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
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.
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.
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
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.