CBSE Class 8 Science Revision Notes Chapter 2 The Invisible Living World: Beyond Our Naked Eye

The invisible living world includes cells and microorganisms that cannot be studied clearly with the unaided human eye. Microscopes help students observe their structure, diversity and importance in life and the environment.

The chapter The Invisible Living World: Beyond Our Naked Eye introduces tiny living structures that remain hidden from ordinary eyesight. It explains how lenses and microscopes helped scientists discover cells and microorganisms.

These CBSE Class 8 Science Revision Notes Chapter 2 cover cell structure, plant and animal cells, levels of organisation and useful microbes. The notes follow the current 2026–27 chapter and support quick, topic-wise revision.

Key Takeaways

• Cell: It is the basic unit of life and forms the body of every living organism.
• Basic cell parts: Most cells contain a cell membrane, cytoplasm and nucleus.
• Levels of organisation: Cell → Tissue → Organ → Organ system → Organism.
• Useful microbes: Microorganisms support decomposition, fermentation, soil fertility, oxygen production and biogas formation.

Access Class 8 Science Chapter 2 The Invisible Living World Notes in 30 Minutes

Revise the chapter in three parts:

First 10 minutes: Microscope, discovery of cells and basic cell structure
Next 10 minutes: Plant and animal cells, cell shapes and levels of organisation
Final 10 minutes: Microorganisms, decomposition, fermentation, Rhizobium and microalgae

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The Invisible Living World in Class 8 Science Chapter 2 Notes

The human eye cannot see objects below a certain size. Lenses and microscopes allow us to observe structures and organisms hidden from normal vision.

How Does a Lens Make Objects Look Bigger?

A lens is a curved piece of glass that bends light and makes an object appear larger. Early lenses were thick in the middle and thin at the edges.

A water-filled round-bottom flask can also make printed letters appear larger. It acts like a simple magnifying glass.

Why Is a Microscope Important?

A microscope magnifies objects that are too small to be seen with the naked eye. It opened a hidden world of cells, bacteria, protozoa, fungi and algae.

Common microscopes may magnify microorganisms about 100 to 400 times. Electron microscopes can provide much higher magnification for studying smaller cell structures.

Robert Hooke and the Discovery of Cells

In 1665, Robert Hooke published a book called Micrographia. He observed a thin slice of cork through a microscope.

Hooke saw many small, empty compartments arranged like a honeycomb. He called each compartment a cell.

His microscope could make objects appear about 200 to 300 times larger.

Antonie van Leeuwenhoek and Microorganisms

Antonie van Leeuwenhoek developed improved lenses during the 1660s. He was the first person to clearly observe and describe tiny living organisms such as bacteria.

He also observed blood cells. His work earned him the title Father of Microbiology.

Cell Structure in CBSE Class 8 Science Chapter 2 Notes

All living organisms are made up of cells. A cell contains different parts that work together to perform life processes.

What Is a Cell?

A cell is the basic structural unit of a living organism. It performs activities needed for growth, survival and proper functioning.

Some organisms consist of one cell. Plants, animals and humans are made up of many cells.

Main Parts of a Cell

Most cells have three main parts:

Cell Membrane

The cell membrane encloses the cytoplasm and nucleus. It also separates one cell from another.

It is porous and allows useful materials to enter. It also permits waste materials to leave the cell.

Cytoplasm

Cytoplasm fills the space between the cell membrane and nucleus. It contains carbohydrates, proteins, fats, mineral salts and other cell components.

Most life processes take place within the cytoplasm.

Nucleus

The nucleus regulates activities occurring inside the cell. It also controls the growth of the cell.

A thin membrane surrounds the nucleus.

Cell Wall

Plant cells have an additional outer covering called the cell wall. It surrounds the cell membrane.

The cell wall gives strength, support and rigidity to the plant cell. It helps plant cells remain firm and closely arranged.

Chloroplasts and Other Plastids

Plant cells contain small structures called plastids. Chloroplasts are green plastids containing chlorophyll.

Chlorophyll helps plants perform photosynthesis. Other plastids may store different substances in non-green plant parts.

Vacuoles

A plant cell usually has a large vacuole. It stores useful substances, collects wastes and helps maintain the cell’s shape.

Animal cells may have small vacuoles or may lack them. When present, they store substances dissolved in water.

Plant and Animal Cells in Class 8 Science Revision Notes Chapter 2

Plant and animal cells share some basic parts, but their shape and structure are not exactly the same. Onion peel and cheek-cell activities show these differences clearly.

Onion Peel Cell Observation

A thin onion peel is stained with safranin and placed on a glass slide. Glycerin prevents it from drying, while a coverslip protects the sample.

Under the microscope, onion peel cells appear nearly rectangular. They are arranged closely with almost no spaces between them.

Visible parts include:

• Cell wall
• Cell membrane
• Cytoplasm
• Nucleus

Human Cheek Cell Observation

Cheek cells can be collected by gently scraping the inside of the mouth. Methylene blue stain makes their structure more visible.

Under the microscope, cheek cells appear thin, flat and polygon-shaped. They form the protective inner lining of the mouth.

Visible parts include:

• Cell membrane
• Cytoplasm
• Nucleus

Cheek cells do not have a cell wall.

Difference Between Plant and Animal Cells Class 8

Cell Shape, Function and Levels of Organisation

Cells differ in shape, size and structure. Their form helps them perform specialised functions efficiently.

How Is Cell Shape Related to Function?

The shape of a cell often matches the work it performs.

Muscle cells in the food pipe contract and relax in waves. This movement pushes food towards the stomach.

Nerve cells are long and branched, which helps them reach different parts of the body and transmit messages quickly.

Levels of Organisation in Living Organisms

The body of a multicellular organism has different levels of organisation.

