CBSE Class 8 Annual Assessment

Chapter 2: The Invisible Living World: Beyond Our Naked Eye

Summary

This chapter opens up the hidden world of cells and microorganisms that the unaided eye cannot see. The invention of the lens and then the microscope let humans observe tiny structures for the first time; Robert Hooke first used the word "cell" while looking at cork, and Antonie van Leeuwenhoek, the Father of Microbiology, first saw bacteria. Through activities with onion peel and human cheek cells, students learn that all living beings are made of cells. A typical cell has a cell membrane, cytoplasm, and a nucleus; plant, fungal, and bacterial cells have an extra cell wall, while bacteria lack a well-defined nucleus and instead have a nucleoid. Cells differ in shape and size to suit their functions, such as long branched nerve cells and spindle-shaped muscle cells. The chapter explains the levels of organisation: cell, tissue, organ, organ system, and organism. Microorganisms may be unicellular (bacteria, Amoeba) or multicellular (some fungi and algae) and live almost everywhere. They are deeply connected to us: decomposers recycle nutrients and make manure, Rhizobium fixes nitrogen in root nodules, yeast and Lactobacillus are used in baking and curd-making, and microalgae produce much of Earths oxygen. Viruses are introduced as acellular agents that multiply only inside host cells.

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