Bioenergetics

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Bioenergetics is a field in biochemistry and cell biology that concerns energy flow through living systems.^[1] This is an active area of biological research that includes the study of the transformation of energy in living organisms and the study of thousands of different cellular processes such as cellular respiration and the many other metabolic and enzymatic processes that lead to production and utilization of energy in forms such as adenosine triphosphate (ATP) molecules.^[2][3] That is, the goal of bioenergetics is to describe how living organisms acquire and transform energy in order to perform biological work.^[4] The study of metabolic pathways is thus essential to bioenergetics.

Overview

Bioenergetics is the part of biochemistry concerned with the energy involved in making and breaking of chemical bonds in the

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; the goal of metabolic and catabolic processes are to synthesize ATP from available starting materials (from the environment), and to break- down ATP (into adenosine diphosphate (ADP) and inorganic phosphate) by utilizing it in biological processes.^[4] In a cell, the ratio of ATP to ADP concentrations is known as the "energy charge" of the cell. A cell can use this energy charge to relay information about cellular needs; if there is more ATP than ADP available, the cell can use ATP to do work, but if there is more ADP than ATP available, the cell must synthesize ATP via oxidative phosphorylation.^[5]

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 hydrolyzed (broken down by water) to adenosine diphosphate and inorganic phosphate. Here it is the thermodynamically favorable free energy of hydrolysis that results in energy release; the phosphoanhydride bond between the terminal phosphate group and the rest of the ATP molecule does not itself contain this energy.^[10] An organism's stockpile of ATP is used as a battery to store energy in cells.^[11] Utilization of chemical energy from such molecular bond rearrangement powers biological processes in every biological organism.

Living organisms obtain energy from organic and inorganic materials; i.e. ATP can be synthesized from a variety of biochemical precursors. 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 produce ATP using light energy, whereas heterotrophs must consume organic compounds, mostly including 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.^[12]

Environmental materials that an organism intakes are generally combined with oxygen to release energy, although some nutrients can also be oxidized anaerobically by various organisms. The utilization of these materials is a form of slow combustion because the nutrients are reacted with oxygen (the materials are oxidized slowly enough that the organisms do not produce fire). The oxidation releases energy, which may evolve as heat or be used by the organism for other purposes, such as breaking chemical bonds.

Types of reactions

• An exergonic reaction is a spontaneous chemical reaction that releases energy.^[4] It is thermodynamically favored, indexed by a negative value of ΔG (Gibbs free energy). Over the course of a reaction, energy needs to be put in, and this activation energy drives the reactants from a stable state to a highly energetically unstable transition state to a more stable state that is lower in energy (see: reaction coordinate). The reactants are usually complex molecules that are broken into simpler products. The entire reaction is usually catabolic.^[13] The release of energy (called Gibbs free energy) is negative (i.e. −ΔG) because energy is released from the reactants to the products.
• An endergonic reaction is an anabolic chemical reaction that consumes energy.^[3] It is the opposite of an exergonic reaction. It has a positive ΔG because it takes more energy to break the bonds of the reactant than the energy of the products offer, i.e. the products have weaker bonds than the reactants. Thus, endergonic reactions are thermodynamically unfavorable. Additionally, endergonic reactions are usually anabolic.^[14]

The free energy (ΔG) gained or lost in a reaction can be calculated as follows: ΔG = ΔH − TΔS where ∆G = Gibbs free energy, ∆H = enthalpy, T = temperature (in kelvins), and ∆S = entropy.^[15]

Examples of major bioenergetic processes

• electron transport chain
  .

• Gluconeogenesis is the opposite of glycolysis; when the cell's energy charge is low (the concentration of ADP is higher than that of ATP), the cell must synthesize glucose from carbon- containing biomolecules such as proteins, amino acids, fats, pyruvate, etc.^[17] For example, proteins can be broken down into amino acids, and these simpler carbon skeletons are used to build/ synthesize glucose.

• citrate.^[18] The remaining eight reactions produce other carbon-containing metabolites. These metabolites are successively oxidized, and the free energy of oxidation is conserved in the form of the reduced coenzymes FADH₂ and NADH.^[19] These reduced electron carriers can then be re-oxidized when they transfer electrons to the electron transport chain
  .

• Ketosis is a metabolic process where the body prioritizes ketone bodies, produced from fat, as its primary fuel source instead of glucose.^[20] This shift often occurs when glucose levels are low: during prolonged fasting, strenuous exercise, or specialized diets like ketogenic plans, the body may also adopt ketosis as an efficient alternative for energy production.^[21] This metabolic adaptation allows the body to conserve precious glucose for organs that depend on it, like the brain, while utilizing readily available fat stores for fuel.

• ATP synthase
  .

• Photosynthesis, another major bioenergetic process, is the metabolic pathway used by plants in which solar energy is used to synthesize glucose from carbon dioxide and water. This reaction takes place in the chloroplast. After glucose is synthesized, the plant cell can undergo photophosphorylation to produce ATP.^[22]

Additional information

• During energy transformations in living systems, order and organization must be compensated by releasing energy which will increase entropy of the surrounding.

• Organisms are open systems that exchange materials and energy with the environment. They are never at equilibrium with the surrounding.

• Energy is spent to create and maintain order in the cells, and surplus energy and other simpler by-products are released to create disorder such that there is an increase in entropy of the surrounding.

• In a reversible process, entropy remains constant where as in an irreversible process (more common to real-world scenarios), entropy tends to increase.

• During phase changes (from solid to liquid, or to gas), entropy increases because the number of possible arrangements of particles increases.

• If ∆G<0, the chemical reaction is spontaneous and favourable in that direction.

• If ∆G=0, the reactants and products of chemical reaction are at equilibrium.

• If ∆G>0, the chemical reaction is non-spontaneous and unfavorable in that direction.

• ∆G is not an indicator for velocity or rate of chemical reaction at which equilibrium is reached. It depends on amount of enzyme and energy activation.

Reaction coupling

Is the linkage of chemical reactions in a way that the product of one reaction becomes the substrate of another reaction.

• This allows organisms to utilize energy and resources efficiently. For example, in cellular respiration, energy released by the breakdown of glucose is coupled in the synthesis of ATP.

Cotransport

In August 1960,

• Bioenergetic systems
• Cellular respiration
• Photosynthesis
• ATP synthase
• Active transport
• Myosin
• Exercise physiology
• Table of standard Gibbs free energies

References