In-text — Levels of Organisation — "The Invisible Living World: Beyond Our Naked Eye"

1. 13 marksCuriosity Grade 8, Chapter 2, Section 2.2, page 14

   What are the levels of organisation in the body of a living organism? Explain each step.

   Hint. The chapter compares the first level to a brick.

   The sequence:

   Cell → Tissue → Organ → Organ system → Organism

   Level	What it is
   Cell	The basic unit of life — just as a brick is the basic unit of a wall
   Tissue	A group of similar cells
   Organ	Different tissues organised together
   Organ system	Several organs working together to perform a major body function
   Organism	All the organ systems together — a complete plant or animal

   The point of the ladder. Each level is built from the one below, so complexity is not something added from outside — it emerges from simple building blocks combining. As the chapter puts it, these levels "help us understand how simple building blocks like cells come together to form a complex living being."

   Watch the word 'similar'. A tissue is a group of similar cells, not just any cells together. The stomach contains muscle cells and juice-producing cells; those are different tissues, and it is the several tissues together that make the organ.

   ✦ Answer: Cell → Tissue → Organ → Organ system → Organism — a group of similar cells forms a tissue, different tissues form an organ, several organs form an organ system, and all the systems together form the organism.

2. 23 marksCuriosity Grade 8, Chapter 2, Section 2.2, page 14

   How does the life of a complex living organism begin, and what does 'multicellular' mean?

   Hint. Every one of those levels has to be built from something.

   It begins with one cell. The life of complex living organisms begins with a single cell — the egg.

   What the egg can do. It has the remarkable ability to divide repeatedly to form a complete living being made of many cells.

   Multicellular organisms are living beings made of many cells in this way. Animals, including humans, and plants are all multicellular.

   Why this is striking. Everything in the previous question — tissues, organs, organ systems — is produced by repeated division of one starting cell. The cells do not stay identical: they become muscle cells, nerve cells, lining cells, each with the shape its job requires. So a single cell gives rise to all the specialised types the body needs.

   The contrast to come. Some microorganisms, such as bacteria and protozoa, remain unicellular and carry out every function necessary for survival within that one cell.

   ✦ Answer: it begins as a single cell, the egg, which divides repeatedly to build a body of many cells; such organisms are called multicellular, and animals including humans, and plants, are all multicellular.

3. 33 marksCuriosity Grade 8, Chapter 2, Ever heard of…, page 15

   What is the largest known cell in the living world, and why do the shell and the white of the egg not count as part of it?

   Hint. Only one part of an egg is a single cell.

   The largest known cell. The yolk — the yellow part — of an ostrich egg. It measures about 130 mm to 170 mm in diameter.

   Why only the yolk. The egg also contains extra non-cellular material: a shell for protection and a white liquid that nourishes the cell during its continued development. Neither of these is itself a cell, so neither counts towards the size of the cell. The single cell is the yolk alone.

   Why this is worth knowing. It corrects the natural assumption that cells are, by definition, microscopic. They are usually microscopic — but not necessarily. A cell 13 to 17 cm across is visible from across a room, and it is still one cell with one membrane around it.

   The pairing to remember. The largest cell is an ostrich yolk at 130-170 mm; at the other end, bacteria need a microscope magnifying 100-400 times before they can be seen at all.

   ✦ Answer: the yolk of an ostrich egg, about 130-170 mm across — the shell and the white are non-cellular material (protection and nourishment), so they are not part of the cell.

Solutions written by the tuition.in editorial team and checked against NCERT Curiosity - Textbook of Science for Grade 8 (hecu102.pdf), Chapter 2 'The Invisible Living World: Beyond Our Naked Eye', pages 8-27, Reprint 2026-27. HAND-WRITTEN throughout. Page count verified against the printed contents page (hecu1ps.pdf p.18): 20 pages, 8-27, matching exactly. Covers the 3 'Probe and ponder' prompts (p.8), all 9 activities (2.1 to 2.9), every in-text section (2.1 to 2.5), the 9 numbered 'Keep the curiosity alive' exercises (pp.25-26) and the 4 'Discover, design, and debate' projects (p.27). EXERCISE Q1 VENN DIAGRAM READ OFF A 500-DPI RENDER of p.25: three circles labelled 'Only in Animal Cell' (top), 'Only in Bacterial Cell' (lower left) and 'Only in Plant Cell' (lower right), with an arrow labelling the central three-way overlap 'Common to all three cells'. Each of those four labelled regions carries two ruled lines; the three PAIRWISE overlap regions carry none. Working from the book's own statements (p.13 'the cell wall in the plant cell provides rigidity'; p.24 'bacteria do not have a well-defined nucleus... instead they have a nucleoid'; and the Snapshots line 'Plant, fungal, and bacterial cells have an extra covering, called a cell wall'), the correct placement is: cytoplasm and cell membrane in the three-way overlap (exactly two items, matching the two ruled lines); chloroplast in plant-only; nucleoid in bacterial-only; NUCLEUS in the animal-plant pairwise overlap (plant cells have nuclei, bacteria do not); CELL WALL in the plant-bacterial pairwise overlap (not plant-only, since the book says bacterial and fungal cells have walls too); and NO listed part is unique to the animal cell. The solution states both traps explicitly. FACTUAL FIGURES TAKEN VERBATIM FROM THE BOOK: Hooke's Micrographia 1665, microscope 200-300x, cork, honeycomb, first use of 'cell'; Leeuwenhoek 1660s, Father of Microbiology; ordinary microscope 100-400x; electron microscope about 10,00,000x; ostrich egg yolk 130-170 mm as the largest known cell; Ananda Mohan Chakrabarty 1938-2020, oil-degrading bacterium 1971, patent 1980; biogas mainly carbon dioxide with a high proportion of methane; one of India's oldest biogas plants late 1850s; microalgae produce more than half of Earth's oxygen; Spirulina over 60 per cent protein by body weight and a source of vitamin B12, harvested after 3-6 weeks. The MNRE biogas-programme project answer deliberately does NOT assert current programme details - it names the programmes to search for and tells the student to verify against the current official MNRE page, since names and subsidy rules change.. Questions are referenced from the NCERT textbook for identification.