You have seen several kinds of plants and animals. However, there are other living organisms around us which we normally cannot see. These are called microorganisms or microbes. For example, you might
have observed that during the rainy
season moist bread gets spoilt and its surface gets covered with greyish white patches. Observe these patches through a magnifying glass. You will see tiny, black rounded structures. Do you know what these structures are and where do these come from? microorganisms or microbes are so small in size that they cannot be seen with the unaided eye. Some of these, such as the fungus that grows on bread, can be seen with a magnifying glass. Others cannot be seen without the help of a microscope. That is why these are called microorganisms or microbes.
Microorganisms are classified into
four major groups. These groups are
bacteria, fungi, protozoa and some
algae. Some of these common microorganisms are shown further in the chapter.
TYPES OF MICROORGANISMS
Bacteria are microscopic single-celled organisms that thrive in diverse environments. They can live within soil, in the ocean and inside the human gut. Humans' relationship with bacteria is complex. Sometimes they lend a helping hand, by curdling milk into yogurt, or helping with our digestion. At other times they are destructive, causing diseases like pneumonia and MRSA.
• Based on the relative complexity of their cells, all living organisms are broadly classified as either prokaryotes or eukaryotes.
• Bacteria are prokaryotes. The entire organism consists of a single cell with a simple internal structure. Unlike eukaryotic DNA, which is neatly packed into a cellular compartment called the nucleus, bacterial DNA floats free, in a twisted thread-like mass called the nucleoid.
• Bacterial cells also contain separate, circular pieces of DNA called plasmids. Bacteria lack membrane-bound organelles, specialized cellular structures that are designed to execute a range of cellular functions from energy production to the transport of proteins. However, both bacterial and eukaryotic cells contain ribosomes. These spherical units are where proteins are assembled from individual amino acids, using the information encoded in a strand of messenger RNA.
• On the outside, bacterial cells are generally surrounded by two protective coverings: an outer cell wall and an inner cell membrane. However, certain bacteria, like the mycoplasmas do not have a cell wall at all. Some bacteria may even have a third, outermost, protective layer called the capsule. Lastly, bacterial surfaces can be covered by whip-like extensions: flagella or pili.
TAKE A LOOK AT THE INSIDE OF BACTERIA.
SECTION OF BACTERIA
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EXAMPLES OF SOME OF THE BACTERIAS
Fungus, plural fungi, any of about 144,000 known species of organisms of the kingdom Fungi, which includes the yeasts, rusts, smuts, mildews, molds, and mushrooms. There are also many funguslike organisms, including slime molds and oomycetes (water molds), that do not belong to kingdom Fungi but are often called fungi. Many of these funguslike organisms are included in the kingdom Chromista. Fungi are among the most widely distributed organisms on Earth and are of great environmental and medical importance. Many fungi are free-living in soil or water; others form parasitic or symbiotic relationships with plants or animals.
EXAMPLES OF SOME OF THE FUNGI
A virus is a chain of nucleic acids (DNA or RNA) which lives in a host cell, uses parts of the cellular machinery to reproduce, and releases the replicated nucleic acid chains to infect more cells. A virus is often housed in a protein coat or protein envelope, a protective covering which allows the virus to survive between hosts.
A virus can take on a variety of different structures. The smallest virus is only 17 nanometers, barely longer than an average sized protein. The largest virus is nearly a thousand times that size, at 1,500 nanometers. This is really small. A human hair is approximately 20,000 nanometers across. This means that most virus particles are well beyond the capability of a normal light microscope.
EXAMPLES OF SOME OF THE VIRUSES
PROTOZOA AND ALGAE
Protozoa also belong to the kingdom "Protista." These organisms are unicellular and are classified by their method of movement. They can swim by using flagella, which are whiplike strands, cilia or pseudopods, which are extensions of the cell that pull it along, or they do not move at all. Amoebas are a type of protozoa that is very familiar. Some protozoans are responsible for human diseases, such as malaria.
The word "algae" refers to a wide variety of organisms that come from many different phyla in the taxonomic system, but all belong to the kingdom "Protista." All algae contain chlorophyll and can create their own energy, like plants, and are considered plant-like. Some are unicellular while others are multicellular, with seaweed being a well-known type of multicellular algae.
Algae and protozoans belong to the same kingdom, Protista, which is the kingdom that is used for many organisms that do not fit neatly into another category. Protists include algae, protozoa and slime molds. The major difference between algae and protozoa is that algae are able to make their own food, as plants do, while protozoa ingest other organisms or organic molecules, as animals do. In scientific terms, algae are "autotrophs" and protozoa are "heterotrophs." The word "protozoa" actually refers to this fundamental difference, with "pro" meaning first and "zoa" meaning animal.