Physiology Sept. 12th
Energetics, Metabolism, and Thermal Biology (Lecture Transcript) - Comprehensive Notes
Quiz recap and key concepts
Regulation vs. control
Regulation: keeping something at a set point (via feedback). In the discussion, it’s emphasized that regulation involves negative feedback.
Control: the ability to change something (make a modification). These concepts are distinct.
Energy units and chemical bond energy
Chemical bond energy is discussed as the “totally broken form of biological energy.”
Kilowatt hours as a total energy amount, not a rate of energy expenditure.
Watts (W) or joules per second are rates, not total energy.
Scaling practice and small changes in mass
A scaling example showed how heart mass comparisons depend on assumed body mass (e.g., when m = 1 kg for both bird and mammal).
The stated comparison: 0.6 is about 20% larger than 0.5 on a particular scaling point, illustrating nonlinearity.
b exponent and allometry
The b exponent describes how a physiological variable scales with body mass.
If a variable is absolutely larger but proportionally smaller as mass increases, that is allometric scaling (negative exponent).
For metabolic rate, the classic relation is , where M is body mass.
Mass-specific metabolic rate scales as , i.e. decreases with mass.
On a log-log plot, the slope for MR vs. M is about 0.75; for mass-specific MR vs. M is about -0.25 to -0.30.
Indirect calorimetry and energy conversion
Indirect calorimetry measures gases (O₂ and CO₂) to estimate energy expenditure.
Energy conversion factors relate gas exchange to energy (kilojoules).
Carbohydrate metabolism
RQ (respiratory quotient) is 1.0 when burning carbohydrates, because carbohydrate oxidation yields equal moles of CO₂ produced per O₂ consumed.
Lipid metabolism
RQ is lower for lipid oxidation due to less CO₂ produced per unit O₂ consumed.
Protein metabolism
Falls between carbohydrate and fat in its RQ; in many exercise studies, protein is ignored for simplicity.
Practical interpretation of RQ values
RQ ≈ 1.00 → predominantly carbohydrate oxidation (e.g., rest/fed state with carbohydrate metabolism).
RQ ≈ 0.70 → predominantly fat oxidation (lipids).
Intermediate RQ (e.g., ≈0.85) → a mix of carbohydrate and fat (roughly 50/50 in some contexts).
Metabolic rates: definitions and measurements
Basal Metabolic Rate (BMR)
For endotherms (warm-blooded animals like birds and mammals).
Measured in fasting, resting state within the thermoneutral zone (no energy spent on warming or cooling).
Standard Metabolic Rate (SMR) for ectotherms
Endpoints adjusted for temperature (e.g., a frog at 25°C or 30°C).
Metabolic rate depends on ambient temperature for ectotherms; needs a defined temperature.
Temperature and “thermoneutral zone”
Thermoneutral zone: the ambient temperature range where no extra metabolic work is needed to maintain body temperature.
Post-absorptive state
Fasted, not actively digesting nutrients; used to measure baseline metabolic rate without SDA confounding digestion.
Specific Dynamic Action (SDA)
The increase in metabolic rate due to digesting and processing food after a meal.
In digestion, the largest part of SDA is the processing of absorbed nutrients (e.g., liver glycogen storage, fat cell lipogenesis).
For protein specifically, roughly 30% of the energy contained in protein can be lost to SDA during amino acid processing.
Example: fish SDA
After a meal, oxygen consumption rises substantially, with larger meals causing a greater rise than smaller meals.
SDA reflects energy costs beyond the mechanical work of digestion; much of SDA stems from hepatic and other post-absorptive processes.
Scaling of metabolic rate with body mass
Resting metabolic rate across vertebrates shows a strong size effect.
Small animals have higher mass-specific metabolic costs than large animals; per gram, small animals expend more energy.
Quantitative comparisons across taxa (rough estimates):
Amphibians are least costly per unit body mass to maintain.
Reptiles cost about 3.6× more energy per day than amphibians.
Mammals cost about 12.5× more energy per day than amphibians.
Birds cost about 17× more energy per day than amphibians.
Conceptual takeaway: higher mass-specific MR in small species makes ectotherms like amphibians relatively energy-efficient per unit mass, contributing to their ecological success under variable conditions.
