Biology is the science that seeks to understand the living world. Cells are the basic unit of life, enclosed by a barrier that separates it from its surroundings. Homeostasis is the process by which organisms maintain a stable internal environment. Sexual reproduction involves cells from two parents uniting to form the first cell of a new organism, while asexual reproduction involves a single parent reproducing alone. Metabolism is the set of chemical reactions through which an organism carries out life processes like building and breaking down materials. A stimulus is a signal to which an organism responds.

1. Studying Life 1.3

2. Biology Science that seeks to understand the living world.

3. CellCollection of living matter enclosed by a barrier that separates the cell from its surroundings; basic unit of all forms of life.

4. Homeostasis Process by which organisms maintain a relatively stable internal environment.

5. Sexual Reproduction Process by which cells from two different parents unite to produce the first cell of a new organism.

6. Asexual ReproductionProcess by which a single parent reproduces by itself.

7. Metabolism Set of chemical reactions through which an organism builds up or breaks down materials as it carries out its life processes.

8. StimulusA signal to which an organism responds.

9. Key ConceptWhat are some characteristics of the living things?Living things share the following characteristics: Made up of units called cellsReproduce Based on a universal genetic code Grow and develop Obtain and use materials and energyRespond to their environment Maintain a stable internal environment Taken as a group, living things change over time

10. Key ConceptHow can life be studied at different levels?Some of the levels at which life can be studied include molecules, cells, organisms, populations of a single kind of organism, communities of different organisms in an area, and the biosphere. At all these levels, smaller living systems are found within larger systems.