The Invisible Living World: Beyond Our Naked Eye

Class 08 Science

The human eye can only see objects that are above a certain size. For a long time, many tiny things around us remained unknown.

Long ago, people discovered that a curved piece of glass could make small things look bigger. The piece of glass was shaped like a lentil seed - thick in the middle and thin at the edge - hence they called it a lens. Over time, lenses were improved to become more powerful. Each new tool, from simple magnifying glasses to microscopes, helped humans see what their eyes could not. The invention of the microscope opened a fascinating hidden world filled with tiny living creatures.

Cell

All living beings are made up of cells. Cells have three main parts - a thin outer lining, a central region, and a small round structure inside it. The outer layer is called the cell membrane. The round structure in the middle is the nucleus, which is also covered by a thin membrane. The space between the cell membrane and nucleus is filled with cytoplasm.

Basic Parts of Cell

1. Cell membrane
2. Cytoplasm
3. Nucleus

Some cells have an extra outer layer called the cell wall.

Cell Membrane

The cell membrane encloses the cytoplasm and nucleus. The cell membrane separates one cell from another. It is porous and allows the entry of materials essential for life processes and the exit of waste material.

Cytoplasm

Cytoplasm contains other components of the cell and compounds, such as carbohydrates, proteins, fats, and mineral salts. Most of the life processes take place within the cytoplasm.

Nucleus

The nucleus regulates all activities that occur within the cell. It also regulates growth.

Cell Wall

The cell wall in the plant cell provides rigidity and strength to plants. This is why all cells are arranged compactly with each other and look firm in structure.

Variation in Cells

A muscle cell is shaped like a spindle, while a nerve cell is very long and has branches. Similarly, some cells are round in shape, while others are long and thin. The number of cells also varies in different organisms.

The unique shape, size, and structure of cells help them carry out their specific functions.

Nerve cells also known as neurons carry messages in our body. The elongated shape and branched structure help them reach different parts of the body and pass on messages quickly.

Plant cells also show variation. In plants, too, cells may be rectangular, elongated, oval, or even tube-like. Some plant cells form long tubes that help carry water throughout the plant.

Levels of Organisation

The body of a living organism is organised in a complex way. Cell is the basic unit of life. A group of similar cells forms a type of tissue. Different tissues are organised to form an organ. Several organs work together to form an organ system that performs a major function of the body. All the organ systems together make up a complete organism - like a plant or an animal.

The levels of organisations are:

Cell → Tissue → Organ → Organ system → Organism

The life of complex living organisms begins with a single cell - egg. The egg of any organism has ability to divide repeatedly to form a complete living being made up of many cells. Such living beings are called multicellular organisms. Animals, including humans, and plants are all examples of multicellular organisms.

Microorganisms

Some living organisms are made up of just one or very few cells. They are so small that they can not be seen with the naked eye. These are called microorganisms.

Some microorganisms, like bacteria and Amoeba, are made of just one cell (unicellular). Others, like some fungi and algae, have many cells (multicellular).

Bacteria, fungi, and protozoa are different kinds of microorganisms. Viruses are also small in size, but they are different from other microorganisms since they reproduce only inside the host organism.

Microorganisms can be beneficial or harmful to us. Some microorganisms decompose the plant and animal waste into simple substances and clean up the environment. Some microorganisms reside in the root nodules of legumes, such as peas, beans, and lentils. They trap nitrogen from the air and increase the soil fertility.

Yeasts are fungi which are used in the process of making breads, cakes, pastries, idlis, dosas, and bhaturas. Lactobacillus is used in the curd formation at home and fermentation process in food industry.