Reasons for classification of living organisms:
- Creates a ‘filing system’- new discoveries easily fit into a group, and relationships between groups can be studied.
- Acts as a universal language that makes international communication easy and efficient
Classifying organisms: classified into sub-groups as follows
All members of a species are able to interbreed to produce offspring of that species. A genus is a group of closely related species, a family is a group of closely related genuses, etc.
A good way to remember how these groups are subdivided is the mnemonic Please Cool Off For Goodness Sake.
Artificial classification: a classification system based on a few, superficial characteristics which are easily observed, e.g. arranging books by title or author.
Natural classification: a classification system based on attempts to show evolutionary relationships by examining physical characteristics, embryology, immunology, biochemistry, cell structure, ecology, behaviour and reproductive strategies.
The 5 Kingdoms of living organisms
- Prokaryotae lack a distinct nucleus and membrane-bound organelles. Includes bacteria and blue-green algae
- Protoctista: includes all protozoa (single-celled organisms) and some algae
- Fungi: eukaryotes with non-cellulose cell walls and hyphal mycelia. They are saprophytic. Includes single-celled yeasts and toadstools.
- Plantae: eukaryotic, multicellular photosynthetic organisms, from multicellular algae to trees. Contain cellulose cell walls and permanent vacuoles. Most have stems, leaves and roots.
- Animalia: non-photosynthetic, multicellular organisms. Growth occurs all over the body surface, whilst co-ordination is both nervous and chemical. Active locomotion occurs at some stage in the life-cycle.
Tissue organisation in animals
- Diploblasts have a 2-layered body plan consisting of the ectoderm, which forms the epidermis and the endoderm, which forms internal structures.
- Triploblasts have a 3-layered body plan consisting of the ectoderm, which forms the epidermis and nervous system; the mesoderm, which forms body organs and systems and the endoderm, which forms the gut and digestive glands.
Symmetry in animals
- Radial symmetry: slow moving or sessile (non-moving) organisms have a point of symmetry. Sensory, feeding and defence structures are spread out around the whole organism, e.g. jellyfish and adult starfish.
- Bilateral symmetry: active animals have an axis of symmetry. They often exhibit cephalisation (all sense organs on a distinct head).
Coelom: a fluid-filled space in the mesoderm of more advanced organisms which divides the gut muscles from those of the body wall. This provides a protective place for specialised organs, which are held in position, supported and lubricated. The main advantages are:
- Separating the gut muscles from the body wall means that digestion can become more efficient (e.g. peristalsis can occur)
- Aids locomotion as muscles and bones can develop. In earthworms, coelomic fluid itself provides a hydrostatic “skeleton”.
- Allows for the development of internal organs
Organs that have a coelom are known as coelomate organisms, all others are acoelomate.
Classification of the animal kingdom reflecting an increase in complexity
- Cnidaria, e.g. Hydra sp. Diploblastic, acoelomate organisms with radial symmetry. Gas exchange takes place by diffusion over the whole body surface, and transport to all cells is by simple diffusion (sufficient because these are small organisms with a large S.A. to volume ratio)
- Platyhelminthes e.g. liver fluke (Fasciola hepatica). Triploblastic, coelomate organisms with bilateral symmetry. Gas exchange takes place by simple diffusion due to large S.A. to volume ratio.
- Annelida e.g. earthworm (Lumbricus terrestris). Triploblastic, coelomate organisms with bilateral symmetry. Gas exchange takes place by diffusion over body surface. Transport is by a primitive circulatory system with blood vessels, hearts, and O2-carrying respiratory pigment.
- Arthropoda e.g. locust (Locusta migratoria). Triploblastic, coelomate organisms with bilateral symmetry. Gases diffuse into tissues from tracheoles. Transport is through blood-filled haemocoel, no O2-carrying pigments, but there are small hearts to keep fluid moving.
Chordata e.g. humans (Homo sapiens). Triploblastic, coelomate organisms with bilateral symmetry. Gas exchange takes place over an internal exchange surface with a large surface area (e.g. alveoli in lungs) to provide for the typically large volume. Transport takes place within a closed circulatory system consisting of a heart and blood vessels.