Chapter 2 — The Invisible Living World: Beyond Our Naked Eye

Chapter 2 — 'The Invisible Living World: Beyond Our Naked Eye' — covers the cell as the basic unit of life, the structure and parts of cells, levels of biological organisation (cell to organism), and microorganisms (bacteria, fungi, protozoa, algae, viruses) including their beneficial roles in food, soil, and the environment.

Robert Hooke first used the word 'cell' in 1665. While observing a thin slice of cork under his microscope (which magnified 200–300 times), he saw small, empty compartments that reminded him of a honeycomb and called each space a cell. He published his findings in a book called Micrographia.

Antonie van Leeuwenhoek, a Dutch scientist working in the 1660s, is known as the Father of Microbiology. He made better lenses to build more powerful microscopes and was the first person to clearly see and describe tiny living things like bacteria and blood cells.

The three main parts of a cell are: (1) the cell membrane — the outer porous layer that separates one cell from another and controls entry of materials needed for life and exit of waste; (2) cytoplasm — the space between the membrane and nucleus that contains carbohydrates, proteins, fats, and mineral salts, and is where most life processes take place; and (3) the nucleus — the round structure in the middle that regulates all cell activities and growth.

Plant cells have an additional cell wall (outside the cell membrane) that provides rigidity and strength, plastids including chloroplasts (which contain chlorophyll for photosynthesis), and a large vacuole for storing substances and maintaining shape. Animal cells generally do not have a cell wall, chloroplasts, or a large vacuole; if vacuoles are present in animal cells, they are usually small.

Unicellular organisms are made of just one cell that carries out all life functions — examples include bacteria and Amoeba. Multicellular organisms are made of many cells that carry out specialised functions and cooperate with each other — examples include plants, animals, and humans. Some microbes can be either: yeast is a unicellular fungus while mould is a multicellular fungus.

The levels of organisation, from simplest to most complex, are: Cell → Tissue (a group of similar cells) → Organ (different tissues organised together) → Organ System (several organs working together to perform a major function) → Organism (all organ systems together making a complete living being).

Rhizobium bacteria live in swollen regions called nodules found in the roots of certain legumes such as beans, peas, and lentils. These bacteria trap nitrogen from the air and make it useful for the plants, helping them grow better without chemical fertilisers. This is why farmers grow legumes in rotation with other crops — it naturally increases nitrogen in the soil.

Yeast is a type of fungus. It respires by breaking down sugar (food) to release energy. During this process it releases carbon dioxide gas, which forms bubbles in the dough and makes it soft and fluffy. Yeast also produces a small amount of alcohol during this process, which gives the dough a slightly different smell. This property is used in making breads, cakes, and similar foods.

Curd is formed by bacteria, mainly Lactobacillus. This bacterium feeds on the sugar in milk (lactose), multiplies, and ferments the milk. Instead of producing alcohol (like yeast), Lactobacillus produces lactic acid, which makes the curd sour. These bacteria grow well in warm conditions, which is why curd forms faster when milk is kept in a warm place.

Microorganisms such as fungi and bacteria break down complex substances in plant waste, fallen leaves, and dead animal bodies into simpler, nutrient-rich substances — a process called decomposition. These nutrients return to the soil and help plants grow better. Certain bacteria can also decompose animal wastes like dung, and some bacteria can even break down oil spills (as shown by Ananda Mohan Chakrabarty's work in 1971). Some bacteria also decompose waste in oxygen-free environments and release biogas (primarily methane and carbon dioxide) that can be used as fuel.

Microalgae are microscopic plant-like organisms that live in water, soil, air, and even on trees. They make their own food using sunlight and, in doing so, release oxygen — producing more than half of the Earth's oxygen supply. They are rich in nutrients and serve as food for many aquatic animals. Some, like Spirulina, Chlorella, and Diatoms, are used as health supplements and medicines. Microalgae are also used to clean water and to make biofuel.

Bacteria do not have a well-defined nucleus or a nuclear membrane. Instead they have a region called the nucleoid that contains their genetic material. This distinguishes bacterial cells from the cells of yeast, protozoa, algae, fungi, plants, and animals, which all have a well-defined nucleus.

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