Speculative bioenergetics

To mantain their structure and functions, living beings need a constant flow of energy from the environment (and to a sink, in the form of heat). Energy can exist as electromagnetic energy, thermal energy, chemical energy, mechanical energy etc., and each one of these forms can be exploited by organisms. Since life is most likely to be based on chemical reactions, the extracted energy will have to be stored as bond energy between the atoms of some simple molecule; every lifeform on Earth uses a sugar, glucose (C6H12O6) which releases this energy when combined with an oxidising agent.

''Note on measurement units: Energy is measured in joules (J) or calories (cal): a calory is equal to 4.18 J, dietary calories (Cal) a thousand times as much. Power is the rate of energy production/consumption per time: one watt (W) is equal to 1 J/s (one joule per second), or 0.24 cal/s. The energy consumption over a day (assuming days of 24 hours) can be measured in kilocalories per day: 1 W = 20.7 kcal/d, 1 kcal/d = 0.0483 W.''

Metabolic chemistry
Depending from the way an organism gets its energy, it can be classified in three different ways: the primary source of energy (phototrophs for light, chemotrophs for chemical compounds, etc.); the source of the electron donor that gets oxidized (organotrophs if it comes from organic compounds, lithothrophs otherwise); the source of the carbon or other basic element (heterotrophs if they obtain them from other organisms, autotrophs if they synthetize the molecules themselves). Thus, plants are photolithoautotrophs, animals and fungi are chemoorganoheterotrophs, sulfate-reducing bacteria are chemolithoautotrophs, and so on.

Earth plants produce glucose through photosynthesis: 6CO2 + 6H2O → C6H12O6 + 6O2. Since the energy stored in glucose comes from light, carbon from an inorganic source (carbon dioxide) and electrons used to reduce it from another inorganic compound (water), plants are photolithoautotrophs. Most ecosystems on Earth ultimately receive their energy from photosynthesis; since this process is so simple, every world with Earth-like biochemistry is likely to develop it.

During synthesis, a reaction occurs in which an electron donor (reducing agent) cedes electrons to the carbon source, thereby reducing it, and being oxidized in return. The inverse reaction is respiration, in which glucose (or the equivalent compound) is combined with an electron acceptor (oxidizing agent) takes electrons back from it, oxidizing the compound and being in turn reduced. The most efficient oxidizing agent is most likely molecular oxygen; see here for possible alternatives. There's a simple form of respiration without electron acceptor, called fermentation, for example, glucose to ethyl alcohol: C6H12O6 → 2C2H5OH +6CO2, which releases 74.6 kJ for each mole(that is, 6.022 × 1023 molecules) of glucose, but it's far less efficient: oxigenation of glucose (C6H12O6 + 6O2 → 6CO2 + 6H2O) releases 2820 kJ per mole of glucose.

It being understood that carbon will still be central to every chemical process with such a biochemistry, glucose can be synthetized with a variety of other reactions. For example, purple sulfur bacteria add hydrogen sulfide to the reagents and replace oxygen with sulfuric acid (their cytoplasm is coloured yellow by the residual sulfur crystals): 6CO2 + 6H2O + 3H2S → C6H12O6 + 3H2SO4. Since sulfuric acid is hydrogen sulfate, this lithotrophic reaction can be identified as sulfide-to-sulfate reduction, with the oxidizing agent being oxygen.

Other examples are hydrogen sulfide synthesis: 6CO2 + 12H2S → C6H12O6+ 6H2O + 12S and methanogenesis: 4H2 + CO2 → CH4 + 2H2O, which does not result in glucose being synthetized. Eponan metabolism is based on the reaction 6H2O + 2N2 → 4NH3 + 3O2, which produces energy but requires carbon to be fixed in a different reaction.



Bond-dissociation energy is the amount of energy needed to build or dissolve a bond between atoms, assuming no participation of external temperature, in each mole of compound. This table contains the bond-dissociation energy of some common bonds, mostly inorganic; see here a table that presents more examples, including the more complex organic bonds. We can se from the table that carbon double- and triple-bounds and nitrogen triple-bounds are very rich in energy.

