Understanding Characteristics and classification of living organisms: IGCSE Biology Explained

What makes something alive?

It sounds like a simple question, but IGCSE Biology demands a precise answer. A candle flame moves, a crystal grows, and a river responds to gravity, yet none of them is alive. The difference lies in seven specific characteristics that every living organism must display. Miss one in an exam answer, and you lose a mark. Get all seven, and you have a watertight definition.

The seven characteristics of life: MRS GREN

The mnemonic MRS GREN gives you the first letter of each characteristic. Learn this sequence and you will never leave one out under pressure.

Common confusion: excretion vs egestion

Excretion removes waste products of metabolism - substances made inside the body's own chemical reactions. Carbon dioxide from respiration, urea from the breakdown of amino acids, and excess water are all excretory products. Egestion is the removal of undigested food that was never absorbed into cells in the first place. Faeces are egested, not excreted. The exam tests this distinction regularly, and a surprising number of candidates still confuse the two.

Why we classify organisms

With an estimated 8.7 million species on Earth, biologists need an orderly system to identify, name, and group living things. Classification organises this vast diversity into manageable categories based on shared features. Organisms that share more features are placed in the same group, which tells us something about their common ancestry.

[Extended/Supplement] At the Extended level, you need to explain that modern classification reflects evolutionary relationships. Evidence comes from three sources: studies of body structure (morphology and anatomy), analysis of DNA base sequences, and comparison of protein molecules. Two species that share 98% of their DNA are almost certainly more closely related than two that share only 60%. Molecular evidence has reshaped classification in recent decades, sometimes overturning groupings that were based on appearance alone.

The binomial system

Every species has a two-part scientific name under the binomial system devised by Carl Linnaeus. The first part is the genus (plural: genera), and the second part is the species. Both parts are always written in italics (or underlined if handwritten), and the genus starts with a capital letter while the species is lowercase.

• Homo sapiens - the genus Homo and the species sapiens
• Panthera leo - the lion, sharing the genus Panthera with Panthera tigris (the tiger)
• Zea mays - maize, a cereal plant in the genus Zea

Defining a species

A species is a group of organisms that can reproduce to produce fertile offspring. A horse and a donkey can mate and produce a mule, but mules are infertile, so horses and donkeys remain separate species. This definition is straightforward for most animals but becomes awkward for organisms that reproduce asexually (such as bacteria) or that hybridise freely (some plants). For the IGCSE exam, the "fertile offspring" definition is what you need.

The five kingdoms

All living organisms are placed into one of five kingdoms. The table below lists the key features the examiner expects you to know for each one.

Vertebrate groups

Animals with a backbone (vertebral column) are vertebrates, and the IGCSE syllabus requires you to distinguish five classes. The distinguishing features examiners test are listed below.

Arthropod groups

Arthropods are invertebrates with jointed legs and an exoskeleton. The IGCSE exam expects you to distinguish four arthropod classes based on the number of legs and body divisions.

• Insects: three pairs of legs (6), three body sections (head, thorax, abdomen), one or two pairs of wings, one pair of antennae. Examples: bee, mosquito, beetle.
• Arachnids: four pairs of legs (8), two body sections (cephalothorax and abdomen), no wings, no antennae. Examples: spider, scorpion, tick.
• Crustaceans: more than four pairs of legs (often 5 pairs of walking legs), hard exoskeleton often calcified, two pairs of antennae. Examples: crab, lobster, woodlouse.
• Myriapods: many pairs of legs (one or two pairs per body segment), elongated body with many segments, one pair of antennae. Examples: centipede, millipede.

Dichotomous keys: a worked example

A dichotomous key is a series of paired statements that guide you through identifying an organism by choosing one statement at each step. "Dichotomous" means splitting into two. You follow one branch at each fork until you reach a name.

