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Cell: The Unit of Life

Biology · Class 11NCERT pp. 85–1022 sets × 25 questions

Key facts and formulas for Cell: The Unit of Life, three worked sample questions, and two full practice sets. Every question has a hint, a reason for every option and a step-by-step solution, with a calculator or animation where one helps. English and Hinglish.

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Key facts for this chapter

IdeaRule or fact
Cell theory peopleHooke named cells (1665); Leeuwenhoek saw live cells; Brown found the nucleus; Schleiden (plants) and Schwann (animals); Virchow: cells come from cells
Prokaryote vs eukaryoteProkaryotes: nucleoid, circular naked DNA, 70S ribosomes, no membrane-bound organelles; Eukaryotes: true nucleus, 80S ribosomes
Ribosome pairs70S = 50S + 30S; 80S = 60S + 40S; S values are not additive
Bacterial envelopeGlycocalyx, cell wall, plasma membrane; mesosomes help in wall formation, DNA replication, respiration, secretion
Cell wall materialsPlants: cellulose (middle lamella: calcium pectate); Bacteria: peptidoglycan; Fungi: chitin
MembraneFluid mosaic model, Singer and Nicolson, 1972; RBC membrane about 52% protein and 40% lipid; sandwich model: Davson and Danielli
TransportPassive: diffusion, osmosis, facilitated diffusion; Active: Na⁺–K⁺ pump, 3 Na⁺ out and 2 K⁺ in per ATP
Endomembrane systemER, Golgi, lysosomes, vacuoles; mitochondria, chloroplasts, peroxisomes are not included
Organelle jobsSER: lipids and steroids; RER: secretory proteins; Golgi: glycoproteins and glycolipids; lysosomes: hydrolases; peroxisomes: catalase
Semi-autonomous organellesMitochondria and chloroplasts: double membrane, circular DNA, 70S ribosomes; F₁ particles on cristae
Plastids and centriolesAmyloplast starch, elaioplast oil, aleuroplast protein, chromoplast carotenoids; centriole 9 triplets (27), axoneme 9+2 (20), basal body 9+0

Three sample questions

Q1. Match: (a) Leeuwenhoek (b) Robert Brown (c) Schleiden (d) Virchow with (i) every cell comes from a pre-existing cell (ii) first to see a live cell (iii) discovered the nucleus (iv) plants are made of cells
  1. a – iii, b – ii, c – i, d – iv
  2. a – ii, b – iv, c – iii, d – i
  3. a – i, b – iii, c – iv, d – ii
  4. a – ii, b – iii, c – iv, d – i
Hint and solution

Hint: Link each name to one discovery: first live cell, nucleus, plants as cells, and cells from cells.

  1. Leeuwenhoek → first live cell — Leeuwenhoek used his own microscope and was the first to see a live cell. So a matches (ii).
  2. Brown → nucleus; Schleiden → plants — Robert Brown discovered the nucleus, so b is (iii). Schleiden studied plants and said they are made of cells, so c is (iv).
  3. Virchow → omnis cellula e cellula — Virchow said every cell arises from a pre-existing cell. So d is (i).
  4. a–ii, b–iii, c–iv, d–i — Combining the four matches gives a-ii, b-iii, c-iv, d-i, which is the fourth choice. Option D.
Answer: D. Leeuwenhoek: live cell. Brown: nucleus. Schleiden: plants. Virchow: cells from cells.
Q2. Assertion (A): The cell theory of Schleiden and Schwann was incomplete. Reason (R): It could not explain how new cells arise.
  1. Both A and R are true and R is the correct explanation of A
  2. Both A and R are true but R is not the correct explanation of A
  3. A is true but R is false
  4. A is false but R is true
Hint and solution

Hint: Ask whether the early theory said anything about how new cells form, and who added that.

  1. Check A: theory was incomplete — Schleiden and Schwann said all organisms are made of cells, but not how cells are formed. So A is true.
  2. Check R: no origin of new cells — Their theory could not explain new cell formation. R is also true.
  3. Does R explain A? — Virchow later said cells arise from pre-existing cells. That missing part is the reason the theory was called incomplete.
  4. Both true, R explains A — So both statements are true and R is the correct explanation. Option A.
Answer: A. Virchow later filled that gap: new cells come from existing cells.
Q3. Which statement about the nucleolus is correct?
  1. It is bounded by a double membrane
  2. It is the site of lipid synthesis
  3. It carries the cristae
  4. It has no membrane and is the site of active rRNA synthesis
Hint and solution

Hint: Recall that the nucleolus is a dense region inside the nucleus, and think about what it makes.

  1. Nucleolus = non-membranous body — It is a spherical structure in the nucleoplasm and is not enclosed by a membrane.
  2. Function: rRNA synthesis — It is the site of rRNA synthesis and ribosome subunit assembly.
  3. Eliminate others — Double membrane, lipid synthesis and cristae belong to other parts of the cell, not the nucleolus.
  4. Answer: no membrane, rRNA site — So the correct statement is the one that says no membrane and rRNA synthesis. Option D.
Answer: D. The nucleolus merges with the nucleoplasm; protein-making cells have larger and more nucleoli.

How to use this chapter

Read the NCERT pages first, then attempt Set 1 without the hints. Note every wrong answer in a mistake list, read the reason for each option, and retry Set 2 two days later. A topic is solid when you can say why each wrong option is wrong.

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