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 ofAmino 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 willMelt 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 calorimetryRespirometry

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, and6 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 Heat produced Heat produced

per O2 consumed per CO2 produce

Carbohydrate 21.1 21.1Lipids 19.8 27.9Proteins 18.7 23.3

Respiratory Quotient Concept: O2 consumed/CO2 producedFor 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 homeothermsthermoneutral zone, fasting, resting

Standard Metabolic Rate

applies to poikilotherms (ectotherms)temperature specificfasting, 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.

Allometric Relationship

Basal Metabolic Rate = a (Weight)b

(whole animal)

Weight Specific Metabolic Rate = a (Weight)(b-1)

“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.

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 hypothalamusreports triglyceride levels > satiety

low leptin > reduced female fertility

Diet Induced Thermogenesis Brown Fat