As soon as you see that you must start writing in full sentences. I havnt seen the mark scheme... But i'm guessing you only need to give 2-3 reasons, and the rest of the marks will come from hitting keywords.
Well what can you tell us about membranes ? What is their importance ?, why is it important for membranes within cells ?
Think about each part of the question. Start writing down, 9 specific and exclusively distinctive facts that are relevant to the question. It might be necessary for you to explain what the membrane is composed of and then work from there.
Good advice for these questions, and also for synoptic: Roughly the number of marks available corresponds to the number of points you have to make.
Briefly make 9 good specific points about function of plasma membranes, and form them into sentences. If you're doing synoptic, just take this one step further by arranging them into paragraphs you can link together.