Characteristics & Classification

Characteristics of Living Organisms

Seven processes together separate living organisms from non-living things. Every organism carries out all seven, even when one is too slow to observe, as in a dormant seed.

Three distinctions decide marks. Respiration releases energy inside cells, whereas breathing only moves air. Excretion removes waste made by metabolism; faeces are undigested food that never entered the cells, so their removal is egestion. Growth needs a permanent gain in dry mass: a tissue that swells with water has changed size but has not grown.

Worked example (naming characteristics): A plant on a windowsill leans towards the glass and, over 14 days, grows from 12 cm to 19 cm tall. Which characteristics does it show? Step 1. The plant detects the direction of light and responds by leaning: sensitivity. Step 2. Height increased by 7 cm and the change is permanent, so the plant has grown: growth. Step 3. The plant also respires and excretes, but nothing in this experiment demonstrates them. Answer: Sensitivity and growth.

Species and Scientific Names

Organisms are classified by the features they share: those with many important features in common go in one group. The basic unit of the system is the species.

What a species is

A species is a group of organisms that can reproduce with one another to produce fertile offspring. Fertility is the test that matters: if the offspring can reproduce as well, the group continues as one breeding population. A horse and a donkey can mate and produce a mule, but a mule is almost always sterile, so horses and donkeys remain separate species.

The binomial system

Each species has a two-part scientific name, agreed internationally so that every country means the same organism whatever its local name. The first part is the genus, the second the species.

• The genus begins with a capital letter; the species name begins with a lower-case letter.

• Printed names are in italics; handwritten names are underlined, each word separately.

Worked example (writing a binomial name): The domestic cat belongs to the genus Felis and the species catus. Write its scientific name. Step 1. Genus first, then species: Felis catus. Step 2. Capital F for the genus, lower-case c for the species. Step 3. The species word alone cannot name an organism: many genera share the same second word. Answer: Felis catus.

Dichotomous Keys and DNA

Building and using a key

A dichotomous key offers two contrasting choices at every step. Each step splits the organisms into two groups and uses a feature that can be observed reliably, such as leg number, never one that varies within a species, such as colour or size. Every organism must end at one name.

Worked example (using a key): A specimen has 14 legs. Use Key 1.1 to name it. Step 1. Fourteen is not six, so statement 1b applies: go to 2. Step 2. Fourteen is not eight, so statement 2b applies: go to 3. Step 3. Statement 3a matches. Colour and size were never needed. Answer: Woodlouse.

ExtendedClassification and DNA

Classification aims to reflect evolutionary relationships: grouped organisms share a more recent common ancestor. Differences build up in DNA base sequences over time, so species with a recent common ancestor have more similar sequences than species whose ancestor is distant.

Vertebrates

Five groups of vertebrates are separated by a small set of features: the body covering, the breathing organs, how the young begin life, and whether body temperature stays constant.

Classifying with several features

One feature is rarely enough, so a classification uses two or three together. The strongest single features are feathers for birds, hair with mammary glands for mammals, gills with fins for fish, and a larval stage for amphibians.

Worked example (classifying an animal): An animal has dry scaly skin, breathes with lungs, lays shelled eggs on land, and its body temperature changes with its surroundings. Name its group. Step 1. No feathers or hair rule out birds and mammals; lungs rule out fish. Step 2. Dry skin and shelled eggs laid on land rule out amphibians, whose skin is moist and eggs jelly-coated. Step 3. Four features agree on one group, so the identification does not depend on any single feature. Answer: Reptile.

Arthropods

All arthropods share jointed legs, a segmented body and an exoskeleton. Four groups are then separated by counting legs, body regions and antennae.

Centipedes have one pair of legs on each body segment; millipedes have two pairs on each segment.

Worked example (classifying an arthropod): An arthropod has five pairs of legs and two pairs of antennae. Name its group. Step 1. Five pairs is more than the three of insects or the four of arachnids. Step 2. Two pairs of antennae rule out insects and myriapods, which have one pair, and arachnids, which have none. Step 3. Both counts point to the same group, so neither needs to be taken on its own. Answer: Crustacean.

The Five Kingdoms

Animals and plants are the two kingdoms every candidate must know. Extended candidates also separate fungi, protoctists and prokaryotes.

Reading the table

Three questions place most organisms. Is there a nucleus? If not, it is a prokaryote. If so, is it many-celled or single-celled? Many-celled organisms are animals, plants or fungi; single-celled ones are usually protoctists. Finally, how does it feed: by making food with chloroplasts, by absorbing it, or by taking it in? Yeast is the exception to remember: a fungus, but single-celled.

ExtendedWorked placement

A microscopic organism has a cell wall, circular DNA and no nucleus. No nucleus means a prokaryote; every other kingdom has one. The cell wall and the loop of DNA agree.

Plant Groups and Viruses

Within the plant kingdom, Extended candidates separate ferns from flowering plants, and flowering plants into monocotyledons and dicotyledons. Viruses fall outside all five kingdoms.

ExtendedViruses

A virus is not a cell. It has just two parts: a coat made of protein, and genetic material inside it, either DNA or RNA. Because it is not made of cells, it is placed in none of the five kingdoms. It can reproduce only inside a host cell.

ExtendedWorked placement

A plant has a single cotyledon in its seed and long, narrow leaves with veins running side by side. One cotyledon and parallel veins both describe a monocot, so the plant is a monocot and the answer does not rest on the leaves alone.