- All cellular living organisms consist of one or more cells.
- The cell membrane, cytoplasm and nucleus are central features of typical plant and animal cells.
- Plant cells have cell walls; bacteria have a nucleoid rather than a membrane-bound nucleus.
- Cells specialise, forming tissues, organs and organ systems.
- Microbes include bacteria, protozoa, fungi and microscopic algae; viruses are acellular.
- Decomposers recycle nutrients and help make manure.
- Yeast produces carbon dioxide in dough; lactic-acid bacteria help form curd.
- Rhizobium fixes nitrogen in legume nodules; microalgae support aquatic food webs.
Use these notes alongside your current textbook. Check important formulas, worked answers and syllabus coverage with your teacher or official learning material.
The Invisible Living World: Beyond Our Naked Eye — NCERT Class 8 Science Chapter 2
Direct answer: Cells are the basic structural and functional units of life. Microscopes reveal cells and microorganisms such as bacteria, protozoa, fungi and microalgae. These organisms support decomposition, food fermentation, nitrogen fixation and ecosystems, although some can cause disease.
What makes bread fluffy, curd sour and fallen leaves disappear into soil? Microscopic living organisms are working around us, even when our eyes cannot see them. This chapter explores their cells, shapes, habitats and useful roles.
Source: NCERT Curiosity Class 8, Chapter 2, reprint 2026–27, printed pages 8–27. Five numbered sections, nine classroom activities and nine numbered Keep the Curiosity Alive exercises are addressed below.
2.1 What Is a Cell?
All living organisms consist of one or more cells. Robert Hooke described compartments in cork in 1665 and called them cells. Antonie van Leeuwenhoek observed microscopic living organisms. The cell membrane regulates exchange with surroundings; cytoplasm is the medium for many processes; the nucleus contains genetic information and helps regulate activities. Plant cells additionally have a cellulose-containing cell wall and typically a large central vacuole; green photosynthetic plant cells have chloroplasts, but onion bulb epidermal cells generally do not.
2.1.1 Variation in shape and structure of cells
Onion peel cells are often rectangular and closely packed; cheek lining cells are flatter and irregular/polygonal. Spindle-shaped muscle cells contract, long branched neurons transmit signals, and some plant cells form tubes to conduct water. Cell structure is related to function, but a drawing is simplified and does not show every organelle.
2.2 Levels of organisation
Cell → tissue → organ → organ system → organism. For example, muscle cells form muscle tissue; several tissues form the stomach; the stomach works with other organs in the digestive system. Unicellular organisms perform essential life processes in one cell; multicellular organisms contain many cooperating specialised cells.
2.3 What Are Microorganisms?
Microorganisms are organisms too small to see clearly with the unaided eye, including bacteria, protozoa, many fungi and microalgae. Amoeba moves with pseudopodia; Paramecium with cilia; some algae contain chlorophyll. Yeast is a unicellular fungus, while bread mould is multicellular. Viruses are acellular and replicate only inside living host cells; they are not ordinary cellular microorganisms.
Microscopes and observation
A water-filled round flask may magnify letters. A microscope uses lenses to show details invisible to the naked eye. For onion peel, a teacher prepares a thin peel, stains it, mounts it under a coverslip and examines cells. A prepared cheek-cell slide reveals a membrane, cytoplasm and nucleus, but no plant-type cell wall. Use only teacher-prepared slides for human samples; avoid collecting body samples independently.
2.4.1 Microbes and decomposition
Decomposer bacteria and fungi break down fruit peels, fallen leaves and other biodegradable waste. The resulting nutrients return to soil; controlled composting requires suitable moisture, aeration and temperature. Anaerobic digestion by specialised microbes can produce methane-rich biogas. Do not touch mouldy waste or open unknown cultures.
2.4.2 Microorganisms and food
Yeast is a fungus that uses sugars and releases carbon dioxide, which makes dough rise. Fermentation may also produce ethanol. Lactic-acid bacteria, including Lactobacillus species, ferment milk sugars and help form curd; warmth generally accelerates the process within a suitable temperature range. Salt, sugar, acidity and refrigeration can inhibit microbial growth, but do not make every food permanently safe.
Root nodules and nitrogen fixation
Rhizobium bacteria live in nodules on roots of certain legumes such as beans, peas and lentils. They convert atmospheric nitrogen into biologically useful nitrogen compounds, helping plant growth. This is one reason legumes are valuable in crop rotations; they do not guarantee that fertilisers are never needed.
2.4.3 Microalgae
Microscopic photosynthetic organisms such as many green algae and diatoms are important producers in aquatic food webs and contribute substantially to global oxygen production. Some are used in research on food, wastewater treatment and biofuels. Not every microalga is safe to eat, and wild-collected algae must not be consumed.
