Biodiversity and Variety of Life notes
MDCAT Biology
Biodiversity is the variety of living organisms and the differences among them. This chapter explains classification and taxonomy, major kingdoms, viruses, bacteria, protists, fungi, plants and animals, along with their important structural, reproductive and evolutionary features.
Classification and Taxonomy
Classification is the arrangement of organisms into groups on the basis of similarities and differences. It makes the study of the enormous variety of organisms easier and helps scientists identify relationships among them.
Taxonomy is the branch of biology concerned with identification, naming and classification. Modern classification considers cell type, number of cells, mode of nutrition, body organisation, reproduction and evolutionary relationships.
- The basic and real unit of classification is the species.
- The main taxonomic categories are species, genus, family, order, class, phylum, kingdom and domain.
- A species consists of similar organisms that can naturally interbreed and produce fertile offspring.
- Binomial nomenclature gives each organism a two-part scientific name. The first part is the genus and the second part is the specific name.
- The five-kingdom system includes Monera, Protista, Fungi, Plantae and Animalia.
- Prokaryotic cells lack a membrane-bound nucleus, while eukaryotic cells possess a membrane-bound nucleus.
- Viruses are acellular and are generally placed separately from the five kingdoms.
Viruses and Acellular Forms
Viruses are non-cellular infectious particles. They contain genetic material surrounded by a protein coat called a capsid. The genetic material may be DNA or RNA, but a virus normally contains only one type of nucleic acid.
Viruses reproduce only inside living host cells. Outside a host cell, they remain inactive and do not carry out independent metabolism. Some viruses have an additional lipid envelope around the capsid.
- A capsid is made of protein subunits called capsomeres.
- Herpes virus has 162 capsomeres in its capsid.
- Poxvirus is a double-stranded DNA virus.
- Tobacco mosaic virus, or TMV, was extracted and crystallised by W. M. Stanley in 1935.
- Viruses may be classified according to their host, shape, nucleic acid and mode of replication.
- Poliomyelitis is caused by poliovirus and mainly affects motor neurons, producing paralysis.
- Virus reproduction is not described as ordinary sexual and asexual reproduction. It occurs by replication inside a host cell.
- Viruses are not included as cellular members of Monera, Protista, Fungi, Plantae or Animalia.
Kingdom Prokaryotae and Bacteria
Kingdom Prokaryotae, also called Monera in the five-kingdom system, consists of prokaryotic organisms. Bacteria are unicellular organisms with simple cellular organisation. They may be autotrophic or heterotrophic.
A typical bacterium has a cell wall, plasma membrane, cytoplasm, ribosomes and a nucleoid. The nucleoid contains the main circular DNA molecule but is not surrounded by a nuclear membrane. Many bacteria also possess a capsule, flagellum or plasmids.
- A typical bacterium may have a capsule, cell wall, plasma membrane, cytoplasm, ribosomes, nucleoid and plasmids.
- Bacterial ribosomes are 70S and are the sites of protein synthesis.
- The bacterial cell wall commonly contains peptidoglycan.
- A capsule protects some bacteria from drying and from engulfment by host cells.
- A flagellum helps in bacterial movement. Pili may help in attachment and genetic exchange.
- Gram-positive bacteria have a thick peptidoglycan wall. Gram-negative bacteria have a thinner wall and an outer membrane.
- Gram-negative bacteria are more likely to produce systemic effects because their outer membrane contains endotoxin and can also reduce antibiotic entry.
- Sterilisation destroys all microorganisms, including resistant forms such as bacterial endospores. Methods include heat, filtration, radiation and chemicals.
- An antiseptic is used on living tissues. It is not a sterilisation method.
- Bacterial population growth has lag, logarithmic, stationary and death phases. In the stationary phase, the birth rate equals the death rate.
Kingdom Protista
Kingdom Protista contains mostly unicellular eukaryotic organisms. Their cells have a true nucleus and membrane-bound organelles. Protists may be animal-like, plant-like or fungus-like.
Protozoans are animal-like protists. Amoeba moves and captures food with pseudopodia. Paramecium moves with cilia. Both use contractile vacuoles for osmoregulation, especially in fresh water.
