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.

The correct order of the level of organization of an individual is as follows: cells, tissue, organs, organ systems, and organism. In other words, tissues form organs. For example, in a human being the order in the musculatory system would be: muscle cell, muscle tissue, bicep muscle, musculatory system, and human being.

DNA—deoxyribonucleic acid—is found in chromosomes within a cell’s nucleus. A complete set of DNA (i.e. 46 chromosomes) is called a genome. DNA contains instructions that make humans different from other species and other individuals. DNA provides instructions for all the proteins that the body makes and is passed from adults to offspring.  DNA cannot get out of the nucleus; however, RNA can. RNA is used to get the instructions from DNA out of the nucleus and into the site of protein synthesis: the ribosomes within the cytoplasm. Proteins are made of amino acids and determine the structure and function of all of the body’s cells; therefore, this process can be simplified into “DNA to RNA to protein.”

The cell theory is the idea that 1) all living things are made of one or more cells, 2) cells are the basic unit of life and 3) all cells come from other cells.

Bacteria, whether infecting humans or foods, originates from other bacterial cells. That illustrates the third point of the cell theory.