For each of the animal groups you have chosen, describe the structures involved in the biological process you have named. In each case, describe how these structures function, and explain how they allow each group to survive in their habitat.
Insects:
Insects have developed an internal gas exchange system. Insects have holes spiracles around the body, these holes are attached to a network of tubes that pass throughout the whole of the body. The tubes get smaller in size and end with a fluid filled tip at the body cells. The circulation system is not required in the transport of gases. The insect has several adaptations to allow an efficient gas exchange rate. The tubes end directly at the body cells which require oxygen and release carbon dioxide. The distance the gases have to travel is therefore small - increasing the rate of diffusion. This also ensures there is a concentration gradient of gases at the gas exchange surface. The gas exchange surface has a large surface area, thus there is a large surface area to volume ratio in an insect. The tips of the tubes are moist - they are fluid filled which increases rate of gas diffusion, fluid from the tube tip is drawn into the body cells further decreasing the distance the gases need to travel.
The insect can increase the rate of ventilation at the gas exchange membranes by body movements that draw air in and out through the spiracles. This allows the insect to have a high metabolic rate - so they can fly. Insects can also open and close their spiracles which allow them to control water loss and allows insect to live in extremely arid conditions
Fish:
Fish live in an aquatic environment. As water contains less oxygen than air they have developed a very efficient gas exchange system. Gas exchange takes place at the gills which are specialised structures. They have several adaptations to increase the rate of gas exchange.
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Quality of writing
Grammar, spelling and punctuation all to a high standard.
Level of analysis
The level of analysis for each type of system named for a group of animals is very well done. Concepts which would have only been done in basic detail at A level have been well researched and the research put into clear and concise information, and the structure of each system clearly related to how it enables the animal to survive in their environment. Describes a clear theme of gas exchange and how it differs between three different groups of animals with differ immensely in their mechanisms of gas exchange. Fish could be a lot better explained, as there is more to the structure and how the fish are adapted to gas exchange than the candidate gives, and there is a distinct difference in mechanisms between fresh water and sea water fish and what happens at the gills. Also would have liked to have seen the relevance of ribs and the diaphragm analysed in the human example. The candidate could also have gone on to explain the mechanism of air breathing in reptiles which differs again, and also the fact that different fish can have different respiratory methods such as a swimbladder or that some fish that live on land also have lungs, only primitive fish have the gill, basic heart and tissue capillaries setup.
Response to question
Response to the question is very well done. No introduction which is missing which could be used to make the essay structure flow better rather than just launching straight into the different animal groups and a certain structure. The main body of the text is very well analysed for this level and concise. The conclusion is well done, and brings together how each animal is similar and how each animal is different and how this adapts them to the environment they live in, however the candidate is wrong in the assumption that fish cannot have any method of ventilating their gills with air other than the water (lungfish, respiratory swimbladders, respiratory intestines and corneas).
