The World of the Living (VI–VIII) • Topic 3 of 5

Cell & Microorganisms

The cell is the favourite single fact of this whole chapter: the cell is the basic structural and functional unit of all living things — first observed by Robert Hooke (in cork) and named 'cell'. Organisms made of one cell are unicellular (Amoeba, Paramecium, bacteria); those made of many are multicellular. Know the parts: the cell membrane encloses the cell and controls what enters and leaves; the cytoplasm is the jelly-like fluid holding the organelles; the nucleus is the control centre carrying the genetic material. The exam loves the plant-vs-animal-cell difference: a plant cell has a rigid cell wall (outside the membrane), chloroplasts (with chlorophyll for photosynthesis) and a large central vacuole — an animal cell has none of these, so it has no cell wall. Microorganisms are living things too small to see with the naked eye, grouped into bacteria, fungi, protozoa and algae, with viruses as a borderline group. They are not all harmful: useful microbes give us curd (Lactobacillus bacteria), bread and idli/dosa batter (yeast, a fungus, by fermentation), antibiotics, and they decompose dead matter and fix nitrogen in the soil. Harmful ones cause disease (cholera, tuberculosis, malaria) and spoil food. The misconception to correct: children assume 'all germs/microbes are bad'. The right framing is friendly vs harmful microbes. How it is tested: 'basic unit of life', a plant-vs-animal cell distinguishing feature, or matching a microbe to its use (yeast -> bread; Lactobacillus -> curd).

✅ Solved examples

1. Which structure is found in a plant cell but NOT in an animal cell, giving the plant cell a fixed shape?
The cell wall. A plant cell has a rigid cell wall outside the cell membrane (plus chloroplasts and a large vacuole); an animal cell has none of these.
2. 'The cell is the basic structural and functional unit of life' — who first observed and named the cell?
Robert Hooke, who observed cells in a thin slice of cork under his microscope and coined the term 'cell'.
3. A teacher wants children to make curd at home. Which microorganism is responsible for setting milk into curd?
Lactobacillus, a bacterium. It ferments the milk sugar and converts milk into curd. (Yeast, by contrast, is used to make bread rise.)
4. Which part of the cell acts as the control centre and contains the genetic material?
The nucleus. It directs all the activities of the cell; the cytoplasm holds the other organelles and the cell membrane controls entry and exit of substances.

✏️ Practice — try these, take hints as needed

1. Amoeba and Paramecium are made of a single cell. What term describes such organisms?
One cell only.
Opposite of multicellular.
Unicellular organisms
2. Which microorganism (a fungus) is used to make bread and idli batter rise by fermentation?
It produces carbon dioxide.
Not a bacterium.
Yeast
3. Name the structure that surrounds the cell and controls the movement of substances in and out of it.
Present in both plant and animal cells.
Just inside the cell wall in plants.
Cell membrane (plasma membrane)
4. Give one example of a disease in humans caused by a microorganism.
Caused by bacteria, virus or protozoa.
e.g. spread by mosquito, or contaminated water.
Any one of: malaria, cholera, tuberculosis (or similar)

📝 Topic test — 8 questions

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