Exchange in animals

• All organisms need to exchange materials with their environment.
• Surface area is calculated by finding the area of each of the shapes faces and adding them all together.
• Volume is found by multiplying a shapes length, width, and height measurements.
• The smaller an organism the larger the surface area: volume ratio. This is why cells are small.
• Animals in cold climates are large to minimise heat loss.
• Specialised exchange surfaces have the following features:

  A large surface area relative to the size of the organism

  Very thin

  Selectively permeable
• Diffusion is proportionate to:

  surface area      x     difference in concentration

                   Length of diffusion path
• Areas where exchange takes place optimise all of these variables in order to achieve rapid diffusion. These areas tend to be vulnerable and protected deep inside an organism.
• Gas exchange in single-celled organisms occurs by diffusion.
• Insects exchange gases by diffusion.
• Insects have an internal network of cells called tracheae
• The tracheae divide into smaller dead end tubes called tracheoles. These tracheoles extend throughout the insect.
• Gas exchange occurs as a result of diffusion between the tracheoles and the insect’s body tissues.
• The diffusion process is helped by muscle contraction when the insect moves.
• Gases enter and leave tracheae through tiny pores called spiracles which can be opened and closed by a valve. These can be controlled to minimise water loss.
• The length of the diffusion pathway limits the size of insects. Insects used to be a lot larger when there was more oxygen in the atmosphere.
• Fish have developed a specialised gas exchange structure called gills.
• Gills are located behind the head of fish. They are made up of gill filaments.
• Gill lamellae are at right angles to the gill filaments.
• The flow of water over the gill lamellae and the flow of blood are in opposite directions. This maximises the opportunities for diffusion of oxygen.