Bioenergetics is the subject of a field of biochemistry that concerns energy flow through living systems. This is an active area of biological research that includes the study of thousands of different cellular processes such as cellular respiration and the many other metabolic processes that can lead to production and utilization of energy in forms such as ATP molecules.
Growth, development and metabolism are some of the central phenomena in the study of biological organisms. The role of energy is fundamental to such biological processes. The ability to harness energy from a variety of metabolic pathways is a property of all living organisms. Life is dependent on energy transformations; living organisms survive because of exchange of energy within and without.
In a living organism, chemical bonds are broken and made as part of the exchange and transformation of energy. Energy is available for work (such as mechanical work) or for other processes (such as chemical synthesis and anabolic processes in growth), when weak bonds are broken and stronger bonds are made. The production of stronger bonds allows release of usable energy.
Living organisms obtain energy from organic and inorganic materials. For example, lithotrophs can oxidize minerals such as nitrates or forms of sulfur, such as elemental sulfur, sulfites, and hydrogen sulfide to produce ATP. In photosynthesis, autotrophs can produce ATP using light energy. Heterotrophs must consume organic compounds. These are mostly carbohydrates, fats, and proteins. The amount of energy actually obtained by the organism is lower than the amount present in the food; there are losses in digestion, metabolism, and thermogenesis.
The materials are generally combined with oxygen to release energy, although some can also be oxidized anaerobically by various organisms. The bonds holding the molecules of nutrients together and the bonds holding molecules of free oxygen together are all relatively weak compared with the chemical bonds holding carbon dioxide and water together. The utilization of these materials is a form of slow combustion. That is why the energy content of food can be estimated with a bomb calorimeter. The materials are oxidized slowly enough that the organisms do not actually produce fire. The oxidation releases energy because stronger bonds have been formed. This net energy may evolve as heat, or some of which may be used by the organism for other purposes, such as breaking other bonds to do chemistry.
Living organisms produce ATP from energy sources via oxidative phosphorylation. The terminal phosphate bonds of ATP are relatively weak compared with the stronger bonds formed when ATP is broken down to adenosine monophosphate and phosphate and then dissolved in water. Here it is the energy of hydration that results in energy release. An organism's stockpile of ATP is used as a battery to store energy in cells, for intermediate metabolism. Utilization of chemical energy from such molecular bond rearrangement powers biological processes in every biological organism.
Types of reactions
- Exergonic is a spontaneous reaction that releases energy. It is thermodynamically favored. On the course of a reaction, energy needs to be put in, this activation energy drives the reactants from a stable state to a highly energetic unstable configuration. These reactants are usually complex molecules that are broken into simpler products. The entire reaction is usually catabolic. The released energy (called Gibbs or Gibbs free energy) is defined as negative because energy is lost from the bonds formed by the products and symbolized by - ΔG
- Endergonic is an anabolic reaction that consumes energy. It is defined as positive + ΔG because energy is required to break bonds.
The Gibbs free energy gained (+ΔG) or lost (-ΔG) in a reaction is defined as:
G(p,T) = U + pV − TS
- U is the internal energy (SI unit: joule)
- p is pressure (SI unit: pascal)
- V is volume (SI unit: m3)
- T is the temperature (SI unit: kelvin)
- S is the entropy (SI unit: joule per kelvin)
- H is the enthalpy (SI unit: joule)
Defining enthalpy as H = U + pV the above equation becomes
G(p,T) = H − TS
Besides H, T, and S, the Gibbs energy in a reaction is also dependent on the concentration of reactants and products. All these variables can be standardized for any possible chemical reaction to produce a constant, the Standard Free Energy (ΔGo’). The standard is set under the assumptions that the reaction has:
- a fixed ratio of products and reactants when the reaction reaches equilibrium;
- the reactants and products are initially present at molar concentrations of 1 M and pH = 7.0;
- T = 298 K (temperature in Kelvin);
- R is the gas constant, 1.987 cal/mol K;
- Keq is the Equilibrium Constant, the ratio of products and reactants when the reaction comes to its normal equilibrium standard T;
- ln = natural logarithm,and -2.303: mathematical constant converting log and ln.
