Beyond Our Naked Eye

1. Seeing the unseen — lens to microscope

The human eye can only see objects above a certain size, so for a long time tiny things stayed unknown. Long ago people found that a curved piece of glass, thick in the middle and thin at the edges (shaped like a lentil seed — that is where the word lens comes from), made small things look bigger.

Over time lenses became more powerful — from a simple magnifying glass to the microscope. In Activity 2.1 a round-bottom flask filled with water acts like a magnifying glass: letters under it look larger. The invention of the microscope finally opened up a fascinating hidden world of tiny living creatures.

• Robert Hooke (1665): in his book Micrographia he looked at a thin slice of cork under his microscope and saw many small empty boxes like a honeycomb. He called each box a cell — the first time the word "cell" was used in science.
• Antonie van Leeuwenhoek (1660s): a Dutch scientist who made better lenses and was the first to clearly see and describe tiny living things like bacteria and blood cells. He is called the Father of Microbiology.

2. What is a cell?

All living beings are made of cells — the basic unit of life. In Activity 2.2 a thin onion-peel layer is stained with safranin (red), mounted in glycerin (to stop drying) and seen under a microscope: you see neat rectangular boxes packed together like bricks in a wall. In Activity 2.3 the inside of the cheek is scraped, stained with methylene blue, and shows flat polygon-shaped cheek cells.

Three basic parts are common to all cells:

• Cell membrane — the outer covering. It encloses the cell, separates one cell from another, and is porous, letting useful materials in and waste out.
• Cytoplasm — the jelly between membrane and nucleus. It holds carbohydrates, proteins, fats, mineral salts and other parts; most life processes happen here.
• Nucleus — the round structure in the middle (covered by a thin membrane). It controls all activities of the cell and regulates growth.

Some cells (like onion-peel and other plant cells) have an extra outer layer, the cell wall, which gives the plant cell rigidity and strength — that is why plant cells look firm and packed.

3. Plant cell vs animal cell

Plant and animal cells share the three basic parts but differ in a few key ways. A few extra parts (from "A step further"):

• Plastids — tiny rod-shaped structures in plant cells. Green ones called chloroplasts contain chlorophyll and carry out photosynthesis (making food).
• Vacuole — a large empty-looking space in plant cells that stores substances, removes waste and keeps the cell's shape. In animal cells vacuoles are usually absent or very small.

4. Cells vary with their job

Cells are not all the same — their shape and structure relate to their function:

• Muscle cell — spindle-shaped; thin, flexible. In the food pipe, muscle cells contract and relax in waves to push food down to the stomach.
• Nerve cell (neuron) — very long with branches; the elongated branched shape helps it carry messages quickly to different parts of the body.
• Cheek cells — thin and flat; form a protective lining inside the mouth.

The same idea applies in plants — some plant cells are long tubes that carry water through the plant.

5. Levels of organisation

The body of a living organism is built up step by step, just as a wall is built from bricks:

• Tissue — a group of similar cells doing one job (e.g. muscle tissue).
• Organ — different tissues working together (e.g. the stomach).
• Organ system — several organs working together (e.g. the digestive system).
• Organism — all organ systems together make a complete living being.

Life of a complex organism begins with a single cell — the egg — which divides again and again to form a body of many cells. Such many-celled organisms are called multicellular organisms (plants, animals, humans).

6. What are microorganisms?

Some living organisms are made of just one or very few cells and are too small to see with the naked eye. These are microorganisms (micro = very small; organism = living being), also called microbes. They are found everywhere — in water, soil, air and even inside our body.

In Activity 2.4 (pond/stagnant water) and Activity 2.5 (soil suspension), a drop is observed under a microscope and tiny moving creatures appear — proof that microbes are all around us. A microscope magnifies them about 100 to 400 times; a foldable low-cost paper microscope also works for many of them.

The four main categories observed (Tables 2.1 & 2.2):

7. Viruses — small but different

Viruses are microscopic and acellular (not made of cells). They are different from other microbes because they multiply only inside a living host cell. They may infect plants, animals or even bacterial cells, and may cause disease. Outside a host they cannot reproduce.

8. Unicellular vs multicellular; the bacterial cell

The cell explains why a cell is the basic unit of life:

• Unicellular (single-celled): the whole organism is one cell that does every life job by itself — e.g. bacteria, protozoa, and yeast (a unicellular fungus).
• Multicellular (many-celled): the body has many cells that share specialised jobs and cooperate — e.g. plants, animals, and mould (a multicellular fungus).

Microbial cells too are surrounded by a cell membrane. Fungi also have a cell wall but no chloroplast, so they cannot make their own food. Importantly, bacteria do not have a well-defined nucleus with a nuclear membrane — instead they have a nucleoid. This is what sets bacterial cells apart from the cells of yeast, protozoa, algae, fungi, plants and animals. (An electron microscope, magnifying about 10,00,000 times, is needed to see finer cell parts.)

9. Microbes that clean the environment

Microbes, especially fungi and bacteria, are key players in keeping nature clean:

• Decomposition: microbes break down complex substances in dead plants, fallen leaves and dead animals into simpler, nutrient-rich substances. This is why decaying leaves disappear over time.
• Manure (Activity 2.7): fruit and vegetable peels buried in soil turn, after 2–3 weeks, into dark manure rich in nutrients that increases soil fertility. This is why gardeners pile dry leaves and plant waste in pits.
• Recycling nutrients: by decomposing waste, microbes return important nutrients to the soil, helping new plants grow. Decomposition needs optimal temperature and moisture.
• Biogas ("A step further"): some bacteria that live in oxygen-free conditions decompose plant/animal waste and release biogas — mostly methane and carbon dioxide — used as fuel for cooking, heating, electricity and vehicles.

