practice — not a real PYQ

  Who first observed and named cells, in a thin slice of cork ?

  • (a)Anton van Leeuwenhoek
  • (b)Robert Hooke
  • (c)Matthias Schleiden
  • (d)Rudolf Virchow

  Answer(b) Robert Hooke — in 1665, publishing the drawing in Micrographia. Leeuwenhoek first saw living cells, Schleiden proposed that plants are made of cells, and Virchow established that every cell comes from a pre-existing cell.

• practice — not a real PYQ

  Which of the following is the correct sequence of levels of biological organisation, from smallest to largest ?

  • (a)Cell → organ → tissue → organism
  • (b)Tissue → cell → organ → organism
  • (c)Cell → tissue → organ → organism
  • (d)Organ → tissue → cell → organism

  Answer(c) Cell → tissue → organ → organism — a tissue is a group of similar cells, an organ is several tissues working together, and the organism is the complete individual.