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Key Idea: Animals use carbon-containing molecules from food to make a variety of other carbon-containing molecules that become part of their body structures.

Students are expected to know that:

  1. Growth, repair, and replacement of body structures involves using carbon-containing molecules (carbohydrates, fats, and proteins) from food to make other carbohydrate, fat, and protein molecules that become part of their body structures.
  2. Growth of animals requires the addition of molecules made up of linked carbon atoms to body structures, and this is the only way that body structures can grow.
  3. The processes by which molecules from food become part of an animal’s body structures involve chemical reactions in which the atoms of the molecules from food (carbohydrates, fats, and proteins) are rearranged to form new molecules of carbohydrates, fats, and proteins that make up the body structures. The carbohydrates, fats, and proteins that animals eat do not get incorporated into body structures without first going through a chemical reaction.
  4. Unlike plants, animals cannot link carbon atoms from carbon dioxide to make sugars or any other molecule made of linked carbon atoms.


  1. “Body structures” include any organ, tissue, or part of an organism with which students are likely to be familiar.
  2. Students are not expected to know that muscles are made largely of protein molecules, fat tissue is made largely of fat molecules, or that the skeletons of insects, lobsters, and crabs are made largely of carbohydrate molecules.
  3. Students are not expected to know the chemical or structural formulas of carbohydrates, proteins, or fats.
  4. This idea does not include the synthesis of carbohydrates, fat, and proteins from their sub-units.
  5. The idea that simple sugars are the “building blocks” of complex carbohydrates, that amino acids are the building blocks of proteins, and that fatty acids are the building blocks of fats are covered in Benchmark 6C/M2 (which is about digestion), not this key idea.
  6. The idea that carbon, because of its small size and four available bonding electrons can join to several other carbon atoms in chains and rings to form large and complex molecules is part of Benchmark 5C/H8, not this key idea.
  7. Students are not expected to know the identity of atoms other than carbon that are contributed by carbohydrates, fats, and proteins from food to the molecules that make up body structures.