Class 8 Science · Chapter 2 NotesThe Invisible Living World: Beyond Our Naked Eye

The human eye can see only objects above a certain size. Long ago people found that a curved piece of glass, shaped like a lentil seed — thick in the middle and thin at the edges — made small things look bigger. This was called a lens. Better lenses led to magnifying glasses and then to microscopes. In 1665 Robert Hooke published Micrographia, a book of drawings of tiny things seen through his microscope, which made objects look 200 to 300 times bigger. Looking at a thin slice of cork, he saw small empty compartments like a honeycomb and called each one a cell — the first use of the word in science. Around the 1660s the Dutch scientist Antonie van Leeuwenhoek made better lenses and was the first to clearly see and describe tiny living things such as bacteria and blood cells. He is known as the Father of Microbiology.

• Lenscurved glass, thick in the middle and thin at the edge
• Robert HookeMicrographia (1665); first used the word cell
• Antonie van Leeuwenhoekfirst to see bacteria and blood cells; Father of Microbiology

All living beings are made of cells. When an onion peel is stained with safranin and viewed under a microscope, closely packed rectangular cells are seen. Cheek cells scraped from the inner lining of the mouth and stained with methylene blue appear polygon-shaped. A typical cell has three basic parts. The cell membrane is the outer covering that encloses the cell, separates it from other cells, and is porous so materials needed for life can enter and waste can leave. The cytoplasm fills the space between the membrane and the nucleus and contains carbohydrates, proteins, fats and mineral salts; most life processes happen here. The nucleus, a round structure covered by a thin membrane, regulates all activities inside the cell and also regulates growth. Plant cells have an extra outer layer, the cell wall, which gives rigidity and strength.

• Cell membraneencloses the cell, porous, controls entry and exit of materials
• Cytoplasmjelly-like filling where most life processes occur
• Nucleuscontrols cell activities and growth
• Cell wallextra covering in plant cells, gives rigidity and strength

Plant cells contain tiny rod-shaped structures called plastids. Some plastids, the chloroplasts, contain the green pigment chlorophyll and help in photosynthesis; in non-green parts they help store substances. Plant cells also have a large vacuole that stores substances, removes waste and helps maintain the cell's shape. Animal cells usually have no vacuole, or only small ones. Cells also vary in shape according to the work they do. Muscle cells are spindle-shaped, thin and flexible, so they can contract and relax in a wave-like manner to push food down the food pipe. Nerve cells, or neurons, are long and branched so they can carry messages quickly to different parts of the body. Cheek cells are thin and flat and form a protective lining. In plants, cells may be rectangular, elongated, oval or tube-like; some form long tubes that carry water.

• Chloroplastgreen plastid that helps in photosynthesis
• Vacuolelarge in plant cells, small or absent in animal cells
• Muscle cellspindle-shaped, contracts and relaxes
• Nerve cell (neuron)long and branched, carries messages

The body of a living organism is organised in levels, just as bricks make a wall. The cell is the basic unit of life. A group of similar cells forms a tissue. Different tissues are organised into an organ. Several organs working together form an organ system, which performs a major function of the body. All organ systems together make up a complete organism, such as a plant or an animal. The order is: cell → tissue → organ → organ system → organism. The life of a complex organism begins with a single cell, the egg, which divides repeatedly to form a many-celled being. Such organisms are called multicellular organisms; animals, including humans, and plants are examples. The yolk of an ostrich egg is a single cell, the largest known cell, about 130 mm to 170 mm in diameter.

• Ordercell → tissue → organ → organ system → organism
• Multicellularmade of many cells; the egg divides repeatedly to form the organism
• Ostrich egg yolklargest known single cell, about 130–170 mm across

Organisms made of just one or a few cells, too small to be seen with the naked eye, are called microorganisms or microbes (micro means very small, organisms means living beings). Some, like bacteria and Amoeba, are unicellular; others, like some fungi and algae, are multicellular. Microbes live everywhere — in water, soil, air, inside our bodies, in hot water springs and snow-cold zones. A drop of pond water or soil suspension under a microscope shows many of them: Amoeba with an irregular shape; Paramecium, which moves using specialised structures; green algae with chlorophyll; fungi such as bread mould, which have branched filaments without chlorophyll; and bacteria, which may be spherical, comma-shaped, spiral or rod-shaped. Viruses are microscopic and acellular; they multiply only when they enter a living cell and may infect plants, animals or bacteria and cause disease.

• Unicellularbacteria, Amoeba, Paramecium, yeast
• Multicellularmoulds, some algae
• Fungibranched filaments without chlorophyll
• Virusesacellular, multiply only inside a living host cell

Microbes are key players in cleaning the environment. When fruit and vegetable peels are buried in soil, fungi and bacteria act on the plant waste and slowly break it into simpler, nutrient-rich manure. This process of breaking complex substances into simpler ones is called decomposition. Fallen leaves and dead animals are decomposed in the same way, and the nutrients return to the soil and help plants grow. Manure formation occurs at optimal temperature and appropriate moisture. Some bacteria and fungi live in an oxygen-free environment and decompose plant and animal waste or household wastewater, releasing biogas — mainly carbon dioxide and a high proportion of methane — used for cooking, heating, generating electricity and running vehicles. The scientist Ananda Mohan Chakrabarty developed a bacterium that could break down oil spills; his discovery received a patent in 1980.

• Decompositionmicrobes break complex waste into simple nutrient-rich substances
• Manuredark nutrient-rich material formed from plant waste
• Biogasmainly carbon dioxide and methane, released by microbes in oxygen-free conditions
• Patenta right that stops others from copying, using or selling an invention without permission

Yeast, a fungus, is used in baking. When flour with a little sugar and warm water is kneaded with yeast and kept warm, the dough rises and becomes soft and fluffy. Yeast respires, breaking down food for energy and releasing carbon dioxide, whose bubbles make the dough fluffy; a small amount of alcohol gives it a different smell. Some bacteria, such as Lactobacillus, ferment batter for idli and dosa and dough for bhatura. In curd formation, Lactobacillus feeds on lactose in milk, multiplies and produces lactic acid, which makes curd sour; it grows well in warm conditions, so warm milk sets into curd while cold milk does not. Rhizobium bacteria live in root nodules of legumes such as beans, peas and lentils, trap nitrogen from the air and make it useful for plants, so farmers grow legumes in rotation with other crops.

• Yeastreleases carbon dioxide, makes bread and cakes soft and fluffy
• Lactobacillusturns milk into curd by producing lactic acid
• Rhizobiumlives in root nodules of legumes and fixes nitrogen from air

Microalgae are microscopic plant-like organisms that live in water, soil, air and even on trees. They make their own food using sunlight and release oxygen, producing more than half of the Earth's oxygen supply. They are rich in nutrients and are food for many aquatic animals; Spirulina, Chlorella and Diatoms are used by humans as health supplements and medicines, and microalgae help clean water and make biofuel. Spirulina is called a superfood: it is rich in protein (more than 60 per cent of its body weight), has little fat and sugar, and is a good source of vitamin B12. The cell is the basic unit of life because every living body is made of cells. Unicellular organisms carry out all life functions in one cell; multicellular organisms have cells that specialise and cooperate. Cells of fungi have a cell wall but no chloroplasts; bacteria have no well-defined nucleus, only a nucleoid.

• Microalgaerelease more than half of Earth's oxygen supply
• Spirulinasuperfood, rich in protein and vitamin B12
• Bacteriano well-defined nucleus; have a nucleoid instead
• Fungihave a cell wall but no chloroplasts, so cannot photosynthesise