11: A Warmer World- Temperature Effects On Chemical Reactions

Types of Thermoregulation (Ectothermy vs. Endothermy) 

Thermoregulation in organisms runs along a spectrum from endothermy to ectothermy. Endotherms generate most of their heat through metabolic processes and are colloquially called “warm-blooded.” Ectotherms use external heat sources to regulate their body temperature. Ectotherms are colloquially called “cold-blooded” even though their body temperatures often stay within the same temperature ranges as warm-blooded animals.

Ectotherm 

An ectotherm, from the Greek (ektós) “outside” and (thermós) “hot,” is an organism in which internal physiological sources of heat are of relatively small or quite negligible importance in controlling body temperature. Since ectotherms rely on environmental heat sources, they can operate at economical metabolic rates. Ectotherms usually live in environments with constant temperatures, such as the tropics or the ocean. Ectotherms have developed several behavioral thermoregulation mechanisms, such as basking in the sun to raise body temperature or seeking shade to lower it.  The common frog is an ectotherm that regulates its body temperature in response to the external environment.

Endotherms 

In contrast to ectotherms, endotherms regulate their body temperature through internal metabolic processes and usually maintain a narrow range of internal temperatures. Animals usually generate heat through normal metabolism, but in extreme cold or during low activity, an endotherm generates additional heat by shivering. Many endotherms have more mitochondria per cell than ectotherms. These mitochondria enable them to generate heat by increasing the rate at which they metabolize fats and sugars. However, endothermic animals must sustain their higher metabolism by eating more food often. For example, a mouse (endotherm) must consume food daily to maintain its high metabolism, while a snake (ectotherm) may only eat once a month because its metabolism is much lower.

Homeothermy vs. Poikilothermy 

Two other descriptors are also used.   A poikilotherm is an organism whose internal temperature varies considerably. It is the opposite of a homeotherm, an organism that maintains thermal homeostasis. Poikilotherms’ internal temperature usually varies with the ambient environmental temperature, and many terrestrial ectotherms are poikilothermic. Poikilothermic animals include many species of fish, amphibians, and reptiles, as well as birds and mammals that lower their metabolism and body temperature during hibernation or torpor. Some ectotherms can also be homeotherms. For example, some tropical fish species inhabit coral reefs with such stable ambient temperatures that their internal temperatures remain constant.  Figure \(\PageIndex{1}\) below shows the energy output vs temperature for a homeotherm (mouse) and a poikilotherm (lizard).

Another term is heterothermy, in which a homeotherm's temperature can vary across body regions (spatially) and over time (daily or seasonally, as in hibernation).  A homeotherm's core is usually warmer than its extremities, allowing it to cool when needed.  Body temperature and metabolic rate decrease during hibernation (or sustained torpor).  

Means of Heat Transfer 

Heat can be exchanged between an animal and its environment through four mechanisms: radiation, evaporation, convection, and conduction. Radiation is the emission of electromagnetic “heat” waves. Heat radiates from the sun and dry skin in the same manner. When a mammal sweats, evaporation removes heat from the skin's surface. Convective air currents remove heat from the surface of dry skin as they pass over it. Heat can be conducted from one surface to another through direct contact, as when an animal rests on a warm rock.

Key Points 

• Processes such as enzyme production can be modified to adapt to varying body temperatures.
• Endotherms regulate their internal body temperature independently of external temperature fluctuations, while ectotherms rely on the external environment to do so.
• Homeotherms maintain their body temperature within a narrow range, while poikilotherms can tolerate wide variations in internal body temperature, usually due to environmental conditions.
• Heat can be exchanged between the environment and animals via radiation, evaporation, convection, or conduction.

Key Terms 

• ectotherm: An animal that relies on the external environment to regulate its internal body temperature.
• endotherm: An animal that regulates its internal body temperature through metabolic processes.
• homeotherm: An animal that maintains a constant internal body temperature, usually within a narrow range of temperatures.
• poikilotherm: An animal that varies its internal body temperature within a wide range of temperatures, usually as a result of variation in the environmental temperature.

These terms are diagrammed below in Figure \(\PageIndex{2}\).

