exchange and transport

As a cube gets bigger its SA;V ratio will get smaller 

substances that organisms need to exchange with their environment= 

• oxygen and nutrients 
• heat
• cells must excrete waste products like carbon dioxide, urea and water 

Passive exchange= no energy required 

Active exchange= energy required 

small orgainisms have a large surface area ; volume ratio - efficient exchange 

large organinsms have a smaller surface area ; volume ratio - makes it harder to exchange subtances  

As a orgainism becomes larger thier volume increases at a faster rate than the surface area. This can cause problems if you are active as diffusion can not happen so we need to develop specific exchange surfaces

Gas exchange in sigle celled organisms and insects 

• large SA:V therefore need no lungs 
• gases can diffuse across cell membrane (and cell wall if present)

insect gas exchange

needs large SA;V but water will also be lost easily 

waterproof exoskeletons and small SA:V 

exoskeletons and size means no simple diffusion through external surface.

Insects have a internal network of tubes called tracheoles. the tracheoles extend into the muscle tissue from the trachea. The trachea has air with oxygen in it, is moved along a diffusion gradient. As the cells respire oxygen is used up. The concentration towards the end of the tracheoles fall. This creates a diffusion gradient. Which makes the gas oxygen diffuse from the atmosphere along the trachea and tracheoles. In the opposite direction is Carbon dioxide. 

Diffusion gradient = cone of oxygen at the end of tracheoles is lower because oxygen is used up by the cells in respiration - oxygen will therefore diffuse towards the end of the tracheoles from the spiracle - vice versa for carbon dioxide 

ventilation=  muscle contractions help movement of air so insects that need a large supply of oxygen will pump the air in this way 

ends of tracheoles filled up with water during exercise cells respire anaerobically which produces lactate.This lowers the water potenial of the cells therfore moves into these cells by osmosis from the tracheoles this decreases the amount of water in them so this means more air can be drawn in