2.7 Going Deeper: Enrichment & Exam Preparation

Microbes in Everyday Life

A Day Full of Microbes Before school you eat curd (set by Lactobacillus) and toast made from risen dough (puffed by yeast). The vegetables came from a field enriched by Rhizobium and decomposer microbes. The oxygen in every breath is more than half made by microalgae. Waste from the kitchen will become manure or biogas, thanks again to microbes. You share your whole day — and your gut — with the invisible living world.

Quick Reference

Microbe Group What it does for us
Yeast Fungus Rises bread/cake dough (releases CO₂)
Lactobacillus Bacterium Sets milk into curd (makes lactic acid)
Rhizobium Bacterium Fixes nitrogen in legume root nodules
Decomposer bacteria & fungi Bacteria/Fungi Break down waste into manure; make biogas
Spirulina / microalgae Algae Release oxygen; food & supplements
Cell part Found in Job
Cell membrane All cells Outer boundary; controls entry/exit
Cytoplasm All cells Jelly where activities happen
Nucleus Plant & animal (not bacteria) Control centre
Cell wall Plant, fungi, bacteria Rigidity & strength
Chloroplast Plant (green parts), algae Photosynthesis
Large vacuole Plant Storage, waste, keeps cell firm
Nucleoid Bacteria Genetic material (no true nucleus)

Memory Tricks & One-Page Revision

Revision Card

  • Plant extras = “Wall, Chloroplast, big Vacuole.” Everything else (membrane, cytoplasm, nucleus, mitochondria) is in both plant and animal cells.
  • Ladder of life: C-T-O-O-O — Cell, Tissue, Organ, Organ system, Organism.
  • Bacteria have a nucleoid, not a true nucleus — that’s their signature.
  • Virus = not a cell; it only multiplies inside a host cell.
  • Microbe jobs: Yeast → bread, Lactobacillus → curd, Rhizobium → nitrogen, decomposers → manure/biogas, microalgae → oxygen.
  • Shape follows function: long branched nerve cell carries messages; flat cheek cell protects; tube-shaped plant cell carries water.
  • Salt/sugar preserve food by stopping microbes from growing.

Spot the Mistake

Find the Flaw

  1. “Animal cells have a cell wall.” — Wrong: only plant (and fungal/bacterial) cells have a cell wall; animal cells have only a membrane.
  2. “Yeast makes dough rise by producing oxygen.” — Wrong: yeast releases carbon dioxide, not oxygen.
  3. “Bacteria have a nucleus like plant cells.” — Wrong: bacteria have a nucleoid, with no well-defined nucleus.

Test Your Reflexes

A quick-fire drill: a microbe appears with a job. Decide whether the pairing is Correct or Wrong. Build a streak!

Microbe match-up
Is the microbe matched to the right job?
Score 0 · Streak 0

Exam Corner: CBSE-Style Practice

Mixed Practice (objective, short, long, HOTS)

Objective type (1 mark each)

  1. The word cell was first used by the scientist ______.
  2. The gas released by yeast that makes dough rise is ______.
  3. Bacteria have a ______ instead of a well-defined nucleus.

Short answer (2 marks each)

  1. Name the three parts present in a plant cell but absent in an animal cell.
  2. Why is curd not formed easily from cold milk kept in the refrigerator?

Long answer (3 marks each)

  1. List the levels of organisation in a multicellular body, in order, and explain each with the brick-and-wall comparison.
  2. Explain how Rhizobium helps a legume crop grow without chemical fertiliser, and why farmers practise crop rotation.

HOTS (Higher Order Thinking)

  1. A balloon is fixed over a test tube of warm sugar solution with yeast. After a few hours the balloon inflates. Explain why, and how you would prove the gas is carbon dioxide.
  2. Pickles and murabbas don’t get spoiled by microbes even when left out. Give the scientific reason.

Assertion–Reason (Choose: (a) both true, R explains A; (b) both true, R doesn’t explain A; (c) A true, R false; (d) A false, R true.)

  1. Assertion (A): A nerve cell is long and branched.   Reason (R): Its shape helps it carry messages quickly to distant parts of the body.
  1. Robert Hooke (in 1665, observing cork).
  2. Carbon dioxide (CO₂).
  3. nucleoid.
  4. Cell wall, chloroplast, and large vacuole.
  5. Lactobacillus bacteria need warmth to multiply and produce the lactic acid that sets curd; in a cold fridge they stay inactive, so the milk does not curdle.
  6. Cell → Tissue → Organ → Organ system → Organism. A cell is a single brick; similar cells make a tissue (a patch of wall); tissues form an organ (a room); organs sharing a job form an organ system (a floor of the house); all systems together make the whole organism (the building).
  7. Rhizobium lives in the root nodules of legumes and fixes nitrogen from the air into a usable form, fertilising the soil naturally. Growing legumes in rotation restores that nitrogen for the next crop, so less chemical fertiliser is needed.
  8. Yeast respires, breaking down the sugar and releasing carbon dioxide, which fills the balloon. To prove it is CO₂, pass the collected gas into lime water — CO₂ turns lime water milky.
  9. The high concentration of salt or sugar acts as a preservative: it prevents microbes from growing, so the food does not spoil.
  10. (a) — Both true, and R correctly explains A: the long, branched shape is exactly what lets a nerve cell reach and signal distant parts quickly.

Glossary

Key Terms

  • Cell: the basic structural and functional unit of life.
  • Cell membrane / cell wall: the membrane bounds every cell; the wall is an extra rigid layer in plant, fungal and bacterial cells.
  • Nucleus / nucleoid: the control centre of a cell; bacteria have a nucleoid instead of a true nucleus.
  • Tissue / organ / organ system: successive levels of organisation built from cells.
  • Microorganism (microbe): a living thing too small to see without a microscope.
  • Decomposition: the breakdown of dead matter into simple, nutrient-rich substances by microbes.
  • Fermentation: the microbe-driven change in food, e.g. milk → curd, dough rising.
  • Nitrogen fixation: trapping nitrogen from the air into a form plants can use (by Rhizobium).
  • Virus: an acellular microbe that multiplies only inside a host cell.