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Cell, An Introduction


Biology is the scientific study of life and is the branch of science that studies living organisms and the way organisms interact with their environments. The subject is vast and includes topics as diverse as acid rain, evolution, and genetically modified foods. In this module, you will investigate the definition of life and explore some of the characteristics of living systems.

Characteristics of Life
There are five distinct qualities used to determine whether or not something is (or was) alive. A living organism is something that displays all these qualities. To be considered alive, something must:

Be made of materials organized in a hierarchical pattern.
Use energy and raw materials to survive.
Sense and respond to changing environments and maintain internal stability, or homeostasis.
Grow, develop and reproduce with the help of DNA.
The cell is the smallest unit that displays all of these characteristics. Because of this, living organisms are often identified based on whether or not they are made of cells. Nonliving things can show several of these characteristics. For example, a rock crystal can “grow” in a simple fashion. However, if even one of these conditions is not met (rock crystals do not reproduce with help from DNA), the object in question cannot be considered alive.

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Classification of Matter Matter is any substance that has mass and takes up space. All matter can be classified in one of two categories: biotic (living) or abiotic (nonliving). Matter is considered biotic if it was ever alive at some point in time. In this sense, a dead human buried underground in a wooden coffin is still biotic, as is the wood used to make the coffin (the wood came from a tree that was once alive). However, not everything within, or made by, a biotic organism is biotic. For example, urea, a chemical component of urine, is an abiotic substance. To be classified as biotic, all of the required conditions for life must be met or have been met in the past. Otherwise, the matter being classified is considered to be abiotic, or nonliving.
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