- A contractile vacuole removes excess water and maintains osmotic balance in Amoeba and Paramecium.
- Amoeba obtains food by phagocytosis and forms food vacuoles.
- Paramecium has cilia, an oral groove, food vacuoles and contractile vacuoles.
- Plasmodium is a parasitic protist that causes malaria and has a complex life cycle involving humans and female Anopheles mosquitoes.
- Plasmodium falciparum causes malignant tertian malaria.
- Ponds and puddles provide breeding places for mosquitoes. Their destruction reduces the spread of malaria but does not directly destroy Plasmodium inside an infected human.
- Algae are photosynthetic protists. They may be unicellular or multicellular and contain chlorophyll.
- Lichens are associations between a fungus and a photosynthetic partner, usually a green alga or cyanobacterium. They are sensitive to sulphur dioxide and indicate SO2 pollution.
Kingdom Fungi
Fungi are eukaryotic, non-green organisms. They lack chlorophyll and obtain food by absorption. Their bodies are usually made of thread-like structures called hyphae. A mass of hyphae is called a mycelium.
The fungal cell wall is mainly made of chitin. Fungi commonly reproduce by spores. Classification of fungi is based on the structure and method of reproduction, especially the type of spores and fruiting bodies.
- Fungi are heterotrophic absorptive organisms and may be saprophytic, parasitic or mutualistic.
- Hyphae may be septate, with cross walls, or aseptate, without cross walls.
- Yeast is a unicellular fungus and belongs to the class Ascomycetes.
- Candida causes vaginal thrush, also called candidiasis.
- Rhizopus is a common bread mould. It reproduces asexually by sporangiospores and sexually by zygospores.
- Lichens represent mutualism because the alga provides carbohydrates while the fungus provides protection, water and minerals.
- The basis for fungal classification includes reproductive structures, method of spore formation, nature of hyphae and type of life cycle.
- Fungal reproduction may be asexual, sexual or both, depending on the group.
Kingdom Plantae
Plants are multicellular eukaryotic organisms. Most plants contain chlorophyll and manufacture food by photosynthesis. Their cells have cellulose cell walls and large vacuoles.
Plant groups show increasing complexity from non-vascular plants to vascular plants and from seedless plants to seed plants. Important divisions include bryophytes, pteridophytes, gymnosperms and angiosperms.
- Bryophytes are non-vascular plants. They lack true roots, stems and leaves and require water for fertilisation.
- Pteridophytes are vascular plants that reproduce by spores and do not produce seeds.
- Gymnosperms produce naked seeds, usually on cones. They do not form fruits.
- Angiosperms are flowering plants. Their seeds are enclosed within fruits.
- Monocotyledons have one cotyledon, parallel venation and commonly fibrous roots.
- Dicotyledons have two cotyledons, reticulate venation and commonly a taproot system.
- An aril is a fleshy covering that develops from the funicle or near the seed attachment. The edible part of litchi is the aril.
- An idioblast is a specialised plant cell that differs from neighbouring cells. It is not a nitrogen-containing tissue or compound by definition.
- Plant reproduction may involve alternation between haploid gametophyte and diploid sporophyte generations.
Kingdom Animalia and Invertebrates
Animals are multicellular eukaryotic organisms. Their cells lack cell walls and chlorophyll. Most animals ingest food and have specialised tissues, organs and organ systems.
Animal classification is based on body symmetry, number of germ layers, body cavity, segmentation, level of organisation and development of organ systems.
- Porifera have pores, a canal system and specialised collar cells called choanocytes. Most are asymmetrical.
- Cnidarians have radial symmetry and specialised stinging cells called cnidocytes.
- Cnidarians occur in polyp and medusa forms. Medusae are the motile zooids, while polyps are generally attached.
- Platyhelminthes are dorsoventrally flattened, bilaterally symmetrical and usually acoelomate.
- Aschelminthes, or roundworms in older classifications, have a complete digestive tract. Their nervous system includes a nerve ring around the pharynx.
- Annelids are segmented worms. Nephridia are their excretory organs.
- Arthropods have jointed appendages, a segmented body and a chitinous exoskeleton.
