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Essay on Bioenergetics :- 1. Definition of Bioenergetics 2. Free Energy and the Laws of Thermodynamics 3. Coupling of Endergonic to Exergonic Processes 4. High-energy Phosphates 5. Inter Conversion of Adenine Nucleotides 6. Metabolism of Pyrophosphate 7. Nucleoside Phosphates Related to ATP and ADP.
Contents:
- Essay on the Definition of Bioenergetics
- Essay on the Free Energy and the Laws of Thermodynamics
- Essay on the Coupling of Endergonic to Exergonic Processes
- Essay on the High-energy Phosphates
- Essay on the Inter Conversion of Adenine Nucleotides
- Essay on the Metabolism of Pyrophosphate
- Essay on the Nucleoside Phosphates Related to ATP and ADP
Essay # 1. Definition of Bioenergetics:
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Bioenergetics or biochemical thermodynamics is the study of energy changes in biochemical reactions. Non-biologic systems use heat energy to accomplish work but biologic systems are iso-thermic and utilise chemical energy for the living process.
Essay # 2. Free Energy and the Laws of Thermodynamics:
i. Free energy (ΔG) is the useful energy also known as the chemical potential.
ii. The first law of thermodynamics states that “the total energy of a system plus its surroundings remains constant”. This is also the laws of conservation of energy. Energy may be transferred from one part to another or may be transformed into another form of energy.
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iii. The second law of thermodynamics states that “the total entropy of a system must increase if a process is to occur spontaneously”.
Entropy represents the extent of disorder of the system and becomes maximum when it approaches true equilibrium. Under constant temperature and pressure, the relationship between the free energy change (ΔG) and the change in entropy (ΔS) is given by the following equation which combines the two laws of thermodynamics.
ΔG = ΔH – TΔS
where ΔH = the change in enthalpy (heat) and T = the absolute temperature. Under biochemical reactions AH is approximately equal to AE.
So the above relationship may be expressed in the following manner:
ΔG = ΔH – TΔS
If ΔG is negative in sign, the reaction proceeds spontaneously with loss of free energy i.e. it is exergonic. On the other hand, if ΔG is positive, the reaction proceeds with the gain of energy i.e. it is endergonic. If the magnitude of ΔG is great, the system is stable. If ΔG is zero, the system is at equilibrium.
Essay # 3. Coupling of Endergonic to Exergonic Processes:
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i. The vital processes (Synthetic reactions, muscular contraction, nerve impulse conduction, and active transport) obtain energy by chemical linkage or coupling to oxidative reactions.
ii. Metabolite A is converted to metabolite B with the release of free energy. It is coupled to another reaction in which free energy is required to convert metabolite C to metabolite D. Some of the energy liberated in the degradative reaction is transferred to the synthetic reaction. The exergonic reactions are termed Catabolism (the breakdown or oxidation of fuel molecules), whereas the synthetic reactions are termed Anabolism.
iii. Reaction shown in Fig. 11.1 has to go from left to right, then the overall process must be accompanied by loss of free energy as heat.
One possible mechanism of coupling is shown:
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A + C → I → B + D.
Some exergonic and endergonic reactions in biologic systems are coupled in this way. An extension of the coupling concept is provided by dehydrogenation reactions which are coupled to hydrogenations by an intermediate carrier (Fig. 11.2).
iv. The alternative process of coupling from an exergonic to an endergonic process is to synthesize a compound of high energy potential in the exergonic reaction and to incorporate this new compound into the endergonic reaction, thus effecting a transference of free energy from the exergonic to the endergonic pathway (Fig. 11.3).
In the living cell, the principal high energy intermediate or carrier compound (designated ~ Ⓔ is ATP).
Role of energy Phosphates in Bioenergetics & energy capture:
a. Autotrophic organisms couple their metabolism to some simple exergonic process in their surroundings e.g., green plants utilize the energy of sunlight, and some autotrophic bacteria utilize the reaction Fe++ Fe+++.
