IGCSE Bio Topic 1

Detailed Notes on Characteristics and Classification of Living Organisms

Characteristics of Living Organisms

1. Definition: Biology is the study of living things (organisms). Organisms are recognized as living if they exhibit seven characteristics:

   • MRS GREN mnemonic:

     • M: Movement - Action causing a change of position or place.

     • R: Respiration - Chemical reactions in cells breaking down nutrient molecules to release energy.

     • S: Sensitivity - Detection and response to environmental changes.

     • G: Growth - Permanent increase in size and dry mass.

     • R: Reproduction - Processes to produce more of the same species.

     • E: Excretion - Removal of metabolic waste and excess substances.

     • N: Nutrition - Intake of materials for energy, growth, and development.

Classification of Living Organisms

1. Purpose: Grouping organisms to simplify study and reflect evolutionary relationships.

2. Species:

   • Defined as organisms that can interbreed to produce fertile offspring.

   • Species sharing similar features are grouped together.

The Binomial System

1. Definition:

   • A scientific naming system agreed internationally.

   • Comprises two parts: Genus (capitalized) and species (lowercase).

   • Always italicized (e.g., Homo sapiens).

2. Benefits:

   • Allows hierarchical classification into smaller groups with more shared features.

Dichotomous Keys

1. Usage:

   • Identify organisms via a branching series of questions based on physical features.

   • Each step presents two choices, guiding towards the correct identification.

2. Example:

   • Organism with "body covered in hair" → Check for "webbed feet" → Correct identification.

Classification Systems and Evolutionary Relationships

1. Traditional Classification:

   • Based on shared physical features.

   • Challenges: Insufficient for detailed classification.

2. DNA Sequencing:

   • DNA base sequences reveal evolutionary relationships.

   • Species with more similar DNA share a more recent common ancestor.

The Five Kingdoms

1. Overview: Living organisms are divided into:

   • Animals:

     • Multicellular, no cell walls/chloroplasts.

     • Nutrition from consuming others.

     • Cell features: Nucleus, cell membrane, cytoplasm.

   • Plants:

     • Multicellular, have cell walls (cellulose) and chloroplasts.

     • Nutrition via photosynthesis.

   • Fungi:

     • Mostly multicellular, cell walls (not cellulose).

     • Nutrition by saprophytic or parasitic feeding.

   • Protoctists:

     • Mostly unicellular, some multicellular.

     • Nutrition through photosynthesis or consuming others.

   • Prokaryotes:

     • Unicellular, no nucleus/mitochondria, have DNA strands/plasmids.

Animal Kingdom: Vertebrates and Invertebrates

1. Vertebrates:

   • Animals with backbones.

   • Five groups:

     • Mammals: Hair, milk production, four-chambered heart.

     • Birds: Feathers, hard-shelled eggs, beak.

     • Reptiles: Scaly skin, rubbery-shelled eggs.

     • Amphibians: Moist skin, gills (larva), lungs (adult).

     • Fish: Scales, gills, fins.

2. Invertebrates:

   • Animals without backbones.

   • Arthropod subgroups:

     • Myriapods: Segmented bodies, jointed legs (e.g., centipedes).

     • Insects: Three-part body, three pairs of legs, wings (e.g., grasshoppers).

     • Arachnids: Four pairs of legs, book lungs (e.g., spiders).

     • Crustaceans: >Four pairs of legs, gills (e.g., crabs).

Plant Kingdom

1. Ferns:

   • Fronds (leaves), reproduce via spores under fronds.

2. Flowering Plants:

   • Reproduce using flowers and seeds.

   • Divided into:

     • Monocotyledons: Parallel leaf veins, multiples of 3 petals, branching roots.

     • Dicotyledons: Branched leaf veins, multiples of 4 or 5 petals, taproots.

Viruses

1. Features:

   • Non-living; not classified in the kingdoms.

   • Composed of genetic material surrounded by a protein coat.

   • Require a host cell to reproduce.

Learning Objectives

1. Core:

   • Describe life characteristics.

   • Define species and binomial naming.

   • Use dichotomous keys.

   • Classify into kingdoms and animal/plant groups.

2. Extended:

   • Explain evolutionary-based classification and DNA sequencing.

   • Classify using broader kingdom distinctions (e.g., ferns vs. flowering plants).

   • Recognize viruses as non-living.