Features of organisms

1.3: FEATURES OF ORGANISMS

LEARNING OBJECTIVES

• State the main features used to place animals and plants into kingdoms.

• State features used to classify vertebrates and arthropods into their main groups.

• Classify organisms using identified animal and plant features.

• 🚀 State features used to place organisms into one of the five kingdoms.

• 🚀 State features used to classify ferns, dicotyledons, and monocotyledons.

• 🚀 Classify organisms using five-kingdom and plant-group features.

• 🚀 State the features of viruses, limited to protein coat and genetic material.

CORE VS EXTENDED GUIDE

• 📌 Core students study only the unlabelled sections.

• 📌 Extended students must study everything, including 🚀 Extended points.

• 📌 Extended = Core + Supplement.

Distinguishing Animals from Plants

Every living organism shares certain characteristics, but animals and plants differ in several key ways that allow scientists to place them into the correct kingdom.

Animals are multicellular organisms that do not have cell walls, do not contain chloroplasts, and do not photosynthesise. They obtain nutrition by feeding on other organisms — this is called heterotrophic nutrition. Animals store carbohydrate as glycogen.

Plants are also multicellular, but their cells have cellulose cell walls and contain chloroplasts. Plants make their own food through photosynthesis, which is autotrophic nutrition. They store carbohydrate as starch.

MisconceptionStudents sometimes state that animals "move" and plants "do not move" as a distinguishing feature. Movement is not a reliable classification feature — some animals are sessile [fixed in place], and some plants show movement responses.

Classifying Vertebrates

The animal kingdom is divided into those with backbones and those without. Animals with backbones belong to the vertebrates, which are divided into five main groups: mammals, birds, reptiles, amphibians, and fish.

Each group is identified using observable features such as body covering, method of reproduction, and method of breathing.

Mammals are the only group that produces milk to feed their young. Birds are the only vertebrates with feathers. Reptiles have dry scales and lay eggs on land with leathery shells, while amphibians have moist, permeable skin and typically lay shell-less eggs in water. Fish breathe using gills throughout their lives and have wet scales.

Examiner InsightQuestions often present an unfamiliar organism and ask students to assign it to a vertebrate group based on given features. Focus on the unique feature of each group — feathers for birds, milk for mammals, moist skin for amphibians.

Classifying Arthropods

Arthropods are invertebrates [animals without a backbone] that have an exoskeleton [a hard outer covering] and jointed legs. The four main groups of arthropods are myriapods, insects, arachnids, and crustaceans. They are classified by their number of legs and body parts.

Insects always have exactly three pairs of legs and a body divided into three sections — head, thorax, and abdomen. Arachnids have four pairs of legs and no antennae, which is the quickest way to distinguish them from insects. Crustaceans have more than eight legs and two pairs of antennae. Myriapods have many segments, each bearing legs, giving them far more than 20 legs in total.

MisconceptionStudents often classify spiders as insects. Spiders are arachnids — they have eight legs and two body sections, not six legs and three body sections. Exam cue: Count the legs first, then check the number of body sections.

Classifying Using Observed Features

Classification means placing organisms into groups based on shared features. By using the features described for animals and plants, vertebrates, and arthropods, any organism can be assigned to the correct kingdom and group.

The process works step by step. First, determine whether an organism is an animal or a plant using features like cell walls, chloroplasts, and nutrition type. Then, if it is an animal, check for a backbone. If vertebrate, use body covering, breathing method, and reproduction to assign it to the correct group. If invertebrate with jointed legs and an exoskeleton, count the legs and body sections to assign it to an arthropod group.

Examiner InsightClassification questions often present a table of features for unknown organisms and ask students to identify the group. Read every feature before deciding — do not rely on a single characteristic.

🚀 The Five Kingdoms

All living organisms are classified into one of five kingdoms: animal, plant, fungus, prokaryote, and protoctist. Each kingdom is defined by a combination of features including cell type, cell structure, nutrition, and number of cells.

• Fungi have cell walls made of chitin (not cellulose), and they feed by saprotrophic nutrition — they secrete enzymes onto dead organic matter and absorb the digested nutrients.

• Prokaryotes are microscopic, single-celled organisms whose cells lack a true nucleus [their genetic material is not enclosed in a nuclear membrane].

• Protoctists are eukaryotic organisms that do not fit neatly into the other kingdoms — some are single-celled, some are multicellular, and they show varied nutrition.

MisconceptionStudents often confuse fungi with plants because mushrooms grow in soil. Fungi do not photosynthesise and do not have cellulose cell walls — their wall is made of chitin and they feed saprophytically. Exam cue: If asked to distinguish fungi from plants, mention chitin vs cellulose and saprotrophic vs autotrophic nutrition.

🚀 Classifying Plant Groups

Within the plant kingdom, organisms can be further divided into groups based on their structural features. Two important groups are ferns and flowering plants. Flowering plants are further divided into dicotyledons and monocotyledons.

Ferns reproduce using spores rather than seeds. They do not produce flowers or seeds, but they do have true roots, stems, and leaves.

Flowering plants reproduce using seeds, which develop inside flowers. The two groups of flowering plants are distinguished by several features:

Dicotyledons have two seed leaves (cotyledons) in their seeds, a network of branching veins in their leaves, and flower parts in multiples of four or five. Examples include roses and sunflowers. Monocotyledons have one seed leaf, parallel veins in their leaves, and flower parts in multiples of three. Examples include grasses and lilies.

🚀 Classifying with Five Kingdoms and Plant Groups

Extended classification combines the five-kingdom system with the subgroups within the plant kingdom. An organism is first placed into one of the five kingdoms using features such as cell type, cell wall material, and nutrition type. If the organism is a plant, further classification determines whether it is a fern or a flowering plant, and if flowering, whether it is a dicotyledon or a monocotyledon.

The same step-by-step approach applies: identify the broadest group first, then narrow down using progressively more specific features. A flowering plant with parallel leaf veins, one cotyledon, and flower parts in threes is a monocotyledon. A plant that produces spores instead of seeds is a fern.

🚀 Features of Viruses

Viruses are not classified in any of the five kingdoms because they are not considered truly living. A virus consists of a protein coat surrounding a core of genetic material (DNA or RNA).

Viruses do not have a cellular structure — they have no cytoplasm, no ribosomes, and no cell membrane. They cannot carry out metabolic processes on their own and can only reproduce inside a living host cell. Because viruses lack the features of living cells, they sit outside the five-kingdom classification system entirely.

MisconceptionStudents sometimes describe viruses as "very small cells." Viruses are not cells — they have no cell membrane, no cytoplasm, and no organelles. They are simply genetic material enclosed in a protein coat. Exam cue: If asked why viruses are not placed in a kingdom, state that they are not cells and do not carry out life processes independently.