''Note: more lines between the atoms show a stronger chemical bond. O = oxygen; H = hydrogen; N = nitrogen; C = carbon; S = sulfur; I = iodine; Cl = clorine; Ge = germanium; Br = bromine; Si = silicon; F = fluorine.''

Energy consumption
As discovered in the 1930s by the biologist Max Kleiber, in most animals the metabolic rate (energy required per unit of time) is proportional to the ¾ power of the mass: that means that a cat, a hundred times heavier than a sparrow, needs an amount of energy 100¾ = 32 times greater (and thus it means that metabolic rate per unit of mass is lower in bigger animals). The precise exponent can vary, though: in plants it's closer to 1. This is a consequence of the surface-volume ratio, since a bigger organism can hold a larger fraction of his weight in reserves.

The basal metabolic rate (consumption of energy at rest) can be estimated with the formula r = KM¾, where M is the body mass in kg and K is a constant that depends from the organism type (in the table above are given two values for each type, one for calculating the rate in kilocalories per day and one for calculating the rate in watts). That allows us to estimate the metabolic rate of a 10-kg reptile in 10·10¾ = 56 kcal/d (or 2.7 W), and that of a 5-kg passerine bird in 129·5¾ = 431 kcal/d (or 20.8 W).

Another value linked to the metabolic rate is the heart rate: it can be inferred from the body mass with the formula r = 10(2.9 - 0.2·logM), where M is the mass in grams. This formula gives 19 beats/minute for a 120-ton whale, 81 b/m for a 90-kg human and 200 b/m for a 1-kg rabbit (the real values are 20, 60 and 205). A 1-gram animal would have a heart rate over 800 beats per minute, and in fact some hummingbirds can have 1200 b/m, but under this size the circulatory anatomy becomes so different that the formula loses its efficacy. Besides, an animal can be expected to live for 1-2 billions of heartbeats.

''Note: lifespan is given both in years and in billions of heart beats (Gb). The number of Gb per lifespan remains constant (between 0.7 and 1.5, except in humans) for animals of very different sizes.''

Given as example a mammal weighing one ton, we can thus conclude that it will consume 12 800 kcal/d (or 619 W), that it will have a heart rate of 50 beats/minute and that it will live for 40-80 (Earth) years.

Thermoregulation
The usage of energy to fuel biological processes necessarily releases waste heat, which often builds up fast: during the glucose oxidation employed by Earth animals, each litre of oxygen consumed (see below), which equals to 0.045 moles, produces 21 kJ (5000 cal). Let's assume that all of this energy is destined to muscular work; since animal muscle has only an efficiency of roughly 25%, that means that 75% of that energy, or 15.8 kJ, are going to be dispersed as heat: that's enough energy to heat up a kg of animal tissue of 4.5°C.