Figure \(\PageIndex{2}\): Thermoregulatory Term.  Buffenstein et al., Biol. Rev. (2021), doi: 10.1111/brv.12791.  Creative Commons Attribution License

We have discussed in the previous chapter sections how temperature can affect macromolecules such as proteins (Chapter 4) and nucleic acids (Chapter 9.1), as well as supramolecular assemblies such as membranes (Chapter 10.3).  Temperature effects on small molecules and ions (such as salts in the Hofmeister series and glycerol, Chapter 4.9) in the environment that regulate the function/activity of these larger molecules and assemblies are also important. In the next section, we'll review and discuss the effects of temperature on these key molecular species.  First, we'll delve deeper into how temperature affects chemical and biochemical reactions.

Temperature Effects on the Rates of Chemical Reactions

To understand how temperature affects metabolic processes, let's first review its effects on ordinary chemical and biochemical reactions.  You may remember the general rule that the rate of a chemical reaction approximately doubles for every 10 °C (10 K) increase in temperature.  How does that arise?  This is generally true in a specific temperature range, as shown below.

The rates of reactions, either endothermic or exothermic, depend on the activation energy (E_a).  Activation energy is the energy required to move from a reactant to the transition state, which can then form the product.

The activation energy can be obtained from the Arrhenius equation (that you learned in introductory chemistry), which shows how the rate of an individual chemical reaction depends on temperature.

\begin{equation}
k=A e^{-E_a / R T}
\end{equation}

where k is the rate constant, E_a is the activation energy, E_a/RT is the average kinetic energy, and A is a constant (the "pre-exponential" factor). 

By taking the natural log (ln) of each side and rearranging the equation, you get a "linearized" equation that is easier for most.

\begin{equation}
\ln k=\ln A-\frac{E_a}{R T}
\end{equation}

A plot of ln k vs 1/T has a slope = E_a/R, from which the activation energy can be calculated.  

An alternative form can be derived:

\begin{equation}
\ln \frac{k_2}{k_1}=\frac{E_a}{R}\left(\frac{1}{T_1}-\frac{1}{T_2}\right)
\end{equation}

Solving for E_a gives

\begin{equation}
E_a=\frac{R \ln \frac{k_2}{k_1}}{\frac{1}{T_1}-\frac{1}{T_2}}
\end{equation}

Let's use this equation to calculate an E_a that will give a doubling of the reaction rate (k₂/k₁ = 2) going from T₁ = 295 K (21.9 °C, 71.3  °F) to T₂ = 305 K (21.9 °C, 89.3  °F), a 10 °C temperature rise. 

\begin{equation}
\begin{aligned}
E_a & =\frac{(8.314)(\ln 2)}{\frac{1}{295}-\frac{1}{305}} \\
& =\frac{\left(8.314 \mathrm{~J} \mathrm{~mol}^{-1} \mathrm{~K}^{-1}\right)(0.693)}{0.00339 \mathrm{~K}^{-1}-0.00328 \mathrm{~K}^{-1}} \\
& =\frac{5.76 \mathrm{Jmol}^{-1} \mathrm{~K}^{-1}}{\left(0.00011 \mathrm{~K}^{-1}\right)} \\
& =52,400 \mathrm{Jmol}^{-1}=52.4 \mathrm{~kJ} \mathrm{~mol}^{-1}
\end{aligned}
\end{equation}

Hence, if a reaction has an activation energy E_a of about 54 kJ/mol, increasing the temperature from 295 to 305 °C, (i.e., by 10 °C), doubles the reaction rate.

Assuming the activation energy is constant, the rate constants increase with temperature because a larger fraction of molecules has the energy (> E_a) required to react.  This is illustrated in Figure \(\PageIndex{3}\) below.

Figure \(\PageIndex{3}\): Plot of a Maxwell-Boltzmann distribution of speeds for different temperatures T=100 K, T=1200 K, T=5000 K.  Points along the curve show (1) most likely speed, (2) average speed, and (3) thermal speed (velocity that a particle in a system would have if its kinetic energy were equal to the average energy of all the particles of the system).  https://commons.wikimedia.org/wiki/F...xis-labels.svg.   Creative Commons Attribution-Share Alike 4.0 International license.

Let's look at the brown vertical line around 950 m/s.  If we take that as the activation energy, very few molecules in the blue distribution have the required kinetic energy > Eact to overcome it. At progressively higher temperatures, a greater fraction (as measured by the area under the curve to the right of the dotted line at 950 m/s) has the required energy; hence, the rates increase with temperature.

