The Fundamental Unit of Life — Practice

Question 1

Who proposed the cell theory and what are its three main points?

Accepted Answer

Step 1 — Attribution.
  • Matthias Schleiden (1838) — all plants are made of cells.
  • Theodor Schwann (1839) — all animals are made of cells.
  • Rudolf Virchow (1855) — all cells come from pre-existing cells.

Step 2 — Three main points.
  1. All living organisms are made of cells.
  2. The cell is the basic structural and functional unit of life.
  3. All cells arise from pre-existing cells.

✦ Answer: Schleiden + Schwann (later refined by Virchow). Three points: cells make up life, cells are the functional unit, cells come from cells.

Question 2

Why are mitochondria called the powerhouse of the cell?

Accepted Answer

Step 1 — Function of mitochondria.
  Site of cellular respiration: oxidise glucose using oxygen → produce ATP (the cell's energy currency).

Step 2 — Analogy.
  Just as a power plant generates electricity for a city, mitochondria generate ATP for the cell. Hence 'powerhouse'.

✦ Answer: Mitochondria carry out cellular respiration and produce ATP — the cell's chemical energy currency. All cellular activities (muscle contraction, nerve impulses, biosynthesis) depend on ATP, so mitochondria essentially power everything.

Question 3

State any two differences between prokaryotic and eukaryotic cells.

Accepted Answer

Step 1 — Pick two clear differences.

1. Nucleus: Prokaryotes have no true nucleus (only a nucleoid). Eukaryotes have a membrane-bound nucleus.
  2. Membrane-bound organelles: Prokaryotes have NONE. Eukaryotes have many (mitochondria, ER, Golgi, lysosomes, etc.).

(Alternative pair: size — prokaryotes 1–10 μm vs eukaryotes 10–100 μm.)

✦ Answer: (i) Prokaryotes have no true nucleus (nucleoid only); eukaryotes have a true membrane-bound nucleus. (ii) Prokaryotes lack membrane-bound organelles; eukaryotes have many.

Question 4

Give three differences between a plant cell and an animal cell.

Accepted Answer

Step 1 — Pick the most distinctive differences.

| Feature | Plant cell | Animal cell |
  |---------|-----------|-------------|
  | Cell wall | Present (cellulose) | Absent |
  | Plastids | Present (incl. chloroplasts) | Absent |
  | Vacuole | One large central vacuole | Small, many |
  | Centrosome | Absent | Present |
  | Shape | Fixed rectangular | Variable |

✦ Answer: Three differences from the table above (any three). Most common picks: cell wall, plastids, large central vacuole — these are the plant cell's signature features.

Question 5

What will happen to (a) an animal cell, (b) a plant cell placed in a hypertonic solution?

Accepted Answer

Step 1 — Hypertonic = MORE solute outside than inside.
  Water flows OUT of the cell → cell loses water and shrinks.

Step 2 — Effect on each cell type.
  (a) Animal cell: shrinks, becomes 'spiky' looking → CRENATION. Cell is damaged.
  (b) Plant cell: the protoplast (cell content) shrinks away from the cell wall → PLASMOLYSIS. The cell wall remains intact, but the cell becomes flaccid. If reversed (cell moved to hypotonic), the plant cell can recover.

✦ Answer: Both cells lose water and shrink. Animal cell undergoes crenation; plant cell undergoes plasmolysis (the protoplast pulls away from the cell wall).

Question 6

Match each organelle with its function: (a) Ribosome, (b) Golgi apparatus, (c) Lysosome, (d) Smooth ER, (e) Chloroplast, (f) Nucleolus.

Accepted Answer

Step 1 — Recall each organelle's function.

(a) Ribosome → PROTEIN SYNTHESIS (translates mRNA into polypeptides).
(b) Golgi apparatus → MODIFIES AND PACKAGES proteins/lipids (the 'post office'). Sends vesicles to destinations.
(c) Lysosome → DIGESTS unwanted material and damaged organelles. 'Suicide bags' when the cell is dying.
(d) Smooth ER → LIPID SYNTHESIS, DETOXIFICATION (also stores Ca²⁺).
(e) Chloroplast → PHOTOSYNTHESIS (captures sunlight → produces glucose).
(f) Nucleolus → SYNTHESISES RIBOSOMES (specifically, the rRNA component).

✦ Answer: As above. Six organelles, six unique functions to memorise.

Question 7

Three raisins are dipped (i) in plain water, (ii) in concentrated sugar solution, (iii) in pure ethanol. Describe what happens in each case and why.

Accepted Answer

Step 1 — Recognise that the inside of a raisin has a high concentration of sugars (so its internal solution is hypertonic).

