IB Biology: Metabolism and Cell Respiration HL

Understanding cell respiration is fundamental to biology because it explains how all living organisms, from bacteria to humans, extract usable energy from their food. For IB Biology HL, you must move beyond memorizing steps and grasp the intricate biochemistry that transforms glucose into ATP, the universal energy currency. This deep dive into glycolysis, the Krebs cycle, and oxidative phosphorylation will equip you to analyze experimental data, tackle complex essay questions, and appreciate the elegant efficiency of this core metabolic pathway.

Glycolysis: The Universal Starting Point

Glycolysis is the first stage of cellular respiration, occurring in the cytosol of all living cells. It is an anaerobic process, meaning it does not require oxygen. The pathway involves ten enzymatic reactions that convert one 6-carbon glucose molecule into two 3-carbon molecules of pyruvate. Crucially, glycolysis requires an initial investment of 2 ATP molecules to phosphorylate glucose, making it more reactive. This investment is later repaid with interest.

The process can be divided into two phases: the energy investment phase (using 2 ATP) and the energy payoff phase. During the payoff phase, substrate-level phosphorylation occurs, directly synthesizing ATP from an intermediate molecule. Furthermore, the coenzyme NAD+ is reduced to NADH by gaining electrons and a proton. Per glucose molecule, the net yield of glycolysis is 2 ATP, 2 NADH, and 2 pyruvate. This relatively small ATP yield highlights why aerobic respiration is so much more efficient.

The Link Reaction and Krebs Cycle: Central Carbon Processing

In the presence of oxygen, pyruvate produced in glycolysis is actively transported into the mitochondrial matrix. Here, the link reaction acts as a gateway. Pyruvate is decarboxylated (loses a CO2), oxidized, and combined with coenzyme A to form acetyl CoA. This reaction, catalyzed by a multi-enzyme complex, also reduces another NAD+ to NADH.

The acetyl CoA then enters the Krebs cycle (also called the citric acid cycle). This is a cyclic series of eight reactions that completely oxidizes the acetyl group. For each acetyl CoA entering the cycle, the key outputs are: 3 NADH, 1 FADH2 (another electron carrier), 1 ATP (via substrate-level phosphorylation), and 2 CO2 molecules. Since one glucose yields two acetyl CoA molecules, the total yield per glucose from the Krebs cycle proper is doubled: 6 NADH, 2 FADH2, 2 ATP, and 4 CO2. The cycle's true importance lies not in direct ATP production, but in harvesting high-energy electrons into NADH and FADH2 for the next stage.

Oxidative Phosphorylation and Chemiosmosis: The ATP Powerhouse

The majority of ATP is generated during oxidative phosphorylation, which couples electron transport with ATP synthesis via chemiosmosis. The NADH and FADH2 produced earlier carry electrons to the electron transport chain (ETC), a series of protein complexes embedded in the inner mitochondrial membrane. As electrons pass from one carrier to the next, they lose energy. This energy is used to pump protons (H+) from the matrix into the intermembrane space, creating a strong electrochemical gradient, or proton motive force.

Chemiosmosis is the process by which this gradient drives ATP synthesis. Protons flow back into the matrix through a channel enzyme called ATP synthase. This flow causes the rotor of ATP synthase to spin, catalyzing the phosphorylation of ADP to ATP, a process known as oxidative phosphorylation. Oxygen acts as the final electron acceptor at the end of the ETC, combining with electrons and H+ to form water, preventing the chain from backing up.

Anaerobic Respiration and Alternative Substrates

When oxygen is absent, the ETC and Krebs cycle cannot operate. Cells must rely on anaerobic respiration to recycle NAD+ from NADH, allowing glycolysis to continue producing its net 2 ATP. In animal cells and some bacteria, pyruvate is reduced to lactate using NADH, regenerating NAD+. In yeast and plant cells, pyruvate is first decarboxylated to acetaldehyde, which is then reduced to ethanol, also regenerating NAD+. These pathways are far less efficient but critical for short-term energy needs.

Glucose is not the only respiratory substrate. Lipids and proteins can also be broken down for energy. Triglycerides are hydrolyzed to glycerol and fatty acids. Fatty acids undergo beta-oxidation in the mitochondrial matrix, producing many acetyl CoA molecules, which then feed into the Krebs cycle. Proteins are deaminated, and their carbon skeletons are converted into Krebs cycle intermediates. The yield of ATP per gram from lipids is more than double that from carbohydrates, explaining their role as long-term energy stores.

Calculating ATP Yield from Glucose Oxidation

A common HL exam challenge is calculating the theoretical maximum ATP yield from the complete aerobic oxidation of one glucose molecule. The yield is not a fixed number, as it depends on the "shuttle" system used to transport cytosolic NADH from glycolysis into the mitochondria. Using the more common malate-aspartate shuttle, the theoretical yield is as follows:

• Glycolysis: 2 ATP (net), 2 NADH → yields 5 ATP via ETC.
• Link Reaction: 2 NADH (one per pyruvate) → yields 5 ATP each, total 10 ATP.
• Krebs Cycle: 6 NADH → yields 15 ATP, 2 FADH2 → yields 3 ATP, 2 ATP (direct).
• Total: 2 + 5 + 10 + 15 + 3 + 2 = 32 ATP.

This yield is theoretical; in reality, some energy is used for proton leakage and transport, making the actual yield slightly lower. The calculation demonstrates the massive efficiency gain of aerobic over anaerobic respiration (32 ATP vs. 2 ATP).

Common Pitfalls

1. Confusing ATP Yield from NADH vs. FADH2: Each NADH fuels the pumping of enough protons to generate approximately 2.5 ATP, while each FADH2 yields about 1.5 ATP. This is because FADH2 donates electrons to the ETC at a lower energy level (Complex II), resulting in fewer protons being pumped.
2. Misunderstanding the Role of Oxygen: Oxygen does not "fuel" respiration. It is the final electron acceptor at the end of the ETC. Without it, the ETC stops, NAD+ cannot be regenerated, and glycolysis halts unless fermentation occurs.
3. Overlooking Location: A frequent error is misidentifying the cellular location of each stage. Remember: Glycolysis (cytosol), Link Reaction & Krebs Cycle (mitochondrial matrix), Electron Transport Chain & Chemiosmosis (inner mitochondrial membrane).
4. Describing Chemiosmosis Vaguely: Avoid saying "the electron transport chain makes ATP." Instead, explain that the ETC creates a proton gradient, and chemiosmosis (the flow of protons through ATP synthase) is the process that synthesizes ATP.

Summary

• Cell respiration is a controlled, multi-step process of oxidizing glucose to carbon dioxide and water to produce ATP, with oxygen as the final electron acceptor.
• Glycolysis in the cytosol nets 2 ATP and 2 pyruvate per glucose, while the Krebs cycle in the matrix completes oxidation, producing electron carriers (NADH, FADH2) and a small amount of ATP.
• The bulk of ATP (approximately 28 out of 32 per glucose) is produced via oxidative phosphorylation, where the electron transport chain creates a proton gradient that drives ATP synthesis by chemiosmosis.
• In the absence of oxygen, anaerobic pathways (lactate or ethanol fermentation) regenerate NAD+ to allow glycolysis to continue, producing a much lower ATP yield.
• Different respiratory substrates (lipids, proteins) enter the pathway at different points, with lipids yielding significantly more ATP per gram than carbohydrates.