22) Some students investigate the effect of the ratio of surface area:volume on the rate of diffusion in animal cells.
They use hydrochloric acid and gelatine cubes stained blue with pH indicator.
They put different sized cubes into a beaker of hydrochloric acid and time how long it takes for the cubes to completely change colour.
The table shows their results.
(a) (i) Calculate the surface area:volume ratio for the cube with sides of 1 cm. 
answer = ………………………. cm-1
(ii) Calculate the rate of colour change for each of the three cubes.
Write your answers in the table below.
Show your answers in standard form.
(iii) Use the results and your calculations in parts (i) and (ii)
Explain why most large multi cellular organisms need transport systems, such as the blood system, but most single celled organisms do not. 
(iv) Explain why using gelatine spheres instead of cubes might be more biologically accurate but suggest why the students used cubes instead. 
(b) Oxygen enters red blood cells by diffusion.
Describe and explain how red blood cells are adapted for the efficient uptake and transport of oxygen.