Characteristics, Classification & Features of Organisms (CIE IGCSE Biology)

Characteristics of Living Organisms

• Living things share seven essential life-processes (use mnemonic MRS GREN):
  • Movement – action causing change of position/place.
  • Respiration – chemical reactions that break down nutrients to release energy for metabolism.
  • Sensitivity – ability to detect/respond to internal or external stimuli.
  • Growth – permanent increase in size & dry mass via cell division and/or enlargement.
  • Reproduction – production of more organisms of the same kind.
  • Excretion – removal of toxic materials, metabolic waste & excess substances.
  • Nutrition – intake of materials for energy, growth, development.

• Plants: need light, $CO<em>2$, $H</em>2O$, ions.

• Animals: need organic compounds, ions, usually water.

Concept & Uses of Classification Systems

• Why classify?
  • Handles millions of species efficiently.
  • Groups organisms with shared features → reflects relatedness & aids communication.
• Species definition – group of organisms that can reproduce to produce fertile offspring.
• Traditional basis – shared morphological & anatomical features (e.g. mammals have hair, mammary glands, external ears).

Linnaeus & the Binomial System

• First formal classifier; introduced Latin two-part names.
  • Genus (capitalised) + species (lower case), always italicised, e.g. Homo sapiens.
• Hierarchical ranks (wide → narrow):

$\text{Kingdom} \rightarrow \text{Phylum} \rightarrow \text{Class} \rightarrow \text{Order} \rightarrow \text{Family} \rightarrow \text{Genus} \rightarrow \text{Species}$

• Mnemonic: “KING PHILIP CAME OVER FOR GRAN’S SPAGHETTI.”

Dichotomous Keys

• Tool for identifying unknown organisms via successive pairs of contrasting statements ("dichotomous" = splits into two).
• Usage guidelines:
  • Start with one organism.
  • Answer each question using provided data/picture.
  • Follow the directed path until a name is reached; restart for next organism.
• Exam focus: You almost always use a key rather than construct one (very common in MCQ paper).

Reflecting Evolutionary Relationships (Extended-tier content)

• Goal: make classification mirror evolutionary history (common ancestry).
• Limitations of morphology/anatomy: convergent evolution, subjective interpretation ⇒ potential mis-grouping.
• Molecular approach:
  • Compare DNA base sequences.
  • Greater sequence similarity ⇒ more recent common ancestor.
  • Amino-acid sequences in proteins provide supportive evidence.
• Example: sequences show Brachinus armiger vs B. hirsutus differ at only one base, thus are closest relatives in dataset.

The Five Kingdoms (first major split)

1. Animals (e.g., lion, human, bird)
2. Plants (e.g., oak tree, rose bush, moss)
3. Fungi (e.g., mushroom, yeast)
4. Protoctists (Protists) (e.g., Amoeba, Paramecium, Algae)
5. Prokaryotes (e.g., bacteria, archaea)

Main Features Per Kingdom

• Animals
  • Multicellular, nucleus present.
  • No cell walls/chloroplasts.
  • Heterotrophic (feed on organic substances).
• Plants
  • Multicellular, nuclei, cellulose cell walls, chloroplasts.
  • Autotrophic by photosynthesis.
• Fungi
  • Usually multicellular (yeast unicellular).
  • Nuclei; cell walls not cellulose (often chitin).
  • No photosynthesis; nutrition by saprophytic or parasitic absorption.
• Protoctists
  • Largely unicellular (some multicellular).
  • All have nucleus; some have cell walls & chloroplasts.
  • Nutrition varies: photosynthetic or heterotrophic.
• Prokaryotes
  • Unicellular.
  • Cell wall (non-cellulose) & cytoplasm but no nucleus or mitochondria.

Animal Kingdom in Detail

Vertebrates (possess backbone)

Invertebrates

• Lack backbone.
• Morphological split: with legs vs legless.
• All with jointed legs belong to Phylum Arthropoda. Sub-groups:
  • Insects – 3 body segments, 6 legs, 1 pair antennae, often wings (e.g., ant, butterfly, beetle).
  • Arachnids – 2 segments, 8 legs, no antennae, simple eyes (e.g., spider, scorpion, tick).
  • Crustaceans – Variable segments, 10+ legs, 2 pairs antennae, gills (e.g., crab, lobster, shrimp).
  • Myriapods – Many segments each bearing legs (e.g., centipedes, millipedes).
• Key distinction reminder: 2 antennae pairs = crustaceans; fur = mammals, etc.

Plant Kingdom in Detail (Extended-tier)

Ferns

• Green fronds (leaves).
• Reproduce by spores on underside of fronds; no flowers (e.g., maidenhair fern, bracken).

Flowering Plants (Angiosperms)

• Reproduce sexually via flowers & seeds (seeds develop inside ovary at base of flower).
• Two major groups:
  1. Monocotyledons (monocots)
     • One cotyledon in seed.
     • Floral parts in multiples of 3.
     • Parallel leaf venation.
     • Leaves narrow, grass-like (e.g. wheat).
  2. Dicotyledons (dicots)
     • Two cotyledons.
     • Floral parts in multiples of 4 or 5.
     • Reticulated (net-like) leaf veins.
     • Broader variety of leaf shapes (e.g. sunflower).
• Exam tip: monochrome MCQs often test flower-part counts & vein patterns.

Fungi, Protoctists & Prokaryotes (Cell Images Recap)

• Representative diagrams (mould hypha, Amoeba, Paramecium, bacterial cell) emphasise nucleus presence/absence, cell wall composition, chloroplast distribution.

Viruses (Outside the 5 Kingdoms)

• Not considered living: cannot perform MRS GREN independently.
• Structure: genetic material (RNA or DNA) enclosed in protein coat (capsid).
• Replicate by hijacking host cell metabolic pathways (e.g., influenza virus, HIV, bacteriophage).

Examiner Tips & Tricks (embedded reminders)

• Use MRS GREN for characteristics of life.
• Mnemonic for taxonomic hierarchy: K P C O F G S.
• Focus on using dichotomous keys; construction rarely examined.
• Distinguish between main vs defining features in questions (e.g. all vertebrates share backbone = main feature; fur = defining for mammals only).
• Monocot vs dicot distinctions (petal number, vein pattern) frequently appear in MCQs.