Energy and Metabolism
Metabolism
The Sum of all chemical reactions in the body
Biochemical Pathway
A series of enzyme-catalyzed chemical reactions
starting with a substrate and ending with a product.
Anabolic Pathways
Chemical pathways that lead to the synthesis of
complex molecules from simpler molecules:
consumes ATP.
Catabolic Pathways
Chemical pathways that hydrolyze complex
molecules and produce simpler molecules: net
production of ATP
Anaerobic Catabolic Pathways
Glycolysis
Starting molecule: glucose
Ending molecules: pyruvate
Net Gains
2 ATP per glucose
2 NADH2
Lactate Fermentation
Starting molecule: pyruvate
Ending molecule: lactate
Recycles NADH2 back to
NAD+
Aerobic Catabolic Pathways
2 Pyruvates
Net Benefits of Aerobic Pathways
• full oxidation of pyruvate to carbon dioxide and water
• extraction of high energy electrons from pyruvate and
acetate: yielding the formation of many NADH2 and
FADH2.
• use of high energy electrons in the electron transport
system to produce at least 32 additional ATP’s from each
pair of pyruvate molecules.
• Requires oxygen as final electron acceptor.
• CO2 produced as waste during the transition reaction and
Kreb’s cycle.
• Water produced as waste product during electron
transport system.
Triglyceride Oxidation
Glycerol
Fatty Acids
Ketone Bodies are acidic and may lead to metabolic acidosis: diabetes, anorexia
Protein Catabolism
Hydrolysis
Deamination of
Amino acids
• Ammonia is basic and can become toxic
• Pyruvate and Kreb’s cycle intermediates provide energy
• “Last Choice” metabolic pathway
Overview of Metabolism
Metabolic Rate: rate at which all metabolic
pathways proceed. (Joules/hour, Calories/hour)
Methods I: Direct Calorimetry (heat production)
One calorie of heat will
Melt one gram of ice to
water.
Methods II: indirect calorimetry (energy balance)
Total amount of energy in food
- Energy in feces and urine
__________________________
= Energy used by Animal
Methods III: indirect calorimetry
Respirometry
C6H12O6 + 6O2 ---> 6CO2 + 6H2O + 2820 kJ/mol
note: 1kcal = 4.186 kJ (kilojoules)
note: if the animal is metabolizing only carbohydrate
6 moles of O2 correspond to 2820 kJ, and
6 moles of CO2 correspond to 2820 kJ
When an animal metabolizes a mixture of foods, the
relationship between gas exchange and energy use will
vary.
Metabolism of lipids and proteins is not as straight-forward as
carbohydrate metabolism. For this reason the ratio of O2
consumed to CO2 produced may vary and is not 1:1.
Foodstuff
Carbohydrate
Lipids
Proteins
Heat produced
per O2 consumed
21.1
19.8
18.7
Heat produced
per CO2 produce
21.1
27.9
23.3
Respiratory Quotient Concept: O2 consumed/CO2 produced
For a mixed diet the respiratory quotient is about 0.81.
Metabolic Rate: the rate of heat production, oxygen
consumption, or carbon dioxide production per unit of time.
Basal Metabolic Rate
applies to homeotherms
thermoneutral zone, fasting, resting
Standard Metabolic Rate
applies to poikilotherms (ectotherms)
temperature specific
fasting, resting
Measures of Metabolic Rate
Whole Body Metabolic Rate: the total volume of
oxygen consumed by the animal per hour (mL/Hr)
Mass Specific Metabolic Rate: the volume of
oxygen consumed per gram of animal per hour
(mL/g.Hr.)
Observation: a meadow vole consumes 5.8 times its weight in food every
week. A rhino consumes only 34% of its body weight per week.
“Resting metabolic rate is an allometric function of body weight in related species”
As the size of an animal increases, the whole body metabolic rate also increases.
However, the total metabolic rate does not increase proportionately to body
weight. Weight-specific metabolic rate decreases as the mass of the animal
increases.
Allometric Relationship
Basal Metabolic Rate = a (Weight)b
(whole animal)
Weight Specific Metabolic Rate = a (Weight)(b-1)
Surface Area to Volume Hypothesis
• As the size of an animal increases, its SA/V
decreases. This mean that there is relatively
less area for heat loss.
• Mammals and birds spend energy to keep
their bodies warm.
• Small animals would lose heat more rapidly
because of the large SA/V. Therefore they
would have to expend more energy to replace
the lost heat.
Problems with the SA/V Hypothesis
• Surface area increases as a square function.
Volume increases as a cubic function.
Therefore the exponent of the allometric
equation should be 0.67 (2/3). In reality it is
closer to 0.75.
• Many invertebrates that are poikilotherms
also show the allometric relationship; but
they do not expend metabolic energy to keep
warm.
Regulation of Metabolism
Thyroid Hormone -----> stimulates thermogenesis; more food
energy goes into food production
Hypothalamus
Appetite center > Neuropeptide Y > stimulates appetite
Satiety center > alpha Melanocyte stimulating Hormone >
suppresses hunger
Adipose tissue and leptins - provide feedback to hypothalamus
reports triglyceride levels > satiety
low leptin > reduced female fertility
Diet Induced Thermogenesis
Brown Fat