2.5 Why Is the Cell a Basic Unit of Life?
A cell is the smallest structural and functional living unit. Bacteria lack a membrane-bound nucleus and have DNA in a nucleoid region. Plant and animal cells are eukaryotic and generally have membrane-bound nuclei; fungal cells also have nuclei and cell walls. Cell membranes are present in all cellular organisms.
Plant, animal and bacterial cell comparison
| Feature | Plant cell | Animal cell | Bacterial cell |
|---|---|---|---|
| Cell membrane | Present | Present | Present |
| Cell wall | Usually cellulose | Absent | Usually peptidoglycan |
| Membrane-bound nucleus | Present | Present | Absent; nucleoid |
| Cytoplasm | Present | Present | Present |
| Chloroplasts | In photosynthetic green cells | Absent | Absent |
| Vacuoles | Often large central | Often small or absent | No typical large central vacuole |
Microorganisms and their roles
| Microbe | Group | Key role or feature |
|---|---|---|
| Amoeba | Protozoan | Unicellular; pseudopodia |
| Paramecium | Protozoan | Unicellular; cilia |
| Yeast | Fungus | Carbon dioxide in dough |
| Bread mould | Fungus | Decomposition |
| Lactobacillus | Bacterium | Lactic acid in curd |
| Rhizobium | Bacterium | Nitrogen fixation in nodules |
| Microalgae | Several algal groups | Photosynthesis; aquatic food webs |
NCERT activities 2.1–2.9 explained
Activity 2.1: water-filled flask as a simple magnifier. 2.2: teacher-prepared stained onion peel slide. 2.3: observe a safely prepared cheek-cell slide. 2.4: pond-water organisms under supervision. 2.5: observe a teacher-prepared soil suspension. 2.6: compare observed organisms with Tables 2.1 and 2.2; record only organisms actually seen. 2.7: observe composting of covered plant waste without handling mould. 2.8: compare yeast-added dough with a control. 2.9: compare curd formation in warmer and cooler conditions; use safe food handling and do not consume experimental samples.
Diagram guide and scientific safety
For Fig. 2.3, label onion-cell wall, membrane, cytoplasm and nucleus. Fig. 2.4: label cheek-cell membrane, cytoplasm and nucleus. Fig. 2.5: compare animal and plant cell organelles. Fig. 2.7: draw cell→tissue→organ→organ system→organism. Fig. 2.13: label bacterial nucleoid. Fig. 2.14 and 2.15: distinguish the yeast sugar flask and limewater test. Never culture unknown microbes at home or smell/taste mouldy materials.
NCERT Keep the Curiosity Alive — all nine numbered questions and subparts
Questions are carefully paraphrased from the official textbook. For the diagram-placement task, use the printed Venn diagram with the explained regions. Activity observations are predictions, not fabricated measurements.
Question 1. The textbook gives six labels—nucleus, cytoplasm, chloroplast, cell wall, cell membrane and nucleoid—and asks you to place them in a Venn-style diagram comparing plant, animal and bacterial cells.
Step-by-step explanation: Cell membrane and cytoplasm occur in all three cell types. Membrane-bound nucleus occurs in plant and animal cells. Chloroplast occurs in photosynthetic plant cells (not all plant cells). Cell wall occurs in plant and bacterial cells. Nucleoid occurs in bacteria. Use the printed diagram's intersection regions for these placements.
Final answer: All three: membrane, cytoplasm; plant+animal: nucleus; plant+bacteria: wall; plant only: chloroplast; bacteria only: nucleoid.
Question 2. Aanandi sets up sugar solution in two tubes, adds yeast only to tube B and fits balloons. After several hours, which explanation best accounts for balloon B inflating?
Step-by-step explanation: Yeast uses sugar and releases carbon dioxide during metabolism. The gas accumulates in the balloon. The other proposed explanations do not explain why the yeast-containing tube inflates preferentially.
Final answer: Option (c): yeast produces gas.
Question 3. In Aanandi's experiment, why transfer the balloon gas to limewater and shake it?
Step-by-step explanation: Limewater is an indicator for carbon dioxide: CO₂ reacts with calcium hydroxide to form insoluble calcium carbonate, causing milkiness. It helps identify the gas produced by yeast.
Final answer: To test whether the gas is carbon dioxide; limewater should turn milky.
Question 4. Why might a farmer growing beans need less added nitrogen fertiliser than a neighbouring farmer growing wheat?
