Introduction: Movement as a Universal Feature of Life
Movement is a fundamental characteristic of living organisms. It takes two broad forms:
Internal movement: Movement occurring within the body of an organism , present in all living things, even sessile ones. It is essential for nutrient distribution, waste removal, and maintaining cellular functions.
Locomotion: The ability of an organism to move from one location to another. It enables organisms to find resources, evade predators, reproduce, and migrate.
Why does locomotion matter?
- Foraging: Predators chase prey; herbivores seek plants.
- Predator evasion: Rapid locomotion reduces the risk of being caught.
- Reproduction: Finding mates and dispersing offspring.
- Migration: Seasonal journeys (e.g., snow geese) to access food and breeding sites.
Internal movement examples:
- Plants: Water and nutrient transport through xylem and phloem.
- Animals: Peristalsis and blood circulation.
- Unicellular organisms: Cytoplasmic streaming moves nutrients and organelles.
Locomotion examples:
- Cheetahs: Sprinting to catch prey.
- Birds: Migrating thousands of kilometres.
- Bacteria: Moving via flagella.
All organisms exhibit internal movement, but only motile organisms show locomotion. Even sessile organisms like barnacles and corals rely on subtle movements (e.g., filter-feeding appendages) and often have motile larval stages for dispersal.
Motile vs. Sessile Organisms
Organisms can be broadly classified based on their capacity for locomotion:
Motile organisms actively move through their environment and show:
- Skeletal and muscular systems: e.g., elongated limbs in cheetahs for speed.
- Energy storage: Migratory birds accumulate fat reserves before long flights.
- Behavioural strategies: Responding to seasonal cues or predator threats.
Sessile organisms remain fixed in one location for most of their lives (e.g., barnacles, corals, many cnidarians) yet still exhibit movement at smaller scales:
- Feeding movements: Barnacles use cirri (modified legs) to filter-feed; corals use tentacles to capture prey.
- Larval motility: Many sessile species produce motile larvae for dispersal before settling.
- Anchorage mechanisms: Adhesive cement in barnacles; calcium carbonate skeletons in corals.
Even sessile organisms rely on subtle movements for feeding, reproduction, and survival. The distinction between internal movement and locomotion is important for IB exam answers.
Movement adaptations across taxonomic groups:
| Group | Mechanism | Example |
|---|---|---|
| Unicellular organisms | Flagella, cilia, pseudopodia | Amoeba (cytoplasmic streaming) |
| Invertebrates (arthropods) | Exoskeleton + internal muscles | Grasshopper jumping |
| Vertebrates | Endoskeleton + skeletal muscles | Human walking |
| Marine mammals | Streamlined body + flippers/flukes | Dolphin swimming |
Think of locomotion adaptations as an organism's "toolkit" for navigating its environment , the toolkit looks very different in a bacterium, an insect, and a whale, but all serve the same fundamental purposes of finding food, avoiding danger, and reproducing.