Suppose you are given five organisms to identify: a spider, a crab, a butterfly, a centipede, and a woodlouse. A key might look like this:

1. (a) Has six legs ............ go to 2
   (b) Has more than six legs ............ go to 3
2. (a) Has wings ............ Butterfly
   (b) Has no wings ............ (not applicable in this set)
3. (a) Has exactly eight legs ............ Spider
   (b) Has more than eight legs ............ go to 4
4. (a) Has two pairs of antennae and a calcified exoskeleton ............ go to 5
   (b) Has one pair of antennae and many body segments ............ Centipede
5. (a) Large body; ten or more walking legs ............ Crab
   (b) Small, oval body; seven pairs of walking legs ............ Woodlouse

When the exam asks you to construct a key, follow these principles: each step must offer exactly two mutually exclusive options, use observable (not hidden) features, and avoid subjective terms like "big" or "colourful." Measurable or countable features work best.

Features used to classify plants

The IGCSE syllabus also requires you to distinguish plant groups. The two main divisions tested are flowering plants (angiosperms) and ferns.

• Flowering plants (angiosperms): produce seeds enclosed in a fruit; have flowers for reproduction; further divided into monocotyledons (one seed leaf, parallel leaf veins, fibrous roots) and dicotyledons (two seed leaves, branching leaf veins, taproot).
• Ferns: do not produce seeds or flowers; reproduce using spores produced in sporangia on the underside of fronds; have roots, stems and leaves.

[Extended/Supplement] At the Extended level, you should also be aware that classification is an evolving discipline. DNA sequencing has revealed that some organisms previously grouped by physical similarity are actually distantly related, while others that look quite different share recent common ancestors. This is why modern classification increasingly relies on molecular evidence alongside traditional morphological study.

Common exam mistakes

1. Defining growth as "getting bigger": the mark scheme requires "a permanent increase in size and dry mass." Temporary swelling (such as a cell absorbing water) is not growth. The word "permanent" is essential.
2. Confusing excretion with egestion: excretion removes metabolic waste (CO₂, urea). Egestion removes undigested food (faeces). The exam almost always includes a question that tests whether you know the difference.
3. Saying respiration is "breathing": respiration is a chemical reaction inside cells. Breathing (ventilation) is the physical movement of air in and out of the lungs. They are related but not the same process.
4. Forgetting to italicise (or underline) binomial names: Homo sapiens must be in italics in print or underlined if handwritten. The genus takes a capital letter; the species does not. Writing "homo Sapiens" or "Homo Sapiens" will cost you a mark.
5. Placing organisms in the wrong kingdom: yeast is a fungus, not a bacterium. Chlorella is a protoctist, not a plant. Amoeba is a protoctist, not an animal. These are tested repeatedly.
6. Using subjective features in dichotomous keys: "is it pretty?" or "is it large?" are not valid key statements. Use measurable, observable features like number of legs, presence of wings, or type of body covering.
7. Confusing ectothermic and endothermic: ectothermic organisms depend on external heat sources to regulate body temperature (reptiles, fish, amphibians). Endothermic organisms generate their own body heat internally (birds, mammals). "Cold-blooded" and "warm-blooded" are imprecise terms the exam avoids.

Self-check questions

1. List all seven characteristics of living organisms using the MRS GREN mnemonic. For each one, write a single-sentence definition that would satisfy the mark scheme.
2. Explain the difference between excretion and egestion, giving one example of a substance involved in each process.
3. A scientist discovers a multicellular eukaryotic organism that has chitin cell walls and absorbs nutrients from decaying wood. Which kingdom does it belong to, and why?
4. An organism has eight legs, two body sections, and no antennae. Identify the arthropod class and name one other organism from the same class.
5. Write a four-step dichotomous key that could be used to distinguish a frog, a lizard, a robin, and a salmon using only the features in the vertebrate classification table above.
6. [Extended] Explain how DNA evidence can change the way organisms are classified, giving one reason why molecular evidence might contradict a classification based on physical appearance.