This question has 3 marks attached to it, therefore the examiner is looking for three clear points. Larger organisms tend to have higher metabolic demands as they are more active. Furthermore, there are large distances required for oxygen to diffuse across into the organism which further enhances the need for a transport system. These large distances account for the same reason that in animals especially, we can see them having a large surface area to volume ratio. This makes diffusion very difficult to take place and therefore a transport system is required to remove waste products and add oxygen around the body.