Step 2 — Analyse each medium.
  (i) Plain water (HYPOTONIC to raisin): water flows IN → raisin swells. After a few hours, raisin is plump and rehydrated.
  (ii) Concentrated sugar solution (HYPERTONIC to raisin): water flows OUT (or stays out) → raisin shrinks further (or stays small).
  (iii) Pure ethanol: ethanol is NOT water. The raisin's membrane is not permeable to ethanol the way it is to water, AND ethanol denatures proteins, damaging the cell. Effectively no osmosis; cellular structures may be destroyed. Raisin doesn't swell.

✦ Answer: (i) Raisin swells (water enters by osmosis); (ii) Raisin stays shrunken (no net water inflow against the concentration gradient); (iii) No swelling — ethanol can't cross the membrane the way water can, and damages cell membranes.

Question 8

Why does a wilted plant recover when watered, but a salted slug doesn't recover when placed in fresh water?

Accepted Answer

Step 1 — Recall what happens during osmotic stress.
  Both lose water in a hypertonic environment, and gain water in a hypotonic one.

Step 2 — Plant cell: walled, recoverable.
  Wilted plant: cells have lost water (plasmolysed). Cell wall is intact and rigid.
  When watered: water enters by osmosis → cells become turgid again → plant recovers. The wall held everything in place during the dehydration.

Step 3 — Slug (animal): no cell wall.
  Salted slug: animal cells lose water → shrink, plasma membrane folds, proteins denature, cells die.
  Damage is IRREVERSIBLE because animal cells have no rigid wall to hold structure during dehydration; once the membrane integrity is lost, water re-entry can't fix it.

✦ Answer: Plant cells have a rigid CELL WALL that maintains structure during dehydration; cells recover their water upon rewatering. Animal cells have NO cell wall — severe water loss destroys their structure permanently. The wall is the reason plants survive transient drought stress while animal tissue doesn't.

Question 9

Why do mitochondria and chloroplasts have their own DNA?

Accepted Answer

Step 1 — State the endosymbiosis theory (Lynn Margulis, 1967).
  Mitochondria and chloroplasts were once free-living prokaryotes that were ENGULFED by an ancestral eukaryotic cell ~ 1.5 billion years ago.
  Instead of being digested, they formed a symbiotic relationship: they kept producing energy (or sugar) and got a safe home.

Step 2 — Consequences observable today.
  • They have their own circular DNA (like bacteria).
  • They have 70S ribosomes (prokaryote-type), not the 80S ribosomes of eukaryotic cytoplasm.
  • They have double membranes (the outer is the host's, the inner is the original bacterium's).
  • They divide by binary fission (like bacteria).

✦ Answer: Mitochondria and chloroplasts were originally free-living bacteria-like organisms engulfed by an ancestral eukaryote. They retained their own DNA and reproduce semi-autonomously inside our cells — a relic of their prokaryotic origin (endosymbiotic theory).

Question 10

How do diffusion and osmosis differ from each other? Give one example of each from your own body.

Accepted Answer

Step 1 — Diffusion definition.
  Movement of any substance (gas, liquid solute) from high to low concentration. No membrane required.

Step 2 — Osmosis definition.
  Specific to WATER moving across a SEMIPERMEABLE membrane, from low solute (high water potential) to high solute (low water potential).

Step 3 — Examples from your body.
  • Diffusion: oxygen moves from alveoli (high O₂) into blood (low O₂) in your lungs.
  • Osmosis: water moves from the lumen of your large intestine into blood (which has higher solute) — keeping you hydrated.

✦ Answer: Diffusion = any substance moving down its concentration gradient. Osmosis = specifically water moving across a semipermeable membrane. Body examples: O₂ uptake in lungs (diffusion); water absorption in the large intestine (osmosis).

Question 11

Compare RER, SER, Golgi apparatus and lysosomes by stating one defining structural feature and one function each.

Accepted Answer

Step 1 — Lay out each in a table.

• Rough ER (RER):
    Structure — flat sheets of folded membrane WITH RIBOSOMES on cytoplasmic side.
    Function — synthesises proteins for export or insertion into membranes.

• Smooth ER (SER):
    Structure — tubular folded membrane WITHOUT ribosomes.
    Function — synthesises lipids/steroids, stores calcium, detoxifies drugs (especially in liver cells).

• Golgi apparatus:
    Structure — stack of flat membrane-bound CISTERNAE (sacs), often near the nucleus.
    Function — modifies, sorts and packages proteins/lipids from the ER into vesicles for export or delivery.

• Lysosomes:
    Structure — small spherical vesicles bounded by a single membrane, containing DIGESTIVE ENZYMES.
    Function — digest waste, foreign particles, worn-out organelles ('cellular recycling').

Step 2 — Mental model of the workflow.
  Protein is made on RER → vesicle carries it to Golgi → Golgi modifies/packages → vesicle ships to destination (membrane, secretion, or lysosome).

✦ Answer: As above. RER has ribosomes (protein synthesis), SER has no ribosomes (lipid synthesis, detox), Golgi packages proteins, lysosomes digest waste.

Question 12

A doctor administers fluids to a dehydrated patient. He chooses 0.9 % saline instead of pure water. Explain his reasoning at the cell level.

