Table of Contents
Determining how much energy an organism uses is central to understanding its metabolism, performance, and environmental needs. In this chapter, the focus is not on how energy is transformed in cells (that belongs to other sections), but on how we measure and describe these energy conversions at the level of whole organisms.
We will distinguish between:
- How we quantify energy turnover (metabolic rate),
- The measurement methods (direct and indirect calorimetry),
- And the different levels of energy expenditure (basal vs. performance metabolism).
These ideas form the bridge between biochemical processes in cells and the physiology and ecology of whole organisms.
Why Determine Energy Conversion?
Knowing how much energy an organism uses over time allows us to:
- Estimate food requirements and nutrient needs.
- Compare activity levels and performance between individuals or species.
- Understand adaptations to environments (e.g., cold vs. warm climates).
- Study growth, reproduction, and survival strategies.
- Design feeding regimes in agriculture and animal husbandry.
- Assess human health, including obesity, malnutrition, and athletic performance.
In all these cases, the central quantity is energy turnover per unit time: the metabolic rate.
Units and Basic Terms
Energy
In biology, energy is usually expressed in:
- Joule ($\mathrm{J}$) – SI unit of energy
- Kilojoule ($\mathrm{kJ}$) – $1\,\mathrm{kJ} = 1000\,\mathrm{J}$
- Kilocalorie ($\mathrm{kcal}$) – commonly used in nutrition
Relationship:
$\,\mathrm{kcal} \approx 4.184\,\mathrm{kJ}$$
Food labels often use kcal; scientific literature prefers kJ.
Power / Rate
Energy use over time is a rate (energy per time):
- Watt ($\mathrm{W}$):
$\,\mathrm{W} = 1\,\mathrm{J/s}$$
In biology we often express metabolic rate as:
- $\mathrm{kJ/day}$ or $\mathrm{kcal/day}$
- $\mathrm{W}$ (Joule per second)
- $\mathrm{ml\,O_2/min}$ (from gas exchange measurements; see indirect calorimetry)
Metabolic Rate
Definition
Metabolic rate is the total rate of energy turnover in an organism:
- It includes all chemical processes that use or release energy: synthesis, breakdown, ion pumping, movement, heat production, etc.
- Mathematically:
$$\text{Metabolic rate} = \frac{\text{Energy used in a given time}}{\text{Time interval}}$$
For example, if a person uses $8400\,\mathrm{kJ}$ in 24 hours:
$$\text{Metabolic rate} = \frac{8400\,\mathrm{kJ}}{24\,\mathrm{h}} = 350\,\mathrm{kJ/h}$$
This is the whole-organism perspective on energy conversion.
Influencing Factors
Metabolic rate is not fixed. It depends on:
- Body size and mass (larger animals use more total energy but less per unit mass)
- Body temperature regulation (warm-blooded vs. cold-blooded)
- Activity level (rest, walking, running, flight)
- Growth, reproduction, lactation
- Nutritional state (fasted vs. well-fed)
- Environmental conditions (temperature, oxygen availability)
Other sections deal with why these factors matter; here they are important mainly because they must be controlled or at least known when measuring energy conversion.
Methods of Determining Energy Conversion
Two fundamental approaches are used:
- Direct calorimetry – measure heat directly.
- Indirect calorimetry (respirometry) – infer energy turnover from gas exchange (mainly oxygen consumption and carbon dioxide production).
Each has its own advantages, limitations, and typical applications.
1. Direct Calorimetry
Principle
Most of the energy released in metabolism eventually appears as heat (even mechanical work ends as heat after friction and dissipation). In direct calorimetry, the organism is placed in an insulated chamber, and the heat flow from its body is measured.
Because:
Heat output per unit time $\propto$ energy turnover per unit time,
measured heat gives a direct estimate of metabolic rate.
Calorimeter Setup (Conceptual)
A direct calorimeter typically includes:
- An insulated chamber to minimize exchange of heat with the environment.
- A heat absorption system, for example:
- Water flowing around the chamber,
- Or heat-sensitive elements (thermocouples, thermistors).
- Temperature sensors that register the temperature rise in the absorbing medium.
From the temperature change and the known heat capacity of the absorbing material (often water), the heat output of the organism per time can be calculated.
Example (conceptual, not a derivation):
- Water flow: $F$ liters per minute,
- Temperature increase: $\Delta T$ in °C,
- Specific heat of water: $c \approx 4.184\,\mathrm{kJ/(kg\cdot ^\circ C)}$.
Then:
$$\text{Heat per minute} \approx F\cdot \rho \cdot c \cdot \Delta T$$
(with $\rho$ as water density, close to $1\,\mathrm{kg/L}$).
This gives energy per minute; dividing by 60 yields Watts.
Advantages
- Measures all heat produced, regardless of metabolic pathways.
- Does not depend on assumptions about substrate use (fat vs. carbohydrate).
- Conceptually straightforward: what you measure is directly related to energy release.
Limitations
- Technically complex and expensive.
- Requires the animal or human to stay in a confined chamber.
- Difficult to use during vigorous movement, exercise, or in natural environments.
- Slow response to rapid changes in metabolism.
For these reasons, direct calorimetry is important for method development and validation, but not practical for most everyday or field measurements.
2. Indirect Calorimetry (Respirometry)
Principle
In aerobic metabolism, nutrients are oxidized using oxygen, producing carbon dioxide and water. For many substrates, the amount of energy released per liter of oxygen consumed is relatively constant.
Therefore:
Measuring oxygen consumption (and often carbon dioxide production) allows us to infer metabolic rate.
This is called indirect calorimetry or respirometry.
