IGCSE Biology: Classification
Seven characteristics have to be met in order to classify an organism as living (MRS GREN).
- Movement - all (living) organisms are able to move using their body (animals) and parts of themselves (plants)
- Reproduction - all organisms are able to make new organisms of the same species as themselves
- Sensitivity - all organisms can sense and react to changes in the environment
- Growth - all organisms can grow from smaller to larger by adding new cells to their bodies
- Respiration - all organisms can break down glucose and other substances to release energy (aerobic and anaerobic)
- Excretion - all organisms can remove unwanted and/or toxic waste products from their body
- Nutrition - all organisms have a source of energy (via food) from the environment to aid in growth
All living organisms are made of cells. These cells are eukaryotic and contain cytoplasm, a cell membrane, DNA, ribosomes, and enzymes.
Classification is a method of grouping organisms to easily identify common features and study them. Organisms can be classified using DNA, morphology, and anatomy.
The classification system used today is known as the Linnaeus system. The system follows this order:
- Kingdom - fits into one of the five kingdoms; animal/animalia, plant/plantae, fungi, protocista, prokaryotes/bacteria
- Phylum - in the animal kingdom, this can be either arthropods or vertebrates; in the plant kingdom, this can be angiosperms (flowering plants) or ferns
- Class - sub-categories of phyla that include mammals, insects, reptiles, monocotyledons, dicotyledons, etc.
- Order - a group of related families
- Family - contains one or more genus
- Genus - several species with similar characteristics
- Species - a group of organisms that can reproduce and produce fertile offspring with one another and their offspring
However, organisms are only named based on their genus and species. This is known as the binomial system.
It is important that you write an organism’s binomial name correctly. It is to be written with the genus capitalised and the species lowercase and always in italics. When written, it can also be written with an underline. For example, the binomial name for a wolf is Canis lupus, or C. lupus for short.
Kingdoms
| Kingdom | Characteristics | Examples |
|---|---|---|
| Animals | - includes multicellular, eukaryotic, heterotrophic organisms \n - cannot prepare their own food \n - lacks cell walls in their cells \n - exhibit all levels of organization in structure \n - reproduce sexually as well as asexually \n - have the ability to regrow missing parts \n - ingest food and digest it in an internal cavity- motility (ability to move) | horse, humans, tapeworm, sponge |
| Plants | - ability to make their own food (autotrophs) \n - reproduce asexually and sexually \n - multicellular eukaryotes \n - contain cell walls in their cells as well as a large central vacuole and chloroplasts | flowers, vegetables, fruits |
| Fungi | - eukaryotic, non-vascular, non-motile, and heterotrophic- may be unicellular or filamentous \n - reproduction by spores- cannot perform photosynthesis- biosynthesis of chitin occurs in fungi- no embryonic stage → develop from spores can infect the host | mushrooms, mould, yeast |
| Protocista | - unicellular eukaryote- may have cilia or flagella for motility- sexual reproduction occurs by cell fusion and zygote formation- can carry out photosynthesis or feed on organic substances by other organisms | algae |
| Prokaryotes | - no membrane-bound organelles such as mitochondria, Golgi body, and chloroplasts- genetic material is present on a single chromosome- cell wall is made of peptidoglycan (carbohydrates and amino acids)- divide asexually by binary fission | E. coli |
It can be argued that viruses are also part of the kingdoms. However, viruses do not meet all the characteristics required to be considered a living organism until it occupies a living cell.
Simple viruses are made up of genetic material (usually RNA) and are surrounded by a protein coat.
Phyla and Classes
Vertebrate
| Class | Body covering | Fertilisation site | Embryo development | Breathing | Temperature | Other features | Examples |
|---|---|---|---|---|---|---|---|
| Fish | scaly skin | external - in water | in soft eggs | gills | ectothermic | find | rainbow trout |
| Amphibian | moist, scaleless skin | external - eggs in water | larva (tadpole) lives in water | larva - gills \n adults - lungs | ectothermic | cold-blooded | frog |
| Reptile | scaly skin | external - eggs with rubbery shells | in rubbery eggs filled with water | lungs | ectothermic | snake, lizard, turtle | |
| Bird | feathers | external - eggs with hard, waterproof shells | in eggs | lungs | endothermic | wings, beak | eagle, stork, penguin |
| Mammal | hair and skin | internal - placenta | a zygote, which results from an ovum being fertilized by a single sperm | lungs | endothermic | heart has four chambers, different types of teeth, a diaphragm | human, cat, dog, lion |
Arthropods
| Class | Body regions | Legs | Breathing | Wings | Examples |
|---|---|---|---|---|---|
| Insects | 3 (head, thorax, abdomen) | 3 pairs | spiracles | 2 pairs | beetles, ladybugs, bees |
| Crustaceans | chalky exoskeleton formed from calcium | 5 pairs | gills | - | crabs, lobsters, wood lice |
| Arachnids | 2 (cephalothorax, abdomen) | 4 pairs | gills (book lungs) | - | spiders, scorpions |
| Myriapods | many segments | up to 750, one pair per segment | trachea | - | centipedes, millipedes |
Angiosperms (Flowering Plants)
Angiosperms have roots, stems, and leaves. They reproduce sexually via flowers and seeds and these seeds are produced inside the ovary of a plant. They can also be further classified as:
- Monocotyledons
- or monocot for short, they have a single cotyledon
- long, narrow leaf
- veins are parallel
- vascular bundles are scattered
- flower parts are in multiples of three
- Dicotyledons
- or dicot for short, they have two cotyledons
- broad leaf
- network of veins
- ring of vascular bundles
- flower parts in multiples of four or five
Ferns
Ferns have roots, stems, and leaves. Their leaves are known as fronds. They reproduce using spores produced on the underside of their fronds.
Dichotomous Keys
Dichotomous keys are used to identify an organism that is unknown using a series of descriptions. You may be given an image and asked to identify the organism using the key by following the descriptions that match the organism in the image until you reach an endpoint.
