BACTERIA
The commonest living things are bacteria. They are too small to see without a microscope. Most are about one to five microns (0.001 to 0.005 millimetres) across. A quarter of a million would fit on the head of a pin. Bacteria are all around us in their billions. They float in air and live on icy mountain-tops, in the scalding water of hot springs, in dark caves and on the bottom of the sea. There are more than 4000 known kinds, and probably many more yet to be identified. They vary in form but there are three main shapes. These are: spheres or balls known as cocci, cylinders or rods, called bacilli, and corkscrew-like spirilli. Most bacteria reproduce simply by splitting in two.
Bacteria belong to the main kingdom of living things known as monerans. A typical bacterium has a tough outer skin, or cell membrane, which contains jelly-like cytoplasm. Tiny blobs, known as ribosomes, float in the jelly and make various substances for the bacterium’s life processes. Also floating in the cytoplasm is a long, coiled-up chemical called DNA, which unravelled would be more than 1000 times longer than the bacterium itself. This is the bacterium’s genes, a “manual” containing every structural detail of the organism. Some bacteria get their energy from light, like plants. Others absorb nutrients through their cell membranes.
Some bacteria are harmful. They get into other living things, including humans, and cause diseases such as anthrax and typhoid. But most bacteria are harmless. Many kinds live in the soil and play a vital role in nature because they cause the decay or rotting of dead plants and animals.
VIRUSES
The smallest living things are viruses. They are “alive” only because they can produce more of their kind if they invade another living thing. Viruses cannot reproduce on their own. They get into another living cell, the host cell, and take over its life processes to make more copies of themselves. In the process they destroy the host cell.
A typical virus has an outer shell or coat made of proteins. Inside is a length of genetic material, usually DNA. Different viruses are shaped like bricks, rods, golf balls and even space rockets. Many can exist in their non-living form for years and be frozen solid, boiled or made into crystals – yet still come alive when host cells are available. Viruses cause diseases in plants, animals and people. These include the common cold, measles and AIDS (caused by the Human Immunodeficiency Virus, HIV).
PROTISTS
Like Bacteria and other monerans, protists are microscopic single cells. But unlike monerans, each protist has its genetic material (DNA) wrapped inside a bag-like membrane to form the nucleus or control centre of the cell. Protists live mainly in water and damp places. Some are like tiny plants, absorbing their energy from sunlight and their raw materials for growth from the water around them. Others move around and consume food particles such as bacteria.
Some protists have a rigid, case-like cell wall around them. The types known as foraminiferans and radiolarians make shells with beautiful shapes and patterns. Others have no rigid case and can take up any shape. A few protists cause diseases, such as plasmodia, which produce malaria.
Picture Credit : Google