Plant and animal kingdoms

Plants and animals are placed in different kingdoms because their cells have different structures and they obtain food in different ways.

Plant kingdom

Plant cells have a cellulose cell wall around the cell membrane and a large permanent vacuole. Many plant cells also contain chloroplasts, where photosynthesis takes place. Not every plant cell has chloroplasts: for example, root cells generally do not.

Plants make their own food by photosynthesis, using light energy to make sugars. Organisms that make their own food are described as autotrophic.

Animal kingdom

Animal cells do not have cell walls or chloroplasts. Animals cannot make food by photosynthesis; they obtain it by feeding on other organisms. Organisms that obtain food this way are described as heterotrophic.

Most animals can move and have nervous systems that coordinate rapid responses to changes around them.

Distinguishing plants from animals

When classifying an organism at this level, look at both its cell structures and how it obtains food. A plant has cellulose cell walls and makes food by photosynthesis, using chloroplasts in its photosynthesising cells. An animal lacks cell walls and chloroplasts and obtains food by feeding.

Arthropod classification

Arthropods are animals with a hard external skeleton, jointed limbs and a segmented body. Their shared body features distinguish them as arthropods, while differences in those features help identify which group an arthropod belongs to.

The main groups can be distinguished by their body sections, legs, antennae and wings:

• Myriapods have many similar body segments and many legs.
• Insects have three body sections and three pairs of legs. They usually have one pair of antennae, and many have wings.
• Arachnids have two body sections and four pairs of legs. They have no antennae.
• Crustaceans usually have more than four pairs of legs and have two pairs of antennae. They are mainly aquatic.

To classify an arthropod, compare several features rather than relying on just one. For example, an animal with three pairs of legs and three body sections is an insect, even if it has no wings.

Classifying organisms by features

Biological classification places organisms into groups by matching their characteristics to the defining features of those groups. To make a classification decision, identify several relevant features, compare them with the group descriptions and state which features support your choice.

Classifying an organism as a plant or an animal

Plants have cell walls and typically have chloroplasts, which contain chlorophyll for photosynthesis. Not every plant cell contains chloroplasts: for example, cells in roots generally do not.

Animals feed on other organisms or organic material. Their cells do not have chloroplasts or cell walls. When classifying an organism, use the combination of features rather than relying on just one.

Classifying vertebrates

An animal with a backbone is a vertebrate. To decide which vertebrate group it belongs to, compare its body covering, reproduction and breathing structures:

• Mammals have hair or fur, breathe with lungs and feed their young with milk. Most give birth to live young.
• Birds have feathers and beaks, breathe with lungs and lay hard-shelled eggs.
• Reptiles have dry, scaly skin and breathe with lungs. Most lay eggs on land.
• Amphibians have moist skin. Their young usually live in water and breathe through gills; adults usually breathe with lungs and through their skin. Most lay eggs in water.
• Fish live in water, have fins and breathe through gills. Most have scales and lay eggs.

For example, an animal with a backbone, feathers and a beak can be classified as a bird. The backbone identifies it as a vertebrate, while the feathers and beak support the more specific classification.

Classifying arthropods

Arthropods are animals with jointed legs and a hard external skeleton. They do not have a backbone. To identify an arthropod group, look at the number of legs, the arrangement of body sections and the number of antennae:

• Insects have six legs, three main body sections and one pair of antennae.
• Arachnids have eight legs, two main body sections and no antennae.
• Myriapods have a long body made of many segments, many legs and one pair of antennae.
• Crustaceans have two pairs of antennae and usually have five or more pairs of legs. Their bodies commonly have two main sections.

For example, an arthropod with six legs, three body sections and one pair of antennae is an insect. These features together justify the decision; having jointed legs alone would identify it only as an arthropod.

Fungus kingdom

Fungi obtain nutrients from organic material rather than making their own food by photosynthesis. Their cells have cell walls, but they do not have chloroplasts, so they cannot photosynthesise.

How fungi obtain nutrients

A fungus releases digestive substances onto organic material outside its body. These break large molecules into small, soluble products, which the fungus then absorbs. This is called external digestion.

The structure of many fungi

Many fungi are made of fine, thread-like structures called hyphae. The hyphae grow through or over the material from which the fungus obtains nutrients. Together, they form a network called a mycelium.

Fungi, plants and animals

Fungi and plants both have cell walls, but photosynthetic plants have chloroplasts and can make food by photosynthesis. Fungi do not have chloroplasts and obtain nutrients by external digestion and absorption.

Animals obtain food by ingestion: they take it into their bodies before digesting it. Fungi instead digest organic material outside their bodies and absorb the soluble products.

Prokaryote kingdom

Prokaryotes are single-celled organisms. Each organism consists of one cell, whose genetic material is not enclosed in a nucleus.

A prokaryotic cell contains cytoplasm surrounded by a cell membrane. Most prokaryotic cells also have a cell wall outside the membrane. Their genetic material is found within the cell but is not separated from the cytoplasm by a nucleus.

This absence of a nucleus distinguishes prokaryotic cell organisation from that of animals, plants, fungi and protoctists, whose cells have nuclei enclosing their genetic material.

Protoctist kingdom

Protoctists are a diverse group of organisms whose cells have nuclei. Most protoctists are single-celled: one cell carries out the activities needed for life.

Protoctists differ in how they obtain food. Some have chloroplasts and make their own food by photosynthesis. Others obtain food from other organisms or from organic material.

The ability to photosynthesise does not, by itself, mean that an organism belongs to the plant kingdom. A single-celled organism with a nucleus may photosynthesise and still be classified as a protoctist.

Ferns and flowering plants

Ferns and flowering plants can both have roots, stems and leaves. These general features help identify them as plants, but they do not distinguish the two groups. The distinction is chiefly in how they reproduce.

Ferns have roots, stems and leaves, but they do not produce flowers or seeds. They reproduce using spores, which can develop into new ferns.

Flowering plants produce flowers and form seeds. A seed can develop into a new plant. A plant does not need to have visible flowers at every moment to belong to the flowering-plant group; it is classified by the reproductive structures it produces, rather than by its general appearance alone.

Monocotyledons and dicotyledons

Flowering plants can be classified into two groups by comparing their seeds, leaves and flowers. The groups are named for the number of cotyledons, or seed leaves, in the seed.

• Monocotyledons have one cotyledon. Their leaves usually have parallel veins, and their flower parts are commonly in multiples of three, such as three or six petals.
• Dicotyledons have two cotyledons. Their leaves usually have veins that branch to form a network, and their flower parts are commonly in multiples of four or five, such as four or five petals.

These features can be used together to identify a plant. For example, if its seed cannot be observed, its leaf veins and flower parts can provide evidence of which group it belongs to. If no flower is visible, the seed and leaves can be compared instead. Leaf-vein patterns and flower-part numbers are usual features, not rules that apply to every plant.

Virus structure

A virus has genetic material enclosed by a protein coat. The genetic material carries the virus’s information, while the coat surrounds and protects it.

These two parts are the basic features used to describe a virus’s structure. Viruses are not classified as members of any of the five organism kingdoms.