Part a I would assume it's talking about things such as the glycoproteins on the cells which bind to its complementary binding sites on the target cell (which would be identical to the cell that is binding) as they help to form connective tissues which effectively forms an organ.
Part b I'm guessing it's because different cells have different functions and so need to be different sizes, despite being on the same organ ie
in the kidney you have the hepatocytes, which make up most of the mass so obviously need to have a large surface area and a large size, and kupffer cells which are elongated as they're macrophages, so need to adapt their shape to engulf microbes.
Those are two pretty generic answers to the questions you asked but you could apply them to the examples you've given. Not sure about the amount of detail you need as I'm not sure if you're doing A Level or a degree - can't really help you with a degree unfortunately!
Also, not sure if they're 100% correct as I'm on my gap year so haven't done biology in a while
Let me know if you need any more help - I should probably start revising soon anyway since I'm retaking.