Metabolism Chapters 5-7

Chemical energy → (Electrical or Mechanical Energy) → Heat Metabolism Sum of all chemical reactions occurring in a living organism Energy Conversion Chemical energy → (Electrical or Mechanical Energy) → Heat Material Conversion Catabolism - break down of complex substances Anabolism - build up of complex substances

Uses of Energy Biosynthesis Maintenance (homeostasis) Replacing body structures Growth Reproduction Storage (Fat, Glycogen) Exported Materials Maintenance (homeostasis) External Work (e.g, movement)

Energy Metabolism Fig 5.1 Law of Entropy (2nd Law of Thermodynamics) all metabolic processes involve a loss of free energy (organized energy  disorganized energy) Animals require a constant input of organized energy (organic chemical bonds) All energy involved in metabolism is eventually lost in the form of heat Fig 5.1

Energy Metabolism Fig 5.4, Box 5.4 Energy usage by an organism Rate at which organized energy is converted into heat Calculation of Metabolic Rate Direct calorimetry measure heat production (kJ or Cal) Indirect calorimetry measure chemical changes C6H12O6 + 6O2  6CO2 + 6H2O + Energy (673 Cal, 2820 kJ) Fig 5.4, Box 5.4

Indirect Calorimetry: Oxygen Consumption Almost synonymous w/ metabolism NOTE: only indicates energy usage through aerobic respiration Accurate measure of energy expenditure through aerobic respiration Roughly equal heat generation per liter O2 by carbohydrates, fats and proteins Table 5.1

Indirect Calorimetry: Carbon Dioxide Production Amount of CO2 formed does not always equal amount of O2 consumed Respiratory Quotient (RQ) Amt CO2 produced/O2 consumed Varies for different energy sources Table 5.2

O2 vs. CO2 CO2 production is not as effective a measure of energy metabolism as O2 consumption Energy yield per ml CO2 produced varies greatly CO2 production can change easily through non-metabolic processes e.g. hyperventilation

What Affects Metabolic Rate? Physical Activity Environmental Temperature Digestive Processing (Specific Dynamic Action) Body Size Age Gender Endocrine Activity Circadian Rhythms Aquatic Salinity (Osmoregulation) Fig 5.5

Measuring “Apples and Apples” Metabolic Rate Define physiological conditions under which metabolism is measured Basal metabolic rate (BMR) – homeotherms Temperature in thermal neutral zone Fasting Resting Standard metabolic rate (SMR) – poikilotherms

Metabolism and Body Size Kleiber’s Rule For eutherian mammals Oxygen Consumption (VO2) = 0.676(Mass)0.75 Specific Oxygen Consumption (VO2/kg) = 0.676(Mass)-0.25 Small animals have relatively higher metabolic rates E.g. shrews have 100x the per-gram VO2 as an elephant Figs 5.6-5.10

Metabolism and Body Size Marsupial Mammals VO2 = 0.409(Mass)0.75 Passerine Birds VO2 = 1.11(Mass)0.72 Non-Passerine Birds VO2 = 0.679(Mass)0.72 Other Organisms Ectothermic vertebrates Invertebrates Protozoa Plants

Why Does Metabolic Rate Scale to Mass0.75? Max Rubner – study on dogs Small and large dogs have same body temperature Heat must be produced in relation to heat loss Heat production per square m2 surface area equal in small and large dogs Large dogs have relatively lower surface areas Rubner’s Surface Rule Metabolic rate (heat production)  surface area

What’s Wrong With This? If metabolism was directly related to scaling of heat loss, it should scale to Mass0.67 If related to heat generation and body temperature maintenance, why is it seen in ectothermic organisms?

Why Does Metabolic Rate Scale to Mass0.75? O2 delivery mechanisms function  Mass0.75 Lung Ventilation  Mass0.75 Lung Volume  Mass1.0 Breathing Rate  Mass-0.25 Cardiac Output  Mass0.75 Heart Mass  Mass1.0 Heart Rate  Mass-0.25 Do these cause metabolism’s scaling, or does metabolism cause their scaling?

Why Does Metabolic Rate Scale to Mass0.75? West et al. model - space filling fractal model Biological distribution networks have a fractal design (branching) Delivery of volumes of material to tissues approximated as spheres Account for number of branchings needed to fill a given body volume (mass) , change in diameter, and delivery, flow to tissues α mass0.75 Supply limitation Fig 5.12a

Why Does Metabolic Rate Scale to Mass0.75? Darveau et al. model - allometric cascade Overall MR = Σ various contributors to ATP turnover (materials supply and energetic demand) b = Σ scaling exponents of these contributors Scaling differs depending upon particular biochemical and physiological pathways activated e.g., SMR - scaling dominated by demand (ATP usage) e.g., MMR - both supply and demand influence scaling (O2 delivery vs. ATP usage) Fig 5.12b

Other Explanations Related to the noncoding DNA content of cells (Koslowski et al. 2003) Larger organisms have more noncoding DNA More noncoding DNA produces larger cells Larger cells have relatively lower MR Mitochondrial function (Porter 2001) Relatively less inner mitochondrial membrane surface in the cells of larger animals Less ATP turnover Less proton leak

Why Does Metabolic Rate Scale to Mass0.75?