Metabolism

Units

Calorie (cal)

1 cal = energy required to raise the temperature of 1g of water from 15 degree C to 16 degree C.

1 Cal = 1 kcal = 1000 cal

Covertion to Joule

1 calorie = 4.1868 J

 

Glucose conversion

1mole of glucose
= 180gram

Glucose level of
180mg/dL = 10mmol/L

Caloric value

Carbohydrate = 4.1 kcal/g

Fat = 9.3 kcal/g

Protein = 4.1 kcal/g
* Outside body, when measured with bomb calorimeter
--> protein = 5.3 kcal/g
* Difference is due to end product being urea and other nitrogenous compounds

Respiratory quotient

Respiratory quotient (RQ) = the ratio of CO2 produced to O2 consumed in steady state.

Respiratory exchange ratio (R) = the ratio of CO2 produced to O2 consumed at any given time, whether or not equilibrium has been reached.

RQ

RQ of carbohydrate = 1.0

RQ of fat = 0.7 (because extra O2 is needed for the formation of H2O)

RQ of protein = 0.82 (average value)

RQ of an organ

... is determined by the metabolic process occurring in them.

RQ of brain = 0.97 to 0.99
* Carbohydrate is the main, but not the only fuel

RQ of stomach = negative
* Negative CO2 output because of acid secretion

Metabolic rate

Isotonic muscle contraction is about 50% efficient.

Essentially all isometric contraction appears as heat and no work is done.

Measurement

Basal metabolic rate is measured:

• in thermoneutral zone
• 12-14 hours after last meal
• at rest

Measured indirectly.

O2 consumption is measured using oxygen-filled spirometer and a CO2 absorber.

Conversion: 4.82 kcal/L of O2 consumed
* [WG21:p284]

Correlates well with body surface area

Factors affecting metabolic rate

• Muscle exertion (including repayment of oxygen debt)
• Food consumption (i.e. specific dynamic actions)
• Environmental temperature
  * U-shaped.
• Body temperature
  * 14% increased for every Celsius degree elevated
• Others: sex, age, reproduction, lactation, emotion, thyroid function, catecholamine levels

Specific dynamic action

[WG21:p284]

In providing 100 cal worth of energy, metabolic rate increase by:

• 30 cal when energy is provided by protein
• 6 cal when energy is provided by carbohydrate
• 4 cal when energy is provided by fat

Energy transfer

High energy phosphate compounds

The most important - adenosine triphosphate (ATP)

Others include:

• [In muscles] creatine phosphate (or phosphorylcreatine)
• thioester
• Acetyl-CoA
  * 1 Acetyl-CoA is equivalent to 1 ATP

Hydrogen acceptors

After picking up hydrogen

• Nicotinamide adenine dinucleotide (NAD+)
  --> Dihydronicotinamide adenine dinucleotide (NADH)
• Nicotinamide adenine dinucleotide phosphate (NADP+)
  --> Dihydronicotinamide adenine dinucleotide phosphate (NADPH)
• Flavin adenine dinucleotide (FAD)
  -> FADH or FADH₂.

NADP+ and NADPH

##20050509-01

• NADP+ and NADPH is only produced by pentose phosphate pathway
• Used for synthesis of fatty acids and steroids
• Also used for removal of H2O2 in RBCs
  * [HH26:p166]