1.Bacteria
Bacteria are unicellular organisms. The cells are described as prokaryotic because they lack a nucleus. They exist in four major shapes: bacillus (rod shape), coccus (spherical shape), spirilla (spiral shape), and vibrio (curved shape).
Most bacteria have a peptidoglycan cell wall; they divide by binary fission; and they may possess flagella for motility. The difference in their cell wall structure is a major feature used in classifying these organisms.
According to the way their cell wall structure stains, bacteria can be classified as either Gram-positive or Gram-negative when using the Gram staining.
Bacteria can be further divided based on their response to gaseous oxygen into the following groups: aerobic (living in the presence of oxygen), anaerobic (living without oxygen), and facultative anaerobes (can live in both environments).
According to the way they obtain energy, bacteria are classified as heterotrophs or autotrophs. Autotrophs make their own food by using the energy of sunlight or chemical reactions, in which case they are called chemoautotrophs. Heterotrophs obtain their energy by consuming other organisms. Bacteria that use decaying life forms as a source of energy are called saprophytes.
Characteristics of Bacteria:
Bacteria are prokaryotic unicellular organisms. They lack organized nucleus, but possess a rigid cell wall comparable to that found in plants. The average size of a bacterium is around 2 µm. The bacteria may be spherical, rod-like, spirally coiled or filament like. Certain bacteria may occur in more than one form.
2. Archaea
Archaea or Archaebacteria differ from true bacteria in their cell wall structure and lack peptidoglycans. They are prokaryotic cells with avidity to extreme environmental conditions.
Based on their habitat, all Archaeans can be divided into the following groups: methanogens (methane-producing organisms), halophiles (Archaeans that live in salty environments), thermophiles (archaeans that live at extremely hot temperatures), and psychrophiles (cold-temperature Archaeans). Archaeans use different energy sources like hydrogen gas, carbon dioxide, and sulphur. Some of them use sunlight to make energy, but not the same way plants do. They absorb sunlight using their membrane pigment, bacteriorhodopsin. This reacts with light, leading to the formation of the energy molecule adenosine triphosphate (ATP).
3.Fungi
Fungi (mushroom, molds, and yeasts) are eukaryotic cells (with a true nucleus). Most fungi are multicellular and their cell wall is composed of chitin. They obtain nutrients by absorbing organic material from their environment (decomposers), through symbiotic relationships with plants (symbionts), or harmful relationships with a host (parasites). They form characteristic filamentous tubes called hyphae that help absorb material. The collection of hyphae is called mycelium. Fungi reproduce by releasing spores.
Characteristics of Fungi:
Fungi cells are usually larger than the bacteria. The sizes may range 1-5 µm in width and 5-35 µm in length. Fungal cells may be elongated or spherical. The fungi are heterotrophic, since they cannot synthesize their own food from the inorganic compounds.
Importance of Fungi:
As is the case with bacteria, fungi are both friends and enemies of humans.
Beneficial aspects of fungi:
The yeasts are useful for the following:
i. Alcohol fermentation e.g. Saccharomyces cerevisiae
ii. Production of vitamins e.g. Ashbya gossypii.
iii. Citric acid fermentation e.g. Candida sp.
iv. Baker’s yeast e.g. S. cerevisiae.
The applications of molds are listed:
i. Production of enzymes e.g. Aspergillus sp.
ii. Citric acid fermentation e.g. Aspergillus niger.
iii. Penicillin production e.g. Penicillium notatum.
iv. Steroid transformation e.g. Rhizopus sp.
v. Gluconic acid production e.g. Aspergillus niger.
Harmful aspects of fungi:
i. Spoilage of foods e.g. moldy bread, rot of fruits and vegetables.
ii. Deterioration of textiles made up of cotton.
iii. Damage to paper.
iv. Diseases caused by fungi e.g. ring worm of the scalp in children caused by Microsporum audouinii.
Control of Fungi:
The fungal growth can be controlled by using phenol and its derivatives e.g. cresol, ethyl phenol, propyl phenol, butyl phenol. Chlorine and chlorine compounds are also useful in this regard. Vegetative cells of yeasts and other fungi can be destroyed by moist heat at 50-60°C for about 5-10 minutes. Spores however require higher temperature 20-80°C.
4. Protozoa
Protozoa are unicellular aerobic eukaryotes. They have a nucleus, complex organelles, and obtain nourishment by absorption or ingestion through specialized structures. They make up the largest group of organisms in the world in terms of numbers, biomass, and diversity. Their cell walls are made up of cellulose. Protozoa have been traditionally divided based on their mode of locomotion: flagellates produce their own food and use their whip-like structure to propel forward, ciliates have tiny hair that beat to produce movement, amoeboids have false feet or pseudopodia used for feeding and locomotion, and sporozoans are non-motile. They also have different means of nutrition, which groups them as autotrophs or heterotrophs.
5.Algae
Algae, also called cyanobacteria or blue-green algae, are unicellular or multicellular eukaryotes that obtain nourishment by photosynthesis. They live in water, damp soil, and rocks and produce oxygen and carbohydrates used by other organisms. It is believed that cyanobacteria are the origins of green land plants.
6.Viruses
Viruses are non-cellular entities that consist of a nucleic acid core (DNA or RNA) surrounded by a protein coat. Although viruses are classified as microorganisms, they are not considered living organisms. Viruses cannot reproduce outside a host cell and cannot metabolize on their own. Viruses often infest prokaryotic and eukaryotic cells causing diseases.
Importance of Viruses:
Viruses are associated with several diseases in humans and plants (Table 63.2). Besides the common diseases such as common cold, and rabies, viruses cause diseases such as AIDS. i.e. human immunodeficiency virus (HIV) is responsible for AIDS, the dreaded and incurable disease.
Titany answered the question on September 13, 2021 at 06:51