Gas Exchange

Theres a lower concentration of oxygen in the water than in the air.

• Water (with o2) enters the mouth and passes through the gills.
• Each gill is made up of gill filaments, which provide a large surface area for gas exchange.
• Gill filaments are covered in lots of lamellae which increase the surface area more.
• Blood flows through the lamallae in one direction and water in the oppostie direction.
• This is the counter current system, in maintains a large concentration gradient between the blood and the water.

• Insects have microscopic air filled pipes called tracheae, used for gas exchange.
• Air moves into the tracheae through spiracles (pores on the surface)
• Oxygen travels down the concentration gradient towards the cells.
• CO2 travels down the concentraiton gradient away from cells to go out the spiracles.
• Tracheae branch off into tracheoles which have thin permeable walls and go to individual cells. This means oxygen diffuses directly into respiring cells.
• They also use rhythmic abdominal movements to move air in and out of the spiracles.

Gas exchage surfaces have two major adaptions. They have a large surface area and are thin which provides a short diffusion pathway. They also maintain a steep concentration gradient. These all increase the rate of diffusion. 
Single celled organisms exhcange gases across their body surface by diffusion. They have a large surface area, a thin and short diffusion pathway so theres no need for a gas exchange system.

Plants exchange carbon dioxide and oxygen at the mesophyll cells, they have a large surface area. They move in and out through the stomata which are controller by guard cells.

Insects and plants can control water loss. 
Insects can close their spiracles using muscles, they also have a waterproof waxy cuticle all over their body and hairs around the spiracles all which reduce evaporation.
In plants if the guard cell becomes flaccid they close the stomata.

Some plants are specially adapted for certain habitats where water loss is a problem - Xerophytes
Stomata sunk in pits to trap moist air (reduces the diffusion gradient).
Curled leaves which protect the stomata from the wind.