In-text — The Cell as the Basic Unit of Life — "The Invisible Living World: Beyond Our Naked Eye"

1. 13 marksCuriosity Grade 8, Chapter 2, Section 2.5, page 23

   What is the difference between unicellular and multicellular organisms? Give examples, including one group that contains both.

   Hint. Fungi are the awkward group — yeast and mould are not alike in this respect.

   Multicellular organisms have bodies made of many cells. All plants and animals are multicellular. Their cells carry out specialised functions individually but also cooperate with each other, which the chapter says increases the chance of survival.

   Unicellular organisms are made of just one cell, and they carry out all the functions necessary for their survival in that single cell. Bacteria and protozoa are unicellular.

   The group that contains both. Fungi. The chapter's example is exact: yeast is a unicellular fungus while mould is a multicellular fungus. Algae likewise are made of one or more cells.

   Organism	Cells
   Bacteria, protozoa (Amoeba)	Unicellular
   Yeast	Unicellular fungus
   Mould	Multicellular fungus
   Algae	One or more cells
   Plants, animals	Multicellular

   The point worth holding on to. A unicellular organism is not an incomplete one. It does everything a living thing must do — inside a single cell.

   ✦ Answer: unicellular organisms are a single cell performing every function needed for survival (bacteria, protozoa, yeast); multicellular organisms are built of many cells with specialised, cooperating roles (plants, animals, mould) — and fungi include both, since yeast is unicellular but mould is multicellular.

2. 24 marksCuriosity Grade 8, Chapter 2, Section 2.5, page 24

   How do the cells of microorganisms compare with plant and animal cells? What single feature sets bacteria apart?

   Hint. The distinguishing feature is about the nucleus.

   What is shared. Like animal and plant cells, the cells of microorganisms are also surrounded by a cell membrane.

   Fungi. Their cells additionally have a cell wall, but they do not have chloroplasts, so they cannot make their own food through photosynthesis. (This matches Table 2.2, which recorded both moulds as filaments without chlorophyll.)

   Bacteria — the distinguishing feature. Bacteria do not have a well-defined nucleus and a nuclear membrane. Instead they have a nucleoid. The chapter is explicit that this feature distinguishes them from the cells of yeast, protozoa, algae, fungi, plants and animals — every other group in the chapter has a proper nucleus, and bacteria alone do not.

   Why the nucleus matters here. The nucleus regulates all the cell's activities and its growth. A bacterium still does these things; it simply organises the material differently, without enclosing it in a membrane.

   How much of the cell we have actually seen. The chapter is candid: only a few basic structures have been shown, and seeing subcellular components needs an electron microscope magnifying about 10,00,000 times.

   ✦ Answer: microbial cells also have a cell membrane; fungal cells add a cell wall but have no chloroplasts, so they cannot photosynthesise; and bacteria alone lack a well-defined nucleus and nuclear membrane, having a nucleoid instead — the feature that distinguishes them from all the other groups.

3. 33 marksCuriosity Grade 8, Chapter 2, Section 2.5, page 23

   Why is the cell considered to be the basic unit of life?

   Hint. The chapter's own analogy is a brick.

   Because everything living is built from cells. The body of all living organisms is made of these tiny building blocks. Plants and animals are made of many; some microorganisms consist of just one. There is no organism made of something else.

   Because a cell is complete in itself. A single cell contains the various components needed to perform the functions that keep an organism alive — and in unicellular organisms it does exactly that, with no help from any other cell. So the cell is the smallest thing that can carry on life: divide it further and you get parts that cannot survive alone.

   The analogy. The chapter compares the cell to a brick, the basic unit of a wall (Fig. 2.3d). A wall is built by repeating one unit; a body is built by repeating and specialising one unit.

   Where the ladder starts. All the levels of organisation — tissue, organ, organ system, organism — begin here. Each is a way of combining cells, so the cell is where the structure of every living body begins.

   ✦ Answer: because every living organism's body is made of cells, because a single cell contains everything needed to carry out life processes — as unicellular organisms demonstrate — and because all higher levels of organisation are built by combining cells, just as a wall is built from bricks.

4. 43 marksCuriosity Grade 8, Chapter 2, Snapshots, pages 24-25

   Summarise what this chapter establishes about microorganisms and cells.

   Hint. The chapter's own 'Snapshots' box does this; check you can reproduce it.

   About microorganisms

   • Small-sized organisms, not visible to the unaided eye
   • They live in all kinds of environments, and even inside the bodies of plants and animals
   • They are either unicellular or multicellular: bacteria and protozoans are unicellular; fungi can be either; plants and animals are multicellular
   • Bacteria, fungi and protozoa are different kinds of microorganisms
   • Viruses are also small, but differ from other microorganisms since they reproduce only inside the host organism
   • They can be beneficial or harmful to us

   About cells

   • The cell is the basic unit of life, and the body of every living organism is made of cells
   • A typical cell is bounded by a cell membrane, filled with cytoplasm, and contains a nucleus
   • Plant, fungal and bacterial cells have an extra covering, the cell wall
   • Bacteria lack a well-defined nucleus
   • Cells differ in shape and size, and their shape is related to the function they perform

   What microbes do for us

   • Some decompose plant and animal waste and clean up the environment
   • Some live in the root nodules of legumes, trapping nitrogen and increasing soil fertility
   • Yeasts are fungi used in making breads, cakes, pastries, idlis, dosas and bhaturas
   • Lactobacillus is used in curd formation and in food-industry fermentation

   ✦ Answer: the cell is the basic unit of life and all organisms are built from cells that vary in shape according to function; microorganisms — bacteria, fungi, protozoa, some algae — live everywhere, may be unicellular or multicellular, are beneficial or harmful, and drive decomposition, nitrogen fixation and food fermentation, while viruses stand apart as acellular and reproduce only inside a host.

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.