Human Metabolism and the First Law of ThermodynamicsThe metabolism of the human body involves converting chemical energy from food into work, heat, and stored energy—primarily in the form of fat. This process provides a rich example of the first law of thermodynamics in action. By treating the body as a thermodynamic system, we can use the first law to analyze how energy is transferred and transformed during activities ranging from rest to intense physical exercise.

One of the key observations is that body temperature remains relatively constant. This constancy implies that the body continuously releases heat to the environment, meaning [latex]Q[/latex] is negative. At the same time, the body performs work on the surroundings—such as lifting objects or moving limbs—so [latex]W[/latex] is positive. According to the first law:

In this equation, a negative [latex]Q[/latex] and a positive [latex]W[/latex] both contribute to a net loss in internal energy [latex](\Delta U)[/latex], unless replenished through food intake.

Calories and Food Energy

Eating restores the body’s internal energy by adding chemical potential energy. From a thermodynamic perspective, food is a form of energy input—functionally equivalent to work done on the body. This energy is commonly measured in Calories. One nutritional Calorie (with an uppercase “C”) is defined as the amount of energy needed to raise the temperature of 1 kilogram of water by 1°C, and it is equivalent to 1 kilocalorie in the scientific context.

When food is metabolized—primarily via oxidation reactions—it releases energy that is either used to do work, transferred as heat, or stored as fat. If energy intake from food equals energy lost to heat and work, the body maintains a constant internal energy, or [latex]\Delta U = 0[/latex]. If intake exceeds energy expenditure, [latex]\Delta U[/latex] becomes positive and fat is stored. Conversely, if intake is less than energy expenditure, [latex]\Delta U[/latex] is negative and the body metabolizes fat to compensate.

Thermodynamics of Dieting

Weight loss occurs when the energy the body expends (through work and heat loss) exceeds what is replaced by food, causing a negative change in internal energy. However, weight regulation is not straightforward. When caloric intake decreases, the body responds by lowering its basal metabolic rate (BMR)—the energy used at rest for essential physiological functions. As a result, the body conserves energy and slows fat loss. Repeated dieting leads to an adaptive reduction in BMR, making subsequent attempts at weight loss more difficult.

Exercise plays a key role in weight management by increasing both the work performed by the body and heat transfer, as well as temporarily raising the BMR. Additionally, human metabolism is relatively inefficient at converting chemical energy into mechanical work, meaning more energy is lost as heat—a helpful factor in calorie expenditure during physical activity.

Irreversibility and Biological Systems

Biological processes such as metabolism and photosynthesis are fundamentally irreversible. This means they occur in one direction only under normal conditions. For example, although stored fat can be metabolized to do work and generate heat, the reverse—using external heat or mechanical work to create body fat—is not possible. You cannot skip lunch by simply sitting in the sun or walking down stairs. Similarly, photosynthesis in plants converts radiant energy into stored chemical energy, but this energy cannot be recovered by shining light on a plant that has already performed photosynthesis.

These processes are governed by the first law of thermodynamics, which tracks energy conservation from start to finish—regardless of the complex biochemical details in between. This law gives us a powerful tool to analyze human physiology and plant biology, as shown in Figure 108.4.

Table 108.1 Summary of Terms for the First Law of Thermodynamics, ΔU=Q−W

Term	Definition
[latex]U[/latex]	Internal energy—the sum of the kinetic and potential energies of a system’s atoms and molecules. Can be divided into many subcategories, such as thermal and chemical energy. Depends only on the state of a system (such as its [latex]P[/latex], [latex]V[/latex], and [latex]T[/latex]), not on how the energy entered the system. Change in internal energy is path independent.
[latex]Q[/latex]	Heat—energy transferred because of a temperature difference. Characterized by random molecular motion. Highly dependent on path. [latex]Q[/latex] entering a system is positive.
[latex]W[/latex]	Work—energy transferred by a force moving through a distance. An organized, orderly process. Path dependent. [latex]W[/latex] done by a system (either against an external force or to increase the volume of the system) is positive.

Section Summary

• The first law of thermodynamics is [latex]\Delta U = Q - W[/latex], where [latex]\Delta U[/latex] is the change in internal energy, [latex]Q[/latex] is net heat transfer, and [latex]W[/latex] is net work done by the system.
• Heat and work are both energy in motion—not stored—while internal energy is a state function that can be stored.
• Internal energy depends only on the system’s current state, not on how that state was achieved.
• Biological processes such as metabolism and photosynthesis illustrate the interplay of heat, work, and internal energy and exemplify irreversible thermodynamic processes.