Bioenergetics is the study of how organisms manage their energy resources.
Remember: First comes cellular work then comes biosynthesis.
Metabolic rate is the total energy used in a unit of time. How is it measured? Since the use of chemical energy creates heat, monitor heat loss; or it can be determined by figuring the amount of oxygen consumed by cellular respiration in that time frame. In the first case, an animal would be put in a calorimeter, a closed insulated chamger with technology capable of measuring heat production.
Factors of Metabolic Rate
Age, gender, size, body temperature, environment temperature, food quality/quantity, hormone and activity levels, time of day, amount of oxygen available at the current position.
An endotherm is an animal that creates body heat from the internal processes of metabolism. An ectotherm does not generate an internal temperature, but absorbs heat from its environment.
Basal Metabolic Rate (BMR)
BMR is the metabolic rate of an endotherm at rest, with nothing in their digestive system and no stress. For humans this number is 1600 to 1800 kilocalories per day for the average male; 1300-1500 kilocalories per day for the average female. This is the same as the energy consumption as a 100 watt light bulb!
The SMR - or standard metabolic rate - is the rate of an ectotherm under the same conditions for the BMR of an endotherm. However, the only difference is that the SMR must be determined at a specific environmental temperature to assure consistent scientific results since ectotherms do not maintain their own body temperature internally.
The maximum metabolic rate of any animal is equal to 5 or 10 times the BMR or SMR.
Body Size as it relates to Metabolic Rate
The amount of energy needed to maintatin each gram of body weight is inversely related to the body size (calories/gram). The theory goes like this: a field mouse uses 10 times the amount of energy of an elephant because it is one-tenth the size. This number is always a lot more than the total energy consumed. This works very well for endotherms like the mouse and elephant due to the surface area to volume ratio problem; however, in theory it shouldn't work for ectotherms...and it does!
In mammals and birds the most important feature in supporting the weight of an animals own body is posture. Posture is defined as the position of the legs relative to the torso, or main body, partially attributed to muscle contraction. For example large animals like elephants have legs in vertical or nearly vertical positions; this prevents undue stress to the body of the elephant because extension of the limbs and joints is very limited. Large animals also have different body proportions: leg size to torso size, etc.