One of the major differences between living and non-living entities, such as rocks or water, lies in the capacity of living organisms to multiply. This amazing ability stems from the replication capability of nucleic acids. The majority of cells within our body have the potential to replicate their genetic material and divide into two daughter cells. Similar to other cellular functions, self-replication requires energy, prompting cells to find an energy source for division. Thus arises the question: from where does this energy originate?

This article aims to not only address this question but also explore additional fascinating aspects, such as:

• Which cellular processes contribute to energy generation?
• Where within the cell is energy produced?
• And more importantly, how is this energy transformed?

To unravel these questions, we will explore the field of bioenergetics.

What is Bioenergetics?

Bioenergetics was originally focused on the energetic events occurring across cellular and organelle membranes [1]. Although those events remain a core focus, bioenergetics is now considered a branch of biochemistry and biology that studies the flow of energy within living systems [2]. More specifically, it elucidates the energy conversion processes in cells. This article explores complex bioenergetic events and links them to other crucial cellular mechanisms such as metabolism to provide a deeper comprehension of the significance of bioenergetics.

ATP: The Energy Currency of the Cell

A single cell hosts a wide range of chemical reactions, which can either generate or consume energy. Breaking chemical bonds is one of the primary energy-generating mechanisms within a cell. When a chemical bond from a given molecule is broken, the energy released can be converted, for instance, into mechanical movement in muscle cells, or into the creation of other chemical bonds to build new molecules. This transformation of molecules within a cell is highly related to metabolism. After all, metabolism includes all the chemical reactions required to extract energy from nutrients and subsequently use it to maintain the necessary cellular processes to sustain biological functions.

This complex web of metabolic reactions, wherein the energy contained in the chemical bonds of a molecule is transferred to build a new molecule, is an energetic transaction. The molecule used as currency in these transactions is adenosine triphosphate (ATP).

What is ATP?

ATP is a nucleic acid present in all life forms. It serves as an energy source for various biological functions like muscle contraction, nerve impulse propagation, and chemical synthesis. ATP is ubiquitously used: an adult human being processes around 50 kg of the molecule every day [3]. Structurally, ATP is a molecule of adenosine (which consists of an adenine and a ribose) attached to three phosphate groups. The chemical bonds of each of the three phosphate groups can be broken to release energy. As these phosphate groups are consumed, ATP is converted into adenosine diphosphate (ADP), adenosine monophosphate (AMP), and finally into adenosine [2].

ATP continuously loses its phosphates, forming ADP and AMP, but these are then phosphorylated to generate new ATP. This cycle happens constantly in cells to power the essential metabolic reactions needed for basic life processes. However, without an external energy source — like food for us humans and animals — this system would collapse.

Let's explore how nutrients get turned into ATP.

Getting energy: Converting nutrients into ATP

Through digestion, humans can obtain nutrients, mainly falling into three groups: carbohydrates (glucose), fatty acids, and proteins (amino acids). Specific metabolic reactions break down the energy-containing chemical bonds from nutrients to fuel the synthesis of different molecules. Broadly, metabolic reactions are divided into two groups: catabolic and anabolic reactions.

• Catabolic reactions: In simple terms, catabolic reactions break down large molecules into smaller molecules, usually generating energy (exergonic reactions) [2].
• Anabolic reactions: In contrast, anabolic reactions — also known as biosynthetic reactions — build large molecules from smaller ones, usually consuming energy (endergonic reactions) [2].

Catabolic reactions break down nutrients and generate energy that can be used to produce ATP. Conversely, anabolic reactions use the energy stored in ATP to build larger molecules. Before delving into the metabolic pathways implicated in the generation of ATP, let’s first discuss redox potential, which plays a pivotal role in a key cellular process: respiration.

Respiration

Physiologically, respiration is the movement of oxygen from the air into cells and the removal of carbon dioxide from cells back to the air. At the cellular level, respiration involves the oxidation of nutrients in the presence of oxygen (O₂), acting as an inorganic electron acceptor. As nutrients like glucose, fatty acids, and amino acids undergo oxidation, other molecules undergo reduction in a sequence of reduction-oxidation reactions (redox reactions), ultimately converting the energy into ATP. Two pivotal molecules act as electron carriers in these redox reactions: nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD). Their oxidized forms are NAD⁺ and FAD, while their reduced forms are NADH and FADH₂, respectively.

