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 create most heat via metabolic processes and are colloquially called “warm-blooded.” Ectotherms use external sources of temperature to regulate their body temperatures. 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 ocean. Ectotherms have developed several behavioral thermoregulation mechanisms, such as basking in the sun to increase body temperature or seeking shade to decrease body temperature.  The common frog is an ectotherm and regulates its body based on the temperature of 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. Heat is usually generated from the animal’s normal metabolism, but under excessive cold or low activity conditions, an endotherm generates additional heat by shivering. Many endotherms have a larger number of 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. Poikilotherm’s 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 as part of hibernation or torpor. Some ectotherms can also be homeotherms. For example, some species of tropical fish inhabit coral reefs with such stable ambient temperatures that their internal temperature remains constant.  Figure \(\PageIndex{1}\) below shows the energy output vs temperature for a homeotherm (mouse) and poikilotherm (lizard).

Another term is heterothermy, in which the temperature of a homeotherm can vary in different body regions (spatially) and at different times (daily or seasonally, as in hibernation).  The core body of a homeotherm is usually warmer than the extremities, which allows cooling when needed.  In hibernation (or sustained torpor), the body temperature and metabolic rates decrease.  

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 a surface with a liquid. Convection currents of air remove heat from the surface of dry skin as the air passes over it. Heat can be conducted from one surface to another during direct contact with the surfaces, such as an animal resting 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 fluctuating external temperatures, while ectotherms rely on the external environment to do so.
• Homeotherms maintain their body temperature within a narrow range, while poikilotherms can tolerate a wide variation in internal body temperature, usually because of environmental variation.
• 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 diagramed 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 previous chapter sections how temperature can affect macromolecules such as proteins (Chapter 4), nucleic acids (Chapter 9.1), and 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. Hence, we'll review and discuss the effects of temperature on these key molecular species in the next chapter section.  First, we'll delve deeper into the general impact of temperature on chemical and biochemical reactions.

Temperature Effects on the Rates of Chemical Reactions

To understand temperature effects on metabolic processes, let's first review temperature effects on ordinary chemical and biochemical reactions.  You may remember the general rule that the rate of a chemical reaction approximately doubles when the temperature is increased 10^o C (10 K).  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).  The activation energy is required to move from a reactant to the transition state, which then can go on to 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 "preexponential" 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 (21.9⁰ C, 71.3^o F) to T₂ = 305 K (21.9⁰ C, 89.3^o F), a 10^oC 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^o C (i.e., by 10^o C) doubles the reaction rate.

Assuming that the activation energy is constant, the rate constants increase with temperature since a larger fraction of the molecules have the energy (> E_a) necessary 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=100K, T=1200K, T=5000K.  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 a 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. At progressively higher temperatures, great fractions (as measured by the area under the curve to the right of the dotted line at 950 m/s) have the required energy; hence, the rates increase with temperature.

When the temperature change is 10^oC, 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 10⁰ C (10 K).  Hence, the Q₁₀ value for the 10⁰ range from 273-283K differs from the Q₁₀ value from 373-383K).  Q₁₀ for many reactions is around 2 (doubling of the reaction rate) - 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^o 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^oC increase.  

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

We will see how this is important in biological settings in a bit.  If Q₁₀ = 1, the reaction is independent of temperature, and a Q₁₀ <1 shows a reaction that is not functioning.  An example might be an enzyme-catalyzed reaction in which the threshold is reached at a higher temperature T₂ = T₁+10, at which the enzymes lose an active conformation and start 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 are also used to describe biological processes like respiration, the speed of neural signal propagation, metabolic rates, and more. Many biological processes are affected by temperature, especially ectotherms, which adjust their temperatures to outside environments, including daily and seasonal temperature shifts. Mammals and birds alter their metabolic rates with temperature, as do hibernating animals. 

The Q₁₀ temperature coefficient can be considered 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.  It is given by the following equation:

\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 the van't Hoff's temperature coefficient.  To help understand Q₁₀, let's consider some examples.

