Mitochondrial structure and substrate preparation
The processes of oxygen-dependent degradation are inextricably linked with the double-membrane structure of mitochondria. The outer membrane delimits the organelle, while the inner membrane forms folds—cristae—where the enzymes of the respiratory chain are located. The intermembrane space serves as a reservoir for proton accumulation, and the inner fluid medium (matrix) hosts the Krebs cycle.
Before the main cycle, pyruvic acid (pyruvate), containing three carbon atoms, enters the matrix from the cytosol. There, oxidative decarboxylation takes place: pyruvate interacts with an enzyme complex, splitting off carbon dioxide and hydrogen, reducing the coenzyme NAD to NADH. As a result of the attachment of coenzyme A, acetyl-CoA is formed—a two-carbon compound with a high-energy bond that enters further reactions.
Tricarboxylic acid cycle (Krebs cycle)
The Krebs cycle is the process of the gradual "combustion" of the acetyl-CoA residue. At the starting stage, the two-carbon acetyl-CoA merges with the four-carbon oxaloacetic acid. As a result of their interaction, six-carbon citric acid is synthesized.
This is followed by a chain of seven sequential enzymatic transformations. During these reactions, citric acid is oxidized, losing two carbon atoms, which are removed as $CO_2$. At the end of the cycle, oxaloacetic acid is regenerated and is ready to accept a new acetyl-CoA molecule.
For one turn of the cycle, the energy of chemical bonds is accumulated in reduced coenzymes: three molecules of $NADH \cdot H_2$ and one molecule of $FADH_2$ are synthesized. Via substrate-level phosphorylation, a GTP molecule is formed, the energy of which is immediately spent on creating ATP. In total, 2 ATP molecules are generated in the matrix.
Respiratory chain and ATP synthesis
The accumulated coenzymes are directed to the inner mitochondrial membrane, where they transfer hydrogen atoms to the proteins of the respiratory chain (electron transport chain). Here the streams divide:
- Electrons are transported along the chain of enzymes to the final acceptor—oxygen.
- Protons are pumped to the outer side of the membrane into the proton reservoir.
Due to the accumulation of protons, the outside of the membrane becomes positively charged, and electrons on the inside merge with oxygen, forming anions ($O_2 + 2e^- \rightarrow O_2^-$). A powerful transmembrane potential arises. When the potential difference reaches a critical mark of 200 mV, the proton channel of the ATP synthase enzyme opens.
Protons rush inward under the action of the electric field. Their kinetic energy is used for the mass synthesis of ATP from ADP and inorganic phosphate. On the inner side, protons combine with oxygen anions, forming water: $O_2^- + 2H^+ \rightarrow H_2O$. During this stage (oxidative phosphorylation), 34 ATP molecules are generated.
General outcomes of energy metabolism
Aerobic respiration refers to processes of dissimilation (catabolism)—carbohydrates and other nutrients are broken down into simple inorganic molecules. This leads to a natural decrease in the organic mass of the cell.
The total amount of energy released at the oxygen stage is 2600 kJ. The process is highly efficient: 55% of this energy is successfully stored in the form of ATP (36 molecules in total), and the remainder is dissipated as heat. Oxygen is consumed during gas exchange, while carbon dioxide and water are released.
Briefly: answers to topical questions
Which enzymes (subunits) and coenzymes are part of the pyruvate dehydrogenase complex?
The pyruvate dehydrogenase complex includes the enzyme pyruvate decarboxylase and several coenzymes.
- Pyruvate decarboxylase (E1) is the enzyme that carries out the first stage (pyruvate decarboxylation).
- Thiamine diphosphate (TDP) is the coenzyme of E1, participating in the transfer of the acetyl residue.
- Lipoic acid (lipoamide) accepts the hydroxyethyl residue from TDP.
- Coenzyme A (HS–CoA) binds to the metabolite to form Acetyl-CoA.
Other subunits of the complex are not mentioned in the text.
Which specific inhibitors (poisons) block electron transfer at complexes I, II, III, and IV of the respiratory chain?
Electron transfer in the respiratory chain is blocked by the following specific inhibitors of enzyme complexes:
- Complex I — barbiturates and rotenone (inhibit NADH dehydrogenase).
- Complex III — antimycin A (inhibits QH2 dehydrogenase).
- Complex IV — carbon monoxide (CO), hydrogen sulfide (H2S), and cyanide (inhibit cytochrome c oxidase).
Information on specific inhibitors of Complex II of the respiratory chain is absent in the provided materials.
What components form the electrochemical proton gradient (driving force) on the inner mitochondrial membrane?
The electrochemical proton gradient is formed due to the active pumping of protons from the matrix into the intermembrane space by transmembrane protein complexes of the respiratory chain.
- Complex I pumps 4 H⁺ per pair of electrons.
- Complex III pumps 2 H⁺ per pair of electrons.
- Complex IV pumps 4 H⁺ per pair of electrons.
As a result, a proton concentration difference and a transmembrane potential arise, which act as the driving force for ATP synthesis.
What is the difference between substrate-level and oxidative phosphorylation?
The difference between these processes lies in the source of energy for the synthesis of ATP/GTP from ADP.
| Characteristic | Substrate-level phosphorylation | Oxidative phosphorylation |
|---|---|---|
| Mechanism | Synthesis of ATP/GTP using the energy of the substrate's high-energy bond, without the participation of the respiratory chain | Synthesis of ATP by the enzyme ATP synthase using the energy of the proton electrochemical potential |
| Where mentioned in sources | Krebs cycle: formation of GTP, the energy of which is used to phosphorylate ADP to form ATP | Inner mitochondrial membrane; ATP synthase, or Complex V |
To remember the start of the Krebs cycle: Pretty Oxen Ate — Oxaloacetate combines with Acetyl-CoA to form citrate (citric acid).
Class and exam questions
How many ATP molecules are produced during aerobic respiration?
During the oxygen stage, 36 ATP molecules are synthesized. Of these, 2 molecules are formed in the matrix during the Krebs cycle, and another 34 on the mitochondrial cristae during oxidative phosphorylation.
Where exactly does the Krebs cycle take place?
The tricarboxylic acid cycle takes place in the matrix—the inner fluid environment of the mitochondria, where the enzymes necessary for the process are located.
Why is oxygen needed in the electron transport chain?
Oxygen acts as the final electron acceptor in the respiratory chain. It combines with the incoming electrons and hydrogen protons, resulting in the formation of a water molecule.
What else to study on this topic
- Structure of ATP synthase and the mechanism of the proton channel
- Comparative characteristics of photosynthesis and aerobic respiration
- Detailed chemistry of the seven sequential reactions of the Krebs cycle
- Anaerobic stage: formation of pyruvic acid (glycolysis)
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