Why is the energy released from glucose controlled by cells precisely rather than combustion that emits all energy in a short time? The secret is that cellular aerobic respiration involves many biochemical reactions catalyzed by specialized enzymes. Only tiny changes occur in each reaction. These reactions are classified into 3 stages: glycolysis, citric acid cycle, and electron transport chain.
Glycolysis: anaerobic breakdown of glucose
The term glycolysis means " sugar splitting." Glucose is split into two three-carbon sugars that lose hydrogen atoms in subsequent steps. The remaining atoms rearrange to form two pyruvate molecules. Glycolysis is divided into two phases: energy investment phase and energy payoff phase. During the energy investment phase, the cell actually consumes 2 ATP to phosphorylate substrate to overcome activation energy barrier. This investment is repaid with interest in the energy payoff phase: 4 ATP and another high-energy compound, NADH, are synthesized. Thus, the net profit is 2 ATP and 2 NADH.
Glycolysis does not depend on oxygen. When oxygen is abundant, pyruvate is further processed in mitochondria. If it is anaerobic condition, the next metabolism is fermentation. Glycolysis has 10 steps that all occur in cytoplasm. Most steps are reversible, but three irreversible steps (1, 3, 10) must overcome energy barriers. The third step is the most important. The very inefficient PFK-1 is rate-limiting enzyme in glycolysis. ATP and citric acid inhibit it, while ADP and AMP activate it. Almost all organisms can use glycolysis to obtain energy from glucose. This indirectly shows that we all share a common ancestor.
Tricarboxylic Acid (TCA) Cycle
Because most of energy is still in pyruvate, pyruvate is transported into mitochondrial matrix under oxygen-rich conditions. Before entering TCA cycle, the multienzyme complex oxidizes pyruvate. A carboxyl group becomes carbon dioxide. The remaining two-carbon fragment loses two hydrogen atoms to transfer energy in 2 NADH. CoA binds with remaining parts to form acetyl-CoA.
Acetyl-CoA carries abundant energy into the citric acid cycle. This cycle was discovered by Krebs in 1930s, and citric acid, an organic acid with three carboxyl groups, is the initial product, so it is also called Krebs cycle or tricarboxylic acid cycle.
A four-carbon compound oxaloacetate is in the starting point of eight-steps TCA cycle. It combines with acetyl-CoA to form a six-carbon compound citrate. In the cyclic metabolic pathway, its carboxyl group becomes carbon dioxide, and hydrogen atoms are transferred to 6 NADH and 2 FADH₂. In the final step, malate is synthesized into oxaloacetate, and the pathway returns to starting point. Substrate-level phosphorylation also produces 2 ATP or 2 GTP. Krebs cycle is like a metabolic transit station, and its intermediates are shared with other metabolisms.
Electron Transport Chain or Respiratory Chain, Chemiosmosis
So far, 4 ATP, 10 NADH, and 2 FADH₂ have been harvested. Most of the energy is stored in these compounds as electric potential energy. Under aerobic conditions, this energy is gradually released via respiratory chain that is located on the inner mitochondrial membrane in eukaryotes. In prokaryotes, it is on plasma membrane. Its main components are four multi-protein complexes. The electrons pass through it easily because it contains conductive iron elements (iron-sulfur clusters and cytochrome c).
Electrons from NADH are captured by Complex I. If FADH₂ provides electrons, the first acceptor is Complex II. Lipophilic molecule coenzyme Q moves freely within the membrane like a truck delivering goods. It shuttles between complexes to transfer electrons from Complex I and II to Complex III. Cytochrome c is located between Complex III and IV. Finally, electrons are taken away from Complex IV by oxygen, and combined with protons to form water molecules. Electron transfer sequence is strictly unchangeable because the next carrier has a stronger affinity for electrons. They strip electrons from the previous carrier and bind with them. This is thermodynamically favorable due to the drop in potential energy. Each complex contains multiple such proteins, so the potential energy of electrons is actually released in many steps gradually. Just like when you walk down stairs from the second floor to the first floor, a little gravitational potential energy is released per step.
Biochemical reactions on the respiratory chain only release energy, so how is ATP produced?
The generally accepted theory is chemiosmosis mechanism. As electrons pass through protein complexes I, III, and IV, its energy decreased is expended in hydrogen ions (protons) transport from mitochondrial matrix to intermembrane space where a higher proton concentration is created. Infolding inner membrane forms many mitochondrial cristae that stabilizes proton gradient and increase surface. The pH value is about 5 in cristae. Protons cannot directly penetrate cristae to reach the matrix. The only pathway for them is ATP synthases span the inner mitochondrial membrane.
ATP synthase consists of subunits F0 and F1. Proton enters from its ion channel and is adsorbed on F0 subunit whose conformational change results in the rotation of the entire device. Electrochemical potential energy stored hydrogen ions is converted into kinetic energy of F0 and F1. The F1 consists of three components with different conformations. Conformation 1 grabs ADP and Pi. After rotating 120°, it turns into conformation 2 to press Pi into ADP. After another 120° rotation, it becomes conformation 3 and releases ATP. Finally, another 120° rotation returns it to conformation 1. Each full rotation discharges about 12 protons to matrix, and 3 ATP are synthesized. Generally, 32 ATP are harvested in one glucose oxidation. Therefore, the electron transport chain is coupled to ATP synthesis by energy temporarily stored in transmembrane proton gradient.
Isn’t this very much like a hydropower plant? Its dam is like inner mitochondrial membrane; its turbine is like ATPase; water stored in reservoir is like proton gradient.
The mechanism in prokaryotes is similar but slightly different. Protons are stored in the space between their plasma membrane and cell wall. There are no developed cristae that is common in mitochondria, so their coupling efficiency between the respiratory chain and ATP synthase is much lower.