Cellular respiration end to end — glycolysis to fermentation

Cellular respiration is one of the topics most often tested on the AP Biology exam, but not by asking you to recite its steps: the course framework explicitly leaves the names of glycolysis steps, Krebs cycle intermediates and most enzymes out of the exam. What it does ask is whether you can follow energy and electrons through the pathway well enough to predict what happens when something is blocked, when oxygen runs out, or when a cell has no mitochondria at all. This material trains exactly that kind of prediction.

The questions are built around experiments. A poison stops electrons from reaching oxygen, and you work out what happens to the proton gradient. A drug blocks ATP synthase while the electron transport chain keeps running, and you predict the change in pH of the intermembrane space. Isolated mitochondria use oxygen slowly until ADP is added, and you explain why ATP synthesis and oxygen consumption are coupled through the proton gradient. You also compare the pH of the matrix with that of the intermembrane space, and state what oxygen does as the terminal electron acceptor and what changes for anaerobic prokaryotes that use other acceptors.

Fermentation gets several questions of its own, because it is where students most often go wrong. Yeast in a sealed flask and an aerated flask are compared for ethanol and carbon dioxide. Muscle cells short of oxygen switch to lactic acid fermentation, and you identify its real purpose: regenerating NAD⁺ so glycolysis can keep making ATP, not producing extra ATP. A mutant bacterium that cannot regenerate NAD⁺ shows what happens when that step fails, and mature red blood cells, which have no mitochondria, show how glycolysis plus lactate formation supplies all the ATP a cell needs.

Two questions address the principles in topic 3.3: why energy-releasing pathways run in many sequential steps rather than a single reaction, and what energy coupling means, from ATP hydrolysis driving an energy-requiring reaction to the proton gradient driving ATP synthesis.

The material offers a quiz and a flashcard deck. The quiz explanations trace the flow of electrons and protons for every prediction. The flashcards review the location of each stage, electron carriers, the terminal electron acceptor, chemiosmosis and ATP synthase, the direction of the proton gradient, energy coupling, the net ATP yield of glycolysis and the purpose of fermentation.

• Locate glycolysis, the Krebs cycle and the electron transport chain in the cell
• Predict the effects of blocking electron transfer to oxygen or blocking ATP synthase
• Explain how the proton gradient couples electron transport, oxygen use and ATP synthesis
• Explain the purpose and products of alcoholic and lactic acid fermentation
• Describe energy coupling and the reason energy pathways proceed in sequential steps

Practice material written by Zestly, based on the College Board AP Biology course framework (topics 3.3 Cellular energy and 3.5 Cellular respiration).