Final answer:
The increased surface area due to cristae in mitochondria and thylakoids in chloroplasts allows for more space to host important energy conversion processes, including cellular respiration and photosynthesis, respectively.
Step-by-step explanation:
The cristae of mitochondria and the thylakoids of chloroplasts both serve to increase the surface area within these organelles, which is beneficial for their functions. For mitochondria, the folding of the inner membrane into cristae provides more space for the electron transport chain and ATP synthesis, which are vital processes in cellular respiration. Similarly, in chloroplasts, the arrangement of membrane sacs into thylakoids allows for a greater area for the embedding of proteins and pigments necessary for photosynthesis to occur efficiently. By maximizing the surface area, these organelles can host a larger number of proteins and complexes that drive the energy conversion processes that are essential for the cell.