Cell — The Fundamental Unit of Life — Practice

Question 1

Who are the three scientists associated with Cell Theory? State what each contributed.

Accepted Answer

Step 1 — Schleiden (1838): All PLANTS are composed of cells.
Step 2 — Schwann (1839): All ANIMALS are composed of cells.
Step 3 — Virchow (1855): All cells arise from PRE-EXISTING cells ('Omnis cellula e cellula').

✦ Answer: Schleiden (plants), Schwann (animals), Virchow (cells from cells).

Question 2

List four differences between a plant cell and an animal cell.

Accepted Answer

Step 1 — Compare key features.

Feature | Plant Cell | Animal Cell
  --------|-----------|------------
  Cell wall | Present (cellulose) | Absent
  Chloroplasts | Present | Absent
  Vacuole | LARGE central vacuole | Small/absent
  Centrioles | Absent | Present

✦ Answer: Plant has cell wall, chloroplasts, large central vacuole. Animal has centrioles. Animal lacks cell wall and chloroplasts.

Question 3

What happens to a plant cell when placed in (a) a hypertonic solution (b) a hypotonic solution?

Accepted Answer

Step 1 — Hypertonic solution (more solute outside):
  Water moves OUT of cell by osmosis. Cell membrane shrinks and pulls away from cell wall — this is PLASMOLYSIS. The cell becomes FLACCID and wilts.

Step 2 — Hypotonic solution (less solute outside):
  Water moves INTO cell by osmosis. Cell swells. The cell wall prevents bursting — the cell becomes TURGID (firm). This is the normal healthy state of plant cells.

✦ Answer: Hypertonic → plasmolysis (wilts). Hypotonic → turgid (firm).

Question 4

Describe the structure and function of any FOUR cell organelles. Include one that is present only in plant cells.

Accepted Answer

Step 1 — NUCLEUS (both cells): Double membrane with pores. Contains chromatin (DNA + proteins) and nucleolus. Controls all cell activities — the 'control centre.'

Step 2 — MITOCHONDRIA (both cells): Double membrane; inner folds = cristae. Site of aerobic respiration: glucose + O₂ → ATP. Own DNA + 70S ribosomes (semi-autonomous). 'Powerhouse of the cell.'

Step 3 — LYSOSOMES (animal cells, mainly): Single membrane. Contain acid hydrolases (pH ~5). Digest worn-out organelles and foreign matter. 'Suicide bags.'

Step 4 — CHLOROPLASTS (plant cells only): Double membrane. Thylakoids (grana) contain chlorophyll. Stroma = Calvin cycle site. Converts CO₂ + H₂O → glucose using sunlight. Own DNA + 70S ribosomes.

✦ Answer: Nucleus (control), Mitochondria (ATP), Lysosomes (digest waste), Chloroplasts (photosynthesis — plant only).

Question 5

Explain why mitochondria and chloroplasts are called 'semi-autonomous' organelles. Give four pieces of evidence for the endosymbiotic theory.

Accepted Answer

Step 1 — Semi-autonomous defined:
  Mitochondria and chloroplasts can PARTIALLY function and reproduce independently — but they still depend on the nucleus for many proteins. They are 'semi' because they cannot survive outside the cell.

Step 2 — Four evidence pieces for endosymbiotic theory:
  (1) OWN CIRCULAR DNA — like bacteria (not linear like eukaryotic nuclear DNA).
  (2) 70S RIBOSOMES — like prokaryotes (not 80S like cytoplasmic ribosomes).
  (3) BINARY FISSION — both divide by binary fission, independently of the cell cycle.
  (4) DOUBLE MEMBRANE — inner = original prokaryote membrane; outer = from engulfment.

✦ Answer: Semi-autonomous = partial independence. Evidence: circular DNA, 70S ribosomes, binary fission, double membrane.

Question 6

Compare prokaryotic and eukaryotic cells in detail. Which type is found in bacteria? In which kingdom(s) do eukaryotic cells appear? Explain why mitochondria support the endosymbiotic theory.

Accepted Answer

Step 1 — PROKARYOTIC CELLS:
  No nuclear membrane (nucleoid instead). No membrane-bound organelles. 70S ribosomes. Cell size 1-10 μm. Reproduce by binary fission. Cell wall (peptidoglycan in bacteria). Examples: bacteria, cyanobacteria, Mycoplasma.

Step 2 — EUKARYOTIC CELLS:
  True nucleus with double membrane and pores. Membrane-bound organelles (mitochondria, ER, Golgi, lysosomes, chloroplasts in plants). 80S ribosomes in cytoplasm. Cell size 10-100 μm. Reproduce by mitosis or meiosis. Examples: plants, animals, fungi, protists.

Step 3 — Comparison table:
  Feature | Prokaryotic | Eukaryotic
  --------|------------|----------
  Nuclear membrane | Absent | Present
  Membrane organelles | Absent | Present
  Ribosomes | 70S | 80S
  Size | 1-10 μm | 10-100 μm
  Division | Binary fission | Mitosis/Meiosis

Step 4 — Bacteria: PROKARYOTIC cells only.
  Eukaryotic kingdoms: Protista, Fungi, Plantae, Animalia (all four).

Step 5 — Mitochondria and endosymbiosis:
  Mitochondria have circular DNA (like bacteria), 70S ribosomes (like prokaryotes), and reproduce by binary fission. The theory: a large anaerobic cell engulfed an aerobic prokaryote ~1.5 billion years ago. Instead of being digested, it became the mitochondrion — powering the host cell with ATP.

✦ Answer: Prokaryotic = bacteria (no true nucleus). Eukaryotic = 4 kingdoms. Mitochondrial circular DNA + 70S ribosomes = prokaryotic legacy.

Practice — Cell — The Fundamental Unit of Life

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