10. Microbes that feed plants — nitrogen fixation

Plants need nitrogen, but they cannot use the nitrogen gas in air directly. Certain bacteria solve this:

• Rhizobium bacteria live in swollen root nodules of legumes (peas, beans, lentils, cowpea). They trap nitrogen from the air and make it usable for the plant.
• This lets legumes grow well without chemical fertiliser, and it enriches the soil with nitrogen for the next crop.
• That is why farmers grow legumes in crop rotation with other crops — a natural way to keep soil fertile.

11. Microalgae — tiny helpers in water

Microalgae are microscopic plant-like organisms living in water, soil, air and on trees. They make their own food using sunlight and, while doing so, release more than half of Earth's oxygen. They are nutrient-rich and serve as food for aquatic animals.

• Spirulina, Chlorella and Diatoms are used by humans as health supplements and medicines.
• Spirulina is a "superfood" — rich in protein (over 60% of its weight) and vitamin B₁₂; it can be farmed in a tank of pond water for livelihood and food security.
• Microalgae also help clean water and are used to make biofuel.
• Pollution, climate change and habitat loss threaten them, so it is important to conserve them to maintain Earth's oxygen balance.

12. Microbes in our food

Yeast (Activity 2.8): Yeast is a fungus that grows well in warm conditions. When dough has yeast, sugar and warm water, the yeast respires and breaks down sugar, releasing carbon dioxide. The CO₂ forms bubbles that make the dough rise, soft and fluffy; a little alcohol is also made, giving the special smell. This is how breads, cakes and pastries are made.

Lactobacillus (Activity 2.9): Curd contains the bacterium Lactobacillus. It feeds on the milk sugar lactose, multiplies, and ferments milk into curd, producing lactic acid (not alcohol) which makes curd sour. It works in warm conditions — that is why warm milk (bowl A) sets into curd but cold milk in the fridge (bowl B) does not.

Some bacteria such as Lactobacillus also help ferment the batter for idli and dosa and the dough for bhatura.

13. Helpful vs harmful microbes — summary

(Diseases caused by microbes are studied in the next chapter — here we focus mainly on the beneficial ones.)

14. Keep the curiosity alive — solved

Q1. Place the cell parts in the diagram.

• Common to all three cells (animal, plant, bacterial): Cytoplasm, Cell membrane.
• Only in animal cell: none of the special ones listed (animal cell has no wall/chloroplast/nucleoid).
• Only in plant cell: Chloroplast (and cell wall, which plants share with bacteria).
• Only in bacterial cell: Nucleoid. (Plant + bacterial share Cell wall; plant + animal share Nucleus.)

Q2. Yeast + sugar with a balloon (test tube B). (i) The balloon on B inflated — correct answer (c): yeast produced a gas (CO₂) inside the tube. (ii) She attaches the gas-filled balloon to lime water and shakes it to test that the gas is carbon dioxide — lime water turns milky with CO₂.

Q3. Wheat farmer adds nitrogen fertiliser, bean farmer does not. Bean (legume) roots have Rhizobium nodules that fix nitrogen from the air naturally, so she does not need fertiliser; wheat has no such bacteria, so the wheat farmer must add nitrogen fertiliser.

Q4. Pit A (peels + dried leaves) vs pit B (peels only). Snehal is testing how the mix affects decomposition / manure formation — comparing whether adding dried leaves helps microbes break down the waste faster into better manure.

Q5. Identify the microorganisms. (i) lives in every environment and in your gut → Bacteria; (ii) makes bread and cakes soft & fluffy → Yeast (fungus); (iii) lives in roots of pulses and gives them nutrients → Rhizobium (bacteria).

Q7. Bread near the sink vs in the fridge. Bread near the warm sink grows mould faster; the refrigerated slice stays fresh longer because cold slows microbial growth.

Q8. Curd left out becomes more sour. The warm air lets Lactobacillus keep multiplying and making more lactic acid; the longer fermentation produces more acid, so the curd turns more sour.

Q9. Warm sugar+yeast (A) connected to lime water (B). (i) The sugar solution in A ferments — yeast releases CO₂ and a little alcohol; (ii) the lime water in B turns milky, because the CO₂ gas reaches it — showing CO₂ is produced; (iii) with no yeast, no gas is made, so the lime water stays clear and nothing happens.

15. Common mistakes to avoid

• Saying animal cells have a cell wall — only plant, fungal and bacterial cells do.
• Calling a virus a "microorganism just like bacteria" — viruses are acellular and multiply only inside a host.
• Saying yeast makes curd — yeast raises bread (CO₂ + alcohol); Lactobacillus makes curd (lactic acid).
• Forgetting that bacteria have a nucleoid, not a true (well-defined) nucleus.
• Thinking Rhizobium lives in all plants — it lives in root nodules of legumes only.
• Mixing up the order of organisation — it is Cell → Tissue → Organ → Organ system → Organism.

16. Quick revision checklist

• Lens (lentil-shaped glass) → magnifying glass → microscope; Hooke named the "cell", Leeuwenhoek = Father of Microbiology.
• Cell parts: membrane (porous covering), cytoplasm (life processes), nucleus (control). Extra in plants: cell wall, chloroplast, big vacuole.
• Microbes = bacteria, protozoa, fungi, algae; viruses are acellular.
• Helpful: decomposition, manure, biogas, Rhizobium (nitrogen), microalgae (oxygen), yeast (bread), Lactobacillus (curd).
• Preservation: salt/sugar/cold stop microbial growth.
• Unicellular (bacteria, protozoa, yeast) vs multicellular (plants, animals, mould).