Cellular Respiration Review

Overview

Cellular respiration is the process cells use to break down glucose and other macromolecules to make ATP, the cell's usable energy currency. For AP Bio, it happens in three connected stages: glycolysis (in the cytosol), the Krebs cycle (in the mitochondrial matrix), and oxidative phosphorylation at the electron transport chain (across the inner mitochondrial membrane). Every form of life carries out respiration or fermentation, including plants and other organisms that also photosynthesize. This sits in Unit 3: Cellular Energetics, which is worth 12-16% of the AP exam.

The exam won't ask you to memorize every enzyme or intermediate. It wants you to know the inputs and outputs of each stage, where each one happens, how electron carriers feed the electron transport chain, and why oxygen matters. Get fluent in predicting what happens when something changes (like running out of oxygen), and you're in good shape.

The Big Picture: Glucose to ATP

Cellular respiration converts the chemical energy in glucose into ATP, releasing carbon dioxide and water along the way. The simplified overall reaction is:

$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{ATP}$

Notice this is essentially photosynthesis run backward. Photosynthesis builds glucose using CO2, water, and light energy; respiration takes that glucose apart to release the stored energy. If you understood photosynthesis, the logic of respiration should click fast.

The three stages depend on each other in sequence. The product of one step becomes the reactant for the next, which is how cells keep energy transfer controlled instead of releasing it all at once as heat.

Image courtesy of WikiMedia Commons.

Here's a quick map of where everything happens:

Glycolysis

Glycolysis is the first stage of cellular respiration, splitting one 6-carbon glucose molecule into two 3-carbon pyruvate molecules in the cytosol. It's the most evolutionarily conserved metabolic pathway, happening in essentially the same way across Archaea, Bacteria, and Eukarya. That shared chemistry is one piece of evidence for common ancestry.

Breaking glucose's bonds releases a small amount of energy that gets captured as a net of 2 ATP (made from ADP and inorganic phosphate). Some electrons released along the way get picked up by the carrier NAD⁺, reducing it to 2 NADH. Those NADH molecules carry electrons to the electron transport chain later.

Glycolysis doesn't need oxygen. That detail matters: it's why glycolysis can keep running during fermentation when oxygen runs out.

Before pyruvate enters the Krebs cycle, it gets oxidized (loses electrons) and converted into a 2-carbon molecule called acetyl-CoA. Each pyruvate that's oxidized releases one CO₂ and reduces another NAD⁺ to NADH.

Krebs Cycle

The Krebs cycle (also called the citric acid cycle) takes place in the mitochondrial matrix and finishes breaking down the carbon from glucose. A series of enzyme-catalyzed reactions strip electrons and carbon away from acetyl-CoA. You don't need to memorize the individual steps, enzymes, or intermediates for the AP exam, so focus on the inputs and outputs.

For each turn of the cycle, three things happen worth remembering. CO₂ is released from the organic intermediates (this is the carbon you eventually breathe out). A small amount of ATP is made directly from ADP and inorganic phosphate. And lots of electrons get loaded onto the coenzymes NAD⁺ and FAD, producing NADH and FADH₂.

The real payoff of the Krebs cycle isn't the ATP. It's the pile of loaded electron carriers (NADH and FADH₂). Those carriers deliver electrons to the electron transport chain, where most of the ATP actually gets made.

Electron Transport Chain and Oxidative Phosphorylation

The electron transport chain (ETC) produces the vast majority of ATP in aerobic respiration, and it works through chemiosmosis just like the light reactions in photosynthesis. The ETC sits in the inner mitochondrial membrane, which is heavily folded into cristae. That folding increases surface area, which means more room for ATP production.

Here's the flow. NADH and FADH₂ deliver their electrons to the chain. As electrons pass through a series of protein carriers in oxidation-reduction (redox) reactions, the released energy pumps protons (H⁺) across the inner mitochondrial membrane. This builds an electrochemical gradient: high proton concentration in the intermembrane space, low concentration inside the matrix. (The matrix ends up with a higher pH than the intermembrane space.)

Those protons want to flow back down their gradient, and the only easy route is through the enzyme ATP synthase. As protons flow through ATP synthase by chemiosmosis, the energy drives the production of ATP from ADP and inorganic phosphate. Because this uses oxygen at the end of the chain, the whole process is called oxidative phosphorylation.

So what does oxygen do? It's the final (terminal) electron acceptor. After electrons reach the end of the chain, oxygen accepts them and combines with hydrogen ions to form water. Aerobic prokaryotes also use oxygen as their terminal acceptor; anaerobic prokaryotes use other molecules instead.

One more thing the ETC accomplishes: it recycles electron carriers. When NADH and FADH₂ drop off their electrons, they go back to being NAD⁺ and FAD, ready to be reloaded in glycolysis and the Krebs cycle. Without this recycling, those earlier stages would grind to a halt.

