Metabolism
Chapters 5-7
Metabolism
•
Sum of all chemical reactions occurring in a living
organism
–
Energy Conversion
Chemical energy → (Electrical or Mechanical Energy) → Heat
–
Material Conversion
1.
2.
Catabolism - break down of complex substances
Anabolism - build up of complex substances
Uses of Energy
• Biosynthesis
–
–
–
–
–
Replacing body structures
Growth
Reproduction
Storage (Fat, Glycogen)
Exported Materials
• Maintenance (homeostasis)
• External Work (e.g, movement)
Energy Metabolism
• 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
Energy Metabolism
• 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)
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
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
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 nonmetabolic 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)
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
– Fasting
– Resting
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 pergram VO2 as an elephant
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
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)
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?
?