Main Features of Animals

Cellular Organisation

• Animals are multicellular eukaryotes.
  • Each cell possesses membrane-bound organelles and a true nucleus.
  • Multicellularity allows specialisation (e.g.
  • $\text{myocytes}$ for contraction,
  • $\text{neurons}$ for information transfer,
  • $\text{epithelial cells}$ for protection & absorption).
  • Contrast with unicellular organisms (many protists, bacteria) that carry out every life function within one cell.

Genetic Material

• Nucleus contains linear DNA arranged in chromosomes.
  • Linear configuration facilitates complex regulation (multiple origins of replication, histone packaging, etc.).
  • During mitosis/meiosis, chromosomes condense, allowing accurate segregation to daughter cells.
• Reminder link to earlier lecture: Plant cells have the same linear‐chromosome arrangement, whereas bacterial DNA is typically circular and localised in a nucleoid, often accompanied by plasmids.

Absence of Cell Wall & Chloroplasts

• Animal cells lack cellulose cell walls.
  • Gives flexibility, enabling diverse shapes (neuronal axons, muscle fibres).
  • But requires alternative structural support: extracellular matrix rich in collagen, elastin, proteoglycans.
• No chloroplasts ➔ animals cannot photosynthesise.
  • Energy must therefore be obtained heterotrophically (see Nutrition below).
• Practical implication: absence of a rigid wall permits phagocytosis (key for immune cells such as macrophages).

Nutrition (Heterotrophy)

• Animals feed on organic substances made by other living things (heterotrophic metabolism).
  • Modes include herbivory, carnivory, omnivory, parasitism, filter-feeding, detritivory.
• Biochemical pathway emphasis:
  • Aerobic respiration: $\text{C}<em>6\text{H}</em>{12}\text{O}<em>6 + 6\,\text{O}</em>2 \rightarrow 6\,\text{CO}<em>2 + 6\,\text{H}</em>2\text{O} + \text{ATP}$.
  • Comparative link: plants produce their own glucose via photosynthesis before entering the same respiratory pathway.

Energy Storage

• Surplus carbohydrates are commonly stored as glycogen (a highly branched polysaccharide).
  • Predominant storage sites: liver and skeletal muscle.
  • Rapid mobilisation allows animals to meet sudden bursts of energetic demand (e.g. flight-or-fight response).
• Contrast: Plants store energy mainly as starch, fungi also store glycogen (useful classificatory clue).

Nervous Coordination

• Animals usually possess nervous systems for rapid communication and coordination.
  • Neurons communicate via electro-chemical signals (action potentials) up to 120\,\text{m·s}^{-1} in myelinated axons.
  • Enables sophisticated behaviours: predator avoidance, complex social interaction, learning & memory.
• Philosophical/ethical angle: Existence of sentience raises welfare considerations in research, agriculture, and conservation.

Locomotion

• Most animals can move freely at some life stage.
  • Structural bases: cilia/flagella in protozoan ancestors, hydrostatic skeletons (worms), exoskeletons (arthropods), endoskeletons (vertebrates).
  • Evolutionary significance: mobility drives predator–prey arms races, niche expansion, and biodiversity.

Comparison with Other Kingdoms

• Plants: multicellular autotrophs, possess cellulose walls & chloroplasts, store starch.
• Fungi: heterotrophs with chitinous cell walls, store glycogen, generally non-motile.
• Bacteria (Prokaryotae): unicellular, peptidoglycan cell wall, circular DNA, no nucleus; some are photosynthetic, many are heterotrophic.
  • Transcript note: “Bacterias contain …” likely refers to plasmids or cell walls; recall they often carry extra-chromosomal plasmid DNA that confers antibiotic resistance.

Real-World Relevance

• Medicine: Understanding animal cell biology informs oncology (loss of multicellular coordination), neurodegenerative research (nervous coordination failures), and metabolic disorders (glycogen storage diseases).
• Biotechnology: Animal cell cultures produce vaccines, monoclonal antibodies, and recombinant proteins.
• Ecology & Conservation: Mobility and heterotrophy place animals as consumers in trophic webs, critical for nutrient cycling and ecosystem stability.

Numerical Snapshot of Animal Diversity

• Estimated described animal species: $\approx 1.5 \times 10^{6}$.
• Insects alone constitute >70\% of known animal diversity.

Study Tips & Connections

• Be able to contrast animal features with those of at least two other kingdoms in essay or MCQ format.
• Draw and label a generalized animal cell, indicating nucleus, mitochondrion, ribosome, Golgi, but highlight absence of chloroplast and cell wall.
• Practice explaining why glycogen’s highly branched structure accelerates enzymatic breakdown compared with amylose (starch).