Cell → Tissue → Organ → Organ system → Organism

• A cell is the basic unit of life.
• A group of similar cells forms a tissue.
• Different tissues combine to form an organ.
• Several organs work together as an organ system.
• All organ systems together form an organism.

For example, muscle cells form muscle tissue. Different tissues form the stomach, which is an organ of the digestive system.

Unicellular and Multicellular Organisms

A unicellular organism is made up of one cell. That single cell performs all the functions required for survival.

Examples include:

• Bacteria
• Amoeba
• Paramecium
• Yeast

A multicellular organism is made up of many cells. These cells carry out specialised functions and work together.

Examples include:

• Plants
• Animals
• Humans
• Moulds and some algae

The life of a multicellular organism begins with a single cell called an egg. The cell divides repeatedly and develops into a complete organism.

Microorganisms in The Invisible Living World Notes

Microorganisms are living organisms too small to be seen with the unaided eye. A microscope is needed to observe them clearly.

They are found in air, water, soil, food and inside living bodies. Some can also survive in hot springs and very cold regions.

Microorganisms in Pond Water

A drop of pond or stagnant water may contain several tiny organisms.

These organisms may move through water and show different shapes and structures.

Microorganisms in Soil Suspension

Soil mixed with water forms a soil suspension. A drop taken from the upper layer may show microorganisms under a microscope.

The presence of these organisms shows that soil contains many living forms invisible to the naked eye.

Major Groups of Microorganisms

The chapter describes microorganisms such as:

• Bacteria
• Protozoa
• Fungi
• Some algae

Microorganisms may be unicellular or multicellular.

Bacteria and protozoa are unicellular. Yeast is a unicellular fungus, while mould is multicellular.

What Are Viruses?

Viruses are microscopic and acellular. This means they are not made up of cells.

They multiply only after entering a living cell. They may infect plants, animals or bacterial cells.

Useful Microorganisms in Class 8 Science Chapter 2

Microorganisms have many useful roles. They help recycle nutrients, prepare food, improve soil fertility and produce fuel.

Microorganisms as Decomposers

Bacteria and fungi break down dead plants, fallen leaves, food peels and animal waste. This process is called decomposition.

Decomposition converts complex materials into simpler, nutrient-rich substances. These nutrients return to the soil and support plant growth.

Formation of Manure

Fruit and vegetable peels buried in moist soil slowly turn into dark-coloured manure. Microorganisms present in the soil carry out this change.

Manure formation requires suitable moisture and temperature. The manure improves soil fertility.

Microbes and Biogas

Some microorganisms live in places without oxygen. They break down plant waste, animal waste and wastewater.

During this process, they release biogas. It contains a high proportion of methane along with carbon dioxide.

Biogas can be used for:

• Cooking
• Heating
• Generating electricity
• Running vehicles

Yeast in Bread and Dough

Yeast is a fungus that grows well in warm conditions. It breaks down sugar to release energy.

During this process, yeast produces carbon dioxide. Gas bubbles make dough soft, fluffy and larger in volume.

Yeast also forms a small amount of alcohol, which gives the dough a different smell. It is used in bread, cakes and similar food items.

Lactobacillus in Curd Formation

Curd contains bacteria, including Lactobacillus. These bacteria feed on lactose, the sugar present in milk.

They produce lactic acid, which turns milk into curd and gives it a sour taste. Warm conditions support faster bacterial growth and curd formation.

Fermentation of Indian Foods

Microorganisms support the fermentation of several food items.

Rhizobium in Legume Roots

Roots of beans, peas and lentils contain swollen structures called root nodules. Rhizobium bacteria live inside these nodules.

The bacteria trap nitrogen from the air and convert it into a form useful for plants. This improves soil fertility.

Farmers may grow legumes in rotation with other crops. This helps increase nitrogen in the soil naturally.

Microalgae as Tiny Helpers

Microalgae are microscopic, plant-like organisms. They live in water, soil, air and on trees.

They make food using sunlight and release oxygen. They produce more than half of Earth’s oxygen supply.

Microalgae also:

• Support aquatic food chains
• Help clean water
• Provide nutrients
• Support biofuel production
• Have uses in health supplements and medicines

Examples include Spirulina, Chlorella and diatoms.

Importance of Spirulina

Spirulina is a nutrient-rich microalga. It contains more than 60 per cent protein by body weight.

It is also considered a useful source of vitamin B12. Spirulina farming can support food security and livelihood opportunities.

Plant, Animal, Fungal and Bacterial Cells

Different organisms have cells with different structures.

Bacteria do not have a well-defined nucleus or nuclear membrane. Their genetic material lies in a nucleoid region.

Fungal cells have a cell wall but do not contain chloroplasts. Therefore, fungi cannot perform photosynthesis.

Why Is the Cell Called the Basic Unit of Life?

Every living organism is made up of one or more cells. Each cell contains structures that perform essential life processes.

In unicellular organisms, one cell carries out all survival functions. In multicellular organisms, specialised cells work together.

Therefore, the cell is called the basic unit of life.

Important Terms in Class 8 Chapter 2 Science Notes

Quick Revision of The Invisible Living World

• Microscopes reveal cells and microorganisms.
• Robert Hooke named cells after observing cork.
• Antonie van Leeuwenhoek observed living microbes.
• The cell membrane, cytoplasm and nucleus form the basic parts of many cells.
• Plant cells also have a cell wall and usually a large vacuole.
• Cell shape is linked with function.
• Cells form tissues, organs, organ systems and organisms.
• Bacteria and protozoa are unicellular.
• Yeast is unicellular, while mould is multicellular.
• Microbes support decomposition, fermentation and nitrogen fixation.
• Microalgae release oxygen and support aquatic life.
• Bacterial cells contain a nucleoid instead of a well-defined nucleus.

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