Step-by-step explanation: Legume roots can form nodules containing nitrogen-fixing Rhizobium bacteria. These supply some usable nitrogen compounds. Wheat generally lacks the same symbiotic nodules. Actual fertiliser needs depend on soil conditions.
Final answer: Beans may gain nitrogen through Rhizobium symbiosis.
Question 5. Snehal adds fruit peels and dry leaves to pit A, but fruit peels alone to pit B, then covers both with soil. What is she testing?
Step-by-step explanation: The intentionally changed condition is mixing dry leaves with wet fruit/vegetable peels. Compare decomposition after the same time while keeping other factors similar. Dry leaves affect carbon balance, texture, moisture and aeration.
Final answer: Whether adding dry leaves changes the rate or quality of decomposition.
Question 6. Identify the microorganism that can live in diverse environments, including the human gut.
Step-by-step explanation: Many bacteria occur in soil, water, air and inside the human intestine. Some gut bacteria help digestion.
Final answer: Bacteria.
Question 7. Identify the microorganism that makes bread and cake mixtures rise and become fluffy.
Step-by-step explanation: Yeast metabolises sugar and releases carbon dioxide, forming bubbles in dough.
Final answer: Yeast, a unicellular fungus.
Question 8. Identify the microorganism that lives in root nodules of pulse crops and helps supply nitrogen.
Step-by-step explanation: Rhizobium forms a symbiotic relationship with certain legumes and fixes atmospheric nitrogen.
Final answer: Rhizobium.
Question 9. Design a fair, safe investigation of the effect of moisture, temperature and air availability on microbial decomposition.
Step-by-step explanation: Use teacher-managed, sealed or safely covered compost models with equal biodegradable material; compare one variable at a time while others remain controlled. Record changes without opening unknown cultures. Air availability can be compared using approved compost setups rather than sealed anaerobic experiments in class.
Final answer: Three controlled comparisons, actual recorded observations and teacher-supervised handling; no home culturing.
Question 10. Two bread slices are kept near a sink and in a refrigerator. What might happen after three days, and why?
Step-by-step explanation: Moist, warmer surroundings often favour mould growth, while refrigeration slows it. Actual results vary with contamination, humidity and temperature. Do not open or smell mouldy bread.
Final answer: More visible mould may appear near the sink; refrigeration generally slows growth.
Question 11. Why might curd left out for a day become more sour? Give two possible explanations.
Step-by-step explanation: Lactic-acid bacteria may continue converting lactose into lactic acid, and warmer conditions can allow faster bacterial activity than refrigeration. Both increase acidity when conditions permit.
Final answer: Continued fermentation and favourable ambient temperature.
Question 12. In a setup with warm sugar solution and yeast in flask A connected to limewater in tube B, what happens to the sugar solution?
Step-by-step explanation: Yeast metabolises available sugars, releasing carbon dioxide; bubbles may form and the mixture can become cloudy or foamy depending on conditions.
Final answer: Yeast fermentation releases gas in flask A.
Question 13. What change is expected in limewater in tube B after several hours, and why?
Step-by-step explanation: Carbon dioxide from the yeast mixture passes into limewater. It reacts to form calcium carbonate precipitate.
Final answer: Limewater turns milky because of carbon dioxide.
Question 14. What if yeast is omitted from the sugar-solution flask?
Step-by-step explanation: Without yeast, the intended fermentation does not occur at a comparable rate. Therefore much less carbon dioxide is produced and the limewater typically remains clear in the controlled demonstration.
Final answer: Little or no milkiness is expected.
Question 15. How does a water-filled round-bottom flask help magnify text?
Step-by-step explanation: The curved water-and-glass surfaces refract light and can act as a simple lens. The apparent letter size depends on geometry and viewing distance.
Final answer: It can make nearby letters appear larger.
Question 16. How do onion-peel and human cheek cells differ under a microscope?
Step-by-step explanation: Onion epidermal cells tend to be rectangular, with a firm wall outside the membrane. Cheek cells are irregular/flat and lack a cell wall. Both have cytoplasm, a membrane and a nucleus.
Final answer: Onion: cell wall and regular shape; cheek: no cell wall and irregular shape.
Question 17. Why do neurons have long branches while muscle cells are elongated?
Step-by-step explanation: Neuron extensions help communicate with distant cells. Muscle-cell structure supports contraction and movement.
Final answer: Cell structure supports specialised function.
Question 18. Arrange cell, organ, organism, tissue and organ system from simplest to most complex.
Step-by-step explanation: Cells group into tissues; tissues form organs; organs cooperate in organ systems; systems form an organism.
Final answer: Cell → tissue → organ → organ system → organism.
Question 19. How does a bacterial nucleoid differ from a nucleus?
