A population is a group of organisms of the same species who live in the same area at the same time. A community is a group of populations living and interacting with each other in an area.

Heat is always transferred from warmer to cooler environments through conduction, convection, or radiation.

Below domain, life is classified into six kingdoms: plants, animals, archaebacteria, eubacteria, and fungi. The last kingdom, protists, include all microscopic organisms that are not bacteria, animals, plants or fungi.  (Archaebacteria and eubacteria are sometimes combined into a single kingdom, monera.)

An acid is a substance that gives up positively charged hydrogen ions (H⁺) when dissolved in water. A base (alkaline) gives up negatively charged hydroxide ions (OH^-) when dissolved in water. pH is a scale that measures of how basic or acidic a solution is. Numbered from 0 to 14, solutions with a pH of 7 are neutral, less than 7 are acidic, more than 7 are alkaline.

Part of the brainstem, the medulla is the connection between the brain and the spinal cord. It controls involuntary actions like breathing, swallowing, and heartbeat.