IGCSE Biology: Characteristics and Classification of Living Organisms — Revision Notes

Condensed for the final weeks. For the full explanation, use the Characteristics and Classification study guide.

The seven characteristics — MRS GREN

Letter	Process	Definition
M	Movement	An action by an organism or part of an organism causing a change of position or place
R	Respiration	The chemical reactions in cells that break down nutrient molecules and release energy for metabolism
S	Sensitivity	The ability to detect and respond to changes in the internal or external environment
G	Growth	A permanent increase in size and dry mass
R	Reproduction	The processes that make more of the same kind of organism
E	Excretion	The removal of the waste products of metabolism and substances in excess of requirements
N	Nutrition	The taking in of materials for energy, growth and development

Two definitions carry the marks and are usually the ones dropped:

• Growth must say permanent and dry mass — a plant that absorbs water gains mass without growing.
• Excretion is waste made by the body’s own reactions. Egestion is undigested food passing out — it was never absorbed, so it is not excretion. Confusing the two is the most common error in the topic.

Classification

Species: a group of organisms that can reproduce to produce fertile offspring.

Binomial system — Genus species. Genus capitalised, species lower case, both italicised or underlined. It is universal, avoiding the confusion caused by common names varying between languages and regions.

(Supplement) Modern classification uses DNA base sequences: the more similar the sequences of two species, the more closely related they are and the more recently they shared a common ancestor. This is more reliable than appearance, and it usually confirms groupings originally based on morphology.

The five kingdoms (Supplement)

Kingdom	Cells	Wall	Nutrition
Animal	Multicellular, no wall	—	Ingest food
Plant	Multicellular, chloroplasts	Cellulose	Photosynthesis
Fungus	Multicellular (mostly), hyphae	Chitin	Saprotrophic — secrete enzymes, absorb
Prokaryote	Single, no nucleus	Peptidoglycan	Various
Protoctist	Mostly single, some multicellular; has a nucleus	Sometimes	Various

The distinction between prokaryote and protoctist is the presence of a nucleus, not size.

Vertebrate groups

Group	Skin	Eggs	Temperature
Fish	Wet scales	In water, no shell	Variable
Amphibian	Moist, smooth	In water, jelly-coated	Variable
Reptile	Dry scales	On land, soft leathery shell	Variable
Bird	Feathers	On land, hard shell	Constant
Mammal	Hair or fur, mammary glands	Live young (mostly)	Constant

Feathers, fur and the shell type are the single features that identify each group without ambiguity.

Arthropods

All have a hard exoskeleton, jointed legs and a segmented body. They are separated by legs and body parts:

Group	Legs	Body	Antennae
Insect	6	3 parts (head, thorax, abdomen)	1 pair
Arachnid	8	2 parts	None
Crustacean	Many, varied	Body parts vary; chalky exoskeleton	2 pairs
Myriapod	Very many, one or two pairs per segment	Many segments	1 pair

Counting legs answers most classification questions. Six means insect; eight means arachnid.

Plant groups (Supplement)

Ferns — have leaves called fronds, reproduce by spores, no flowers or seeds. Flowering plants — reproduce by seeds produced in flowers.

• Monocotyledons — one cotyledon, parallel leaf veins, flower parts in threes.
• Dicotyledons — two cotyledons, branching (net-like) leaf veins, flower parts in fours or fives.

Dichotomous keys

A series of paired statements. At each step choose the one that applies and follow it to the next pair or to a name.

When constructing one: use observable, contrasting features only, phrase each pair so exactly one must apply, and avoid anything relative such as “large” without a comparison. Features like colour or behaviour that vary with age or season make unreliable keys.

Exam traps

• Defining growth without “permanent” or without “dry mass”.
• Calling egestion excretion.
• Saying respiration is breathing — respiration is a chemical reaction inside cells.
• Getting the fungal cell wall wrong: chitin, not cellulose.
• Forgetting that reptile eggs have soft shells and bird eggs hard ones.
• Building a key on features that are not clearly contrasting.

Self-test

1. Give MRS GREN in full, with the definitions of growth and excretion.
2. What is the difference between excretion and egestion?
3. (Supplement) Name the five kingdoms and the key feature of each.
4. How do you tell an insect from an arachnid?
5. (Supplement) Give two differences between monocotyledons and dicotyledons.

Answers: 1. Movement, respiration, sensitivity, growth, reproduction, excretion, nutrition; growth is a permanent increase in size and dry mass; excretion is the removal of the waste products of metabolism and substances in excess of requirements. 2. Excretion removes waste produced by the body’s own chemical reactions; egestion removes undigested food that was never absorbed. 3. Animal (multicellular, ingests food), Plant (chloroplasts, cellulose wall, photosynthesis), Fungus (chitin wall, saprotrophic), Prokaryote (no nucleus), Protoctist (has a nucleus, mostly single-celled). 4. Insects have six legs and three body parts; arachnids have eight legs and two body parts and no antennae. 5. One cotyledon versus two; parallel versus branching leaf veins (also flower parts in threes versus fours or fives).