Organisms deal in different ways with the problem of excess heat, often simply letting it disperse in the environment. They can be classified in two different ways, one of them according to the tolerance to internal temperature variation. The other parameter is the ability of organisms to change their own temperature: The most obvious morphological difference between ectotherm and endotherm organisms will be their skin cover: ectotherms generally have naked skin, cuticle, thin scales, etc., while endotherm usually have an insulating fibrous coat, such as hair, wool or feathers.
 * Poikilotherm organisms have aHomeothermy-poikilothermy.png variable internal temperature; they can have several different enzyme systems that work at different temperatures. Since they don't spend much energy to produce or remove heat, they need as little as a tenth of the food homeotherms eat. Given these characteristics, it's likely that they'd be precluded from energy-consuming activities such as large-scale powered flight, high intelligence or prey pursuing: poikilothermic predators (spiders, mantises, frogs, etc.) prefer ambushes. They often invest most of their energy in reproduction, produncing quickly a large offspring; they have short lives and are usually preyed upon by homeotherms.
 * Heterotherm organisms are intermediate between poikilotherms and homeotherms. Temporal heterotherms change their temperature during the day: the smallest homeotherms (bats and hummingbirds) save energy reducing their metabolism and lowering their temperature when they rest. Regional heterotherms have a different temperature in different parts of the body: tunas and many penguins and seals have retia mirabilia that heat up blood incoming from the appendages and cool down blood coming from the heart, thus reducing heat loss.
 * Homeotherm organisms, such as birds and mammals, keep constant their internal temperature, often with a precision of a tenth of a degree, to allow the optimal functioning of enzymes. This adaptation requires a large amount of energy, especially in a medium with a low specific heat capacity (see the table here), such as argon, sulfur oxides, nitrogen and carbon dioxides etc., which absorb heat quickly. Gigantothermy is a particular form of homeothermy that isn't produced by metabolic activity but by size: larger organisms don't disperse heat as easily, they tend to have a reliably high internal temperature (organisms in cold environment tend thus to be bigger than those in warm environments). Gigantotherms include sharks, great sea turtles, ichthyosaurs and mosasaurs, and perhaps the largest sauropod and ornithopod dinosaurs.
 * Ectotherm organisms include all the poikilotherms: they're those who regulate their temperature through external means. There are, though, many ectothermic homeotherm: water is such a good insulant that allows to animals without an internal thermoregulation system to keep their temperature constant. Many mammals also thermoregulate themselves with both internal and external means. These include:
 * Convection. Warm air rises, cold air falls: an animal can move to meet air at the right temperature.
 * Conduction. Touching warm or cold materials; many lizard bask on sun-heated rocks.
 * Radiation. Large surfaces such as wings, crests, dewlaps or membranes of exposed skin can absorb or leak heat. Dimetrodon's "sail" and stegosaurid crests had probably this function.
 * Evaporation. Evaporating water is an excellent heat carrier. Humans sweat (up to 10-12 litres per day in some conditions); dogs, rabbit and many birds pant, losing water through the oral mucosa; bats, rodents and several marsupials lick themselves; pigs, hippos and elephants wallow in water or mud.
 * Endotherm organisms keep their temperature stable by internal mechanisms: a simple method is shivering (quick vibration of muscles), which is though very inefficient, as most of the energy is turned in useless cyclical movement; brown adipose tissue, a type of fat found in most mammals, performs a series of reactions that produce more heat than usual cellular respiration. To avoid dispersing the produced heat through the surface, endotherms cannot be very small: the smallest weigh at least a few grams. Endotherms, at least in cold environments, also need some form of insulation, given by a layer of matter that doesn't carry heat well, such as fat (animals that need fat as a food storage but need to avoid overheating concentrate it in a small area, as camels do), or air: fur and feather retain air close to the body, limiting the cooling. Furahan woolly haired shuffler has skin with the fur on the inside, making the insulation more effective.

Autotrophy
Autotrophs produce complex organic molecules (carbohydrates, fats, proteins, nucleic acids) from simple chemicals absorbed from the environment. As said above, autotrophs perform at least a double reaction: first they produce energy-rich molecules such as glucose (both to store energy and to produce more complex chemicals) and then they break them down to extract the energy within. Notes: most of the table derived from one in Cosmic Biology ; the solar irradiation considers the amount of photons that hit Earth's Equator as an average, night included; the energy derived from wind and water flow is proportional to the 3rd power of its velocity, and directly proportional to its density.

Phototrophy
Also see: Productivity

Light should be abundant on the surface of any planet, even more so if it's located inside the water habitable zone. An extremely thick and opaque atmosphere (perhaps rich in fluorine, chlorine, nitrogen dioxide and/or methane) could effectively block most of the light at the ground level, but we'd still expect to see a layer of phototrophic aeroplankton at the upper boundary of the atmosphere. Oceans completely covered in ice, such as those on Europa, abyssal ecosystems and deep caves would also be among the few environments devoid of light.

Phototrophs use compounds called photosynthetic pigments that capture energy from light of certain wavelengths, reflecting the light of the other wavelengths, and appearing of that colour. For example, chlorophyll absorbs mostly blue and red light; therefore, it reflects green light, and thus it tinges leaves green. Other plausible photosynthetic pigments, many of them used by Earth organisms (see here) can appear black, brown, red, orange, yellow, blue or purple; the colour of an alien "plant" should be a wavelength that it has no reason to exploit. Darker colours absorb more light: plants on a planet orbiting a red dwarf could be dark purple or black, while on a planet orbiting an extremely bright star they could be silver or white to reflect excessive radiation and heat.

