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Classification of Living Organisms

How we organise the living world

MRS GREN — 7 Life Processes

Movement — action causing change of position

Respiration — chemical reactions releasing energy

Sensitivity — detecting and responding to changes

Growth — permanent increase in size and dry mass

Reproduction — making more of the same kind

Excretion — removal of metabolic waste products

Nutrition — taking in materials for energy and growth

Classification

Organisms are classified into groups by the features they share

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

Binomial naming: Two-part scientific name — Genus + species
Example: Homo sapiens (Homo = genus, sapiens = species)

The binomial system is internationally agreed so scientists worldwide use the same names

The Five Kingdoms

Kingdom	Cell Type	Examples
Animals	Eukaryotic, no cell wall	Humans, insects, fish
Plants	Eukaryotic, cellulose wall	Trees, ferns, mosses
Fungi	Eukaryotic, chitin wall	Mushrooms, yeast, mould
Protoctists	Eukaryotic, varied	Amoeba, Plasmodium
Prokaryotes	Prokaryotic, no nucleus	Bacteria

Animal Classification

Vertebrates (have backbone): Fish, Amphibians, Reptiles, Birds, Mammals

Arthropods (jointed legs, exoskeleton): Insects (6 legs), Arachnids (8 legs), Crustaceans, Myriapods

Key features to classify:

Mammals → hair/fur, produce milk, live young
Birds → feathers, lay hard-shelled eggs, beak
Reptiles → dry scales, lay soft-shelled eggs
Amphibians → moist skin, lay eggs in water
Fish → scales, gills, fins, lay eggs in water

Plant Classification

Ferns: Have leaves (fronds), stems and roots. Reproduce by spores. No flowers or seeds.

Flowering plants (Angiosperms): Have roots, stems, leaves AND flowers. Reproduce by seeds.

Flowering plants split into:
Monocotyledons — one seed leaf, parallel veins, fibrous roots (e.g., grass, rice)
Dicotyledons — two seed leaves, net veins, tap root (e.g., roses, beans)

Dichotomous Keys

A series of paired questions used to identify organisms

Each question has TWO options, leading to either another question or an identification

Example:
1a. Has wings → Go to 2
1b. No wings → Ant
2a. Has 2 wings → Fly
2b. Has 4 wings → Go to 3
3a. Wings coloured → Butterfly
3b. Wings transparent → Dragonfly

DNA and Classification

Modern classification also uses DNA base sequences

Organisms that share a more recent ancestor (more closely related) have DNA base sequences that are more similar

Classification systems aim to reflect evolutionary relationships between organisms

This is more reliable than appearance alone — some organisms look similar but aren't closely related (convergent evolution)

Exam Tips

✓ Know all 7 life processes (MRS GREN) with exact definitions

✓ Don't confuse excretion (metabolic waste) with egestion (undigested food) — excretion is a life process, egestion is not

✓ Practice using dichotomous keys — common in Paper 6

✓ Know the 5 kingdoms and key features of vertebrate groups

✓ DNA base sequences are used for classification — more similar DNA = more closely related

Key Takeaways

✅ MRS GREN = 7 characteristics of life

✅ Species = can reproduce to produce fertile offspring

✅ Binomial naming: Genus + species (italicised)

✅ 5 Kingdoms: Animals, Plants, Fungi, Protoctists, Prokaryotes

✅ Dichotomous keys use paired questions to identify organisms

✅ DNA base sequences reflect evolutionary relationships

Now test yourself in the study guide! 💪🧬