4 Energy and Thermodynamics in Biology

Energy in biological systems

Biological systems use energy to move, maintain posture, pump blood, breathe, and carry out cellular processes. Thermodynamics helps explain how energy is transferred and transformed, and why some of it becomes heat rather than useful mechanical work.

Work, power, and heat

Work is energy transferred when a force causes displacement. For a constant force in the direction of motion, the relationship is

A muscle does external work when it lifts a weight. Muscles also consume energy to maintain posture, even when there is little visible movement.

Power describes how quickly work is done:

Power is measured in watts, where $1\,\mathrm{W} = 1\,\mathrm{J/s}$. Climbing the same stairs quickly or slowly may involve similar work, but climbing quickly requires greater average power.

Heat is energy transferred because of a temperature difference. It can move between a patient and the surroundings by conduction, convection, radiation, or evaporation. For example, sweat evaporating from wet skin carries heat away, while a warming blanket transfers heat to a patient. Temperature describes a system’s thermal state; heat describes energy in transit.

Systems and the laws of thermodynamics

A thermodynamic system is the part being considered; everything outside it is the surroundings. An organism is an open system: it takes in food, water, and oxygen and releases waste, carbon dioxide, and heat. This exchange matters when accounting for an organism’s energy.

The first law states that energy is conserved. For a closed system, using the convention that heat added is positive and work done by the system is positive:

Here, $\Delta U$ is the change in internal energy, $Q$ is heat added, and $W$ is work done by the system. For an organism, a complete energy balance also accounts for energy entering and leaving with matter. Nutrient energy is transformed into cellular work, stored chemical energy, and heat; metabolism does not create energy.

The second law states that the total entropy of an isolated system does not decrease. Entropy relates to how energy is distributed and how much is unavailable for useful work. An organism can maintain local organization because it exchanges matter and energy with its surroundings. In doing so, it releases heat and waste, increasing the entropy of its surroundings.

Free energy and cellular work

For many biochemical reactions at constant temperature and pressure, the change in Gibbs free energy is

A negative $\Delta G$ indicates a thermodynamically favorable direction under the specified conditions, but it does not tell how quickly the reaction occurs.

Cells couple reactions that release free energy, often ATP hydrolysis, to reactions that require energy. ATP transfers usable free energy between metabolic reactions and cellular tasks. It is not an independent source of energy: its production depends on food or other energy inputs.

Metabolism, temperature, and clinical use

Metabolism converts chemical energy in nutrients into forms the body can use. Some energy supports cellular processes, some may contribute to external mechanical work, and some is released as heat. During exercise, increased muscle energy use accompanies greater heat production. The body helps regulate core temperature by balancing heat production with heat loss.

Indirect calorimetry estimates energy expenditure from oxygen consumption and carbon dioxide production. Temperature monitoring and warming measures in surgery help limit unintended heat loss. Guidance cited in this material recommends checking that a patient’s temperature is at least $36.0\,^{\circ}\mathrm{C}$ before anesthesia induction, unless urgent clinical circumstances require otherwise.

Takeaway: Thermodynamic principles connect nutrient use to cellular activity, mechanical work, heat production, and clinical approaches to measuring metabolism and managing body temperature.