REA's Essentials provide quick and easy access to critical information in a variety of different fields, ranging from the most basic to the most advanced. As its name implies, these concise, comprehensive study guides summarize the essentials of the field covered. Essentials are helpful when preparing for exams, doing homework and will remain a lasting reference source for students, teachers, and professionals. Biology I includes the chemical and molecular basis of life, cellular organization, cellular metabolism and energy pathways, nutrition in plants and animals, gas exchange in plants and animals, circulatory systems of animals, excretion and homeostasis, and hormonal control.