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Fungi range in form and size from unicellular yeasts to large mushrooms, yeasts are unicellular, and they do not have flagella and reproduce asexually by budding or transverse fission or sexually by spore formation. Multicellular forms such as moulds have long, branched, threadlike filaments called hyphae, which aggregate together to form a tangled mycelium. 

Many fungi are dimorphic, that is, they exist in two distinct forms. Fungi that cause human infections can change from the yeast form in the human to a mycelial form in the environment in response to changes in nutrients, and environmental factors such as Carbon (iv)oxide CO2 and temperature. 

One of the features that caused taxonomist finally to remove fungi from the plant kingdom was the distinctive chemical nature of the fungal cell wall. Whereas plants and algal cells have walls composed of cellulose, the cell wall of fungi is made up principally of chitin, a strong but flexible polysaccharides that is also found in the exoskeleton of insects.


The algae are mostly aquatic plants, making up a highly diverse group of over 30,000 species. They display a wide variety of structure, habitats and life - cycle, ranging from single -celled forms to massive seaweeds tens of metres in length. Most algae share a number of common features which caused them to be grouped together. Among these are possessions of pigment chlorophyll, deriving energy from the sun by means of oxygenic photosynthesis, fixing carbon from CO2 or dissolved bicarbonate. 

All algae are eukaryotic, and therefore contain the internal organelles, that is, nuclei, mitochondria, endoplasmic reticulum, ribosomes, Golgi body and in most instances, chloroplasts. With the exception of one group (the Euglenophyta) all have cellulose cell wall, which is frequently modified with other polysaccharides, including pectin and alginic acid. In some cases, the cell wall may be fortified with deposits of calcium carbonate or silica. This is permeable to small molecules and ions, but impermeable to macromolecules.


A protozoan can be defined as a usually motile eukaryotic unicellular Protist. Protozoa are directly related only on the basis of a single negative charao they are not multicellular. However all demonstrate the basic body plan of a single protistian eukaryotic cell. Protozoa grow in a wide variety of moist habitats. Moisture is absolutely necessary for the existence of protozoa because they ate susceptible to desiccation. Most protozoa are eukaryotic cells, in many respects their morphology and physiology are the same as the cells of multicellular animals.

However, because all of life's various functions must be performed within the Individual protozoan, some morphological and physiological features are unique to protozoan cells. In some species the cytoplasm immediately under the plasma membrane is semisolid or gelatinous, giving some rigidity to the cell body. It is term ectoplasm. The plasma membrane and the structures immediately beneath it are called the pellicle. Inside the ectoplasm is the area referred to as endoplasm, which is more fluid and granules in composition and contain most of the organelles. Some protozoa have one nucleus, while others have two or identical nuclei, still other protozoa have two distinct types of nuclei -a micronucleus and one or more micronuclei. 

The macronucleus, when present is typically larger and associated with trophic activities and regeneration processes. The macronucleus is diploid and involved in both genetic recombination during reproduction and the regeneration of the micronucleus. One or more vacuoles ate usually present in the cytoplasm of protozoa. These ate differentiated into contractile, secretory, and food vacuoles. The contractile vacuoles functions as osmoregulatory organelles in those protozoa that lives in a hypotonic environment, such as free water lake. Osmotic balance is maintained by continuous water expulsion. Most marine protozoa and parasitic species are isotonic to their environment and lack such vacuoles. Secretory vacuoles usually contain specific enzymes that perform various functions.


All viruses are obligate intracellular parasites (parasite that cannot complete their life cycle without it host cell, without this they cannot reproduce)

They inhabit a no-man's land between the living and the non-living worlds, and possess characteristics of both. They are known to differ radically from the simplest true organisms, bacteria, in a number of respects: they cannot be observed using a light microscope.

They have no internal cellular structure.
They contain either DNA or RNA, but not both.
They are incapable of replication unless occupying an appropriate living host cell.
They are incapable of metabolism.
Individuals show no increase in size.

When inside a host cell, viruses show some of the features of a living organism, such as the ability to replicate themselves, but outside the cell they are just inert chemical. Compared to even the most primitive cellular organism, viruses have a very simple structure. Most viruses are smaller than even the smallest bacterial cells. The genetic material of a virus may be either RNA or DNA, and either of these may be single stranded or double - stranded.


Bacteria are classified as unicellular organisms. The cells are depicted as prokaryotic because they lack a nucleus. Bacterial exist in four major shapes: bacillus (rod shape), coccus (spherical shape), spirilla (spiral shape), and vibrio (curved shape). The majority of bacteria have a peptidoglycan cell wall; they are divided by binary fission; and they may have 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.

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