• If T₂-T₁=10^o, Q₁₀ = k₂/k₁ for the specified temperature pairs separated by a 10⁰ 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 > 10⁰ C, the the measured ratio k₂/k₁ is a factor > 1 x Q₁₀
• If the temperature range is < 10⁰ 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,  k₂ at T₁+10 = k₁ at T₁, 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 temperature difference of 10^o C.   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

The soil temperature, T_s, was measured at a 10-cm depth. Open circles are from January to June; closed circles are from 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 by a logistic model we used in Chapter 5.7 for fitting ELISA data. R_s is significantly different between the first and second half of the year.  

The soil respiration rate, R_s, at 10 cm depth was strongly affected by temperature, with an annual Q₁₀ value of 2.76.   Daily estimates of Q₁₀ averaged 2.04 and decreased with increasing T_s^. A study of seagrass showed that the Q10 values are affected by plant tissue age and that Q₁₀ varied significantly with the initial temperature and temperature ranges.  

The use of Q₁₀ values from the Arrhenius equation is based on the assumption that the chemical/biochemical processes are exponential functions of temperature.  For complex processes like the decay of organic matter, it would be better to model the whole system by looking at the individual enzymes involved.  One problem with using Q₁₀ values for very complex systems is the choice of the base temperature value for rate comparisons.  The anaerobic decomposition of organic matter is generally a linear function of temperature between 5°C and 30°C, which shows that a Q₁₀ modeling system is not ideal.  A more complex systems biology approach using programs like Vcell and COPASI would be better and less likely to cause errors in predicted CH₄ emissions from the decomposition process.

Getting Back to Proteins

Chapter 6.1 explored the mechanisms enzymes use to 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 include actual bond breaking in the substrate, dissociation of product, and conformational change 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 the accessibility of reactant to and dissociation of product from the active site.  These all may be influenced by temperature, with localized conformational flexibility 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 enzymatic 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 the 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.  On ligand binding, the loops move a few angstroms to make the active site more closed, 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.  If mutated to alanine, the k_cat decreases greater than 10X, indicating a change in the rate-determining conformational motion. The enzyme is still very active compared to the uncatalyzed reaction.  His 48 appears to regulate coupled motions in the protein that are rate-limiting.

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 apo-RNase A (magenta, no ligand, 1FS3) on conversion to the substrate-bound form (cyan, ligand in sticks, 1U1B).

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

Summary

This chapter explores hydrogen as a potential clean fuel alternative, emphasizing its biochemical and catalytic foundations. Hydrogen, when combusted, produces only water—a stark contrast to fossil fuels that emit carbon dioxide—making it an attractive energy carrier due to its high energy content per unit mass. The chapter details various production methods, highlighting that most industrial hydrogen currently comes from fossil fuel processes such as steam reforming, while emerging “green” methods like water electrolysis (powered by renewable energy) and biological production via hydrogenases are being investigated.

A key focus is on the role of hydrogen in biochemical redox reactions. The reversible interconversion of H₂ and H⁺, which underpins processes such as mitochondrial electron transport and photosynthetic water oxidation, is explained. The chapter further delves into the diverse classes of hydrogenases ([NiFe], [FeFe], and [Fe]-hydrogenases), which catalyze the formation and oxidation of hydrogen in microorganisms. These enzymes not only illustrate nature’s elegant solutions to energy conversion but also inspire the design of biomimetic catalysts for sustainable hydrogen production.

Additionally, the text discusses electron bifurcation—a mechanism by which certain hydrogenases distribute electrons between different redox pathways, thereby efficiently coupling endergonic and exergonic reactions. Despite its potential, biological H₂ production faces challenges, including oxygen sensitivity of hydrogenases, which can lead to enzyme inactivation and affect overall process efficiency.

Finally, the chapter considers environmental and practical aspects. While hydrogen itself is not a greenhouse gas, its leakage can indirectly influence atmospheric methane and ozone levels by perturbing the balance of hydroxyl radicals. Overall, this chapter integrates biochemical principles, enzyme mechanisms, and industrial considerations to provide a comprehensive understanding of hydrogen's promise and challenges as a clean energy fuel.