Characteristics of Life Explore the characteristics and needs of living things. Tools A B Stimulus Signal that causes an organism to react Metabolism Chemical reactions that occur in living things Ingestion Taking in food or producing food Respiration Taking in oxygen and using it to produce energy Life Span Maximum length of time an organism can be expected to live Excretion Process of getting rid of waste materials Sexual Reproduction Requires two parent cells Asexual Reproduction Requires one parent cell Digestion Process by which food is broken down into simpler substances Response Action, movement, or change in behavior caused by a stimulus Organism Any living thing Food Needed by living things to grow, develop, and repair body parts Warmblooded Animals Maintain a constant body temperature Coldblooded Animals Body temperatures change with the environment Enzymes Special types of proteins that regulate chemical activities Compound Two or more elements chemically combined Elements Pure substance that cannot be broken down into simpler substances Organic Compounds Found in living things and contain carbon Inorganic Compounds May or may not contain carbon Amino Acids Building blocks of proteins Carbohydrates Main source of energy for living things such as sugar and starch Nucleic Acids Store information that helps the body make proteins it needs Proteins Used to build and repair cells; made of amino acids Lipids Energy-rich compounds made of C, O, and H