The energy content of light increases proportionally to its frequence (and thus inversely to wavelength): it's lowest in radio waves, it increases through microwaves, infrared, visible light, ultraviolet and X-rays, and it's highest in gamma rays; still, light above ultraviolet is unlikely to be exploited, because they're both dangerous to molecular structures (though examples of adaptation to strong radiations are known) and blocked by most likely atmospheres. The light most likely to be employed includes infrared, visible and ultraviolet, roughly between 100 and 10 000 nm of wavelength.

Tree-, bush- and grass-like forms are highly likely for any extraterrestrial phototroph: wide and thin parts such as leaves are ideal to absorbe light without adding unnecessary mass, a large number of small leaves is more resistent to mechanical stress than few large ones, and "plants" will likely compete in height to overshadow their neighbours, as far as structural constraints allow them to do so. As analyzed on the Furaha blog, multiple layers of leaves are useful if the upper ones don't cast too much shadow: in Earth-like conditions, the vertical length of the umbra (the dark zone where other leaves shouldn't be placed) is about 108 times the width of the upper leaf, though this value is inversely proportional to the apparent diameter of the star in the sky. Smaller, branched, indented leaves would cast a smaller umbra and allow a multi-layered canopy.

Chemotrophy
Chemotrophs obtain energy from the oxidation of reducing chemicals in their environment. Animals are chemoorganoheteroptrophs, as they oxide organic molecules (-organo-) extracted from other organisms (-hetero-). The greatest diversity in chemotrophic diet is found in chemoautotrophic bacteria, which oxide hydrogen sulfide, elemental sulfur, ferrous iron (FeO), hydrogen and ammonia (since these are inorganic molecules, these bacteria are considered chemolithoautotrophs); they're most common around extreme environments such as hydrothermal vents, which emit iron, manganese, sulfur and other useful elements. For example, certain microbial organisms, such as Mariprofundus ferrooxydans and Acidithiobacillus ferrooxidans, survive by oxidizing iron.

When one of the reducing compounds is oxidized, it releases one or more electrons (increasing their oxidation number, for example from Mn2+ to Mn4+, since each electron has negative charge). Some examples of synthesis of organic molecules, which can be driven by chemical energy as well as by light, are given above.

Thermotrophy
A very simple form of autotrophy based on heat. A thermotroph organism would sit on the boundary between a very cold and a very hot area, getting its energy from the flux of heat towards the colder zone. The best location for thermotrophy is a hydrothermal vent, though it could exist - with less effectiveness - in lesser temperature gradient (for example at the boundary of the atmosphere), provided there is always a colder heat sink, such as the space.

The book Life in the Universe provides some examples of thermotrophs that could inhabit an alien hydrothermal vent (which presumes a planet with significant tectonic activity, radioactive decay and/or tidal flexing, see here), migrating up and down, flexing an appendage or transferring a vacuole inside their body. Given that water has a heat capacity of 4.4 J/kgK at the most common temperature and pressure conditions, cooling a kg of water by 1°C allows to extract 4.4 joules. Let's suppose that 1 kg of water has to be lifted from the vent for 20 cm to be cooled by 1°C: at Earth's gravity, lifting 1 kg for 0.2 m requires 9.8*1*0.2 = 1.96 J, and since the cooling gives off 4.4*1*1 = 4.4 J, the net gain is 4.4-1.96 = 2.44 J = 0.58 calories.

While easy to exploit, heat flow is a very inefficient source of energy. It's likely to evolve on environments devoid of light, such as oceanic abyss or seas entirely covered in ice, such as those on Europa.

Osmotrophy
As speculated in Cosmic Biology, life-sustaining work might also be obtained from osmotic gradients - the difference in salt concentration (tonicity) between layers on liquid. Europa-like oceans can have a seafloor that ejects large amounts of minerals and a surface diluted by the melting ice on top: in the salt-rich (hypertonic) bottom, the interior of a cell or sac would expel water (or other solvent) or draw ions inside, while in the salt-poor (hypotonic) top the cell would draw water inside or expel ions. This motion could power chemical reactions or directly alter the structure of energy-storing molecules. Osmotrophs might want to have a large surface area to exchange more water/ions, though a greater volume would defend them against extreme variations of their internal chemical environment.