Heat: the amount of energy flowing spontaneously from one body to another due to differences in temperature (gradient, kilocalorie= the heat required to raise the temperature of one kg of water one degree celsius. Energy return on investment (eroi: eroi= gross energy/ energy used to obtain energy. How surplus energy is used: maintenance, growth, storage, reproduction, protection. Maintenance- the fight against disorder/entropy: removing wastes, producing new blood cells, thermoregulation. Ectothermy: use environmental (external-ecto) energy to maintain body temperature. Can endure wider range of body temperature. Endotherms: use own (internal-endo) energy to maintain body temperature. Storage: energy storage for periods when food less available. Most energy is stored as carbohydrates (easier to move around through fluids) Most energy is stored as fats (higher energy storage per unit of weight) Break down chlorophyll and move this energy to store in roots, in spring move this energy and matter back to their branches from their roots as sap.