Cellular Respiration

Cellular respiration is the process of extracting energy from food, primarily glucose (C6H12O6), which reacts with six oxygen molecules to produce six carbon dioxide and six water molecules, releasing a significant amount of energy (exergonic reaction). Some energy is lost as heat, while some is captured as ATP (adenosine triphosphate), the cell's energy currency. ATP provides energy for various cellular activities through the transfer of a phosphate group. ATP stores energy by converting ADP to ATP (endergonic), and releases energy when it loses a phosphate group (exergonic). ATP is more efficient than glucose for energy transfer as it releases smaller, manageable amounts of energy, minimizing heat loss, similar to how a car engine uses small explosions rather than one large one.

ATP consists of three subunits: a five-carbon ribose sugar, an adenine nitrogenous base, and three phosphate groups. Cellular respiration is divided into four main stages: 1. Glycolysis: Glucose is split into two molecules of pyruvate. This occurs in the cytosol. 2. Pyruvate Oxidation: Pyruvate is oxidized into acetyl coenzyme A, occurring in the mitochondria. 3. Krebs Cycle (Citric Acid Cycle): Acetyl coenzyme A is oxidized into carbon dioxide, occurring in the mitochondrial matrix. 4. Electron Transport Chain: NADH and FADH2 release electrons, which pass through the chain, producing ATP. This occurs in the inner membrane of the mitochondria.

In glycolysis, glucose (a six-carbon molecule) is converted into two three-carbon pyruvate molecules. This process also converts two ADP to two ATP (substrate-level phosphorylation) and reduces two NAD+ to two NADH. Oxidation and reduction reactions are key, with oxidation involving loss of electrons/hydrogen or gain of oxygen, and reduction being the opposite. Glycolysis has an 'investment phase' where two ATP are consumed and a 'payoff phase' where four ATP are produced, resulting in a net gain of two ATP molecules and two NADH per glucose molecule. Key enzymes include kinase (transfers phosphate groups), isomerase (rearranges molecules), and dehydrogenase (removes hydrogen).

Pyruvate oxidation is the second stage, where pyruvate is converted into acetyl coenzyme A. This involves a decarboxylation reaction, where a carbon dioxide molecule is lost, and an oxidation reaction, where NAD+ is reduced to NADH. The enzyme pyruvate dehydrogenase catalyzes this conversion. This process occurs in the mitochondrial matrix.

The Krebs cycle, occurring in the mitochondrial matrix, begins with acetyl coenzyme A combining with oxaloacetate (four carbons) to form citric acid (six carbons). The two-carbon acetyl group is then oxidized into two molecules of carbon dioxide. Electron carriers NAD+ and FAD pick up electrons, forming NADH and FADH2. One turn of the Krebs cycle produces three NADH, one FADH2, and one ATP (via GTP). Since one glucose molecule yields two pyruvate and thus two acetyl coenzyme A, one glucose molecule results in two turns of the cycle, yielding six NADH, two FADH2, and two ATP. It's noted that succinate dehydrogenase (complex II), involved in FADH2 production, is located in the inner mitochondrial membrane, not the matrix.

The mitochondria has an outer membrane, an inner membrane, an intermembrane space between them, and a mitochondrial matrix inside. The Krebs cycle happens in the matrix, while the electron transport chain (ETC) occurs in the inner membrane. In the ETC, NADH donates hydrogen and electrons to Complex I (NADH dehydrogenase), which then pass to ubiquinone (Q), a mobile electron carrier. Ubiquinone carries electrons to Complex III (cytochrome reductase), which transfers them to cytochrome c (another mobile carrier). Cytochrome c then delivers electrons to Complex IV (cytochrome oxidase). Finally, oxygen acts as the last electron acceptor, combining with electrons and hydrogen ions to form water, a product of cellular respiration.

FADH2, produced in the Krebs cycle, is part of Complex II (succinate dehydrogenase) in the inner membrane. It releases its electrons there, which are picked up by ubiquinone and then proceed through Complexes III and IV, similar to electrons from NADH. As electrons move through these complexes, protons are pumped from the mitochondrial matrix into the intermembrane space, creating a proton gradient and an electrical force. This electrochemical gradient drives protons through ATP synthase, an enzyme that synthesizes ATP by combining ADP and phosphate, a process called chemiosmosis. The combination of the electron transport chain and chemiosmosis is known as oxidative phosphorylation. NADH activates three proton pumps, yielding three ATP, while FADH2 activates two pumps, yielding two ATP.

For one glucose molecule: Glycolysis yields 2 net ATP and 2 NADH (6 ATP in ETC). Pyruvate oxidation yields 2 NADH (6 ATP in ETC). The Krebs cycle yields 2 ATP, 6 NADH (18 ATP in ETC), and 2 FADH2 (4 ATP in ETC). The total theoretical maximum ATP yield is 38 ATP. However, transporting the two NADH from glycolysis into the mitochondria costs 2 ATP, resulting in a net yield of 36 ATP. This is an ideal scenario; in biological systems, efficiency is less than 100%, and some protons may leak, leading to a lower actual yield.

When oxygen is not present (anaerobic conditions), fermentation occurs. Glycolysis, which does not require oxygen, still converts glucose to pyruvate, generating two ATP and two NADH. To allow glycolysis to continue, NAD+ must be regenerated from NADH. In lactic acid fermentation (e.g., in muscle cells during strenuous exercise), pyruvate is converted to lactate, oxidizing NADH back to NAD+. In ethanol fermentation (e.g., in yeast cells), pyruvate is first decarboxylated to acetaldehyde and then reduced to ethanol by NADH, regenerating NAD+ and releasing carbon dioxide (which causes bread to rise).

The video presents practice problems. Key takeaways include: Carbon dioxide and water are primary products of cellular respiration. Ethanol fermentation is not part of traditional cellular respiration but an anaerobic process. Glycolysis produces a net of two ATP and two NADH but no lactic acid in its direct products. ATP synthase is key for ATP production in oxidative phosphorylation. Oxygen is the final electron acceptor in aerobic respiration. NADH and FADH2 are electron carriers, with NADH having a lower electron affinity than subsequent components of the electron transport chain. Kinase enzymes transfer phosphate groups, dehydrogenase enzymes remove hydrogen, and isomerases rearrange molecules. Complex II (succinate dehydrogenase) is synonymous with FADH2 participation. Chemiosmosis, not the electron transport chain itself, is where the greatest number of ATP molecules are directly produced. Water is produced only in the electron transport chain, while carbon dioxide is produced in pyruvate oxidation and the Krebs cycle. Glucose is a reactant, not a product, of cellular respiration.

This section tests understanding with true/false questions: Electrons are donated to the ETC by NADH and FADH2 (True). The investment phase of glycolysis yields four ATP molecules (False, net two ATP). Oxidation is loss of electrons, reduction is gain of electrons (True). Oxidation is loss of oxygen (False, it's gain of oxygen or loss of hydrogen/electrons). Cellular respiration is a redox reaction (True). The Krebs cycle generates six ATP per acetyl CoA (False, one ATP). The Krebs cycle generates two FADH2 per glucose (True). In aerobic respiration, glucose is oxidized and oxygen is reduced (True). Oxaloacetate combines with acetyl CoA to form citrate in the Krebs cycle (True). Four CO2 are produced in the Krebs cycle per glucose (True). Two CO2 are produced during pyruvate oxidation for every pyruvate ion (False, one CO2 per pyruvate).