Ch 2 Biological Classification - 1

Dinoflagellates

• Dinoflagellates are mostly marine and photosynthetic organisms
• They appear in different colors (yellow, green, brown, blue, or red) depending on the main pigments present in their cells
• They have a cell wall with stiff cellulose plates on the outer surface
• Most dinoflagellates have two flagella, one longitudinal and one transverse, in a furrow between the wall plates
• Red dinoflagellates (e.g., Gonyaulax) can undergo rapid multiplication, causing the sea to appear red (red tides), and releasing toxins that can kill other marine animals like fish

Euglenoids

• Euglenoids are mostly fresh water organisms found in stagnant water
• They have a protein-rich layer called pellicle instead of a cell wall, making their bodies flexible
• They have two flagella, a short and a long one
• Although they are photosynthetic in the presence of sunlight, they can behave like heterotrophs by preying on smaller organisms when deprived of sunlight
• Their pigments are identical to those found in higher plants
• Example: Euglena

Slime Moulds

• Slime moulds are saprophytic protists
• They move along decaying twigs and leaves, engulfing organic material
• Under suitable conditions, they form an aggregation called plasmodium, which can grow and spread over several feet
• During unfavourable conditions, the plasmodium differentiates and forms fruiting bodies bearing spores at their tips
• The spores have true walls and are extremely resistant, surviving for many years even under adverse conditions

Protozoans

• All protozoans are heterotrophs and live as predators or parasites
• They are believed to be primitive relatives of animals
• Some are the source of antibiotics, e.g., Penicillium

Fungi

• Fungi are cosmopolitan and occur in air, water, soil, and on animals and plants
• They prefer to grow in warm and humid places
• Fungi have bodies consisting of long, slender thread-like structures called hyphae
• The network of hyphae is known as mycelium
• Some hyphae are continuous tubes filled with multinucleated cytoplasm – these are called coenocytic hyphae
• Others have septae or cross walls in their hyphae
• The cell walls of fungi are composed of chitin and polysaccharides
• Most fungi are heterotrophic and absorb soluble organic matter from dead substrates, hence they are called saprophytes
• Some fungi are parasites, depending on living plants and animals
• They can also live as symbionts – in association with algae as lichens and with roots of higher plants as mycorrhiza
• Reproduction in fungi can take place by vegetative means – fragmentation, fission, and budding
• Asexual reproduction is by spores, and sexual reproduction is by oospores, ascospores, and basidiospores
• The various spores are produced in distinct structures called fruiting bodies

Basidiomycetes

• Commonly known forms of basidiomycetes are mushrooms, bracket fungi, or puffballs
• They grow in soil, on logs, and tree stumps, and in living plant bodies as parasites, e.g., rusts and smuts
• The mycelium is branched and septate
• Asexual spores are generally not found, but vegetative reproduction by fragmentation is common
• The sex organs are absent, but plasmogamy is brought about by fusion of two vegetative or somatic cells of different strains or genotypes
• The resultant structure is dikaryotic, which ultimately gives rise to basidium
• Karyogamy and meiosis take place in the basidium, producing four basidiospores
• The basidiospores are exogenously produced on the basidium
• The basidia are arranged in fruiting bodies called basidiocarps

Deuteromycetes

• Commonly known as imperfect fungi because only the asexual or vegetative phases of these fungi are known
• When the sexual forms of these fungi were discovered, they were moved into classes they rightly belong to
• It is also possible that the asexual and vegetative stage have been given one name (and placed under deuteromycetes) and the sexual stage another (and placed under another class)

Viruses

• Viruses are obligate parasites
• In addition to proteins, viruses also contain genetic material, which could be either RNA or DNA
• No virus contains both RNA and DNA
• Viruses are nucleoproteins, and the genetic material is infectious
• Viruses cause diseases like mumps, small pox, herpes, influenza, and AIDS in humans
• In plants, symptoms can be mosaic formation, leaf rolling and curling, yellowing, and vein clearing, dwarfing, and stunted growth

Viroids

• Viroids are infectious agents that are smaller than viruses and cause diseases like potato spindle tuber disease
• They lack the protein coat that is found in viruses
• The RNA of the viroid is of low molecular weight

Prions

• Prions are infectious agents consisting of abnormally folded protein
• They are similar in size to viruses
• They cause diseases like bovine spongiform encephalopathy (BSE) in cattle and its analogous variant Creutzfeldt-Jakob disease (CJD) in humans