CK-12 maintains an online life science textbook. These textbooks are intended for middle school students, but the site also offers an online biology textbook aimed at high school students. All of CK-12's textbooks and interactive learning activities are available for free.
CK-12's life science curriculum is available as both a student textbook and a teacher's edition. It is created by teachers and based on textbooks currently in use in schools. It includes sections on general life science, cell biology, genetics and microbiology, ecology and evolution. Other sections include plants and animals, bacteria and fungi and human biology. A guide to teen health is available from the same section of the site.