What do you mean by human metabolism?

Being described as the most complicated machine ever encountered by humans, the human body and the cell metabolism is nothing less than a wonder. Metabolism is the set of life-sustaining chemical reactions within organisms.

The function of human metabolism

Human metabolism can be classified into three main processes. Humans require energy to perform various functions. Food contain various micro and macronutrients such as carbohydrates, fats, protein, etc. During digestion, food is converted into simpler form. The process by which food is broken down into energy at the cellular level is the first purpose of human metabolism. The conversion of food into energy is not sufficient for the cellular system of our body. Hence, breaking down this energy into carbohydrate, proteins, lipids, and other cellular requirements is the next function of metabolism. Every conversion reaction will have by-products. It can be considered as cellular waste and removal of cellular waste is the final purpose of human metabolism. Because of the occurrence of a conversion reaction, the whole process can be explained using the first law of thermodynamics.


The branch of physics, that deals with the interconversion of heat and energy is known as thermodynamics. The whole concept of heat, work, and energy is explained in four laws. Energy can be converted into one form to another. Thermodynamic laws explain the nature of this conversion and help to find the unknown entity.

Laws of thermodynamics

Zeroth law: Two systems will be in thermal equilibrium with each other if they are in thermal equilibrium with another common system.

First law: Energy is neither created nor destroyed. It is transferred from one entity to another in various forms.

Second law: The entropy of a body will increase when it is in isolated conditions.


  • Q is the heat 
  • U is the internal energy
  • W is the work done

Third Law: Entropy reaches a minimum value as the temperature of a body drops to absolute zero.

Energy conversions in the human body

The human body is a work cycle of heat and energy. These create spontaneous and irreversible conditions that can affect the equilibrium of the body. All food intake is metabolized by the body and with respect to the complexity of the food, the sets of chemical reactions will be different.

The internal energy of the human body changes with the consumption of food. When food reaches the digestive system, the metabolism process starts from the mouth. The food is broken down into cellular energy with an oxidation process. Hence, the digestive process increases the chemical potential energy. Intake of the food itself can be considered as the work done by the body. Human body process heat as the by-product of digestion. Hence, the digestion process increases the heat output of the body.

Food can be categorized with respect to the energy provided to the human body.  A calorie is the unit of energy. It is the energy needed to increase the temperature of one gram of water by one degree Celsius. The total internal energy of the body is exhausted throughout the day-to-day activities of a human being. The internal energy is lost in heat transfer that happens as a part of various life-sustaining processes. Work done while digestion, respiration, cell repair, growth, waste removal, and other processes also consume internal energy. Another by-product of digestion is fat, storing this fat will consume internal energy. The consumption and restoration of internal energy in the human body can be classified as the internal energy cycle.

Energy flow in human metabolism

The human body is always required to maintain an equilibrium in its various cycles. Thus, maintaining the right amount of food intake will ensure that the average internal energy of the human body can be kept constant. The difference in consumption and restoration of the internal energy can be denoted as U. This value will be positive when a person is over-eating. Overeating causes abundant production of internal energy. The excess internal energy is stored as fat in the human body. In the longer run, this can cause health issues such as cholesterol and obesity. When the value of U turns out to be negative, that will denote a situation where the person is under-eating. Hence, the internal energy that is consumed by the body to maintain its temperature and to do work is not replenished. The body can run for a few days in this condition as it will burn the existing fat to create more internal energy. After that, this situation can lead to malnutrition and other major health issues.

The rate at which food intake is converted into heat energy and the rate at which work is done using the energy is known as the basal metabolic rate of the human body. The metabolic rate depends upon eating habits and external conditions.

The entropy of the human body

Entropy is a state function that can be explained as the amount of heat loss divided by the absolute temperature. Total entropy change of the human body is the sum of the entropy change of the surroundings and the body itself. The entropy of the human body can be calculated as the ratio of basal metabolic rate and average body temperature.

Being a thermodynamic quantity, entropy is used to explain the type of the process. To maintain equilibrium, the change in entropy should be equal to 0. Positive change denotes the process as a spontaneous process and negative change denotes that the process is non-spontaneous. Various researchers have worked on finding the entropy flow of a human body in basal conditions. It was found that the external condition and the clothing of the human body had no effect regarding the entropy change. Scientists have ruled out the dependency of the human body entropy on environmental factors.

Free energy of the human body

Free energy is produced as a part of the digestive system. Food that is broken down into cellular energy produces the energy currency known as adenosine triphosphate or commonly known as ATP. This is the main way by which free energy is stored in our bodies. Since free energy is the energy that is available for work to be done, ATP is readily available to the cellular mechanisms to perform work. Being a conserved quantity, energy is transferred in various forms throughout the human metabolism. The efficiency of the body to convert this chemical potential energy available from food intake to useful work that can be done using free energy is known as body efficiency.

Context and Applications

This topic is taught in graduate and postgraduate courses like

  • Bachelor in Science in Physics
  • Master in Science in Physics
  • Bachelor in Science in Food technology
  • Master in Science in Food technology
  • Bachelor of Medicine and Bachelor of Surgery (MBBS)

Practice Problems

1. Which of the following is the unit of energy?

  1. Calorie
  2. Joule
  3. both a and b
  4. none of these

Answer: c

Explanation: Calorie and Joule are the units of energy.

2. What is the ratio of basal metabolic rate and average body temperature?

  1. Enthalpy
  2. Entropy
  3. Energy
  4. All of these

Answer: b

Explanation: The entropy of the human body is the ratio of basal metabolic rate and average body temperature.

3. What is the value of change in entropy at equilibrium?

  1. Zero
  2. Positive
  3. Negative
  4. Infinity

Answer: a

Explanation: The value of change in entropy at equilibrium is zero.

4. What is the form of free energy in the human body?

  1. Fat
  2. Glucose
  3. Protein
  4. ATP

Answer: d

Explanation: ATP is the form of free energy in the human body.

5. What is the temperature of the body when entropy reaches minimum value?

  1. Zero
  2. negative
  3. absolute zero
  4. None of these

Answer: c

Explanation: Entropy reaches a minimum value as the temperature of a body drops to absolute zero.

Common Mistakes

Human metabolism is a complicated topic that is still to be fully studied by scientists. Common mistakes can be made while relating the concept of metabolism to thermodynamic laws. Intake and outflow of energy need to be understood with proper signs and mistakes made while determining the flow of energy can reflect in the calculation of metabolic rate and body efficiency. Care must be taken to avoid mistakes in SI units of temperature and energy measurements.

  • Digestive System
  • Thermodynamics
  • Statistical thermodynamic laws

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