Cellular respiration can be divided into three primary phases. The first phase involves the oxidation of basic nutrients (carbohydrates, fatty acids, and amino acids) into acetyl-CoA. The second phase, referred to as the citric acid cycle or the Krebs cycle, further oxidizes acetyl-CoA into carbon dioxide and water. Notably, some key products of the citric acid cycle are NADH and FADH₂, pivotal in generating APT by serving as electron donors in the third phase, also known as oxidative phosphorylation.

Nutrient Catabolism

The first phase of cellular respiration involves the catabolization of basic nutrients. In this section, we will briefly cover the oxidation process of glucose, fatty acids, and amino acids.

Glycolysis

Glycolysis (from the Greek glykys, meaning “sweet” or “sugar,” and lysis, meaning “splitting”) entails a series of enzyme-driven reactions that break down a single molecule of glucose into two molecules of pyruvate. Glycolysis stands as one of the first metabolic pathways to be described and remains one of the most comprehensively studied. It serves as the only energy source for certain mammalian tissues and cell types, such as erythrocytes, the brain, and sperm. This pathway for glucose breakdown is nearly universal, as it is present in almost all living organisms. Throughout the sequential glycolytic reactions, the energy contained in glucose is preserved in the form of 2 ATP and 2 NADH molecules, alongside 2 pyruvate molecules [2].

For the majority of eukaryotic cells and many aerobic bacteria, glycolysis merely represents the starting point in glucose breakdown. Instead of converting into lactate, ethanol, or other byproducts via fermentation, the pyruvate generated by glycolysis undergoes further oxidation, producing water and carbon dioxide through cellular respiration [2]. To achieve this, the generated pyruvate must first convert into acetyl-CoA. Remember this name, as acetyl-CoA holds a significant role in this narrative!

Catabolism of Fats

Besides glucose, cells can obtain energy from fat catabolism. In specific mammalian tissues like the heart and liver, fatty acid oxidation generates 80% of the total energy consumed under physiological conditions. A long-chain fatty acid stores twice the energy per weight compared to carbohydrates or amino acids. This high energy capacity, combined with the low reactivity of fatty acids due to their hydrophobic nature, makes them efficient energy reservoirs [2].

The initial step of fatty acid oxidation is called β-oxidation. In each β-oxidation reaction, a two-carbon unit is removed from the end of the chain, obtaining an acetyl-CoA. For instance, palmitic acid, a 16-carbon fatty acid, yields 8 acetyl-CoA molecules after 8 rounds of β-oxidation. Similar to glycolysis, the final product of β-oxidation is acetyl-CoA.

When glucose is not available, acetyl-CoA generated from fatty acid breakdown can transform into ketone bodies. These molecules are then transported to tissues (like the brain) that primarily utilize glucose for energy [2].

Catabolism of Amino Acids

Amino acid catabolism is complex and goes beyond this article’s scope. In essence, similar to glucose or fatty acids, amino acids can be converted into pyruvate and acetyl-CoA, serving as energy sources [4].

The Citric Acid Cycle

We have observed how carbohydrates, fatty acids, and amino acids break down to generate acetyl-CoA. In a way, acetyl-CoA funnels the oxidation of those macromolecules into the second phase of cellular respiration: the citric acid cycle. This cycle stands as a pivotal process in cell metabolism and bioenergetics.

In 1937, Hans Adolf Krebs elucidated and detailed the citric acid cycle or tricarboxylic acid cycle [5]. It is also known as the Krebs cycle in his honor. Despite facing initial rejection by the prestigious journal Nature, Krebs’ discoveries eventually earned him the Nobel Prize in Physiology or Medicine in 1953 [6].

The citric acid cycle consists of a sequence of reactions that oxidize acetyl-CoA into water (H₂O) and carbon dioxide (CO₂). Most of the energy of these oxidation reactions is retained in the form of electron donors NADH and FADH₂. Each cycle produces 1 ATP, 3 NADH, and 2 FADH₂. Subsequently, these NADH and FADH₂ molecules proceed to the next phase of cellular respiration: oxidative phosphorylation.

