Editing Energy (section)

=== Biology ===
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{{Main|Bioenergetics|Food energy}}
[[File:Energy and life.svg|thumb|Basic overview of [[Bioenergetics|energy and human life]]]]
{{anchor|Biology}}In [[biology#Energy|biology]], energy is an attribute of all biological systems, from the biosphere to the smallest living organism. Within an organism it is responsible for growth and development of a biological [[Cell (biology)|cell]] or [[organelle]] of a biological organism. Energy used in [[respiration (physiology)|respiration]] is stored in substances such as [[carbohydrate]]s (including sugars), [[lipid]]s, and [[protein]]s stored by [[Cell (biology)|cells]]. In human terms, the [[human equivalent]] (H-e) (Human energy conversion) indicates, for a given amount of energy expenditure, the relative quantity of energy needed for human [[metabolism]], using as a standard an average human energy expenditure of 6,900&nbsp;kJ per day and a [[basal metabolic rate]] of 80 watts.

For example, if our bodies run (on average) at 80 watts, then a light bulb running at 100 watts is running at 1.25 human equivalents (100 ÷ 80) i.e. 1.25 H-e. For a difficult task of only a few seconds' duration, a person can put out thousands of watts, many times the 746 watts in one official horsepower. For tasks lasting a few minutes, a fit human can generate perhaps 1,000 watts. For an activity that must be sustained for an hour, output drops to around 300; for an activity kept up all day, 150 watts is about the maximum.<ref>{{cite web |url=http://www.uic.edu/aa/college/gallery400/notions/human%20energy.htm |title=Retrieved on May-29-09 |publisher=Uic.edu |access-date=2010-12-12 |url-status=live |archive-url=https://web.archive.org/web/20100604191319/http://www.uic.edu/aa/college/gallery400/notions/human%20energy.htm |archive-date=2010-06-04 }}</ref> The human equivalent assists understanding of energy flows in physical and biological systems by expressing energy units in human terms: it provides a "feel" for the use of a given amount of energy.<ref>Bicycle calculator – speed, weight, wattage etc. {{cite web |url=http://bikecalculator.com/ |title=Bike Calculator |access-date=2009-05-29 |url-status=live |archive-url=https://web.archive.org/web/20090513091201/http://bikecalculator.com/ |archive-date=2009-05-13 }}.</ref>

Sunlight's radiant energy is also captured by plants as ''chemical potential energy'' in [[photosynthesis]], when carbon dioxide and water (two low-energy compounds) are converted into carbohydrates, lipids, proteins and oxygen. Release of the energy stored during photosynthesis as heat or light may be triggered suddenly by a spark in a forest fire, or it may be made available more slowly for animal or human metabolism when organic molecules are ingested and [[catabolism]] is triggered by [[enzyme]] action.

All living creatures rely on an external source of energy to be able to grow and reproduce – radiant energy from the Sun in the case of green plants and chemical energy (in some form) in the case of animals. The daily 1500–2000&nbsp;[[kilocalorie|Calories]] (6–8&nbsp;MJ) recommended for a human adult are taken as food molecules, mostly carbohydrates and fats, of which [[glucose]] (C<sub>6</sub>H<sub>12</sub>O<sub>6</sub>) and [[stearin]] (C<sub>57</sub>H<sub>110</sub>O<sub>6</sub>) are convenient examples. The food molecules are oxidized to [[carbon dioxide]] and [[water (molecule)|water]] in the [[Mitochondrion|mitochondria]]
<chem display="block">C6H12O6 + 6O2 -> 6CO2 + 6H2O</chem>
<chem display="block">C57H110O6 + (81 1/2) O2 -> 57CO2 + 55H2O</chem>
and some of the energy is used to convert [[Adenosine diphosphate|ADP]] into [[Adenosine triphosphate|ATP]]:
{{block indent|em=1.6|text=ADP + HPO<sub>4</sub><sup>2−</sup> → ATP + H<sub>2</sub>O}}
The rest of the chemical energy of the carbohydrate or fat are converted into heat: the ATP is used as a sort of "energy currency", and some of the chemical energy it contains is used for other [[metabolism]] when ATP reacts with OH groups and eventually splits into ADP and phosphate (at each stage of a [[metabolic pathway]], some chemical energy is converted into heat). Only a tiny fraction of the original chemical energy is used for [[Work (physics)|work]]:<ref group=note>These examples are solely for illustration, as it is not the energy available for work which limits the performance of the athlete but the [[power (physics)|power]] output (in case of a sprinter) and the [[force (physics)|force]] (in case of a weightlifter).</ref>
: gain in kinetic energy of a sprinter during a 100&nbsp;m race: 4&nbsp;kJ
: gain in gravitational potential energy of a 150&nbsp;kg weight lifted through 2&nbsp;metres: 3&nbsp;kJ
: daily food intake of a normal adult: 6–8&nbsp;MJ

It would appear that living organisms are remarkably [[Energy conversion efficiency|inefficient (in the physical sense)]] in their use of the energy they receive (chemical or radiant energy); most [[machine]]s manage higher efficiencies. In growing organisms the energy that is converted to heat serves a vital purpose, as it allows the organism tissue to be highly ordered with regard to the molecules it is built from. The [[second law of thermodynamics]] states that energy (and matter) tends to become more evenly spread out across the universe: to concentrate energy (or matter) in one specific place, it is necessary to spread out a greater amount of energy (as heat) across the remainder of the universe ("the surroundings").<ref group=note>[[Crystal]]s are another example of highly ordered systems that exist in nature: in this case too, the order is associated with the transfer of a large amount of heat (known as the [[lattice energy]]) to the surroundings.</ref> Simpler organisms can achieve higher energy efficiencies than more complex ones, but the complex organisms can occupy [[ecological niche]]s that are not available to their simpler brethren. The conversion of a portion of the chemical energy to heat at each step in a metabolic pathway is the physical reason behind the pyramid of biomass observed in [[ecology]]. As an example, to take just the first step in the [[food chain]]: of the estimated 124.7&nbsp;Pg/a of carbon that is [[carbon fixation|fixed]] by [[photosynthesis]], 64.3&nbsp;Pg/a (52%) are used for the metabolism of green plants,<ref>Ito, Akihito; Oikawa, Takehisa (2004). "[http://www.terrapub.co.jp/e-library/kawahata/pdf/343.pdf Global Mapping of Terrestrial Primary Productivity and Light-Use Efficiency with a Process-Based Model.] {{webarchive|url=https://web.archive.org/web/20061002083948/http://www.terrapub.co.jp/e-library/kawahata/pdf/343.pdf |date=2006-10-02 }}" in Shiyomi, M. et al. (Eds.) ''Global Environmental Change in the Ocean and on Land.'' pp.&nbsp;343–58.</ref> i.e. reconverted into carbon dioxide and heat.