Gas exchange • across the body surface of a single-celled organism • in the tracheal system of an insect (tracheae and spiracles) • across the gills of a fish (gill lamellae and filaments including the countercurrent principle) • by leaves of dicotyledonous plants (mesophyll and stomata). Candidates should be able to use their knowledge and understanding of the principles of diffusion to explain the adaptations of gas exchange surfaces. Structural and functional compromises between the opposing needs for efficient gas exchange and the limitation of water loss shown by terrestrial insects and xerophytic plants.

Single-celled organism
High surface area to volume ratio means that things can diffuse across the organism quickly. This is why they do not need systems, but can just rely on diffusion through them.

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Insects
Low surface area to volume ratio and waterproof coverings (to stop water loss) mean that insects cannot rely on diffusion over their body.
They have developed a tracheal system: air enters via spiracles (controlled by valves) into trachea and then into tracheoles.
Tracheoles go throughout the body meaning that gasses diffuse through them into the all of the insect.
Sometimes insects pulse or move muscles to ventilate the system.
Reducing water loss; water proof outer layer; reduced surface area to volume ratio (small); they can close spiracles.

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Fish
Low surface area to volume ratio and non-permeable outer means they need a specialised system for gas exchange: gills.
Gills consist of gill filaments (which are like strands of a feather) and gill lamellae (which are like discs strung along the filaments). Blood is running through the gills.
A fish pumps water over the gills by opening an closing its mouth- this pushes water out of the mouth, over the gills and then out the sides of the fish.

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The water and the blood are flowing in opposite directions- this is called countercurrent flow and is explained in this post: http://hannahhelpbiologya.blogspot.co.uk/2014/03/the-countercurrent-principle.html

Leaves
Leaves have a large surface area to volume ratio so they do not need a gas exchange system.
However there are adaptations to make diffusion easier, for example the flat shape meaning there is a short diffusion distance and spaces in-between meysophyll cells for gasses to diffuse through easily.
Leaves have stomata on them (which are basically just holes) so they can control diffusion. This means that when they are at risk of loosing too much water the guard cells (that control the stomata) can close the hole to prevent water from evaporating from the leaf- this is an example of a compromise made between the need to exchange gasses and preserve water.

Xerophytic plant adaptations
These plants have developed adaptations to limit the water loss due to the fact that transpiration exceeds root uptake of water.

• Spines for leaves mean that there is a reduced surface area to volume ratio so less surface area from water to be lost from; but they have a thick stem to provide a large surface area for photosynthesis. (cacti)
• Having hairy leaves traps moist air around the plant reducing the water potential gradient.
• Stomata that are sunken into groves also will have moist air trapped around them.
• Leaves roll up to trap moist air around the stomata as well. (marram grass)
• A thick cuticle (waxy layer) which is waterproof stops water being evaporated through the top of the leaf.