Standard Free Energy, under the above assumptions, is given by:
ΔGo’ = -2.303RT log keq = -RT ln keq.
The more negative (-) ΔGo’, the more energy is released in achieving Keq.
The more positive the ΔGo’, the more difficult the reaction is and the more energy is required to enter the system to achieve Keq.
In August 1960, Robert K. Crane presented for the first time his discovery of the sodium-glucose cotransport as the mechanism for intestinal glucose absorption. Crane's discovery of cotransport was the first ever proposal of flux coupling in biology and was the most important event concerning carbohydrate absorption in the 20th century.
One of the major triumphs of bioenergetics is Peter D. Mitchell's chemiosmotic theory of how protons in aqueous solution function in the production of ATP in cell organelles such as mitochondria. This work earned Mitchell the 1978 Nobel Prize for Chemistry. Other cellular sources of ATP such as glycolysis were understood first, but such processes for direct coupling of enzyme activity to ATP production are not the major source of useful chemical energy in most cells. Chemiosmotic coupling is the major energy producing process in most cells, being utilized in chloroplasts and several single celled organisms in addition to mitochondria.
- Cellular respiration
- ATP synthase
- Active transport
- Table of standard Gibbs free energies.
- FAO, Calculation of the Energy Content of Foods—Energy Conversion Factors
- The symbol o’ has no mathematical meaning. The o signifies the Gibbs Standard and the apostrophe signifies use of the standard temperature, 298K.
- Robert K. Crane, D. Miller and I. Bihler. “The restrictions on possible mechanisms of intestinal transport of sugars”. In: Membrane Transport and Metabolism. Proceedings of a Symposium held in Prague, August 22–27, 1960. Edited by A. Kleinzeller and A. Kotyk. Czech Academy of Sciences, Prague, 1961, pp. 439-449.
- Wright, Ernest M., Turk, Eric (2004). "The sodium glucose cotransport family SLC5". Pflügers Arch 447 (5): 510–8. doi:10.1007/s00424-003-1063-6. PMID 12748858. "Crane in 1961 was the first to formulate the cotransport concept to explain active transport . Specifically, he proposed that the accumulation of glucose in the intestinal epithelium across the brush border membrane was coupled to downhill Na+
transport cross the brush border. This hypothesis was rapidly tested, refined and extended [to] encompass the active transport of a diverse range of molecules and ions into virtually every cell type."
- Boyd, C A R (2008). "Facts, fantasies and fun in epithelial physiology". Experimental Physiology 93 (3): 304. doi:10.1113/expphysiol.2007.037523. PMID 18192340. "the insight from this time that remains in all current text books is the notion of Robert Crane published originally as an appendix to a symposium paper published in 1960 (Crane et al. 1960). The key point here was 'flux coupling', the cotransport of sodium and glucose in the apical membrane of the small intestinal epithelial cell. Half a century later this idea has turned into one of the most studied of all transporter proteins (SGLT1), the sodium–glucose cotransporter."
- Peter Mitchell (1961). "Coupling of phosphorylation to electron and hydrogen transfer by a chemi-osmotic type of mechanism". Nature 191 (4784): 144–8. Bibcode:1961Natur.191..144M. doi:10.1038/191144a0. PMID 13771349.
- Lehninger, Albert L (1971). Bioenergetics: The Molecular Basis of Biological Energy Transformations (2nd ed.). Addison-Wesley. ISBN 0-8053-6103-0.
- Nicholls, David G., Ferguson, Stuart J. (2002). Bioenergetics (3rd ed.). Academic Press. ISBN 0-12-518124-8.
- Green DE, Zande HD (September 1981). "Universal energy principle of biological systems and the unity of bioenergetics". Proc. Natl. Acad. Sci. U.S.A. 78 (9): 5344–7. Bibcode:1981PNAS...78.5344G. doi:10.1073/pnas.78.9.5344. PMC 348741. PMID 6946475.