Krebs Cycle and Oxidative Phosphorylation HL

Understanding the Krebs cycle and oxidative phosphorylation is essential for mastering cellular respiration, the process that converts the energy in food into the universal energy currency, ATP. For IB Biology HL, you must move beyond memorizing steps to analyzing the elegant chemistry and physics that allow a single glucose molecule to be systematically dismantled, its energy harvested through redox reactions, and finally used to power ATP synthesis via a remarkable proton gradient.

From Pyruvate to Acetyl CoA: The Gateway Step

Before the Krebs cycle can begin, the end-product of glycolysis, pyruvate, must be transported from the cytoplasm into the mitochondrial matrix. Here, a critical linking reaction occurs, catalyzed by the multi-enzyme pyruvate dehydrogenase complex. This step is not part of the Krebs cycle itself but is an irreversible commitment.

In this oxidative decarboxylation, a single pyruvate molecule (3 carbons) loses one carbon as carbon dioxide ($C{O}_2$). The remaining two-carbon acetyl group is attached to coenzyme A (CoA), forming acetyl CoA. Crucially, during this process, dehydrogenation occurs: two hydrogen atoms (with their electrons) are removed and used to reduce NAD+ to NADH + H+. This step highlights two key themes: carbon skeletons are progressively shortened by decarboxylation, and energy is captured in the reduced electron carriers NADH and FADH2.

The Krebs Cycle: A Central Metabolic Hub

The Krebs cycle, also known as the citric acid cycle, is a closed loop of eight enzyme-catalyzed reactions. Its primary function is to complete the oxidation of the acetyl group from acetyl CoA, harvesting high-energy electrons and producing precursor molecules for biosynthesis.

Cycle Initiation: The two-carbon acetyl CoA combines with the four-carbon oxaloacetate to form the six-carbon citrate. CoA is released and recycled.

Energy Harvesting Steps: The cycle then undergoes two major types of reactions:

1. Decarboxylation Reactions: Carbon atoms are removed as $C{O}_2$. This happens twice per turn, converting the six-carbon citrate back to a four-carbon molecule.
2. Dehydrogenation Reactions: Hydrogen atoms (electrons) are removed. These redox reactions are where most of the energy capture occurs:

• NAD+ is reduced to NADH at three separate steps.
• FAD is reduced to FADH2 at one step (during the conversion of succinate to fumarate).
• These carriers now hold the potential energy originally in the acetyl group.

Substrate-Level Phosphorylation: One step in the cycle directly produces ATP (or GTP in some cells) through substrate-level phosphorylation. When succinyl CoA is converted to succinate, the released energy is used to phosphorylate GDP to GTP, which can then transfer a phosphate to ADP.

For every acetyl CoA entering the cycle, the net yield is: 3 NADH, 1 FADH2, 1 ATP (via GTP), and 2 $C{O}_2$. Since one glucose yields two acetyl CoA molecules, the cycle turns twice per glucose.

The Electron Transport Chain: Setting the Stage for Chemiosmosis

The reduced carriers (NADH and FADH2) produced in the previous stages now deliver their high-energy electrons to the electron transport chain (ETC), a series of protein complexes embedded in the inner mitochondrial membrane. This is not a direct synthesis of ATP but an energy conversion process.

Electron Flow and Proton Pumping: The electrons are passed from one carrier to the next (e.g., from Complex I to Coenzyme Q to Complex III to Cytochrome c to Complex IV) in a series of redox reactions. Each carrier has a progressively higher affinity for electrons, ensuring the downhill flow. The key is that the energy released from this exergonic electron transfer is used by three of the complexes (I, III, and IV) to actively pump protons ($H^{+}$) from the matrix into the intermembrane space.

Establishing the Electrochemical Gradient: This pumping creates two interdependent gradients across the inner membrane:

1. A chemical gradient (more $H^{+}$ in the intermembrane space).
2. A charge (voltage) gradient (the intermembrane space becomes more positive).

Together, this is the proton motive force, a form of stored potential energy. The final electron acceptor is molecular oxygen ($O_2$), which combines with electrons and matrix $H^{+}$ to form water. This makes the process aerobic.

