Cellular Respiration Steps

Why This Matters

Cellular respiration is the metabolic backbone of nearly every living cell—and it's absolutely central to the AP Biology exam. You're being tested on your understanding of energy transformations, enzyme regulation, membrane structure, and the laws of thermodynamics all wrapped into one elegant pathway. When you see questions about mitochondria, ATP yield, or why organisms need oxygen, they're really asking: do you understand how cells harvest energy from glucose step by step?

Don't just memorize that glycolysis makes 2 ATP or that the ETC needs oxygen. Know why each step exists, where it happens (and why location matters), and how electron carriers connect everything together. The exam loves to test whether you can trace a carbon atom through the pathway or explain what happens when oxygen is absent. Master the mechanisms, and you'll crush both multiple choice and FRQs.

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Glucose Breakdown: Setting the Stage

The first stages of cellular respiration focus on breaking carbon-carbon bonds in glucose and capturing that released energy in electron carriers. These reactions happen before the big ATP payoff and don't require oxygen.

Glycolysis

• Occurs in the cytoplasm—this is the only stage that happens outside the mitochondria, making it evolutionarily ancient and universal to all cells
• Splits one 6-carbon glucose into two 3-carbon pyruvates—the net yield is 2 ATP and 2 NADH per glucose molecule
• Anaerobic process—no oxygen required, which is why it can continue during fermentation when oxygen is unavailable

Pyruvate Oxidation

• Transition step in the mitochondrial matrix—pyruvate enters the mitochondria and loses one carbon as $CO_2$, forming 2-carbon acetyl-CoA
• Produces 2 NADH per glucose—one from each pyruvate molecule, adding to the electron carrier pool
• Links glycolysis to the citric acid cycle—acetyl-CoA is the fuel that enters the next stage; without this conversion, the cycle can't proceed

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The Citric Acid Cycle: Completing Carbon Oxidation

The citric acid cycle (also called the Krebs cycle) finishes extracting energy from carbon bonds and loads up electron carriers for the big ATP payoff. By the end, all six carbons from glucose have been released as $CO_2$.

Citric Acid Cycle (Krebs Cycle)

• Occurs in the mitochondrial matrix—acetyl-CoA combines with 4-carbon oxaloacetate to form 6-carbon citrate, which is progressively oxidized
• Produces 3 NADH, 1 $FADH_2$, and 1 ATP per turn—since each glucose yields 2 acetyl-CoA, double these numbers for total yield per glucose
• Regenerates oxaloacetate—this cyclical design means the pathway can run continuously as long as acetyl-CoA keeps entering

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Electron Carriers: The Energy Shuttle System

NADH and $FADH_2$ don't make ATP directly—they carry high-energy electrons to the electron transport chain, where the real ATP production happens. Think of them as rechargeable batteries.

NAD⁺ and FAD as Electron Carriers

• $NAD^+$ and $FAD$ accept electrons during oxidation reactions—they become NADH and $FADH_2$, storing energy in their electrons for later use
• Deliver electrons to the ETC—NADH feeds into Complex I, while $FADH_2$ enters at Complex II (yielding slightly less ATP)
• Must be regenerated for respiration to continue—if $NAD^+$ runs out, glycolysis stops; this is why fermentation exists

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The Electron Transport Chain: Building the Gradient

The ETC is where electrons flow through protein complexes, releasing energy that pumps protons across the inner mitochondrial membrane. This creates the electrochemical gradient that drives ATP synthesis.

Electron Transport Chain

• Located in the inner mitochondrial membrane—the cristae (folds) increase surface area for more ETC complexes and greater ATP production
• Electrons pass through protein complexes (I → II → III → IV)—each transfer releases energy used to pump $H^+$ ions into the intermembrane space
• Creates a proton gradient—this chemiosmotic gradient stores potential energy, like water behind a dam

Oxygen as Final Electron Acceptor

• Oxygen accepts electrons at Complex IV—it combines with $H^+$ to form water ($O_2 + 4H^+ + 4e^- \rightarrow 2H_2O$)
• Keeps the ETC running—without oxygen to accept electrons, the chain backs up and ATP production via oxidative phosphorylation stops
• Explains why we breathe—oxygen's role as the final acceptor is the entire reason aerobic organisms require it

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ATP Synthesis: Cashing In the Gradient

All that electron transport was building toward this: using the proton gradient to power ATP synthase. This is where the majority of ATP is actually made.

Oxidative Phosphorylation

• ATP synthase uses the proton gradient—$H^+$ ions flow back through this enzyme, causing it to rotate and catalyze $ADP + P_i \rightarrow ATP$
• Produces ~26-28 ATP per glucose—this accounts for the vast majority of cellular respiration's ATP yield
• Chemiosmosis couples electron transport to phosphorylation—the proton-motive force is the critical link; disrupt the gradient, and ATP production collapses

Substrate-Level Phosphorylation

• Direct transfer of phosphate to ADP—no electron transport chain or proton gradient involved
• Occurs in glycolysis and the citric acid cycle—produces 4 ATP total per glucose (2 in each stage, though glycolysis has a net gain of 2)
• Works without oxygen—this is the only ATP-generating mechanism available during anaerobic conditions

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Total Energy Yield: Putting It Together

Understanding the complete ATP budget helps you see why aerobic respiration is so much more efficient than anaerobic alternatives.

ATP Production Summary

• Total yield: ~30-32 ATP per glucose—includes 2 from glycolysis, 2 from the citric acid cycle, and ~26-28 from oxidative phosphorylation
• Energy stored in ATP's phosphate bonds—hydrolysis of ATP to ADP releases energy for cellular work
• Tightly regulated—cells adjust respiration rate based on ATP demand; high ATP levels inhibit key enzymes through feedback inhibition

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Anaerobic Alternatives: When Oxygen Is Absent

Fermentation isn't a replacement for aerobic respiration—it's a survival mechanism that regenerates $NAD^+$ so glycolysis can continue producing at least some ATP.

Fermentation (Anaerobic Pathways)

• Regenerates $NAD^+$ without oxygen—pyruvate accepts electrons from NADH, freeing $NAD^+$ to return to glycolysis
• Lactic acid fermentation in animals—pyruvate becomes lactate; occurs in muscle cells during intense exercise
• Alcoholic fermentation in yeast—pyruvate becomes ethanol and $CO_2$; used in brewing and baking

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Quick Reference Table

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Self-Check Questions

1. Which two stages of cellular respiration release $CO_2$, and where in the cell does each occur?

2. Compare the ATP yield from substrate-level phosphorylation versus oxidative phosphorylation. Why is the difference so dramatic?

3. If a cell's mitochondria are damaged but the cytoplasm is intact, which stages of cellular respiration can still occur? What would happen to total ATP production?

4. Explain why NADH generated in glycolysis yields slightly less ATP than NADH generated in the mitochondrial matrix. (Hint: think about membrane transport.)

5. A student claims that fermentation produces energy without any ATP being made. Identify the error and explain the actual role of fermentation in energy metabolism.