Bioenergetics is the captivating field of biochemistry and cell biology that unravels the intricate dance of energy flow within living systems. It encompasses the transformation of energy in organisms, including processes like cellular respiration and the production of adenosine triphosphate (ATP) molecules. In essence, bioenergetics aims to describe how living beings acquire and transform energy to perform vital biological work. The study of metabolic pathways is essential to this fascinating realm, where bonds are made and broken, and ATP serves as the universal “energy currency.” Whether you’re a sun-loving plant harnessing sunlight or a banana-eating human, we’re all part of this cosmic relay race of energy exchange.

Concept of free energy

Free energy (often denoted as Gibbs free energy, G) is a thermodynamic potential that measures the maximum reversible work a system can perform at constant temperature and pressure.

Usable Energy: It represents the amount of usable energy within a system—energy that can do work. Think of it as backstage passes to the energy concert happening within cells.

Change in Gibbs Free Energy (ΔG): When a chemical reaction occurs, ΔG tells us the maximum usable energy released (or absorbed) as the system transitions from initial reactants to final products. Its sign (positive or negative) indicates whether the reaction happens spontaneously (without added energy).

Equation: We express ΔG as:


ΔH: Change in enthalpy (internal energy + pressure-volume work)

ΔS: Change in entropy (disorder or randomness)

T: Temperature

Practical Assumptions: While working with Gibbs free energy, we assume constant temperature and pressure—conditions that hold true for cells and living systems.

Endergonic Reactions

  • In an endergonic reaction, energy is absorbed from the surroundings.
  • These reactions are also known as unfavorable or nonspontaneous reactions because they require an input of energy to proceed.
  • The products (final state) have more free energy than the reactants (initial state).

Examples of endergonic reactions include:

  • Photosynthesis: Plants absorb sunlight energy to convert carbon dioxide and water into glucose and oxygen.
  • Na⁺/K⁺ Pump: Essential for muscle contraction and nerve conduction, this pump actively transports ions across cell membranes.
  • Protein Synthesis: Building proteins from amino acids consumes energy.
  • Dissolving Potassium Chloride (KCl) in Water: The dissolution process absorbs energy.

Exergonic Reactions

  • In contrast, an exergonic reaction releases energy to the surroundings.
  • These reactions are also known as favorable, exoergic, or spontaneous reactions.
  • The products (final state) have less free energy than the reactants (initial state).

Examples of exergonic reactions include:

  • Cellular Respiration: Cells break down glucose to produce adenosine triphosphate (ATP), releasing energy.
  • Combustion: Burning wood or fossil fuels releases energy.
  • Rusting of Iron: Oxidation of iron in the presence of oxygen liberates energy.

The following table succinctly captures the essence of how these reactions are energetically and functionally distinct from each other.

FeatureEndergonic ReactionsExergonic Reactions
Energy ChangeAbsorb energy (require input of energy)Release energy (spontaneous)
Gibbs Free Energy (ΔG ))Positive (ΔG> 0 )Negative (ΔG < 0 )
SpontaneityNon-spontaneous (not favorable without energy input)Spontaneous (favorable)
ExamplesPhotosynthesis, Na⁺/K⁺ pump, protein synthesisCellular respiration, combustion, rusting of iron


The relationship between free energy, enthalpy, and entropy is a central concept in thermodynamics and is described by the Gibbs free energy equation:



– ΔG is the change in Gibbs free energy,

– ΔH is the change in enthalpy (total heat content),

– T is the absolute temperature in Kelvin,

– ΔS is the change in entropy (disorder or randomness).

Here’s how they relate:

  • Enthalpy (ΔH): This term reflects the total energy of a system, including internal energy and the product of pressure and volume. It represents the heat absorbed or released during a reaction at constant pressure.
  • Entropy (ΔS): Entropy measures the degree of disorder or randomness in a system. The second law of thermodynamics states that for any spontaneous process, the total entropy of a system and its surroundings always increases.
  • Free Energy (ΔG): Gibbs free energy combines enthalpy and entropy to determine whether a process will occur spontaneously at constant temperature and pressure. If (ΔG) is negative, the process is spontaneous; if positive, it’s non-spontaneous; if zero, the system is at equilibrium.

In essence:

– A reaction with a negative (ΔH) (exothermic) and positive (ΔS) (increased disorder) will likely be spontaneous (ΔG < 0).

– A reaction with a positive (ΔH) (endothermic) and negative (ΔS) (decreased disorder) will likely be non-spontaneous (ΔG > 0).

Redox potential

Redox potential, also known as oxidation-reduction potential (ORP), is a measure of the tendency of a chemical species to acquire electrons and thus be reduced. It is expressed in volts (V) or millivolts (mV).