Basic Idea
For example, complete oxidation of a typical mixture of carbohydrates and fats in humans yields about:
- $\approx 20\,\mathrm{kJ}$ per liter of $O_2$ consumed (value depends on substrate mix).
Thus, if a person consumes $0.3\,\mathrm{L\,O_2/min}$ at rest:
- Energy per minute $\approx 0.3 \times 20 = 6\,\mathrm{kJ/min}$
- Per hour: $\approx 360\,\mathrm{kJ/h}$
- Per day: $\approx 8640\,\mathrm{kJ/day}$
(Actual conversion factors are more precise and substrate-dependent.)
Measuring Gas Exchange
In practice, respirometry can be done by:
- Closed systems:
The organism breathes in a sealed container; oxygen decline (and CO$_2$ increase) is monitored. Oxygen can be replenished and CO$_2$ absorbed by chemicals (e.g., soda lime) to keep conditions stable. - Open-flow systems:
Fresh air flows through a chamber or a mask. Differences in $O_2$ and CO$_2$ concentrations between inflow and outflow air, together with flow rate, give the rates of $O_2$ consumption and CO$_2$ production.
For humans, this is often done using:
- Face masks or mouthpieces with nose clips.
- Sensors for oxygen and carbon dioxide concentration.
- Flow meters to measure ventilation volume.
Respiratory Quotient (RQ)
The respiratory quotient is defined as:
$$\text{RQ} = \frac{\text{CO}_2 \text{ produced}}{\text{O}_2 \text{ consumed}}$$
- Pure carbohydrate oxidation: $\text{RQ} \approx 1.0$
- Pure fat oxidation: $\text{RQ} \approx 0.7$
- Protein: intermediate values.
Knowing RQ helps to:
- Estimate which substrates (carbohydrates, fats) are being used.
- Select the correct energy-per-liter-O$_2$ conversion factor.
Advantages
- Technically easier and more flexible than direct calorimetry.
- Suitable for:
- Resting metabolism,
- Exercise tests,
- Field studies (using portable devices).
- Provides insights into substrate use (fats vs. carbohydrates).
Limitations
- Only valid for aerobic metabolism; during strong, short-term workloads, anaerobic processes contribute, which are not directly captured by gas exchange.
- Assumes steady-state conditions during measurement.
- Requires corrections for leaks, sensor drift, and humidity in precise experiments.
Basal Metabolic Rate and Performance Metabolism
Metabolic rate varies with activity and physiological state. To compare organisms or individuals, standard conditions are defined.
Basal Metabolic Rate (BMR)
Basal metabolic rate is the minimum energy turnover of an organism that is:
- Awake
- At complete mental and physical rest
- In a post-absorptive state (no active digestion; often after fasting overnight)
- At a thermally neutral environmental temperature (no shivering or sweating needed)
- Free from acute stress or disease
BMR reflects the energy needed to maintain:
- Ion gradients across membranes
- Circulation and breathing
- Basic neuronal activity
- Basic metabolic processes in cells
Measurement:
- Usually determined by indirect calorimetry, under strict standard conditions.
- Power is often expressed as $\mathrm{kJ/day}$, $\mathrm{kcal/day}$, or $\mathrm{W}$.
BMR is useful for:
- Comparing species or individuals when controlling for size.
- Studying adaptations (e.g., energy-saving strategies in small mammals or desert animals).
- Serving as a reference point for daily energy expenditure.
Resting Metabolic Rate (RMR)
In practice, it is often difficult to meet all strict criteria for BMR. Therefore, a slightly less strict measure is widely used: resting metabolic rate (RMR).
- Conditions are similar to BMR but somewhat more relaxed (e.g., shorter fasting period).
- Values are usually slightly higher than true BMR.
- RMR is commonly used in clinical and nutritional settings.
Performance (Activity) Metabolism
When an organism is active, energy demand rises above basal or resting levels. The additional energy expenditure is called performance metabolism or activity metabolism.
It includes:
- Locomotion (walking, running, flying, swimming).
- Work (muscle exertion, shivering for heat).
- Specific dynamic action (extra metabolism after a meal due to digestion and nutrient processing).
- Growth and reproduction, which may significantly raise average daily energy expenditure.
We often distinguish:
- Total daily energy expenditure (TDEE) = BMR (or RMR) + activity + digestion + other components.
- Activity factor:
$$\text{TDEE} \approx \text{BMR} \times \text{activity factor}$$
where the activity factor is >1 and depends on lifestyle or experimental conditions.
Field Metabolic Rate (FMR)
In many ecological and behavioral studies, researchers are interested in an animal’s average energy turnover in its natural environment. This is often called field metabolic rate (FMR).
- Typically measured over days using specialized techniques (e.g., doubly labeled water; details belong to specialized methods).
- Reflects real-life activity patterns, environmental variation, and feeding behavior.
FMR is key for:
- Understanding ecological niches and energy budgets.
- Estimating food requirements and carrying capacity of habitats.
- Comparing wild and captive animals.
Interpreting Measurements of Energy Conversion
Measured metabolic rates can be:
- Normalized to body mass (e.g., $\mathrm{W/kg}$) to compare animals of different sizes.
- Related to time (short-term vs. daily averages).
- Used to derive:
- Energy budgets (how energy intake is partitioned into maintenance, growth, reproduction, storage),
- Thermoregulatory costs (extra energy in cold or heat),
- Performance limits (maximum sustainable work rates).
These interpretations are crucial for connecting biochemical energy conversion to physiology, ecology, and evolution, but they all rely on the measurement principles outlined in this chapter:
- Heat production (direct calorimetry),
- Gas exchange (indirect calorimetry),
- And standardized conditions for basal vs. performance metabolism.