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Bioenergetics studies energy flow in living systems. This is an active area of biological research that includes the study of energy transformation in living organisms and thousands of different cellular processes, such as cellular respiration and metabolic and enzymatic processes that lead to production and use of adenosine triphosphate (ATP) molecules. Bioenergetics describes how organisms acquire and transform energy to perform biological work. Bioenergetics studies metabolic pathways.
Bioenergetics
Bioenergetics studies the energy required to form and break chemical bonds in biological molecules. It’s also the study of energy relationships in living organisms. The ability to harness energy from a variety of metabolic pathways is a property of all living organisms that contains earth science. Growth, development, anabolism and catabolism are some of the central processes in the study of biological organisms, because the role of energy is fundamental to such biological processes. Life is dependent on energy transformations; living organisms survive because of exchange of energy between living tissues/ cells and the outside environment. Some organisms, such as autotrophs, can acquire energy from sunlight (through photosynthesis) without needing to consume nutrients and break them down. Other organisms, like heterotrophs, must intake nutrients from food to be able to sustain energy by breaking down chemical bonds in nutrients during metabolic processes such as glycolysis and the citric acid cycle. Importantly, as a direct consequence of the first law of thermodynamics, autotrophs and heterotrophs participate in a universal metabolic network—by eating autotrophs (plants), heterotrophs harness energy that was initially transformed by the plants during photosynthesis.
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.
Adenosine triphosphate (ATP) is the main “energy currency” for organisms; metabolic and catabolic processes create ATP from environmental starting materials and break it down (into ADP and inorganic phosphate) for use in biological functions. “Cell energy charge” is the ratio of ATP to ADP concentrations. If there is more ATP than ADP, the cell may utilise ATP to conduct work; otherwise, it must synthesise ATP by oxidative phosphorylation.
Through oxidative phosphorylation, living organisms make ATP from sunlight or O2. ATP’s terminal phosphate bonds are weak compared to those produced when ATP is hydrolyzed to adenosine diphosphate and inorganic phosphate. The phosphoanhydride link between the terminal phosphate group and the remainder of ATP does not contain this energy. ATP stores energy in cells. Every living creature uses chemical energy from molecular bond rearrangement.
Reactions
Exergonic reactions release energy spontaneously. Negative G indicates thermodynamic favorability (Gibbs free energy). Activation energy moves reactants from a stable state to a highly energetically unstable transition state to a more stable, lower-energy state (see: reaction coordinate). Complex reactants break into simpler products. Usually catabolic. Because reactants have more energy than products, Gibbs free energy is negative (G 0).
A chemical process that consumes energy is called endergonic. Opposite of exergonic. It has a positive G because H > 0, which indicates it takes more energy to break reactant bonds than product bonds, i.e. products have weaker bonds than reactants. Endergonic reactions are thermodynamically favourable and won’t occur at constant temperature. Endergonic reactions are anabolic.
G = H TS where G = Gibbs free energy change, H = enthalpy change, T = temperature (in kelvins), and S = entropy change.
Bioenergetic examples
Glycolysis breaks down glucose into pyruvate, yielding 2 ATP per glucose molecule. When a cell has more ATP than ADP (i.e. a high energy charge), it cannot execute glycolysis to release energy from glucose. Pyruvate is a glycolysis product that may be shuttled into other metabolic processes (gluconeogenesis, etc.). Glycolysis also creates NADH, which donates electrons to the electron transport chain
When the cell’s energy charge is low (the concentration of ADP is larger than that of ATP), it synthesises glucose from carbon-containing molecules such as proteins, amino acids, lipids, pyruvate, etc.
Proteins may be broken down into amino acids, which are needed to make glucose
In the citric acid cycle, acetyl coenzyme A is initially reacted with oxaloacetate to generate citrate. Eight processes yield carbon-containing metabolites. These metabolites are sequentially oxidised, and the oxidation energy is preserved as FADH2 and NADH. When they transmit electrons to the electron transport chain, these reduced electron carriers are re-oxidised
Ketosis uses ketone bodies as cell energy (instead of using glucose). When glucose levels are low, such during hunger, cells switch to ketosis for energy
Oxidative phosphorylation releases energy from O2‘s weak double bonds in the electron transport chain. NADPH, FADH2 and NADH can contribute electrons to redox processes in electron transport chain complexes. Redox reactions occur in mitochondrial enzyme complexes. These redox processes carry electrons “down” the proton-driven electron transport chain. This difference in proton concentration drives ATP production via ATP synthase
Photosynthesis uses sun energy to generate glucose from carbon dioxide and water in plants. Chloroplasts respond. After synthesising glucose, plant cells can create ATP by photophosphorylation. Human Vitamin D production results in ATP
Conclusion
The field of biochemistry known as “bioenergetics” studies how cells use adenosine triphosphate (ATP) to store, produce, and release energy (ATP). The majority of cellular metabolism and, by extension, all life depends on bioenergetic activities like cellular respiration and photosynthesis.