When the temperature change is 10°C, the ratio of the rate constants (or rates), k₂/k₁ is often called Q₁₀, the temperature coefficient (unitless).   Q₁₀ is not a constant since it depends on the two temperatures that differ by 10 °C (10 K).  Hence, the Q₁₀value for the 100⁰ range from 273-283 K differs from the Q₁₀ value for the 373- 383 K range.  For many reactions, Q₁₀ is around 2 (doubling the reaction rate) to 3 (tripling the reaction rate) at physiological temperature.  Q₁₀ =2 for a given E_a only at one set of temperatures that differ by 10 °C.  The variation in Q₁₀ values is illustrated in Table \(\PageIndex{1}\) below for a reaction in which E_a = 44.5 kJ/mol. Q₁₀ decreases from 2 as the temperatures T₁ and T₂ = T₁ + 10 °C increase.  

Table \(\PageIndex{1}\):  Q₁₀ = k₂/k₁ values at different temperatures T₁ and T₂ that differ by 10 ^o C.

We will see why this matters in biological settings in a bit.  If Q₁₀ = 1, the reaction is temperature-independent, and a Q₁₀ < 1 indicates that the reaction is not functioning.  For example, an enzyme-catalyzed reaction might reach its threshold at a higher temperature, T2 = T1 + 10, where the enzyme loses its active conformation and begins to unfold.

The same equation and the Q₁₀parameters apply to enzyme-catalyzed reactions.  The activation energies (E_a) for four enzymes involved in the degradation of lignocellulose in the surface soil and subsoil are shown in Table \(\PageIndex{2}\) below.  The enzymes include two hydrolases, β-glucosidase (BG) and cellobiohydrolase (CB), which cleave cellulose, and two oxidases, peroxidase (PER) and phenol oxidase (POX), which help degrade lignin. The average Ea for these enzymes is about 44.7 kJ/mol, similar to the example in Table 1 above.

Table \(\PageIndex{2}\): Activation Energies (E_a, kJ mol⁻¹) for extracellular soil enzymes involved in the degradation of lignocellulose.Adapted from Steinweg JM et al. (2013) PLOS ONE 8(3): e59943. https://doi.org/10.1371/journal.pone.0059943.  Creative Commons CC0 public domain

Q₁₀temperature coefficients also describe biological processes such as respiration, neural signal propagation, and metabolic rates. Temperature affects many biological processes, especially in ectotherms, which adjust their body temperatures to their environments, including daily and seasonal temperature shifts. Mammals and birds alter their metabolic rates with temperature, as do hibernating animals. 

The Q₁₀ temperature coefficient is the factor by which the reaction rates (k or R) increase (factor of 2, 3, 1.5, etc) for each 10-degree K or C temperature increase.  The following equation gives it:

\begin{equation}
Q_{10}=\left(\frac{k_2}{k_1}\right)^{10^{\circ} \mathrm{C} /\left(T_2-T_1\right)}
\end{equation}

It is also called van't Hoff's temperature coefficient.  To help you understand Q10, consider these examples.

• If T₂ - T₁ = 10 °C, Q₁₀ = k₂/k₁ for the specified temperature pairs separated by a 10 °C range (T₁ and T₂=T₁+10).  Remember that Q₁₀ is not a constant;  it depends on the temperature pairs and decreases with increasing temperature.
• If the temperature range is > 10 °C, the the measured ratio k₂/k₁ is a factor > 1 x Q₁₀
• If the temperature range is < 10 °C, the the measured ratio k₂/k₁ is a fraction of Q₁₀

This equation can be converted to 

\begin{equation}
k_2=k_1 Q_{10}^{\left(T_2-T_1\right) / 10^{\circ} \mathrm{C}}
\end{equation}

where the rate constant k₂ is related to a "base" rate k₁ at a base temperature of T₁.  Figure \(\PageIndex{4}\) below shows an interactive graph of the above equation.

Figure \(\PageIndex{4}\): Interactive graph of k₂ (rate 2) vs. delta T at different base rates k₁. 

Change the base rate constant, k₁, at a base temperature of T₁ and Q₁₀ coefficient to see how they affect k₂.   

Note that if Q₁₀ =1, then k₂ at T₁ + 10 = k₁ at T₁, so the rate is independent of the temperature.

For most biological systems, the Q₁₀ value is ~ 2 to 3 under physiologically relevant conditions.  The ratios of the rates (R₂/R₁) for different Q₁₀ values are shown in Figure \(\PageIndex{5}\) below.

​​​​​​

Figure \(\PageIndex{5}\):  Idealized graphs showing the dependence on temperature of the rates of chemical reactions and various biological processes for several different Q10 temperature coefficients. The dots on the graph show how the rate changes with a 10 °C change in temperature.   Wikipedia. https://en.wikipedia.org/wiki/Q10_(t...e_coefficient).  CC BY-SA 4.0

Again, this hypothetical graph shows the general meaning of Q₁₀ values.

