CLASS 8 SCIENCE · CHAPTER 2 · STUDY NOTES
The Invisible Living World: Beyond Our Naked Eye
Understand the invisible living world: beyond our naked eye through short explanations, labelled models and original practice. Read one topic, explain it in your own words, then check.
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A microscope reveals what our eyes miss
Explain why a tiny cell can be present even when you cannot see it.Small does not mean absent
- Microscope
- An instrument that makes tiny objects appear larger so that we can study them.
A magnifying lens enlarges a small object. A microscope lets us see much smaller details, including many cells and microbes.
In 1665, Robert Hooke saw the empty compartments in cork and called them cells. Leeuwenhoek later described tiny living organisms using improved lenses.
Take a moment. What do you think?
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Question
A drop of clear pond water looks empty. Does that prove it has no living things?
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Find the parts of a cell
Point to the membrane, cytoplasm and nucleus, then explain what each does.Two boundaries or one?
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- Cell
- The smallest living unit that can carry out life processes.
- Cell membrane
- The boundary that controls which materials enter and leave a cell.
- Cytoplasm
- The material inside the membrane where many life processes happen.
- Nucleus
- A membrane-covered structure that holds genetic information and helps control cell activities.
Plant cells also have a firm cell wall outside the membrane. It supports and protects the cell. Animal cells have a membrane but no cell wall.
A teacher can prepare a stained onion peel or cheek-cell slide. Stain adds contrast; it does not create a new cell part.
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Question
A cell has a firm outer wall, a membrane and a nucleus. Is this enough to call it an animal cell? Explain.
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A cell’s shape helps it do its job
Connect a cell’s visible shape with its function.Picture from the NCERT book
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- Specialised cell
- A cell whose structure helps it perform a particular job.
- Chloroplast
- A structure containing chlorophyll that helps a green plant cell make food using light.
- Vacuole
- A storage space in a cell; the large plant vacuole also helps support its shape.
A nerve cell has branches for connections and a long extension for carrying messages. Muscle cells contract to make movement possible.
Green plant cells may have chloroplasts. An underground onion-bulb cell does not need chloroplasts to capture sunlight. Do not assume every plant cell is green.
Let’s look a little closer.
Go deeper: How different cells cooperate
Muscle cells in the food pipe contract and relax to push food along. Other cells in the stomach make digestive juices. Movement and digestion therefore need different cells working together.
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Question
Why would a long, branched shape be useful for a nerve cell?
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Many cells work together
Follow the difference between a cell, tissue, organ, organ system and organism.Build up the levels of life
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- Tissue
- A group of cells that work together for a shared function.
- Organ
- A body part made of different tissues working together.
- Organ system
- A group of organs that work together for a major function.
- Unicellular organism
- An organism made of one cell, which carries out its life processes.
Similar working cells form a tissue. Different tissues make an organ such as the stomach. Several organs make a system such as the digestive system. The systems work together in a whole organism.
A bacterium or Amoeba can live as one cell. A human has many specialised cells. A larger body does not mean that each individual cell is larger.
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Question
Is the stomach a tissue or an organ? Give the reason.
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Microbes are a varied living world
Distinguish bacteria, protozoa, fungi and microscopic algae.Look for differences, not just size
- Microorganism
- A living organism too small to study clearly with our unaided eyes.
- Bacterium
- A single-celled organism without a membrane-covered nucleus.
- Virus
- A tiny acellular entity that can multiply only inside a living host cell.
Bacteria may be round, rod-shaped or spiral. Protozoa such as Amoeba are single-celled. Yeast is a single-celled fungus, while mould grows as many branching threads. Microscopic algae contain pigments for photosynthesis.
Microbes live in soil, water and many other places, including our bodies. Some cause harm; many do useful jobs. Viruses are acellular: do not draw them as ordinary cells.
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Question
A microscopic organism has no membrane-covered nucleus. Which group in these notes fits that clue?
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Microbes return nutrients to the soil
Explain how decomposition links dead material to new growth.A nutrient loop
- Decomposition
- The breakdown of dead organic material into simpler substances.
- Decomposer
- An organism, such as many bacteria or fungi, that carries out decomposition.
Decomposers break down dead leaves and food scraps. Nutrients return to the surroundings and can support new plant growth. Suitable moisture and temperature help the microbes work.
In oxygen-free conditions, some microbes break down waste to produce biogas, which contains methane and carbon dioxide. This is a managed process, not a gas experiment to try at home.
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Why do fallen leaves help a forest rather than pile up forever?
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Why yeast makes dough rise
Explain the visible link between yeast, gas bubbles and rising dough.Follow the gas into the dough
- Fermentation
- A process in which microbes break down substances such as sugars, producing new substances.
Yeast uses sugar and releases carbon dioxide during fermentation. The gas becomes trapped in stretchy dough, so the dough expands. Warmth supports this process, but very high temperature can kill the yeast.
Some food batters are fermented by a community of microbes. Fermentation can change texture and flavour. Food preparation still needs clean equipment and suitable ingredients.
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Question
Two clean dough samples have the same ingredients. Only one has yeast. Why might that one become bigger?
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Milk changes into curd
Explain why adding a little curd can help warm milk set.What changes during curd making
- Lactic acid
- An acid produced when certain bacteria use milk sugar; it helps milk proteins form curd.
A little curd supplies bacteria, including Lactobacillus. In suitable warm conditions they use lactose, the sugar in milk, and produce lactic acid. This helps the milk proteins set.
Cold conditions usually slow the bacteria. Boiling-hot milk can harm them. Warm does not mean as hot as possible. Use normal safe food-preparation methods with an adult.
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Question
Why is adding a little curd useful, even though it is much less than the milk?
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Plants have microbial partners
Explain what Rhizobium supplies and why legumes are useful in crop rotation.Look below a bean plant
- Nitrogen fixation
- Changing atmospheric nitrogen into forms that living organisms can use.
- Root nodule
- A small swelling on a legume root where helpful nitrogen-fixing bacteria may live.
Rhizobium lives in nodules on the roots of legumes such as peas and beans. It makes nitrogen available in usable forms; the plant supplies food to the bacteria.
Growing legumes in a crop rotation can support soil fertility. Microalgae also make food by photosynthesis and release oxygen. They are important aquatic producers; not every green water organism is safe to eat.
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Question
A bean plant and Rhizobium live together. Explain one benefit for each.
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