Aerobic Respiration: Process, Stages and Energy Production

Geology22 Oct 20264.73 KB

Aerobic Respiration

The aerobic respiration is a type of metabolic energy process in which energy is extracted from organic molecules such as glucose, through a series of reactions in which oxygen acts as the final oxidant. In other variants of respiration, very rare, distinct from oxygen, another oxidant is used (anaerobic respiration). Aerobic respiration is the process responsible for most of the energy that living organisms obtain through aerobic respiration with oxygen. Aerobic respiration takes place in the cells of the body in general, in eukaryotes and in some types of bacteria.

Oxygen, like any gas, crosses biological membranes without obstacles. Oxygen passes through the plasma membrane and then the membranes of the mitochondria, reaching the mitochondrial matrix where it accepts electrons and protons (which together form hydrogen atoms), forming water. This is the final oxidation step in the chain, and the energy previously obtained is used for ATP phosphorylation. In the presence of oxygen, pyruvic acid, obtained during glycolysis or anaerobic glycolysis, is fully oxidized to provide energy, producing carbon dioxide and water. This sequence of reactions is known as aerobic respiration.

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (ATP)

Glycolysis

During glycolysis, the first molecule of glucose is oxidized and split into two molecules of pyruvic acid (pyruvate). In this metabolic route, a net gain of 2 ATP and 2 molecules of NADH is obtained. The number of carbon atoms remains constant (6 in the initial glucose molecule, 3 in each of the two pyruvic acid molecules). This entire process takes place in the cell cytosol.

Glycerol (glycerin) from the lipolysis of triglycerides is incorporated into glycolysis at the level of glyceraldehyde 3-phosphate.

Oxidative deamination also converts some amino acids into pyruvate, whose metabolic fate is the same as that obtained through glycolysis.

Oxidative Decarboxylation of Pyruvic Acid

Pyruvic acid enters the mitochondrial matrix, where the pyruvate dehydrogenase enzyme complex carries out the oxidative decarboxylation of pyruvate. Decarboxylation begins by removing one carbon from the three-carbon pyruvic acid (released as CO₂), and oxidation occurs as two hydrogen atoms are removed (oxidation by dehydrogenation), which are captured by NAD⁺, reducing it to NADH. The pyruvate becomes an acetyl radical (-CO-CH₃, an acetyl group with a hydroxyl), which is captured by coenzyme A (forming acetyl-CoA), ready to enter the Krebs cycle.

This process occurs twice for each molecule of glucose, once for each pyruvate molecule obtained.

Krebs Cycle

The Krebs cycle is a cyclic metabolic pathway that takes place in the mitochondrial matrix and in which the acetyl groups of acetyl-CoA, provided by pyruvate, are oxidized, producing 2 molecules of CO₂ per turn and generating energy in usable forms: reducing coenzymes (NADH, FADH₂) and GTP.

Each glucose molecule completes two turns of the Krebs cycle, since two acetyl-CoA molecules are produced from the previous step. These two turns yield 2 GTP and release 4 molecules of CO₂. These 4 molecules, together with the 2 released in the oxidative decarboxylation of pyruvate, add up to a total of 6, which is the number of CO₂ molecules produced in aerobic respiration (see the general equation).

Respiratory Chain and Oxidative Phosphorylation

Aerobic respiration consists of several stages that have two basic purposes:

To reoxidize the coenzymes (NADH and FADH₂) produced in the previous stages, so they can accept electrons and protons from new substrates to be oxidized.
To produce usable energy in the form of ATP.

These two functions are closely related and coupled. Production occurs in a series of enzyme complexes located in the inner mitochondrial membrane. Four complexes carry out the oxidation of the coenzymes, transferring electrons and using their energy to pump protons from the mitochondrial matrix into the intermembrane space. These protons can later re-enter the matrix through ATP synthase, the enzyme that uses the electrochemical gradient to phosphorylate ADP to ATP. This process is known as oxidative phosphorylation.

The electrons and protons involved in this process ultimately combine with O₂ to form water. This is why the oxygen obtained from atmospheric air through pulmonary ventilation is essential: it serves as the final electron and proton acceptor in aerobic respiration.

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