The "Q" model has been used to fit complex reaction systems, not just individual reactions.  Figure \(\PageIndex{6}\) below shows the daily mean soil respiration rate as a function of soil temperature. In these graphs, the x-axis is Temperature, not ΔT.

Figure \(\PageIndex{6}\): Relationships between daily mean soil respiration (R_s) and soil temperature (T_s).  Jia X et al.,  PLoS ONE 8(2): e57858. https://doi.org/10.1371/journal.pone.0057858.  Creative Commons Attribution License

Soil temperature (T_s) was measured at a depth of 10 cm. Open circles represent January to June; closed circles represent July to December. The solid lines use a Q₁₀ model, in which the observed R_s vs. T_s data are fit with an equation that optimizes the Q₁₀ parameter.  The dashed lines are fitted using a logistic model we used in Chapter 5.7 to fit ELISA data. R_s is significantly different between the first and second halves of the year.  

Soil respiration rate (R_s) at a depth of 10 cm was strongly affected by temperature, with an annual Q₁₀ value of 2.76.   Daily Q₁₀ estimates averaged 2.04 and decreased as Ts increased^_. A study of seagrass showed that Q₁₀ values are affected by plant tissue age and vary significantly with initial temperature and temperature range.  

Using Q₁₀ values from the Arrhenius equation assumes that chemical and biochemical processes are exponential functions of temperature.  For complex processes such as the decomposition of organic matter, it is better to model the entire system by accounting for the individual enzymes involved.  One problem with using Q₁₀ values for very complex systems is choosing the base temperature for rate comparisons.  The anaerobic decomposition of organic matter is generally a linear function of temperature between 5 °C and 30 °C, suggesting that a Q₁₀-based model is not ideal.  A more complex systems biology approach using programs such as Vcell and COPASI would be preferable and less likely to lead to errors in predicted CH₄ emissions from decomposition.

Getting Back to Proteins

Chapter 6.1 explored how enzymes catalyze chemical reactions. These included general acid/base catalysis, metal ion (electrostatic) catalysis, covalent (nucleophilic) catalysis, and transition state stabilization. Some physical processes included intramolecular catalysis and strain/distortion.  The rate-limiting step in enzyme-catalyzed reactions can involve bond breaking in the substrate, product dissociation, and conformational changes required to facilitate binding, catalysis, and dissociation.  A rate-limiting conformational change may occur not in the active-site pocket but in nearby loops that modulate reactant accessibility to the active site and product dissociation from it.  Temperature may influence all of these, with localized conformational flexibility being especially important.  

An interesting example of localized conformational changes affecting enzyme activity is RNase A.  His 48, 18 Å from the enzyme active site, is involved in the rate-limiting step involving product release.

Figure \(\PageIndex{7}\) shows an interactive iCn3D modelof bovine pancreatic Ribonuclease A in complex with 3'-phosphothymidine (3'-5')-pyrophosphate adenosine 3'-phosphate (1U1B)

Figure \(\PageIndex{7}\): Bovine pancreatic Ribonuclease A in complex with 3'-phosphothymidine (3'-5')-pyrophosphate adenosine 3'-phosphate (1U1B). (Copyright; author via source).  Click the image for a popup or use this external link: https://structure.ncbi.nlm.nih.gov/i...SnrGYXSVXcLCk6

The substrate is shown in spacefill.  The active site side chains and distal His 48 are shown as sticks and labeled.  Two flexible loops, Loop 1 (magenta) near His 48 and Loop 4 (cyan) near the active site, are highlighted.  During ligand binding, the loops move by a few angstroms, closing the active site and inhibiting product release. Product release is associated with mobile regions, including Loops 1 (20 Å from the active site) and 2.  Loop 4, near the active site, is involved in the specificity for purines 5' to the substrate cleavage site.  His 48 is conserved in pancreatic RNase A.  Mutating it to alanine decreases k_cat by more than 10X, indicating a change in the rate-determining conformational motion. The enzyme remains highly active relative to the uncatalyzed reaction.  His 48 appears to regulate the rate-limiting coupled motions in the protein.

Figure \(\PageIndex{8}\) below shows the subtle shift in the conformation of apo-RNase A (magenta, no ligand, 1FS3) to the substrate-bound form (cyan, ligand in sticks, 1U1B). Note the small motion in His 48, shown in the sticks at the bottom of the animated image.

