Biology classes have specific requirements in the kinds of writing you're likely to do, just like every other subject you enroll in during college.
In particular, though, most writing assignments you'll get tasked with will fall under one of these categories:

• Long, research essays.
  These are in-depth evaluations of new findings and recent information on selected biology topics.
  A hefty amount of research will be required beyond primary reports and reviews, so expect to dig up plenty of literature on the chosen subject.
  
• Short essays.
  Usually confined to one or two pages, these papers often focus on scientific topics that are either part of the course or are in the media spotlight.
  
• Short reports.
  Here, you review literature or findings on specific topics, usually staying within primary sources.
  
• Lab reports.
  These papers document your laboratory projects and usually follow a specific format.
  

As with all forms of writing, they will need to be written as free of spelling and grammar errors as possible, so use your English grammar program.
Since you will be composing scientific papers, always opt for directness, over flowery language and elaborate metaphors, as well.
Most importantly, though, you'll need to concisely and clearly present the subject matter, in terms that the target audience (whether it is your peers or a more general readership) can understand.
That means, giving a clear statement of the problem, performing a concise survey of the available materials, a complete description of the methods of study and a detailed treatment of the results, coupled with incisive discussion that puts everything in proper perspective.