Energy Transfer

There is a flow of energy within a living body continually from one form to another. A kilocalorie (kCal) is a measure of a food’s potential energy trapped/stored within its chemical structure, which the muscular system changes into mechanical energy and finally to heat energy. Such energy transfer follows the tenets of the first law of thermodynamics, which states that energy cannot be created or destroyed but transforms from one form to another.

A bomb calorimeter permits direct quantification of the energy value of food, by measuring the heat dissipated/liberated/released as the food completely burns. The average heat released (food’s total energy value) while burning protein equals 5.65 kCal per gram; carbohydrate equals 4.2 kCal per gram; lipid equals 9.4 kCal per gram.

However, there exists a difference in the food’s total energy value and the net energy available to the body. For example, the nitrogen atoms in the protein combine with hydrogen to form urea and are excreted in the urine. Such loss of hydrogen (approximately 19% of protein’s potential energy) reduces protein’s total energy value (i.e., heat released while combustion) from 5.65 kCal per gram to 4.05 kCal per gram. There is not much change in the total energy values of carbohydrates and lipids.

Another important factor in energy transfer is the coefficient of digestibility, which represents the percentage of ingested food that is digested and absorbed to meet body’s energy demands.

Nutrients Total energy value (kCal/gram) Coefficient of digestibility (%) Net energy (kCal/gram)
Lipids 9.4 95% 8.93
Proteins 5.65 92% 4.05
Carbohydrates 4.2 97% 4.03

Bioenergetics refers to the capacity of a living body to extract energy from food nutrients and transfer it to the energy requiring systems in the body. Such energy transfers help synthesise new biologic tissues and also help meet the cellular energy needs. It is important to note that energy transfers are always on a two-way continuum (energy uptake and release). And, the potential energy always tends to degrade to kinetic or heat energy (second law of thermodynamics). For example, food (a source of potential energy) always tends to degrade as it decomposes through the regular oxidative process. Another example, all biochemical reactions in the body proceeds in a direction that favours irreversible disintegration ultimately producing a loss of potential energy.

Energy transfer stages:

I. Photosynthesis:
6CO2 + 6H2O —–sunlightchlorophyll—–> 6O2 + stored energy (protein, glucose, lipids)

II. Reverse Photosynthesis (Cellular respiration):
Glucose (food energy consumption) + 6O2 ———-> 6CO2 + 6H2O + ATP

III. Biologic Work:
ATP —–enzymes & co-enzymesoptimal pH & temperature—–>

Mechanical Work (skeletal muscle contraction) +
Chemical Work (synthesize cellular molecules/catalyze cellular reactions) +
Transport Work (catalyzing active transport between Intra & Extra cellular fluid)
[Catalysts (enzymes & co-enzymes) that work at optimal pH and temperature are involved in this stage]

1. Merrill, A.L., Watt, B.K. “Energy value of foods: basis and derivation”. Washington, DC: United States Department of Agriculture. 1973.
2. McArdle, W.D., Katch, F.I. and Katch, V.L., 2010. Exercise physiology: nutrition, energy, and human performance. Lippincott Williams & Wilkins.