I'm applying for 4 biology courses and 2 chemistry courses.
How will I write my statement?
Can I star by saying how I chose one and then go on to say why I chose the other?

I've thought about saying.. 'I've always had interest in science and science based subjects, love for natural/living world has led me to me becoming increasingly interested in biology.' Then go on to say I find chemistry challenging and exciting (how it's involved in many parts of everyday life) and have loved it as a subject throughout school and becasue of this have taken it to advanced higher level.
Then I'll go on to talk about science in general.

I'm so stuck!!

why dont you vary the content for each different course. for example for the biology ones go in depth for the biology bit and just touch on the chem side. then for the chem courses do vica versa.