Curvilinear scaling on linear axes vs. linear on log-log axes
On linear axes, MR vs. mass appears curvilinear; on log-log axes, it becomes a straight line with slope around 0.75 for MR vs. M.
Mass-specific metabolic rate (MR/M) declines with body mass, with slope around -0.25 to -0.30 on a log-log scale.
Practical implication: the energy cost per unit body mass decreases as animals get larger, making large animals energetically more economical per gram of tissue.
Mitochondrial density and turnover
Smaller animals tend to have higher mitochondrial densities in muscles to sustain higher mass-specific MR.
Visual representation: data show amphibians, reptiles, mammals, and birds sharing similar scaling exponents for MR/M across groups; the cross-taxa pattern supports a general allometric rule rather than a strict phylogenetic constraint.
Locomotion: power curves and costs of movement
Power curve: energy expenditure (power) as a function of speed for a given locomotion mode.
General principle: power increases with speed; the relationship differs by mode due to drag and mechanics.
Swimming (aquatic locomotion)
Drag is the dominant force increasing with speed; power rises roughly exponentially with speed because drag increases with velocity.
Neutrally buoyant swimmers still pay energy to overcome drag.
The rate at zero speed (P ≈ SMR) represents baseline metabolic rate when not moving.
Walking/running (terrestrial locomotion)
The power curve is less curved on linear axes because drag is less significant at typical speeds; locomotion costs are driven by lifting and braking rather than overcoming drag.
Flying (avian/bat/insect flight; also relevant to aircraft)
The power curve is U-shaped with speed: high power required at very low speeds (overcome gravity) and high speeds (overcome drag), with a minimum power speed in between.
The minimum power speed is the cheapest flight speed (lowest power expenditure to stay airborne).
Maximum range speed (tangent to cost-of-transport curve) yields the most efficient travel over long distances and corresponds to typical jet-aircraft range planning.
Cost of locomotion and the cost of transport (COT)
Cost of transport: energy per unit distance per unit body mass (e.g., J kg⁻¹ m⁻¹ or mL O₂ g⁻¹ s⁻¹ converted to energy).
When plotted on log-log axes, COT curves align across taxa for each mode, showing that swimming generally costs the least per distance, followed by flying, then walking.
Key takeaways:
Swimming is the cheapest way to move large distances in many aquatic and semi-aquatic species.
Flying costs are lower than walking for distance travel, but higher than swimming.
Walking is the most energy-intensive way to cover distance per unit body mass.
Across modes, similar scaling exponents apply within groups (e.g., all swimmers, all flyers, all walkers tend toward similar COT scaling with mass).
Metabolic ceilings and sustainability of energy expenditure
A proposed metabolic ceiling suggests a sustainable daily metabolic rate around 4–5× the basal metabolic rate (BMR) across animals, balancing energy for activity and lifespan.
Real-world examples and constraints:
Tour de France cyclists can operate at ~7–10× their BMR on common days, but this is not typically sustainable over long periods without consequences.
Arctic explorers may sustain ~10–15× BMR during extreme expeditions, indicating a toll on energy balance and potential long-term costs.
The takeaway: there is a practical ceiling to sustained energy expenditure beyond BMR, shaped by physiology, reproduction, and lifespan considerations.
Temperature, thermal biology, and heat exchange (thermoregulation)
Why temperature matters
Temperature strongly influences physiological processes and energy balance, especially under climate change scenarios.
For ectotherms (animals that rely on external sources to regulate body temperature), environmental warming directly affects metabolism and energy budgets.
Temperature vs heat
Temperature is a measure of the average kinetic energy (mean speed) of molecules in a system.
Heat is the total energy carried by those molecular motions (the integrated energy content).
Heat transfer direction is governed by temperature, not by the amount of heat alone: heat flows from higher temperature to lower temperature.
Modes of heat exchange with the environment
Radiation: heat transfer via electromagnetic waves (input from sun; output as infrared radiation from the animal).
The sun emits across a spectrum; the animal absorbs some wavelengths and emits infrared radiation depending on its own temperature.
Radiation is a two-way street: you can gain or lose heat through radiative exchange depending on relative temperatures.
