Which organism would be the best subject for a study of both photosynthesis and cellular respiration?

Believe it or not, regular old plants are nominal candidates. Plants actually utilize both Photosynthesis and Cellular Respiration. 

Fructose, sucrose, and starch are all examples of carbohydrates. There are two classifications of carbohydrates and those are simple carbohydrates and complex carbohydrates. The simple carbohydrates are generally sugars like fructose and sucrose whereas starch would be classified as a complex carbohydrate.