- Molluscs generally have a soft body, muscular foot and mantle. Echinoderms have a water vascular system and adult radial symmetry.
Chordates, Vertebrates and Evolutionary History
Chordates possess a notochord, dorsal hollow nerve cord, pharyngeal slits and a post-anal tail at some stage of life. In vertebrates, the notochord is replaced partly or completely by a vertebral column.
Vertebrates include fishes, amphibians, reptiles, birds and mammals. These groups show adaptations for life in water, on land and in the air.
- Fish are aquatic vertebrates that usually respire through gills and have fins for movement.
- Amphibians can live in water and on land. In male frogs, the ureters act as urinogenital ducts because they carry urine and reproductive products.
- Reptiles have dry, keratinised skin and reproduce with shelled eggs in most groups.
- Birds have feathers, wings and lightweight skeletons. They are warm-blooded and lay shelled eggs.
- Mammals possess mammary glands and hair. They are warm-blooded and generally give birth to young ones.
- Monotremes, such as the duck-billed platypus and echidna, lay eggs but possess mammary glands and hair. They show features of both reptiles and mammals.
- The Cenozoic Era is known as the age in which mammals dominated.
- Evolutionary classification compares shared characteristics and common ancestry among organisms.
Key terms
- Biodiversity
- The variety of living organisms and the differences among them.
- Taxonomy
- The branch of biology concerned with identifying, naming and classifying organisms.
- Species
- The basic real unit of classification whose members can naturally interbreed and produce fertile offspring.
- Prokaryote
- An organism whose cell lacks a membrane-bound nucleus.
- Nucleoid
- The non-membrane-bound region of a prokaryotic cell containing its main DNA.
- Capsid
- The protein coat surrounding the genetic material of a virus.
- Capsomere
- A protein subunit that forms the capsid of a virus.
- Protist
- A mostly unicellular eukaryotic organism placed in Kingdom Protista.
- Contractile vacuole
- An organelle that removes excess water and performs osmoregulation in many freshwater protists.
- Hypha
- A thread-like structure that forms the body of a fungus.
- Mycelium
- A mass of branching hyphae forming the main body of a fungus.
- Ascomycetes
- A fungal class in which sexual spores are produced in sac-like structures called asci.
- Lichen
- A mutual association between a fungus and a photosynthetic alga or cyanobacterium.
- Aril
- A fleshy seed covering that develops from the funicle or nearby seed tissues.
- Cnidocyte
- A specialised stinging cell found in cnidarians.
- Nephridium
- An excretory organ of annelids.
- Notochord
- A flexible supporting rod present along the back of chordate embryos or some adult chordates.
- Monotreme
- An egg-laying mammal, such as the platypus or echidna.
Test yourself on Biodiversity and Variety of Life
Free Biodiversity and Variety of Life MCQs with an explanation on every answer. No account needed.
More for Biodiversity and Variety of Life in the MDCAT pack
- A one-page revision sheet for this chapter
- 5 Biodiversity and Variety of Life mnemonics
- Chapter-wise Ratta Cards and a Quiz Builder for your own tests
Biology shortcuts
Recognising acellular organisms
If an entity has no cellular organisation and depends on a host for replication, identify it as a virus. Viruses are non-cellular and are not placed among cellular organisms.
- Check whether it has cytoplasm, ribosomes and independent metabolism.
- Viruses contain nucleic acid inside a protein coat but lack cellular structure.
- Example: An infectious particle without cytoplasm or ribosomes is a virus.
- Answer: Non-cellular virus.
This shortcut does not apply to bacteria, which are cellular prokaryotes.
Separating prokaryotic and eukaryotic cells
Use ribosomes as the common feature. Both cell types have ribosomes, but only eukaryotes have a membrane-bound nucleus and membrane-bound organelles.
- Look for the feature present in both groups.
- Ribosomes occur in prokaryotes and eukaryotes for protein synthesis.
- Example: Which structure is common to both? Ribosomes.
- Answer: Ribosomes.
Do not use mitochondria, chloroplasts or a membrane-bound nucleus as common features.
15 more Biology shortcuts are in the MDCAT pack. Already have it? See all shortcuts