In essence:

– A positive redox potential indicates a greater affinity for electrons, meaning the substance is a good oxidizing agent and can readily accept electrons.

– A negative redox potential means the substance is more likely to donate electrons, acting as a reducing agent.

Redox potential is crucial in various fields, including biochemistry, where it helps understand electron transfer processes such as those occurring in the mitochondria during cellular respiration.

Energy rich compounds

Energy-rich compounds, also known as high-energy compounds, are molecules that store and provide energy for various biochemical processes. Here’s a detailed classification:

High-Energy Phosphate Compounds

  • Adenosine Triphosphate (ATP): The primary energy currency of the cell, providing energy for most cellular processes.
  • Adenosine Diphosphate (ADP): Formed from ATP after energy release, can be rephosphorylated to ATP.
  • Creatine Phosphate: Stores energy in muscle cells, quickly donating a phosphate group to ADP to regenerate ATP during intense physical activity.
  • Guanosine Triphosphate (GTP): Similar to ATP, involved in protein synthesis and signal transduction.


  • Acetyl-Coenzyme A (Acetyl-CoA): Central to metabolism, carries acetyl for the citric acid cycle and fatty acid synthesis.
  • Succinyl-CoA: An intermediate in the citric acid cycle, involved in the synthesis of porphyrins.


  • Phosphoenolpyruvate (PEP): Has a high phosphoryl-transfer potential; involved in glycolysis.
  • 1,3-Bisphosphoglycerate (1,3-BPG): Also participates in glycolysis, providing energy for ATP synthesis.


  • Glucose: A primary source of energy in cells, broken down via glycolysis and oxidative phosphorylation.
  • Glycogen: A storage form of glucose in animals, readily converted back to glucose when needed.


  • Triacylglycerols (Triglycerides): The main form of stored energy in animals, providing more than twice the energy per gram compared to carbohydrates or proteins.
  • Phospholipids: While not primarily used for energy storage, they can also be metabolized to provide energy.

Nucleoside Diphosphate Sugars– These are activated forms of sugars used for biosynthesis of polysaccharides and glycoconjugates.

Cyclic Adenosine Monophosphate (cAMP)– A second messenger used for intracellular signal transduction.

These compounds are classified based on their chemical structure and the type of bond that stores energy. The phosphate bonds in ATP or the thioester bond in acetyl-CoA are examples of high-energy bonds that release energy when broken during metabolic reactions. This energy is then used for various cellular functions such as muscle contraction, nerve impulse propagation, biosynthesis, and transport across membranes.

Biological significances of ATP and cyclic AMP

ATP (Adenosine Triphosphate) and cAMP (Cyclic Adenosine Monophosphate) are pivotal molecules in cellular biology:

Adenosine Triphosphate (ATP)

  • Energy Transfer: ATP is often referred to as the “molecular unit of currency” of intracellular energy transfer. It transports chemical energy within cells for metabolism.
  • Muscle Contraction: It provides the energy for muscle fibers to slide past one another during contraction.
  • Nerve Impulse Propagation: ATP is necessary for the propagation of nerve impulses, which allows for thought, sensation, and movement.
  • Biosynthesis: It supplies the activation energy needed for biosynthetic reactions, such as protein and nucleic acid synthesis.
  • Transport Work: ATP powers the active transport of molecules across cell membranes against their concentration gradient.
  • Mechanical Work: It fuels cellular movements like the beating of cilia and flagella.

Cyclic Adenosine Monophosphate (cAMP)

  • Signal Transduction: cAMP acts as a second messenger in many hormone and neurotransmitter signalling pathways, amplifying the signal from a first messenger.
  • Regulation of Metabolic Pathways: It regulates metabolic pathways like glycogenolysis and lipolysis, which are critical for energy balance.
  • Gene Expression: cAMP influences gene expression by activating protein kinase A (PKA), which can phosphorylate transcription factors.
  • Cellular Response to Environment: It plays a role in the cellular response to environmental changes, such as the presence of certain hormones or other signals.

Both ATP and cAMP are integral to life’s processes, with ATP serving as an immediate source of energy for cellular functions and cAMP acting as a key component in signal transduction pathways. They exemplify the intricate molecular dance that sustains life at a cellular level.


In biochemistry basics, we discussed on endergonic vs. exergonic reactions—how they handle energy and what makes them go or not, all tied to Gibbs free energy. Then, there was a look at how free energy, enthalpy, and entropy play together to predict reaction outcomes. Redox potential got a shout-out as a way to gauge a substance’s electron vibe—grabbing or giving. Energy-rich compounds like ATP and pals were sorted by how they stash energy for body work. And to wrap up, ATP’s rep as the cell’s go-to energy wallet and cAMP’s gig as a key messenger in cell signals got some spotlight. It’s like getting a peek at the molecular magic that fuels life.

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