Conduction: direct contact heat transfer with a solid surface.
Example: a fox resting on hot ground will lose heat to the ground if the ground is cooler than the fox’s feet.
Convection: heat transfer with a moving fluid (air or water).
Wind speed and fluid properties affect the rate of heat exchange; faster winds increase heat loss when the environment is cooler than the animal.
Evaporation: heat loss via phase change of water (cooling mechanism).
Can only result in heat loss (evaporative cooling).
Includes respiratory evaporation (breathing) and cutaneous evaporation (skin).
Heat budgets and plant analogies
The heat budget diagram applies to plants as well, where transpiration (evaporation of water from leaves) helps cool tissues.
Example: a leaf with higher evaporation is cooler than a leaf with less evaporation (illustrated by a thermal image showing one leaf cooler due to evaporation).
Latent heat of evaporation
When water evaporates, it requires a large amount of energy: the latent heat of vaporization.
Typical reference value (as noted in the lecture): about
This large energy cost of evaporation underpins evaporative cooling as an effective thermoregulation strategy in many organisms.
Endotherms, homeotherms, and ectotherms
Endotherms (warm-blooded) generate a significant portion of their heat metabolically and tend to maintain a relatively constant body temperature (homeothermy) across many conditions.
Ectotherms rely more on environmental heat sources and have metabolism that is temperature-dependent; their body temperature tracks ambient temperature more closely.
Relevance to climate change
Temperature effects on metabolism become increasingly important for predicting organism responses to warming environments, shifts in activity windows, and energy balance.
Practical connections and real-world relevance
Indirect calorimetry techniques are standard tools for estimating energy expenditure in animals and humans, linking gas exchange to caloric energy use.
Allometric scaling informs ecological and evolutionary questions: why larger animals have different energy budgets, how energy constraints shape life histories, and why certain locomotion modes dominate in nature (e.g., migration by swimming or flying).
Understanding SDA and post-absorptive metabolism helps distinguish baseline metabolic rate from the costs of digestion and nutrient processing, which has implications for nutrition, performance, and health.
The concept of a sustainable metabolic ceiling helps reconcile observations of extreme endurance events with long-term fitness and lifespan trade-offs.
Quick reference: key formulas and values
Metabolic rate scaling with mass:
Mass-specific metabolic rate scaling:
Respiratory quotient (RQ):
RQ interpretations:
Carbohydrates:
Lipids:
Proteins: between 0.70 and 1.00 (context-dependent)
Post-absorptive and SDA (conceptual)
SDA: energy cost associated with digestion, absorption, and processing of nutrients; a large portion of SDA arises from hepatic glycogen storage, lipogenesis, and amino acid processing.
For protein, about can be lost due to SDA during amino acid processing.
Latent heat of evaporation (evaporative cooling):
End-of-lecture note on terminology
Endotherm: warm-blooded animals that generate heat metabolically and typically maintain a relatively constant body temperature (homeothermy).
Ectotherm: animals whose body temperature is largely determined by the environment, with metabolism strongly influenced by ambient temperature.
Thermoneutral zone: the temperature range where the organism does not need to expend extra energy to maintain thermal balance.
Post-absorptive: a physiological state in which the digestive tract is largely empty and nutrient absorption is not actively occurring, used to measure baseline metabolism.
Quick study tips based on these notes
Remember the big three energy concepts: MR ∝ M^0.75, RQ as a substrate indicator (1.0 for carbs, ~0.7 for fats), and SDA as a digestion-related MR increase.
Practice converting gas exchange data to energy using RQ values and conversion factors; be able to interpret what an RQ near 0.85 means in terms of substrate use.
Be comfortable with the idea that larger animals have lower mass-specific energy costs, which explains ecological patterns like why large mammals and birds can travel long distances more efficiently per unit mass.
Use the heat exchange framework (radiation, conduction, convection, evaporation) to reason about thermoregulation in different environments and for different taxa.
Note on transcript context
The transcript includes practical lecture examples (e.g., a 1 g vs 1 kg comparison, specific RQ examples, and a fish SDA graph) and emphasizes applying these concepts to interpret metabolic and thermal biology data in a variety of animals.