**amino acids**: Simple molecules that occur naturally in plant and animal tissues and that are the basic building blocks of proteins.
**bat**: A type of winged mammal comprising more than 1,400 separate species — or one in every four known species of mammal.
**carbon dioxide**: (or CO2) A colorless, odorless gas produced by all animals when the oxygen they inhale reacts with the carbon-rich foods that they’ve eaten. Carbon dioxide also is released when organic matter burns (including fossil fuels like oil or gas). Carbon dioxide acts as a greenhouse gas, trapping heat in Earth’s atmosphere. Plants convert carbon dioxide into oxygen during photosynthesis, the process they use to make their own food.
**cattle**: Also known as bovines (because they’re members of the subfamily known as Bovinae), these are breeds of livestock raised as a source of milk and meat. Although the adult females are known as cows and the males as bulls, many people refer to them all, generally, as cows.
**chemistry**: The field of science that deals with the composition, structure and properties of substances and how they interact. Scientists use this knowledge to study unfamiliar substances, to reproduce large quantities of useful substances or to design and create new and useful substances. (about compounds) Chemistry also is used as a term to refer to the recipe of a compound, the way it’s produced or some of its properties. People who work in this field are known as chemists.
**diet**: (n.) The foods and liquids ingested by an animal to provide the nutrition it needs to grow and maintain health. Sometimes this is a specific food-intake plan.
**ecological**: An adjective that refers to a branch of biology that deals with the relations of organisms to one another and to their physical surroundings. A scientist who works in this field is called an ecologist.
**fat**: A natural oily or greasy substance occurring in plants and in animal bodies, especially when deposited as a layer under the skin or around certain organs. Fat’s primary role is as an energy reserve. Fat also is a vital nutrient, though it can be harmful if consumed in excessive amounts.
**fuel**: Any material that will release energy during a controlled chemical or nuclear reaction. Fossil fuels (coal, natural gas and petroleum) are a common type that liberate their energy through chemical reactions that take place when heated (usually to the point of burning).
**mammal**: An animal distinguished by possessing hair or fur, the secretion of milk by females for the feeding of their young, and (typically) the bearing of live young. They also are warm-blooded (or endothermic).
**nutrient**: A vitamin, mineral, fat, carbohydrate or protein that a plant, animal or other organism requires as part of its food in order to survive.
**oxygen**: A gas that makes up about 21 percent of Earth's atmosphere. All animals and many microorganisms need oxygen to fuel their growth (and metabolism).
**physiologist**: A scientist who studies the branch of biology that deals with how the bodies of healthy organisms function under normal circumstances.
**protein**: A compound made from one or more long chains of amino acids. Proteins are an essential part of all living organisms. They form the basis of living cells, muscle and tissues; they also do the work inside of cells. Antibodies, hemoglobin and enzymes are all examples of proteins. Medicines frequently work by latching onto proteins.
**ratio**: The relationship between two numbers or amounts. When written out, the numbers usually are separated by a colon, such as a 50:50. That would mean that for every 50 units of one thing (on the left) there would also be 50 units of another thing (represented by the number on the right).