As bio

You can't rely solely on revision guides to cover everything. Use it as a starting point, then once you've grasped it, move on to the textbook. Some things are difficult to understand so better to learn them verbatim. Use mark schemes and past papers to see the depth of knowledge required to get good marks.

Is it ok to revise from Mark schemes

No. Like I said - they're good to see the depth of knowledge required. There are discreet differences each year to what is required. Plus, the questions aren't always identical.

Learn the concepts and apply the knowledge is the safest way.

So would it be fine if I just learnt all the concepts from the textbook and left out the bits that ramble on and aren't in the specification.

If you learn the 'core' content you'll get a grade that reflects that - maybe a C/B borderline. To get a top mark you need to know more than core and have an ability to apply the knowledge.

Biology is enormous when it comes to content. You can't learn too much in preparation for exams.