The Invisible Living World Class 8 PPT & Notes Free

Bacteria, mould, amoeba and algae live in pond water, soil, air and even in our bodies, but they are too small for the naked eye. This chapter shows how lenses and microscopes opened up this invisible living world, what cells are, and how microorganisms harm us and help us.

A lens is thick in the middle and has thin edges, and it makes things look bigger. In Activity 2.1 a round-bottom flask is filled with water and corked, and then placed over a page of a book. The letters look larger, because the flask of water acts like a magnifying glass.

A magnifying glass shows the details of a tiny object, such as the body of an ant, clearly.

• In his book Micrographia (1665), Robert Hooke described what he saw in a thin slice of cork under an old microscope
• The cork looked like a honeycomb of small boxes
• He named each box a cell

Antonie van Leeuwenhoek used better lenses and was the first to see bacteria and blood cells. He is called the Father of Microbiology.

All living beings (a child, a dog, a plant, a tree or a fish) are made up of cells, the basic unit of life.

1. Peel a thin onion layer with forceps
2. Stain it with safranin (a red drop)
3. Rinse it in water
4. Place it on a glass slide with a glycerin drop
5. Lower a coverslip at 45° with a needle
6. Observe it under the microscope

Under the microscope the onion cells fit together like the bricks of a wall. You can see the cell wall, the cell membrane, the cytoplasm and the nucleus.

1. Rinse your mouth
2. Gently scrape the inside of your cheek
3. Spread the scraping on a slide
4. Add methylene blue
5. Add glycerin
6. Place a coverslip
7. Observe it under the microscope

Human cheek cells are thin, flat cells that line the inside of the mouth.

Animal cell versus plant cell: both have a cell membrane, cytoplasm, a nucleus and mitochondria. Plant cells also have a cell wall, chloroplasts and a big vacuole.

• A muscle cell is spindle shaped
• A nerve cell (neuron) is long and branched and carries messages
• Shape is related to function

In digestion, muscle cells push the food along in a wave motion, and the lining cells of the stomach make digestive juice and acid.

The yolk of an ostrich egg is one single cell, about 130 to 170 mm wide, and the shell protects it.

Cell → Tissue → Organ → Organ system → Organism. For example: a digestive cell → stomach tissue → the stomach (an organ) → the digestive system → a full human being.

• Unicellular organisms are made of one cell: amoeba, and bacteria (often found as single cells)
• Multicellular organisms are made of many cells: plants, animals and humans

• Microorganisms are found in water, soil, air and our body
• They are too small for the naked eye and are seen only with a microscope

Place a drop of pond water on a slide and look at it under a microscope: you can see tiny moving creatures. For soil, make a soil suspension in water and place a drop of it on a slide to observe it in the same way.

The four types are bacteria, fungi, protozoa and algae. Some are one cell, and some are many cells.

• Viruses are acellular, that is, not made of cells
• They multiply only inside a living cell
• They can cause disease in plants, animals and bacteria

A bacterial cell has a cell wall and cytoplasm, but a nucleoid instead of a true nucleus.

Microbes are found in air, water and soil. When they land on food, they grow, for example as fuzzy or powdery mould, and the food rots.

Pickles contain high salt, and murabba contains high sugar. Both act as preservatives, so microbes cannot grow.

• Activity 2.7: peels and soil kept together for 2 to 3 weeks become dark, rich manure, because microbes break the waste down
• Microbes cause decomposition of fallen leaves, so nutrients return to the soil and plants grow better
• In a biogas tank, waste is kept without oxygen and bacteria release biogas (methane and carbon dioxide), which is used as fuel for cooking and power

Ananda Mohan Chakrabarty made a bacterium to clean oil spills (1971) and got a patent in 1980.

The Vedas mention krimi, and there is a reference in the Atharvaveda. Drishya means visible and adrishya means invisible.

Bowl A, with yeast, becomes fluffy and risen. Bowl B, without yeast, stays flat. Yeast is a fungus: it eats sugar and makes carbon dioxide (CO₂) bubbles. The bubbles make the dough soft and fluffy, and yeast is used for breads and cakes.

• Lactobacillus bacteria ferment batter and dough to make idli, dosa and bhatura
• In Activity 2.9, Bowl A kept in a warm place forms curd, and Bowl B kept cold in the fridge does not. Warmth helps the bacteria work
• Lactobacillus eats lactose (milk sugar) and makes lactic acid, so milk turns into sour curd

• Rhizobium bacteria live in the root nodules of legumes and trap nitrogen from the air
• Plants grow without chemical fertiliser
• Farmers grow legumes in crop rotation: legumes add nitrogen to the soil and keep it healthy for the next crop

• Microalgae release oxygen (over half of Earth’s oxygen), make food from sunlight, and are food for aquatic animals
• They are used for biofuel and for cleaning water
• Spirulina is rich in protein (over 60%), is used as a health supplement and is grown in tanks

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