This really isn't too bad at all, but the structure needs work.
Some points:
Be careful with terminology and definitions. For instance, you've used the word 'assimilates' as a noun, but not defined it. You must always define a term like this. However, you've used a lot of good terminology, and used it correctly so well done there.
You need to structure your answer much better to ensure you get maximum marks. You've written this a bit like breathless prose, all cramped into one badly-punctuated paragraph. You need to split this all up, and organise it better. This, I think, shows that you don't plan your essays before writing them - this really is a crucial step. List the examples and points you want to make, how you will expand/describe them, and how do they answer the question? For instance, in the piece above, you've got:
Ammonia
Water movement into roots
Photosynthesis
Respiration
Amino acid synthesis
Trophic levels..
...all in one paragraph!! Split it up! Each of these topics needs a separate paragraph, a good amount of on-topic discussion and then a link (if possible) to the next topic. This will make the writing flow much better and, for the examiner, make it much easier to mark.
So, a good effort. But you must structure your writing much more clearly and resist the temptation to just 'write everything you know on topic X'. And, most importantly
read the question and make sure everything you write answers it.