Energy metabolism

• Energy Metabolism

  The overall use of chemical energy (CE) by animals to power their functions
• Food provides energy
• Animals require chemical energy (CE) for their functions
• Oxidation of foodstuff provides energy
• Measure of energy metabolism

  Amount of O2 consumed
• CE can be obtained without use of O2 (e.g. anaerobic metabolism)
• O2 consumption is not always a measure of energy metabolism
• Energy requiring processes & reactions in living organism use ATP
• ATP is formed in oxidation of foodstuff & in anaerobic energy-yielding processes
• Metabolic Rate (MR)


  The energy metabolism per unit time
• Measuring Metabolic Rate


  1. Calculating the difference between the energy value of all food taken in and the energy value of all excreta

  2. Measuring the total heat production of the animal

  3. Determining the amount of O2 used in oxidation processes
• Determination of O2 consumption is easy and commonly used for estimation of MR
• O2 consumption is not a fully accurate measure of MR as a fully anaerobic organism has a zero O2 consumption but its MR is definitely not zero
• Respiratory Quotient (RQ)

  The ratio between CO2 formed in metabolism and O2 used
• RQ near 0.7 suggests primarily fat metabolism; an RQ near 1.0 suggests primarily carbohydrate metabolism
• CO2 production cannot be used for determinations of MR as accurately as O2 consumption due to the large pool of CO2 in the body that changes easily and the different caloric equivalents of CO2 for different fuels
• Basal Metabolic Rate (BMR)


  The standardized measure of metabolism for homeotherms, measured when the animal is in its thermoneutral zone, resting, and fasting
• Standard Metabolic Rate (SMR)


  The standardized measure of metabolism for poikilotherms, measured when the animal is resting and fasting at its prevailing body temperature
• Routine Metabolic Rate refers to reasonably quiet animals exhibiting only small, spontaneous movements, while SMR refers to animals that have been coaxed to a truly minimal level of activity
• When activity is truly minimal under standard or basal conditions, the MR approximates the rate necessary for simple physiological maintenance of life
• Calorie


  A unit of energy, specifically the amount of heat required to raise the temperature of one gram of water by 1 degree Celsius
• Kilocalorie (kcal)


  The amount of heat (energy) needed to increase the temperature of one kg of water by 1 degree Celsius, exactly 1000 small calories, or about 4.184 kJ
• Joule


  The SI unit of energy, defined as the work done, or energy expended, by a force of one newton moving one meter along the direction of the force
• Metabolic Rate & Body Size

  The larger the diving mammal, the longer lasting the dives it can perform (whales → couple of hrs.; water shrew → rarely exceeds 30 seconds)
• The rate of O2 consumption relative to body size

  Is much higher in the small than in the larger mammal
• The elephant is much larger than the shrew & its total O2 consumption obviously must be much higher
• Specific O2 Consumption (SOC)


  O2 consumption per unit Body Mass, decreases consistently with increasing body size (e.g. 1g shrew tissue has O2 cons. rate some 100-fold as great as 1g elephant tissue)
• This necessitates that the O2 supply & hence blood flow to 1g tissue be 100X > in the shrew than in the elephant. Heart function, respiration, food intake, etc. are similarly affected
• Regression line


  Represents the generalization that O2 consumption of mammals/unit body mass decreases regularly with increasing body size; it also gives a quantitative expression of the magnitude of the decrease
• Equation for regression line

  VO2/Mb = 0.676 X Mb^-0.25, where VO2/Mb = SOC (l O2 kg-1 h-1) & Mb = body mass (kg)
• Equation in log form

  log VO2/Mb = log 0.676 - 0.25 log Mb
• Equation for total O2 consumption

  VO2 = 0.676 X Mb^0.75
• Equation for total O2 consumption in log form

  log VO2 = log 0.676 + 0.75 log Mb
• Data have been compiled for verts and inverts & in general most have rates of O2 consumption that fall on the same or similar regression lines
• Some exceptions: e.g. some insects, pulmonate snails, & a few other groups exhibit lines with a slope closer to 1.0 (O2 consumption directly proportional to body mass)
• O2 consumption rates of micro-organisms & even some trees fall on lines with a similar slope → indicating that this represents a general biological rule
• The fact that cold-blooded verts & many inverts (& at least some plants) have regression lines with the same slope excludes the possibility that temperature regulation is a primary cause of the regularity of the regression lines
• A large number of physiological processes are surface-related: uptake of O2 in lungs & gills, diffusion of O2 from blood to tissues occurs across capillary walls, food uptake in intestine, etc.
• It is easy to understand that overall metabolism cannot be independent of surface considerations; it is more difficult to explain why it deviates in such a regular fashion (0.75 or very close to this value)
• Passerine birds

  Sparrows, finches, crows, etc. have somewhat higher Metabolic Rates than non-passerine birds