Chemiosmosis and ATP Synthase: Harnessing the Gradient

Chemiosmosis is the process by which the energy stored in the proton gradient is used to drive ATP synthesis. It links the exergonic flow of protons down their gradient to the endergonic phosphorylation of ADP.

ATP Synthase: The Molecular Turbine: The enzyme ATP synthase provides a channel for protons to flow back into the matrix. This flow drives the rotation of part of the enzyme complex (the rotor stalk), much like water flowing through a turbine. This mechanical rotation induces conformational changes in the catalytic head (the $F_1$ subunit), which binds ADP and inorganic phosphate ($P_i$), forcing them together to form ATP. This mechanism, where a proton gradient couples electron transport to ATP synthesis, is called the chemiosmotic theory.

Crucially, the $H^{+}$ used to make ATP are not the same atoms that were part of the original substrates; they are simply vehicles for transferring energy via the gradient.

Calculating Theoretical ATP Yield

For IB HL, you are expected to calculate the maximum theoretical yield of ATP per oxidized glucose molecule. This requires careful bookkeeping.

1. Glycolysis (in cytoplasm):

• Net ATP from substrate-level phosphorylation: 2 ATP
• NADH produced: 2 NADH. These must be shuttled into the mitochondrion, which can cost energy. Using the more efficient shuttle (malate-aspartate), they yield about 2.5 ATP each, for a total of 5 ATP (or 3 ATP each with the glycerol phosphate shuttle).

1. Link Reaction (x2 per glucose):

• NADH produced: 2 NADH. Each yields ~2.5 ATP, for 5 ATP.

1. Krebs Cycle (x2 per glucose):

• ATP from substrate-level phosphorylation: 1 ATP x 2 = 2 ATP
• NADH produced: 3 NADH x 2 cycles = 6 NADH. 6 x 2.5 = 15 ATP
• FADH2 produced: 1 FADH2 x 2 cycles = 2 FADH2. Each yields ~1.5 ATP, for 3 ATP

Maximum Theoretical Total (using efficient shuttle): 2 + 5 + 5 + 2 + 15 + 3 = 32 ATP per glucose molecule.

Exam Tip: Be prepared to explain why the actual yield in a cell is often lower (e.g., due to the cost of shuttling, proton leak, or using the less efficient shuttle). The key is to show the reasoning process.

Common Pitfalls

1. Confusing the Location of Processes: A common error is placing glycolysis in the mitochondrion or the Krebs cycle in the cytoplasm. Remember: Glycolysis occurs in the cytoplasm. The link reaction, Krebs cycle, and oxidative phosphorylation occur in/on the mitochondria (matrix and inner membrane).

1. Misunderstanding the Role of Oxygen: Oxygen is not used in the Krebs cycle. It is the final electron acceptor at the end of the ETC (Complex IV). Without oxygen to accept electrons, the ETC backs up, NAD+ cannot be regenerated, and the Krebs cycle and glycolysis halt.

1. Miscalculating ATP Yields: The biggest mistake is forgetting to double the outputs from the link reaction and Krebs cycle (because they occur twice per glucose). Also, confusing the ATP yield from NADH (2.5) vs. FADH2 (1.5) will lead to an incorrect total.

1. Overlooking the Chemiosmotic Principle: Stating that "the ETC makes ATP" is incorrect. The ETC builds the proton gradient. Chemiosmosis, powered by the flow of protons through ATP synthase, is the process that synthesizes ATP.

Summary

• The Krebs cycle completes the oxidation of acetyl CoA, generating $C{O}_2$, ATP (via GTP), and large quantities of reduced electron carriers (NADH and FADH2) through decarboxylation and dehydrogenation reactions.
• The electron transport chain uses the energy from electrons (donated by NADH and FADH2) to pump protons across the inner mitochondrial membrane, creating a proton motive force (electrochemical gradient).
• Chemiosmosis is the coupling mechanism where protons flow back into the matrix through ATP synthase, driving the phosphorylation of ADP to ATP.
• The theoretical maximum ATP yield from one glucose molecule is approximately 32 ATP, derived from careful accounting of substrate-level phosphorylation and the chemiosmotic yield from each NADH (~2.5 ATP) and FADH2 (~1.5 ATP).
• Success in IB HL requires moving beyond memorization to a functional understanding of how these processes are integrated, spatially organized, and regulated to efficiently extract energy from organic molecules.