Feel free to try out some of these Apps from the iTunes App Store. All are free to download.
This is an amazing App for nearly all topics covered in Biology. Khan Academy has done a great job making videos that help clarify and explain multiple topics. The videos are not downloaded to your cell phone, tablet, or computer. They are simply run through the App.
Khan Academy is one of the better teaching apps on the market. There are Khan Academy Apps for many other subjects as well. (Reviews below)