Step-by-step explanation: Bacterial DNA occupies a nucleoid region without a surrounding nuclear membrane. A typical plant or animal nucleus is enclosed by a nuclear envelope.
Final answer: A nucleoid is not membrane-bound; a nucleus is.
Question 20. Why does composting help soil fertility?
Step-by-step explanation: Decomposer microbes break down organic material into simpler compounds; nutrients are recycled into soil and organic matter improves soil properties.
Final answer: Decomposition returns nutrients and organic matter to soil.
Question 21. Why is a virus not described as a typical unicellular organism?
Step-by-step explanation: A virus lacks cellular organisation and cannot reproduce independently; it requires a host cell.
Final answer: Viruses are acellular and host-dependent.
Question 22. What happens when yeast is added to warm dough with sugar?
Step-by-step explanation: Yeast uses available sugars and produces carbon dioxide; gas bubbles are trapped by the dough structure, increasing volume. The effect depends on yeast viability, temperature and ingredients.
Final answer: Dough may rise and become airy.
Question 23. Why does warm milk with curd starter often set faster than refrigerated milk?
Step-by-step explanation: Lactic-acid bacteria generally grow and ferment lactose more quickly at suitable warm temperatures. Cooling slows metabolism and acid production.
Final answer: Faster lactic-acid fermentation in suitably warm conditions.
Question 24. How do microalgae support aquatic ecosystems?
Step-by-step explanation: Photosynthetic microalgae fix carbon dioxide, release oxygen and provide food at the base of many aquatic food webs.
Final answer: Primary production, oxygen generation and food supply.
Question 25. What is the difference between unicellular yeast and multicellular bread mould?
Step-by-step explanation: Yeast consists of individual cells, whereas common bread mould grows as networks of filaments (hyphae). Both are fungi.
Final answer: Yeast is unicellular; bread mould is multicellular.
Question 26. Why must students avoid touching or inhaling mould grown on bread?
Step-by-step explanation: Unknown mould can release spores or irritants and may pose health risks. Observations should be made without opening contaminated samples; an adult should dispose of them safely.
Final answer: Avoid exposure to unknown microbes and spores.
Quick revision, HOTS and MCQs
MCQ: Which cell structure is present in plants and animals but not bacteria? (A) membrane (B) nucleus (C) cytoplasm. Answer B. HOTS: Why can a cell wall not be used alone to identify a plant cell? Because fungi and most bacteria also have cell walls. Application: Explain why composting and curd formation are both examples of beneficial microbial processes but use different metabolic pathways. Exam tip: Never label the bacterial nucleoid as a membrane-bound nucleus.
Beyond the exercise: Discover, Design and Debate
Research India’s biogas initiatives from current official government sources; survey locally eaten fermented foods and identify their ingredients and microbes; study mushroom structure only with a teacher in a laboratory; interview a mushroom farmer about cultivation practices. These are open research tasks, so their answers depend on actual findings and should not be invented.
Official reference and accuracy notes
Read the official NCERT Curiosity Chapter 2 PDF for Figures 2.1–2.15. Some figures are illustrative and cannot be recreated exactly in text; descriptions and label guidance are supplied. The textbook simplifies some scientific details; for example, chloroplasts occur in photosynthetic plant cells, not all plant cells, and bacterial walls differ chemically from plant walls. Experimental outcomes may vary.
Frequently asked questions
What is the invisible living world?
Microscopic cells and organisms that are not normally visible with the naked eye.
Who named cells?
Robert Hooke used the term cell after observing cork in 1665.
What are the main parts of a typical plant or animal cell?
Cell membrane, cytoplasm and nucleus.
What is the difference between plant and animal cells?
Plant cells generally have a cell wall and often a large vacuole; animal cells lack a cell wall.
What is a nucleoid?
The DNA-containing region of a bacterial cell without a surrounding nuclear membrane.
What are microorganisms?
Organisms too small to see clearly with the unaided eye, such as many bacteria, protozoa, fungi and algae.
How does yeast make bread fluffy?
Yeast produces carbon dioxide during sugar metabolism, and gas bubbles make dough rise.
Why does milk turn into curd?
Lactic-acid bacteria ferment milk sugar into lactic acid, causing souring and setting.
How do microorganisms help the environment?
Decomposers recycle organic waste into nutrients and contribute to soil fertility.
What does Rhizobium do?
It fixes atmospheric nitrogen in nodules on certain legume roots.
What are microalgae?
Microscopic photosynthetic organisms important in aquatic food webs and oxygen production.
How many numbered NCERT exercise questions are solved?
All nine Keep the Curiosity Alive questions, including their subparts, are addressed.
