The biochemistry of energy metabolism

The biochemistry of energy metabolism

Energy is essential for life, and organisms use various mechanisms to obtain and utilize it. From cellular respiration and photosynthesis to the breakdown of macronutrients, energy metabolism is a complex process that involves various biochemical reactions. In this article, we will explore the biochemistry of energy metabolism and the different pathways involved.

ATP and energy storage

Adenosine triphosphate, or ATP, is the primary energy storage molecule in cells. ATP is composed of adenosine, a nucleotide, and three phosphate groups. Its hydrolysis, or breakdown, releases energy that cells can use for various metabolic processes.

ATP can be synthesized in cells through two mechanisms: substrate-level phosphorylation and oxidative phosphorylation. Substrate-level phosphorylation occurs during glycolysis and the Krebs cycle, where enzymes transfer a phosphate group from a substrate molecule to ADP, creating ATP. In contrast, oxidative phosphorylation occurs in the mitochondria, where electrons from oxidative reactions are used to create a proton gradient across the mitochondrial membrane, which drives ATP synthesis.

Biosynthesis and energy utilization

Organisms use energy to fuel various biosynthetic processes, including the synthesis of proteins, nucleic acids, and other macromolecules. These processes require energy in the form of ATP and other high-energy molecules.

Biosynthesis occurs through various enzyme-catalyzed reactions that require specific substrates and cofactors. For instance, amino acids are synthesized from intermediates of glycolysis and the Krebs cycle. Similarly, nucleotides are synthesized from the precursor molecule ribose-5-phosphate, which is derived from the pentose phosphate pathway.

Once these molecules are synthesized, they can be utilized for various cellular processes, including energy production, maintenance, and cellular signaling.


Glycolysis is the metabolic pathway that breaks down glucose to produce ATP in the cytosol of the cell. The process requires a series of enzymatic reactions that convert glucose into pyruvate. During glycolysis, glucose is first phosphorylated by ATP to produce glucose-6-phosphate. This molecule is then rearranged and phosphorylated again to produce fructose-6-phosphate.

The pathway continues with a series of enzymatic reactions that produce pyruvate and ATP. In total, glycolysis produces two ATP molecules and two pyruvate molecules per glucose molecule.

The Krebs cycle

The Krebs cycle, also known as the citric acid cycle, is a complex metabolic pathway that occurs in the mitochondria. The process begins with the conversion of pyruvate into acetyl-CoA, which then enters the Krebs cycle.

During the cycle, acetyl-CoA is oxidized and broken down into various intermediates, including citrate, isocitrate, alpha-ketoglutarate, and succinate. Oxidative reactions during the cycle produce NADH and FADH2, which are used in oxidative phosphorylation to produce ATP.

Oxidative phosphorylation

Oxidative phosphorylation is the process of ATP synthesis that occurs in the mitochondria. This pathway utilizes electron transport chains, which are composed of various protein complexes and electron carriers, including NADH and FADH2.

The electron transport chain operates by moving electrons through a series of redox reactions, which create a proton gradient across the mitochondrial membrane. This gradient drives ATP synthesis through the action of ATP synthase.


In summary, energy metabolism is a complex process that involves various biochemical reactions. From ATP synthesis and storage to glycolysis, the Krebs cycle, and oxidative phosphorylation, energy metabolism is a critical aspect of life. By understanding the biochemistry of energy metabolism, we can gain insights into the mechanisms that fuel cellular processes and maintain cellular homeostasis.