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b. Heterotrophic organisms obtain free energy by coupling their metabolism to the breakdown of complex organic molecules in their environment.
c. In all these processes, ATP plays an important role in the transfer of free energy from the exergonic to the endergonic processes. ATP is a nucleotide consisting of adenine, ribose, and three phosphate groups. In its reaction in the cell, it functions as the Mg++ complex.
d. ATP was considered to be a means of transferring phosphate radicals in the process of phosphorylation. Lipmann introduced the concept of “high-energy phosphates” and the “high-energy phosphate bond”:
Essay # 4. High-energy Phosphates:
Lipmann introduced the symbol ~ ℗, indicating high-energy phosphate bond. The term group transfer potential is preferred to “high-energy bond”. Thus, ATP contains 2 high-energy phosphate groups and ADP contains one. The phosphate bond in AMP is of the low energy type, since it is a normal ester link (Fig. 11.4).
Role of High-energy Phosphates as the “Energy Currency” of the Cell:
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i. ATP is the donor of high-energy phosphate and ADP can accept high-energy phosphate to form ATP. ATP/ADP cycle connects these processes which generate ~ ℗to those processes that utilize ~ ℗.
iii. There are three major sources of taking part in energy conservation or energy capture:
(a) Oxidative phosphorylation:
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This is the greatest quantitative source of ~℗ aerobic organisms. The free energy comes from respiratory chain oxidation within mitochondria.
(b) Glycolysis:
A net formation of 2 ~ ℗ results from the formation of lactate from one molecule of glucose, generated in two reactions catalyzed by phosphoglycerate kinase and pyruvate kinase.
(c) Citric acid cycle:
One ~ ℗is generated directly in the cycle at the succinyl thiokinase step.
iii. Another group of compounds (Phosphagens) act as storage forms of high-energy phosphate. These include creatine phosphate in vertebrate muscle and brain, arginine phosphate in invertebrate muscle.
iv. In physiologic conditions, phosphagens permit ATP concentrations to be maintained in muscle when ATP is being rap idly used as a source of energy for muscular contraction. When ATP is abundant, its concentration can cause the reverse reaction to take place and allow the concentration of creatine phosphate to increase abundantly so as to act as a store of high-energy phosphate.
When ATP acts as a phosphate donor to form those compounds of lower free energy of hydrolysis, the phosphate group is invariably converted to one of low energy e.g.:
Essay # 5. Inter Conversion of Adenine Nucleotides:
The enzyme Adenylate Kinase (myokinase) is present in most cells. It catalyzes the inter-conversion of ATP and AMP on the one hand and ADP on the other.
The reaction has three functions:
i. It allows high-energy phosphate in ADP to be used in the synthesis of ATP.
ii. It allows AMP to be recovered by re-phosphorylation to ADP.
iii. It allows AMP to increase in concentration when ATP becomes depleted and act as a metabolic signal to increase the rate of catabolic reactions which, in turn leads, to the generation of more ATP:
Essay # 6. Metabolism of Pyrophosphate:
Inorganic pyrophosphate is formed as occurs in the activation of long chain fatty acids when ATP reacts to form AMP:
This reaction is accompanied by loss of free energy as heat which ensures that the activation reaction will go to the right. This is further aided by the hydrolytic splitting of PPi by inorganic Pyrophosphatase. The activation via the pyrophosphate pathway results in the loss of 2 ~℗rather than one ~ ℗as occurs when ADP and Pi are formed.
Essay # 7. Nucleoside Phosphates Related to ATP and ADP:
Nucleoside triphosphates similar to ATP but containing an alternative base to adenine can be synthesized from their diphosphates by means of the enzyme nucleoside diphosphate kinase, e.g.:
All these triphosphates take part in phosphorylation’s in the cell.
Similarly, nucleoside monophosphate kinases catalyze the formation of nucleoside diphosphates from the corresponding monophosphates:
Thus, Adenylate kinase is a specialized monophosphate kinase.
Clinical Aspects:
i. Energy production from food is the fundamental role of normal nutrition and metabolism.
ii. Death results from starvation when energy reserves are depleted and types of malnutrition are associated with energy imbalance (marasmus).
iii. The rate of energy release is controlled by the thyroid hormones whose malfunction is a cause of disease.
iv. Obesity is caused by the storage of surplus energy.