Accepted Answer

Step 1 — Identify what 0.9 % saline is.
  0.9 % NaCl is ISOTONIC to human blood. Same solute concentration as the inside of red blood cells.

Step 2 — What would happen with pure water?
  Pure water is HYPOTONIC to RBC contents. Massive osmotic water inflow into RBCs → RBCs swell → BURST (haemolysis). The patient's red blood cells would be destroyed and they could die.

Step 3 — Why saline is safe.
  Being isotonic, saline doesn't cause net water flow across RBC membranes → no swelling, no shrinking. The cells are bathed in a friendly fluid, rehydrating tissues without osmotic damage.

Step 4 — Real-world.
  This is why IV (intravenous) drips routinely use 'normal saline' (0.9 % NaCl) or Ringer's lactate — both isotonic with blood.

✦ Answer: Pure water is HYPOTONIC to red blood cells — administering it intravenously would cause RBCs to swell and burst (haemolysis). 0.9 % saline is ISOTONIC, so cell volume is preserved while rehydrating the patient.

Question 13

If lysosomes burst by accident inside a healthy cell, what would happen? Why are they called 'suicide bags'?

Accepted Answer

Step 1 — Recall what's inside lysosomes.
  Powerful digestive enzymes (proteases, lipases, nucleases) optimised to work at acidic pH (~ 5).

Step 2 — If a lysosome bursts inside the cytoplasm (pH ~ 7.4)…
  The released enzymes would digest the cell's own proteins, lipids and nucleic acids → cytoplasm and organelles would be broken down → cell DIES.

Step 3 — Why 'suicide bags'.
  The nickname captures their potential to destroy the very cell that hosts them. Under normal conditions, the lysosomal membrane safely contains the enzymes. But when a cell is damaged beyond repair, the lysosomes intentionally rupture, dismantling the cell in a process called AUTOLYSIS — programmed cell death by self-digestion.

Step 4 — Why this is useful.
  Apoptosis (programmed cell death) is essential for normal development (e.g., separating fingers from webbing during embryonic growth), removing infected cells, eliminating cancerous cells.

✦ Answer: Burst lysosomes release digestive enzymes that destroy the host cell from inside — a controlled cell-suicide mechanism essential for development and removing damaged or infected cells.

Question 14

Why are cells small in size (typically 10–100 μm)? Give two scientifically-grounded reasons.

Accepted Answer

Step 1 — Reason 1: Surface area to volume ratio (SA:V).
  As a cell grows, volume increases as r³ but surface area as r² — so SA:V DECREASES.
  Small cells: high SA:V → easy gas exchange (O₂, CO₂), nutrient uptake, waste removal by diffusion.
  Big cells: low SA:V → the interior would starve / suffocate — diffusion from the membrane wouldn't reach the centre fast enough.

Step 2 — Reason 2: Coordination and signaling.
  Cellular machinery (mRNA travelling from nucleus to ribosomes, vesicles travelling from ER → Golgi → membrane) is efficient over micrometre distances. Over millimetres, transport would be impossibly slow.

Step 3 — Reason 3 (bonus): Diffusion as a transport mechanism.
  Cells rely on diffusion for many transport tasks. Diffusion time scales as distance². Doubling the cell size → 4× slower diffusion to the centre — unsustainable.

✦ Answer: (i) Small cells maximise surface area-to-volume ratio, ensuring fast diffusion of gases, nutrients and wastes. (ii) Intracellular transport (mRNA, vesicles) is efficient only over micrometre distances. Diffusion's r² scaling makes large cells unable to feed their centres.

Question 15

You're shown a microscope image of a cell. It has a rigid, brick-shaped boundary, a single large clear central cavity, and several green oval bodies. Identify the cell type and the named organelles.

Accepted Answer

Step 1 — Brick shape + rigid boundary → suggests a PLANT cell with a cell wall.
  Animal cells are round/irregular without a fixed shape.

Step 2 — Large clear central cavity = LARGE CENTRAL VACUOLE.
  Up to 90 % of plant cell volume; stores water, salts, sugars; provides turgor pressure.

Step 3 — Green oval bodies = CHLOROPLASTS (a type of plastid).
  Contain chlorophyll; site of photosynthesis. The green colour comes from chlorophyll.

Step 4 — Probable identity.
  A photosynthetic plant cell — most likely from a leaf (e.g., mesophyll cell).

✦ Answer: Plant cell (specifically a leaf mesophyll cell). Organelles named: cell wall, central vacuole, chloroplasts. The clue 'green oval bodies' is a dead giveaway — chloroplasts are the only large green organelles in cells.

Practice — The Fundamental Unit of Life

Sign in to save attempts to your dashboard and track your accuracy over time

Find a verified tutor

Join a live course

Browse coaching centres

About

Categories

Support

Documentation

Get Study Tips & Updates

Download Our App

Download for iOS

Download for Android

Popular Cities