IGCSE Biology: Life Traits, Classification & Kingdoms

The mnemonic MRS GREN summarizes the seven traits that define life: Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion and Nutrition.
- Movement is any action that changes an organism’s position.
- Respiration consists of cellular chemical reactions that break down nutrients to release usable energy.
- Sensitivity lets an organism detect and respond to environmental changes.
- Growth is a permanent increase in size and dry mass.
- Reproduction creates more individuals of the same kind.
- Excretion removes metabolic waste and excess substances.
- Nutrition supplies the materials needed for energy, growth and development.

Classification Systems

Species and Binomial Nomenclature

A species groups organisms that can reproduce and yield fertile offspring. The internationally accepted binomial system names each species with a capitalized genus followed by a lowercase species epithet, both italicized (e.g., Homo sapiens).

Dichotomous Keys

Dichotomous keys guide identification through a series of paired, contrasting statements, forcing the user to choose between two alternatives at each step. The term “dichotomous” literally means “branching into two.”

Evolutionary Relationships and DNA

Modern taxonomy incorporates DNA sequencing. When two species share more similar base‑pair sequences, they are considered more closely related and share a more recent common ancestor. This molecular evidence refines traditional morphology‑based groupings.

The Five Kingdoms

Life is organized into five major kingdoms, each distinguished by cellular structure, nutrition mode, and other key traits:

• Animals – multicellular, nuclei present, no cell walls or chloroplasts, heterotrophic.
• Plants – multicellular, nuclei present, cellulose cell walls, chloroplasts, autotrophic via photosynthesis.
• Fungi – usually multicellular, nuclei present, cell walls lacking cellulose, obtain nutrients saprophytically or parasitically.
• Protists – mostly unicellular, nuclei present, variable cell walls and chloroplasts, diverse nutritional strategies.
• Prokaryotes – typically unicellular, lack a true nucleus and mitochondria, have non‑cellulose cell walls, possess DNA strands and plasmids.

Classification of Animals

Vertebrates (backbone)

Vertebrates are split into five classes:

• Mammals – fur or hair, milk production, four‑chambered heart.
• Birds – feathers, hard‑shelled eggs, beaks.
• Reptiles – scaly skin, leathery eggs.
• Amphibians – moist skin, aquatic larvae that metamorphose into terrestrial adults.
• Fish – scales, gills, fins.

Invertebrates – Arthropods

Arthropods possess jointed legs and are grouped into four major sub‑orders:

• Myriapods – many body segments, numerous legs.
• Insects – three distinct body regions and three pairs of legs.
• Arachnids – four pairs of legs, book lungs.
• Crustaceans – more than four leg pairs, gills for respiration.

Classification of Plants

• Ferns reproduce via spores on the underside of fronds and lack flowers.
• Flowering plants reproduce through flowers and seeds.

Among flowering plants, two major groups are distinguished:

• Monocotyledons – have branching (fibrous) roots, parallel leaf veins, and flower parts in multiples of three.
• Dicotyledons – feature a taproot system, net‑like leaf veins, and flower parts in multiples of four or five.

Viruses

Viruses are excluded from any kingdom because they cannot independently perform the seven life processes. Structurally, a virus consists of genetic material (DNA or RNA) encased in a protein coat and lacks cellular organization.