Biochemical Processes
Almost all life on Earth ultimately depends upon the Sun for its energy.   The process of photosynthesis converts the Sun's energy to sugars which living things may use as an energy source.   These sugars are converted to a form living things can use by a process called respiration. 

Thousands of chemical reactions occur in living things.   These reactions are aided by compounds called enzymes.   Enzymes and some other kinds of molecules have specific shapes which allow them to function. 

Disease
Homeostasis in an organism is constantly threatened.  Failure to respond effectively can result in disease or death.  Disease is a disturbance of homeostasis or steady state within an organism.   Many organisms, such as viruses, bacteria, fungi, and parasites may cause disease.   Disease also results from factors which are not living organisms.

The immune response is the defensive reaction of the body to foreign substances or organisms.  The immune system also protects against some cancer cells which may arise in the body.

Feedback Mechanisms
Dynamic equilibrium or homeostasis results from the ability of organisms to detect and respond to stimuli.   Feedback mechanisms are specific ways which have evolved in different living things to respond to internal or external environmental changes and maintain homeostasis.   A feedback mechanism is a process where the level of one substance or activity of an organ or structure influences another substance or structure in some manner.

This page and the links at the left are designed to aid students in reviewing the following topics which will appear on the New York State Living Environment Regents Examination; biochemical processes, disease and immunity, and maintenance of homeostasis in living things.   In addition, students may test their knowledge of the material presented here by accessing multiple-choice questions from past Regents Exams.