Blood is categorized into four different types (A, B, AB, and O) based on the type of antigens found on the outside of the red blood cells. Additionally, each type can be negative or positive based on whether or not the cells have an antigen called the Rh factor.

Small quantities of certain minerals like iron, calcium, magnesium, and salt are important for nutrition and health.

Primary consumers (herbivores) subsist on producers like plants and fungus. Examples are grasshoppers, cows, and plankton.

The narrowest classification of life, species, contains organisms that are so similar that they can only reproduce with others of the same species.