Proteins are synthesized through dehydration synthesis reactions, which is the removal of water between two amino acids. In this case, two hydrogen atoms are removed from the amine group and one oxygen is removed from the carboxyl group, forming a peptide bond between the carbon atom of one amino acid and the nitrogen atom of the other amino acid.

The sequence of amino acids is called a protein's primary structure. Each protein has a unique sequence of amino acids. A difference of just one amino acid in a chain of hundreds can be deadly to the organism. For example, mutation leading to a single amino acid change is responsible for sickle cell anemia.

The tertiary structure of a protein is created by various interactions between the R groups in the chain. This makes the protein fold three-dimensionally.

One example would be ionic bonds forming between a positively charged R group and a negatively charged R group. Covalent disulfide bonds will create a tertiary shape between two cysteine amino acids. Hydrophobic R groups will also contribute to the structure, bending toward one another to avoid contact with the aqueous environment.

A quaternary protein structure is made up of two or more polypeptide chains that fit together based on their shape and polarities, like a jigsaw puzzle. The subunits also fit together by forces of attraction between positively charged regions and negatively charged regions. Not all proteins have quaternary structure.

Active transport is used to carry amino acids from the lumen of the small intestine into the blood stream. Active transport requires the use of energy (ATP) in order to transport molecules. In this case, amino acids are large, blulky, and frequently polar. All of these characteristics make them hard to transport across a plasma membrane.

Steroids are derived from cholesterol, which is a lipid.

Many hormones are are classified as peptide hormones, and are made of protein. Keratin is a hard structural protein found in hair and nails. Enzymes are catalytic proteins that facilitate biochemical reactions in the body.

When a protein is exposed to extreme conditions, such as high temperature or acidity, the interactions that provide protein structure are disrupted. The result is denaturation, or the loss of structure.

When conditions are returned to optimal levels, the protein can often return to its normal structure and remain functional. Denaturation affects the interactions that give the protein shape, but does not affect the peptide bonds that hold amino acids together.

The cell theory has three basic parts: all living things are made of cells, the cell is the smallest unit of life, and all cells come from preexisting cells. Prokaryotic cells are indeed cells and they do not contradict any tenets of the cell theory. However, they do not contain nuclei; rather, their genetic material is contained in the cytoplasm.

The pH of a solution is defined as the the concentration of H+ ions in the solution.  The pH of a solution declines  as H+ concentration increases.  On the pH scale, ranging from 0 to 14, 7 is neutral.  Substances below 7 are more acidic, 0 being the most acidic.  Substances above 7 are more basic, 14 being the most basic. 

“Composed of plutonium” is not a characteristic of a living organism. The other three choices are characteristics of living organisms. Organisms use energy in their metabolic processes. Organisms reproduce either asexually or sexually to make offspring for the next generation. Organisms from the simplest prokaryote to the human species contain DNA—the universal genetic code.