Revision Notes: Class 8 Science Chapter 2 The Invisible Living World

Class 8 Science Chapter 2: The Invisible Living World — Quick Revision Notes

• Cell: the basic unit of life. Every cell has a cell membrane (controls entry/exit of substances), cytoplasm (site of most life processes), and a nucleus (controls cell activities and growth).
• Extra structures: plant, fungal and bacterial cells also have a cell wall; bacteria lack a true nucleus and instead have a nucleoid (unbound genetic material).
• Unicellular vs multicellular: bacteria and protozoa are unicellular; fungi and algae can be either; plants and animals are multicellular. Organisation: Cell → Tissue → Organ → Organ system → Organism.
• Discovery history: Robert Hooke (1665) first observed and named ‘cells’ in cork using a compound microscope; Antonie van Leeuwenhoek (1670s) built stronger single-lens microscopes and first observed bacteria — called the ‘Father of Microbiology’.
• Microorganisms (microbes): living things too small to see unaided — bacteria, protozoa, fungi, algae, and viruses (viruses are acellular, multiplying only inside a host cell).
• Beneficial roles: yeast ferments dough (releases CO₂ + alcohol) for bread and cakes; Lactobacillus ferments milk into curd (produces lactic acid); Rhizobium in legume root nodules fixes atmospheric nitrogen, naturally fertilising soil; various microbes decompose waste into manure and recycle nutrients; some produce biogas from waste, or help clean oil spills.
• Food preservation: pickles/murabbas use high salt or sugar concentration, which draws water out of microbial cells by osmosis, preventing spoilage.
• Important note: this chapter covers only beneficial microorganisms; disease-causing microbes are covered in the next chapter.

Frequently Asked Questions

What are microorganisms, and why can they not be seen with the naked eye?
Microorganisms are living organisms, such as bacteria, viruses, fungi, and protozoa, that are so small they can only be observed using a microscope, since their size is far below the resolving power of the human eye.

Last Updated: September 23, 2026

Are all microorganisms harmful to humans?
No, while some microorganisms cause diseases, many others are beneficial, such as those used in making curd and bread, those that help decompose waste, and those that enrich soil fertility by fixing nitrogen.

Written by Satish

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