Features of OrganismsCambridge IGCSE Biology: Revision notes
Section 1
How are organisms placed into the five kingdoms?
All living organisms are classified into one of five kingdoms based on their fundamental characteristics. These kingdoms are: Animal, Plant, Fungus, Prokaryote, and Protoctist.
The main features used to distinguish between kingdoms include:
- Presence or absence of a nucleus: Prokaryotes lack a true nucleus; all other kingdoms have one
- Method of nutrition: Plants are autotrophic (photosynthetic); animals, fungi and many protoctists are heterotrophic (consume organic matter)
- Cell wall composition: Plants have cellulose cell walls; fungi have chitin cell walls; prokaryotes have peptidoglycan cell walls; animals lack cell walls
- Storage carbohydrate: Plants store starch; fungi store glycogen (like animals)
- Ability to move: Most animals can move; plants are fixed; fungi and protoctists vary
Organisms are placed into a kingdom by identifying which combination of these features they possess.
When classifying an organism into a kingdom, identify at least two key features—examiners expect you to justify your classification with specific evidence rather than just naming the kingdom.
Students often assume all living things are either plants or animals. Remember: fungi are NOT plants—they have chitin cell walls, store glycogen, and are heterotrophic.
Section 2
What are the main features used to classify animals?
Animals are divided into two main groups based on the presence or absence of a backbone:
Vertebrates (animals with backbones):
- Fish: Covered in scales; breathe through gills; have fins; lay eggs in water; cold-blooded (ectothermic)
- Amphibians: Smooth, moist skin; breathe through lungs and skin; lay eggs in water; have a tadpole larval stage; cold-blooded
- Reptiles: Covered in dry scales or plates; breathe through lungs; lay eggs with leathery shells; cold-blooded
- Birds: Covered in feathers; have wings; breathe through lungs with air sacs; lay eggs with hard shells; warm-blooded (endothermic); have hollow bones
- Mammals: Covered in fur or hair; breathe through lungs; possess mammary glands for feeding young; warm-blooded; most give birth to live young
Arthropods (animals with jointed legs and exoskeletons):
- Insects: Six legs; three body segments (head, thorax, abdomen); one pair of antennae; breathe through spiracles
- Arachnids: Eight legs; two body segments (cephalothorax and abdomen); no antennae; includes spiders and scorpions
- Crustaceans: Ten or more legs; two pairs of antennae; mostly aquatic; includes crabs and prawns
- Myriapods: Many body segments; one pair of legs per segment; includes centipedes and millipedes
To classify a spider: it has 8 legs, two body segments, and an exoskeleton—therefore it is an arachnid. To classify a salmon: it has scales, gills, fins, and lays eggs in water—therefore it is a fish.
Learn the key distinguishing feature for each vertebrate class first (e.g. feathers for birds, fur for mammals), then add secondary features. Examiners test identification based on these characteristic features.
Section 3
How are organisms classified within the plant kingdom?
The plant kingdom is divided into two main groups: ferns and flowering plants.
Ferns:
- Do not produce seeds; instead reproduce using spores
- Reproduce sexually using sperm and eggs
- Require moist environments for reproduction
- Have roots, stems and leaves
- Lack flowers and fruits
Flowering plants (Angiosperms): Flowering plants are further divided into two main groups based on cotyledon number (number of seed leaves):
| Feature | Dicotyledons | Monocotyledons |
|---|---|---|
| Cotyledons (seed leaves) | Two | One |
| Leaf venation | Branched/reticulate | Parallel |
| Stem vascular tissue | Arranged in rings | Scattered throughout |
| Flower parts | Usually in fours or fives | Usually in threes |
| Root system | Taproot (one main root) | Fibrous (many fine roots) |
| Examples | Beans, sunflowers, apple trees | Grasses, lilies, cereals, maize |
All flowering plants produce seeds enclosed in fruits and reproduce sexually using flowers.
