organisms exchange substances with their environment

• organisms larger than 1mm are multicellular
• larger organisms have all evolved adaptations with a large sa
• most larger organisms have specialised exchange systems
• a flattened shape increases sa and reduces diffusion pathway
• external gills increase sa however are unprotected so are easily damaged
• internal gills are highly vascularised internal gills (ventilation mechanism draws water over the gill surfaces)
• within tracheal tubes gas exchange takes place at the ends of terminals
• insects have exoskeletons
• insects' exoskeletons are composed of hard fibrous material called chitin(protection) and covered by a lipid rich layer (to prevent water loss)
• insects do not have lungs instead they have tracheal system
• insects tracheal system consists of a series of small holes in the exoskeleton called spiracles
• spiracles open to a network of fine tubes called trachae (lined with rings of chitin to keep them open)
• tracheae then sub-divide into tubes of smaller diameter called tracheoles (unlined)
• tracheoles end at body cells
• gaseous exchange is carried out across the walls of the tracheoles by diffusion