Bacteria

The information that can be gained from studying bacteria is so vast that it has its own branch of biology called bacteriology.

Bacteria are prokaryotes that lack an organised nucleus – they are uni-cells. Bacteria can be found everywhere including soil, air, water, our bodies and they can be essential to supporting life on earth. These prokaryotes tend to be bio-chemically very active and fast growing. Their shapes are usually a telltale sign of identification, with the main ones being spherical, rod-like, spiralled or filamentous.

Despite their difference of shape, certain features are common to them all. These include a plasma membrane which is a fluid-mosais membrane of lipid and proteins that protects the cell. A ring of DNA is usually attached to the plasma membrane. These cells also contain stored food reserves that include lipids, glycogen and inorganic phosphate salts. Essential to all cells is a cell wall. This is a strong, rigid container that gives the cell shape and protection and is composed of a mix of polysaccharide and polypeptides called murein. The wall is permeable to water and other small molecules and to ions. Protein and nucleic acids, however, can’t pass.

Cytoplasm is another must, providing the organelles within the cell with something to be suspended in. The cytoplasm contains the ribosomes which are smaller than eukaryotic cell ribosomes. Other certain bacteria may have added features such as plasmid, flagellum, a slime layer and pili.
Bacteria are dependent on availability of adequate water and appropriate food supply in order to grow. PH is an important factor as most bacteria prefer slightly alkaline conditions but a few can tolerate extremes of acidity or alkalinity. A temperature range of 25 – 45oC is usually favourable for most bacteria, although some grow at temperatures as low as 0oC or above 80oC.

Most bacteria tend to flourish in air (aerobes), yet many can survive with no oxygen (facultative anaerobes). Some even require the absence of oxygen (obligate anaerobes).

Fungi

Fungi are usually aerobic and are saprophytes in the soil or water. Fungi, like bacteria, contribute significantly to the decomposition of matter and recycling nutrients.
By using extra cellular enzymes such as cellulases and pectinases, fungi are the primary decomposers of the hard parts of plants that can’t be digested by most animals.
The study of fungi is called mycology. The fungi include yeasts, mold and fleshy fungi. Yeasts are uni-cellular organisms, molds are multi-cellular, filamentous organisms such as mildews, rusts and smuts. Fleshy fungi include multi-cellular mushrooms, puffballs and coral fungi.
A colony of mold or fleshy fungi consists of long filaments called hyphae. In most molds, the hyphae contain cross walls which divide the hyphae into distinct, uninucleate cell-like units. In a few species however, the hyphae contain no septa and look like continuous long cells with many nuclei.

Many of them may look plant-like, but fungi do not make their own food from sunlight like plants do. Fungi digest food outside their bodies: they release enzymes into the surrounding environment, breaking down organic matter into a form the fungus can absorb. Fungi absorb nutrients from living or dead organic matter (plant or animal stuff) that they grow on. They absorb simple, easily dissolved nutrients, such as sugars, through their cell walls. They give off special digestive enzymes to break down complex nutrients into simpler forms that they can absorb.

Fungi reproduce by releasing spores from a fruiting body. The fruit, called a mushroom, releases spores into the air, and the wind carries the spores off to start the next generation. Around 100,000 species of fungi are divided into five phyla, based mainly on the characteristics of their reproductive organs.
Fungi usually grow best in environments that are slightly acidic (PH 5). They can grow on substances with very low moisture. Fungi live in the soil and on your body, in your house and on plants and animals, in freshwater and seawater.

Algae

Algae are photosynthetic autotrophs. By using the energy produced in photosynthesis, they convert the carbon dioxide in the atmosphere into carbohydrates - oxygen is a by-product of photosynthesis. The chlorophyll involved in photo synthesis is responsible for the colour of most algae. Some algae are single cells; - others exist as colonies of hundreds or thousands of cells.