Think of dicotyledons as having 'split' seeds (like peanut halves) and monocotyledons as having single seeds (like a grain of rice)—this seed structure directly links to their structural features.
Identify a maize plant: it has one cotyledon, parallel leaf veins, scattered vascular bundles, and flower parts in threes—therefore it is a monocotyledon. Identify a bean plant: it has two cotyledons, branched leaf veins, and a taproot—therefore it is a dicotyledon.
Section 4
How do you classify organisms using identified features?
To classify an organism, follow this systematic approach:
- Observe the organism carefully and identify its structural features
- Start with the broadest classification (which kingdom?) using fundamental characteristics
- Narrow down the classification step by step using specific features of that group
- Compare the organism's features against the key distinguishing features of each subgroup
- Select the group that matches all or most of the organism's characteristics
Example classification process for an unknown animal:
- Observation: It has fur, produces live young, has lungs, is warm-blooded
- Kingdom: Animal (has these vertebrate features)
- Class: Mammal (fur, live birth, mammary glands, warm-blooded)
- Further identification depends on other features (e.g. hooves, horns, number of toes)
Example classification process for an unknown plant:
- Observation: It has flowers, seeds in a fruit, parallel leaf veins, flower parts in threes
- Kingdom: Plant (autotrophic, cellulose cell wall, starch storage)
- Group: Flowering plant (has flowers and seeds)
- Subgroup: Monocotyledon (parallel veins, flower parts in threes)
Always justify your classification by stating which specific features led you to that conclusion.
Always name at least two features that support your classification decision. Examiners award marks for stating the organism's features AND explaining why those features place it in that group.
Section 5
What are the features of viruses?
Viruses are not classified into any of the five kingdoms. They are unique biological entities with the following features:
Structure of viruses:
- Protein coat (capsid): A protective layer made of protein surrounding the genetic material
- Genetic material: Either DNA or RNA (but not both); this is the hereditary material of the virus
Key characteristics:
- Viruses are non-living outside a host cell (they do not metabolise or move independently)
- They are parasitic: they can only reproduce by infecting host cells and using the host's cellular machinery
- They are much smaller than cells (typically 20–300 nanometres)
- They are obligate intracellular parasites: they cannot survive or reproduce without entering a living host cell
- Once inside a host cell, they take over the cell's machinery to replicate their genetic material and produce new viral particles
- They cause disease by damaging or destroying host cells
Why viruses are not in the five kingdoms: Viruses lack the fundamental characteristics of living organisms. They do not have cells, do not carry out independent metabolism, do not grow, and cannot reproduce without a host cell. Therefore, they occupy a separate classification outside the traditional five-kingdom system.
Students often think viruses are 'very small cells' or classify them as prokaryotes. Viruses are not cells at all—they lack ribosomes, cannot metabolise independently, and are therefore not alive outside a host.
Must Know
- Five kingdoms: Animal, Plant, Fungus, Prokaryote, Protoctist—distinguished by nucleus presence, nutrition method, cell wall composition, and movement
- Vertebrate classes: Fish (scales, gills), Amphibians (moist skin, gills + lungs), Reptiles (dry scales, lungs), Birds (feathers, wings, warm-blooded), Mammals (fur, mammary glands, warm-blooded)
- Arthropod classes: Insects (6 legs, 3 body segments), Arachnids (8 legs, 2 body segments), Crustaceans (10+ legs, 2 antennae), Myriapods (many legs, one per segment)
- Plant groups: Ferns (reproduce by spores, no flowers/seeds) and Flowering plants (produce seeds in fruits)
- Flowering plants: Dicotyledons (2 cotyledons, branched leaf veins, flower parts in 4s/5s, taproot) vs. Monocotyledons (1 cotyledon, parallel leaf veins, flower parts in 3s, fibrous roots)
- Viruses: Non-living particles consisting of a protein coat and genetic material (DNA or RNA); only reproduce inside host cells; not classified in the five kingdoms
That's the notes covered.
Carry on to the next subtopic.