You don't need exact ATP totals for the exam, just the relationship: oxidative phosphorylation makes far more ATP than glycolysis and the Krebs cycle combined.

Bonus fact worth knowing: if oxidative phosphorylation gets uncoupled from electron transport, the energy gets released as heat instead of captured as ATP. Endothermic ("warm-blooded") organisms use this to help regulate body temperature.

Image courtesy of WikiMedia Commons.

Fermentation: Respiration Without Oxygen

Fermentation lets glycolysis keep making ATP when oxygen isn't available. Without oxygen, the Krebs cycle and the electron transport chain stall, because there's no terminal electron acceptor to keep electrons moving. So cells need another way to recycle their NADH back into NAD⁺.

That's the whole point of fermentation: it regenerates NAD⁺ so glycolysis can continue. Cells pass electrons to another organic molecule instead of to oxygen, producing waste products like lactic acid or ethanol (plus CO₂). Yeast and some bacteria use alcoholic fermentation, which is how beer and wine are made. Your muscle cells switch to lactic acid fermentation during intense exercise when you can't get oxygen fast enough, and the buildup contributes to that next-day soreness.

The trade-off is energy. During fermentation, glycolysis alone makes ATP, so the yield is much lower than in full aerobic respiration. Cells get fast energy, but not much of it.

Key Concepts and Vocabulary

• Cellular respiration is the process that breaks down macromolecules to synthesize ATP; it occurs in all forms of life.
• ATP is the cell's usable energy currency, made from ADP and inorganic phosphate.
• Glycolysis splits glucose into two pyruvate in the cytosol, producing a net 2 ATP and 2 NADH; it needs no oxygen.
• Pyruvate is the 3-carbon product of glycolysis that gets oxidized to acetyl-CoA before entering the Krebs cycle.
• Krebs cycle (citric acid cycle) runs in the mitochondrial matrix, releasing CO₂ and loading electrons onto NADH and FADH₂.
• Electron carriers (NADH and FADH₂) shuttle electrons from glycolysis and the Krebs cycle to the electron transport chain.
• Electron transport chain (ETC) is the series of redox reactions in the inner mitochondrial membrane that pumps protons to build a gradient.
• Oxidation-reduction (redox) reactions transfer electrons; oxidation is losing electrons, reduction is gaining them.
• Proton (electrochemical) gradient is the difference in H⁺ concentration across the inner mitochondrial membrane that stores energy.
• Chemiosmosis is the flow of protons down their gradient through ATP synthase, which drives ATP production.
• ATP synthase is the membrane enzyme that makes ATP from ADP and inorganic phosphate as protons flow through it.
• Oxidative phosphorylation is ATP production driven by the ETC and chemiosmosis, using oxygen as the terminal electron acceptor.
• Terminal electron acceptor is the final molecule that accepts electrons; in aerobic respiration it's oxygen, which forms water.
• Fermentation regenerates NAD⁺ so glycolysis can continue without oxygen, producing lactic acid or ethanol with low ATP yield.

Common Mistakes

• Thinking only animals do cellular respiration. Every organism does it, including plants. Plants photosynthesize AND respire; they make glucose in the chloroplast and break it down in the mitochondria.
• Confusing where each stage happens. Glycolysis is in the cytosol, not the mitochondria. The Krebs cycle is in the matrix, and the ETC is in the inner mitochondrial membrane. Mixing these up is a classic point loss.
• Saying oxygen "powers" or "starts" respiration. Oxygen's job is to be the terminal electron acceptor at the very end of the ETC, picking up spent electrons to form water. Without it, the ETC backs up and stops.
• Thinking fermentation makes ATP directly. Fermentation makes no ATP by itself. Its only job is to recycle NAD⁺ so that glycolysis (which makes the ATP) can keep running.
• Memorizing molecules instead of understanding flow. The exam rewards explaining how a change (like losing oxygen or denaturing ATP synthase) affects ATP output, not reciting intermediates. Practice predicting cause and effect.
• Forgetting carrier recycling. If NADH and FADH₂ can't drop off electrons and return to NAD⁺ and FAD, glycolysis and the Krebs cycle stall. The carriers must cycle for the whole system to keep running.

Practice and Next Steps

Lock this in by predicting outcomes, not just labeling diagrams. Try answering: what happens to ATP production if a poison blocks the ETC? Why does CO₂ get released, and where does it come from? How is this process the reverse of photosynthesis? If you can explain those, you understand respiration the way the AP exam wants.

Put it to work with guided MCQ practice and build your free-response skills with FRQ practice that scores you instantly or the full FRQ question bank. Review the rest of Unit 3: Cellular Energetics, then connect respiration to organismal energy use in the Fitness and Natural Selection guide. When you're ready to gauge your readiness, take a full-length practice exam and check your projected score with the AP score calculator.