Exoskeleton

In the syllabus and the exam

Exoskeleton is introduced in support and movement as a contrasting example to the human endoskeleton, usually through insects or other arthropods, so that candidates understand that not all animals support their bodies with internal bone. The exoskeleton is a hard external covering, made mainly of chitin, that supports the body, protects internal organs from damage, and, because it is largely waterproof, reduces water loss from the body surface. Muscles are attached to the inner surface of the exoskeleton rather than to internal bones, which is a detail candidates are expected to state when comparing the two skeletal systems in Paper 2. A commonly tested limitation is that a rigid exoskeleton cannot grow with the animal, so arthropods must periodically shed and replace it in a process called moulting in order to increase in size; candidates who omit this limitation when asked to compare exoskeletons with endoskeletons often lose marks. A frequent misconception is describing the exoskeleton as living tissue that grows continuously in the way bone does; once hardened, the exoskeleton itself does not grow, which is exactly why moulting becomes necessary. Candidates should also be able to state at least one disadvantage of an exoskeleton beyond the need to moult, such as the extra energy cost of building a new, larger covering each time, which examiners sometimes contrast with the continuous, gradual growth possible with an internal endoskeleton. This is also why newly moulted arthropods appear soft and vulnerable for a short period, since the new exoskeleton has not yet hardened fully, leaving the animal at greater risk from predators until the fresh covering sets.

Example

Comparing support systems, a candidate writes: 'Unlike a human's internal endoskeleton, an insect's exoskeleton covers the outside of the body, providing support and protection while also preventing water loss through its hard, waterproof surface.'

Related terms

Antagonistic musclesTendonAdaptation

Source:SRC-DSKP-EN