Example 1:

Amoeba is a unicellular organism and the stickleback is a multicellular organism. Therefore it consists of one cell. a single cell respiration and a respiration of a multicellular organism is different. While the unicellular organism needs only to provide oxygen to one cell, multicellular organisms have multiple organs within the body that need respiration, as well as the cells on the surface.

Example 2:

Compared to stickleback, amoeba has a higher surface area and volume ratio and thinner surface. So the rate of diffusion would be higher. So it could get sufficient oxygen by diffusion.

Example 3:

amoeba have thin cell membrane that increases the rate of diffusion while
stickle the membrane are complicated

(b) how the gills and the circulating blood will increase the diffusion of oxygen into the cells of the stickleback.

Thin feathery structure of gills gives a large surface area to volume ratio which increases the efficiency of diffusion of gases between the water and the blood by allowing more particles to diffuse at any one time.

By muscular action, water is pushed across the gills and it increases rate of oxygen absorption by maintaining steep concentration gradient between water and blood. The circulating blood means that the blood absorbs the oxygen, and then carries it around the body to every cell where it is needed and removes metabolic waste. This maintains steep concentration gradient at exchange surfaces in gills which increase diffusion even further.