well i am assuming this comes from a local ecosystem module? or either u have done it? well from that module we know that one abiotic factor that varies in aquatic and terrestrial environment is light intensity. Light does not penetrate as much through water and so when it reaches the plants its intensity is very low.
By having the photosynthetic cells outside theplants light does not have to pentrate through as much. If it is deep inside, the light that reaches it is very weak and does not provide efficient photosynthesis. Also as water is readily outside, osmosis occurs quicker as well as it does not have to go further in. Similarly carbon dioxide diffuses faster as well. Like wat matt2233 said.
It basically makes photosynthesis more efficient.
This question is asking to explain so relate cause and effect. Similar to wat i said but it needs reworking on expression. hope that helps
