Cellular respiration - Catabolism

What is catabolism?

The catabolism is a set of metabolic processes in which large and complex molecules (carbohydrates, lipids…) are broken down into smaller and simpler molecules (glucose, fatty acids…), releasing energy in the process. Among the catabolic processes, aerobic cellular respiration stands out as it generates high amounts of energy.

This energy is released in the form of ATP and is used by cells to perform various vital functions, such as growth, tissue repair, muscle contraction, and the maintenance of body temperature.

As ATP molecules are broken down, energy is released for the synthesis of proteins, DNA, RNA and other essential macromolecules.

What is cellular respiration?

Cellular respiration is a set of metabolic processes that cells use to convert nutrients, mainly glucose, into usable energy in the form of adenosine triphosphate (ATP). This process is fundamental for the functioning of cells and, consequently, for living organisms.

Cellular respiration can be aerobic or anaerobic. In this article, we will focus on aerobic cellular respiration, which consists of the following steps:

1. Glycolysis: degrades glucose into a simpler molecule (pyruvate) which later becomes acetyl-CoA. This molecule is degraded in the Krebs cycle, obtaining large amounts of energy.
2. Pyruvate Decarboxylation: each pyruvate molecule is converted into acetyl-CoA, producing CO₂ and NADH.
3. Krebs Cycle: the set of reactions that form this cycle is crucial in cellular metabolism as it releases highly energetic molecules from acetyl-CoA molecules.
4. Electron Transport Chain: the electrons from the molecules generated in the previous step are released, producing high amounts of energy in the form of ATP.

Glycolysis. The beginning of cellular respiration.

Glycolysis occurs in the cytoplasm of the cell and consists of the degradation of glucose, resulting in two pyruvate molecules, two ATP molecules, and two NADH molecules as final products.

Glycolysis is a central pathway in energy metabolism, as it provides quick energy in the form of ATP and generates intermediates that can be used in other metabolic routes, such as the Krebs cycle, thanks to the decarboxylation of pyruvate to acetyl-CoA.

This process consists of two phases:

1. The energy investment phase: in this phase, two ATP molecules are consumed to phosphorylate glucose and convert it into more reactive intermediates.
2. The energy generation phase: In this phase, the previously generated molecules are converted into pyruvate, producing two ATP molecules and two NADH molecules.

The general equation is:
Glucose + 2NAD⁺ + 2ADP + 2Pi → 2Pyruvate + 2NADH + 2H⁺ + 2ATP + 2H₂O

Pyruvate decarboxylation

Pyruvate decarboxylation is a crucial metabolic process in which pyruvate, the final product of glycolysis, is converted into acetyl-CoA. This process is a transitional stage that connects glycolysis with the Krebs cycle.

This process occurs in the mitochondrial matrix in eukaryotic cells where pyruvate (3-carbon molecule) loses a carboxyl group in the form of carbon dioxide, resulting in the formation of a two-carbon molecule.

The resulting two-carbon molecule is oxidized, and the electrons released in this reaction are transferred to NAD+ to form NADH. The two-carbon molecule, now in the form of an acetyl group, binds to coenzyme A (CoA) to form acetyl-CoA.

General equation:
Pyruvate + NAD + CoA → Acetyl-CoA + CO₂ + NADH + H⁺

Krebs cycle. Crucial process of cellular respiration

The Krebs cycle, also known as the citric acid cycle or tricarboxylic acid (TCA) cycle, is a series of chemical reactions that occur in the mitochondrial matrix of eukaryotic cells and in the cytoplasm of prokaryotes.

It is a crucial part of cellular metabolism and plays a central role in energy production through the complete oxidation of acetyl-CoA molecules. This set of reactions releases energy in the form of ATP, NADH, and FADH2 molecules.

Its main function is to produce energy in the form of ATP and high-energy electrons carried by NADH and FADH2 molecules, which subsequently generate more ATP molecules in the electron transport chain.

General equation:
Acetyl-CoA + 3NAD + FAD + GDP + Pi + 2H₂O → 2CO₂ + 1GTP + 3NADH + FADH₂ + 3H⁺

Electron transport chain

The electron transport chain (ETC), also known as the respiratory chain, is the final stage of aerobic cellular respiration and takes place in the inner membrane of the mitochondria in eukaryotic cells. Its main function is to produce ATP through oxidative phosphorylation, using the electrons donated by NADH and FADH₂, which are generated in glycolysis, pyruvate decarboxylation, and the Krebs cycle.

The high-energy electrons from NADH and FADH₂ are transferred through a series of protein complexes in the inner mitochondrial membrane. The energy released in these steps is used to pump protons (H⁺) into the intermembrane space, creating a proton gradient. The protons flow back into the mitochondrial matrix through ATP synthase, driving the synthesis of ATP.

The general equation is:
NADH + H⁺ + O₂ → NAD + H₂O + 3ATP
FADH₂ + O₂ → FAD + H₂O + 2ATP

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP (36−38 molecules)

C₆H₁₂O₆+ 6O₂→ 6CO₂ + 6H₂O + ATP (36-38 molecules)