Dinoflagellates are uni-cellular, free-floating algae. Their cell walls consist of many individual plates made of cellulose and silica. Certain marine Dinoflagellates produce toxins that cause paralytic shellfish poisoning, which spreads to humans when large numbers are eaten by clams or mussels.
Euglenoids are uni-cellular, flagellated algae with a semi-rigid cell membrane.

The brown algae, or kelp, are macroscopic with some reaching 50 meters in length and are found mostly on the coastal waters. Kelp has a phenomenal growth rate; - some grow at rates exceeding 20cm a day. Most red algae are delicately branched, multi-cellular organ isms that live at depth greater than other algae. The red pigments allow the red algae to absorb the blue light that penetrates the deep.
The body of multi cellular algae is called the thallus. Thalli of larger multi-cellular algae have branched ‘holdfasts’, which anchor the alga to rocks etc. Algae do not support themselves, instead, the pressure of the surrounding water supports the thallus – some are also buoyed by a floating, gas-filled bladder.

Algae can reproduced asexually, - the multi-cellular and the filamentous form fragments – each piece is able to form a new thallus. When a uni-cellular alga divides, its nucleus first divides (by mitosis), then the two nuclei move to opposite parts of the cell. It then divides into two complete cells.

Protozoa

Protozoa’s inhabit water and soil, and feed upon bacteria and small particulate nutrients. Some protozoas form part of the normal flora for animals. For example, the gastrointestinal tracts of termites and cows contain protozoas that help them digest cellulose.

Protozoas have been classifies into phyla on the basis of how they move. Amoebas for example move by extending ‘lobe-like’ projections of cytoplasm called pseudo-pods. The amoeba extends these pods and the rest of the cell will flow toward them.

Other examples of how protozoa move include flagella. Flagella are capable of whip like movements that pull the cell through its environment. Cilia are similar to but shorter than flagella and usually cover the whole outside of the cell – they move in a way that propels the cell.
All protozoas live in areas with a large food supply and water. Some transport food across the cell membrane. However, some have a protective covering called the pellicle and require specialised structures to take food in.
The ciliates take food in by waving their cilia toward a mouth like opening called the cytosome.
The amoebas engulf food by surrounding it with pseudo-pods and phagocytising it. In protozoas, digestion takes place in membrane-bounded vacuoles and waste may be eliminated through the cell membrane or through a specialised anal pore.

Viruses

Viruses are entities that contain a single type of nucleic acid, either DNA or RNA, they contain a protein coat that surrounds the nucleic acid, and viruses also multiply inside living cells using the synthesising machinery of the cell. Viruses are also capable of causing the synthesis of specialised elements that can transfer the viral nucleic acid to the other cells.

In order to multiply, viruses must take over the metabolic machinery of the host cell, - this means that most drugs created to destroy the virus would also affect the functioning of the host cell, - making them too toxic for safe use. Viruses multiply only in cells of particular species and are therefore, divided into three main classes – animal, bacterial and plant viruses.

The nucleic acid of a virus is surrounded by a protein coat called the capsid which accounts for most of the mass of a virus. Each capsid is composed of protein subunits called capsomeres. The arrangement of capsomeres is characteristic of a particular virus. Some viruses have a capsid covered by an envelope; - this is usually made up of a combination of lipids, proteins and carbohydrates. Depending on a virus, the envelope may or may not be covered by spikes, which are carbohydrate-protein complexes. Some viruses use these spikes to attach themselves to a host cell. These spikes can even be used in identifying viruses, for example, the influenza’s ability to clump red blood cells is associated with spikes.
Viruses that have capsids not covered by an envelope are known as naked viruses and their capsids protect the nucleic acid from nuclease enzymes in biological fluids and promotes the virus’ attachment to susceptible host cells.

Helical viruses resemble long rods that may be rigid or flexible. Surrounding the nucleic acid, their capsid is a hollow cylinder with a helical structure.
Polyhedral viruses are many sided. Their capsid is in the shape of a polyhedron with twenty triangular faces and twelve corners.