Figure \(\PageIndex{8}\):  Conformational changes in apo-RNase A (magenta, no ligand, 1FS3) on conversion to the substrate-bound form (cyan, ligand in sticks, 1U1B).

We will explore the effects of temperature on protein structure and function further in the next chapter section.

Summary

(Summary written by Claude, Anthropic)

This chapter establishes the physical chemistry and biological context for understanding how temperature — and therefore climate change — affects the rates of chemical and biochemical reactions, providing the quantitative foundation for the two subsequent sections on protein adaptation and ecological consequences.

Thermoregulation and climate vulnerability. Organisms regulate body temperature along a spectrum from endothermy (internal metabolic heat generation, as in mammals and birds) to ectothermy (dependence on external heat sources, as in fish, amphibians, and reptiles). Endotherms maintain narrow internal temperature ranges and require frequent feeding to sustain high metabolic rates; ectotherms operate more economically but closely track environmental temperatures. These strategies are further characterized as homeothermy (a narrow internal temperature range) versus poikilothermy (a wide tolerance). Critically, ectothermic poikilotherms — including most marine invertebrates, fish, and insects — have internal temperatures that closely track environmental conditions, making their biochemistry highly sensitive to climate warming. Two overarching questions motivate the rest of this chapter section: Can organisms adapt to rising temperatures fast enough, and are many species already operating close to their thermal limits?

The Arrhenius equation and activation energy. The rate constant of any chemical reaction depends exponentially on temperature according to the Arrhenius equation: k = Ae^−Ea/RT. The physical basis is the Maxwell-Boltzmann distribution of molecular kinetic energies: at higher temperatures, a larger fraction of molecules has kinetic energy exceeding the activation energy threshold Ea, resulting in exponentially more frequent productive collisions. A linearized form (ln k = ln A − Ea/RT) allows activation energies to be extracted from the slope of ln k versus 1/T plots. A two-temperature form of the equation lets you directly calculate how a rate constant changes between any two temperatures, given Ea. For a reaction with Ea ≈ 54 kJ/mol, a 10 °C temperature increase near room temperature doubles the rate constant — the origin of the familiar "doubling rule."

The Q₁₀ temperature coefficient. When the temperature interval is exactly 10°C, the ratio k₂/k₁ is defined as Q₁₀, the temperature coefficient. For most biochemical reactions at physiological temperatures, Q₁₀ falls between 2 and 3. A Q₁₀ of 1 indicates complete temperature independence; a Q₁₀ below 1 signals a process being impaired at the higher temperature — for example, an enzyme losing its active conformation. Crucially, Q₁₀ is not a constant: it decreases as absolute temperature increases (demonstrated numerically for Ea = 44.5 kJ/mol across 273–483 K). This means biochemical processes are most temperature-sensitive at low baseline temperatures—directly relevant to the vulnerability of cold-adapted and polar species to modest warming. Q₁₀ reasoning extends beyond individual reactions to complex biological processes, including soil respiration (measured annual Q₁₀ ≈ 2.76 in one study, with daily averages of 2.04 that decline with increasing temperature), seagrass metabolism, and whole-organism metabolic rates. Activation energies for real extracellular soil enzymes — β-glucosidase (avg Ea 44.9 kJ/mol), cellobiohydrolase (42.0 kJ/mol), peroxidase (33.2 kJ/mol), and phenol oxidase (58.9 kJ/mol) — are consistent with Q₁₀ ≈ 2, confirming that warming will accelerate lignocellulose decomposition and soil CO₂ release in a positive climate feedback. However, for highly complex processes such as anaerobic decomposition (which is linear in temperature over the range 5–30°C), Q₁₀ models are inadequate, and mechanistic systems biology approaches are preferable.

Temperature, conformational dynamics, and enzyme function. Temperature affects enzyme-catalyzed reactions not only by changing rate constants but also by altering protein conformational flexibility, which modulates the rate-limiting step. RNase A provides a detailed example: His 48, located 18 Å from the active site, participates in the rate-limiting product-release step via coupled conformational motions in Loops 1 and 2. Mutation of His48 to alanine reduces kcat by more than 10-fold, confirming that distal residues that regulate protein dynamics can be as important for catalysis as active-site residues. Loop 4, near the active site, controls substrate specificity. These localized dynamic processes are temperature-sensitive, linking molecular flexibility directly to the temperature dependence of enzyme activity — a theme fully developed in the following chapter section on the thermal adaptation of proteins.