When designing an experiment which is the hardest part to design and why

The controls of the experiment. In a well controlled experiment, there are positive and negative controls, and the controls need to show that the experimental results are due to a specific reason and/or experimental conditions, otherwise, the credibility of the results or the causative/ relevance of factors cannot be deduced accurately.

Of course, this is a subjective question and different people might find different parts of the experiment hard to design. This is thus just my opinion.

photosynthesis makes the glucose that is used in cellular respiration to make atp. the glucose is then turned back into carbon dioxide, which is used in photosynthesis. while water is broken down to form oxygen during photosynthesis, in cellular respiration oxygen is combined with hydrogen to form water.