Living VS. Non-Living
Complex organisms, such as humans, require many systems for their life processes.   Less complex living things may lack the complex systems of more complex organisms, but they still carry on the basic life activities.   While non-living things may carry on some of these life processes, they do not carry on all of them, or these activities do not interact in a manner allowing the non-living thing to reproduce itself.

The components of living things in humans and other organisms, from organ systems to cell organelles, interact to maintain a balanced internal environment. This balanced internal environment is called dynamic equilibrium or homeostasis.  To successfully accomplish this, organisms possess many control mechanisms that detect internal changes and correct them to restore the internal balance of the organism.   If an organism fails to maintain homeostasis, this may result in disease or death.   Non-living things possess few control mechanisms to maintain homeostasis.

Population Diversity
The greater the diversity, or number of different species of organisms in an ecosystem, the ecosystem is more stable and likely to last.  These ecosystems contain many different kinds of organisms carrying on a variety of different nutritional modes.  Organisms can be categorized by the function they serve in an ecosystem. Each species in an ecosystem has a role for which it is best suited. In general, no two species have the same role in an ecosystem.  This allows different species to coexist successfully and helps maintain the stability of the ecosystem.   Ecosystems are stable due to the interactions among the many different populations. These interactions contribute to the overall maintenance and continued existence of the ecosystem.

Organizational Levels
Important levels of organization for structure and function of living things include cells, tissues, organs, organ systems, and whole organisms. The organs and systems of the body help to provide all the cells with their basic needs to carry on the life functions. The cells of the body are of different kinds and are grouped in ways that help their function.

All living things are composed of one or more cells, each capable of carrying out the life functions.   The organelles present in single-celled organisms often act in the same manner as the tissues and systems found in many celled organisms.  Single-celled organisms perform all of the life processes needed to maintain homeostasis, by using specialized cell organelles.

Cell Structure
Cells have particular structures or organelles that perform specific jobs. These structures perform the life activities within the cell. Just as body systems are coordinated and work together in complex organisms, the cells making up those systems must also be coordinated and organized in a cooperative manner so they can function efficiently together.

Inside the cell a variety of cell organelles, formed from many different molecules, carry out the transport of materials, energy capture and release, protein building, waste disposal, and information storage. Each cell is covered by a membrane that performs a number of important functions for the cell as well.

Life Functions
Humans and many other organisms require multiple systems for digestion, respiration, reproduction, circulation, excretion, movement, coordination, and immunity. The systems collectively perform the life processes.  

Once nutrients enter a cell, the cell will use those raw materials for energy or as building blocks in the synthesis of compounds necessary for life.  The energy we initially obtain must must be changed into a form cells can use. A type of protein called an enzyme allows for these changes to occur within the cell.

Cellular Communication
Neurotransmitters and hormones allow communication between nerve cells and other body cells as well.  If nerve or hormone signals are changed, this disrupts communication between cells and will adversely effect organism homeostasis.   Additionally, the DNA molecule contains the instructions that direct the cell’s behavior through the synthesis of proteins.

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; characteristics of living vs. non-living things, some aspects of population diversity, organizational levels in living things, cell structure and function, life functions, and cellular communication.   In addition, students may test their knowledge of the material presented here by accessing multiple-choice questions from past Regents Exams.