Study the details you forgot, then try again the next day with another blank sheet of paper. Memorization is just the first step . Memorizing the parts of a car's engine won't give you a full understanding of how it works, and certainly won't prepare you build your own car. There is a large quantity of information that you'll have to memorize to succeed in biology classes, but memorizing is not the same a learning. In the typical introductory biology class you'll learn as many new words as in a foreign language class.
Some people make the mistake of relying on flash cards as their only study method.
Flash cards may help, but only to a point. Reading is hard . A college biology text book can be quite challenging to read. The book goes into a lot of detail, and a lot of depth. Reading a chapter from front to back, and marking with a highlighter is, for most people, not the best way to learn biology.