Oxidative Phosphorylation

The third phase of cellular respiration takes place within the inner mitochondrial membrane. Here, NADH and FADH₂, derived from the citric acid cycle, transfer electrons to the electron transport chain — a group of proteins situated within the inner mitochondrial membrane. As these electrons move through the proteins, they propel the pumping of protons from the mitochondrial matrix to the intermembrane space. This action creates a high concentration of protons in the intermembrane space, establishing a proton gradient between this space and the mitochondrial matrix.

Thanks to the gradient created by the electron transport chain, protons travel back to the mitochondrial matrix through a protein known as ATP synthase, which uses the energy created by the proton gradient to generate ATP. Each NADH molecule generates around 3 ATP molecules, while each molecule of FADH₂ yields around 2 ATP molecules.

The Mitochondria: The Energy Plant of the Cell

Mitochondria are essential organelles that host most of the processes described in this article. For instance, the citric acid cycle primarily occurs in the mitochondrial matrix, while oxidative phosphorylation takes place within the inner mitochondrial membrane. Consequently, the cell’s main energy production hub lies within the mitochondria.

Mitochondria possess unique characteristics. Similar to prokaryotic cells, they contain circular DNA (known as mitochondrial DNA) and have two membranes. In 1966, Lynn Margulis proposed the endosymbiotic theory, suggesting that mitochondria (and chloroplasts in plants) originated from prokaryotic cells that were engulfed by eukaryotic cells at a certain point during evolution [7]. Therefore, mitochondria are ancient prokaryotic cells adapted to live symbiotically within eukaryotic cells, playing a crucial role in cellular bioenergetics.

Bioenergetics in Health Optimization Medicine and Practice (HOMe/HOPe)

Medical and professional schools teach energy metabolism and bioenergetics, but without clinical application. Their focus is mostly to memorize the cycles and pathways.

The Bioenergetics module of the HOMe/HOPe Essential Course was developed to teach healthcare practitioners how to optimize their clients’ bioenergetics using metabolomics.

Through skillful interpretation of metabolomic data, the practitioner can easily measure all stages of energy metabolism, from macronutrient utilization to citric acid cycle intermediates, to the efficiency of oxidative phosphorylation.

No longer do these bioenergetic pathways need to be left in medical school textbooks to gather dust. We bring them back to life in this module to help our clients optimize their health.

Check out the Bioenergetics module in the HOMe/HOPe Essential Course here.

Conclusion

The second law of thermodynamics posits that all systems drift to entropy and disorder [8]. Cells and organisms, however, seem to challenge this principle by preserving order and structure in a seemingly chaotic environment. Yet bioenergetics reveals that cells and organisms perpetuate themselves by following the same rules as the rest of the universe. Bioenergetics is a beautiful bridge between biology and the fundamental principles outlined by chemistry and physics.

Written by Ferran Riaño-Canalias, PhD

References

1. Nicholls DG. Bioenergetics. Academic Press; 2013.

2. L Nelson D, Michael M C. Lehninger Principles of Biochemistry 8th Edition. Published online 2021.

3. Wilkens S. ATP Synthesis, Chemistry of. In: Wiley Encyclopedia of Chemical Biology. John Wiley & Sons, Ltd; 2008:0-0. doi:10.1002/9780470048672.wecb648

4. Reitzer L. Catabolism of Amino Acids and Related Compounds. EcoSal Plus. 2005;1(2):10.1128/ecosalplus.3.4.7. doi:10.1128/ecosalplus.3.4.7

5. Krebs HA, Johnson WA. The role of citric acid in intermediate metabolism in animal tissues. FEBS Lett. 1980;117 Suppl:K1-10. doi:10.4159/harvard.9780674366701.c143

6. Shah J, Smart P. An author’s guide to submission, revision and rejection. Ann R Coll Surg Engl. 2015;97(8):546-548. doi:10.1308/rcsann.2015.0046

7. Sagan L. On the origin of mitosing cells. J Theor Biol. 1967;14(3):225-IN6. doi:10.1016/0022-5193(67)90079-3

8. Demirel Y, Sandler SI. Thermodynamics and bioenergetics. Biophys Chem. 2002;97(2-3):87-111. doi:10.1016/s0301-4622(02)00069-8