Hydrocarbons and Heat

• Most hydrocarbons are used as fuels.• Knowing how much energy a fuel provides,

can tell us if it is useful for a certain application.• For example, the amount of energy a food

releases when burned, can tell us about it’s caloric content (fats release lots of energy).

• Heat energy released during combustion can be measured with a calorimeter.

• A “bomb calorimeter” is shown. It includes water in a heavily insulated container, a stirrer, valve, bomb chamber, ignition wires, & a thermometer .

How to measure energy in food• Direct Calorimetry

– Direct measurement of heat produced– Bomb calorimeter

• Calculate– Calories/g: 4 (cho), 9 (fat) ,4 (prt) and 7 (alcohol)– based: lab analysis of food composition

• Calorie chart or nutrient database

Bomb CalorimeterBased on Hess law: energy given off in a

chemical reacn is same,no matter by which intermediate steps it is carried out.

• Food is ignited electrically in the presence of oxygen

• Heat of combustion is measured from a rise in water temperature

Energy needsMeasurement of Energy Intake

Bomb Calorimeter measures heat produced when food is burned

Text view of bomb calorimeter

Specific heat capacity balloon demo

The heat needed to the temperature of 1 g of a substance by 1 C. Symbol: c, units: J/(gC).

Heat capacityThe heat needed to the temperature of an object by 1 C. Symbol: C (=c x m), units: J/C

Heat of reactionThe heat released during a chemical reaction. Symbol: none, units: J.

Specific heat (of reaction)The heat released during a chemical reaction per gram of reactant. Symbol: h, units: J/g.

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Heat Calculations

• To determine the amount of heat a substance produces or absorbs we often use q = cmT

• q: heat in J, c: specific heat capacity in J/(gC), m: mass in g, T: temperature change in C,

• This equation makes sense if you consider units

J = JgC

x g x C

Sample problem: (must know water = 4.18 J/gC)When 12 g of a food was burned in a calorimeter,

the 100 mL of water in the calorimeter changed from 20C to 33C. Calculate the heat released.

= 4.18 J/(gC) x 12 g x 13C __ kJq=cmT

Bomb Calorimeter• Heat of combustion represents the gross

energy of the food• Energy lost during digestion and absorption• Affected by illness

Energy needsMeasurement of Energy Intake

Physiological and gross values

Carbohydrate 4kcal/gm(body) 4.1(bomb cal)

Fats 9 9.4

Proteins 4 5.4

direct calorimetry• Measurement of heat loss

Energy needsMeasurement of Energy Output

Indirect calorimetry• Utilisation of oxygen• Oxygen consumption is proportional to ATP

synthesis• Use oxygen consumption to determine heat

production

Energy needsMeasurement of Energy Output

Indirect calorimetry• Glucose oxidation

• Starch oxidation

C6H12O6 + 6O2 6H2O + 6CO 2

15.5 kJ/g of energy+

Energy needsMeasurement of Energy Output

(C6H12O5)n + 6nO2 5nH2O + 6nCO 2 17 kJ/g of energy+

Measurement of Energy Output

Energy needs

Indirect calorimetry • Respiratory quotient (RQ)

• CO 2 Produced / O 2 Consumed

• RQ for Carbohydrates = 1.0

• RQ for fats = 0.71 (average)

C6H12O6 + 6O2 6H2O + 6CO 2

Indirect calorimetry• RQ value can be used to find the amount

energy produced per litre of oxygen consumed

Energy needsMeasurement of Energy Output

Energy Balance

• Sources of fuel for energy– Input from diet: carbs, fat, prot, alcohol– Stored energy: glycogen, fat, muscle

• Energy outgo from:– Basal metabolism– Physical activity– “Dietary thermogenesis”

Energy Out: Basal Metabolism

• Largest daily energy output• Definition: “the sum total of minimal activity of all

tissue cells of the body under steady state conditions” or energy expended by the body when voluntary activity is at minimal.

• The subject should be awake,at complete physical and mental rest, 12 hrs after the last meal and at equable temp.

Basal Metabolic Rate

• BMR = number of calories would need daily simply to stay alive if were totally inactive, in bed, awake for 16 hours & slept for 8 hours

1) Basal Metabolic Rate• 50-70% Energy Expenditure• Maintain basic metabolic processes

Cells Muscles Temperature regulationGrowth

• Osmotic pumps• Protein synthesis• Heart• Respiratory system• Digestive tract• Individual variation

10%

Factors affecting BMR

• 1) Body Size & Composition– Lean tissue BMR– Body weight wt lean tissue (but also fat)

Adipose tissue is not as metabolically active as lean body mass.

• 2) Age:– age Lean tissue

• 3) Sex: Men lean• 4) Activity: Exercise lean tissue

Factors affecting BMR

• 5) BMR is higher in people living in cold regions to compensate for loss of heat from the body in cold.

• 6) Fasting/starvation: BMR(adaptive response). Also bcoz of decrease in lean body mass.

• 7) Fever/stress BMR. In fever BMR rises by 11% for every 1C rise in body temp.

Reason is that chemical reactions occur more rapidly at higher temp .

8)Endocrinal State: BMR inc in hyperthyroidism becoz Thyroid hormones have a stimulatory effect on metabolism.In hypothyroidism there is 40% decrease in BMR so wt gain.

9) Lifestyle: Atheletes have higher.Stress and anxiety inc BMR which is due to secretion of epinephrine.

10. Smoking and coffee inc. BMR.