Cellular respiration: how cells turn glucose into ATP.

What cellular respiration is

Cellular respiration is the set of enzyme-catalyzed reactions a cell uses to release the energy stored in glucose and other biological macromolecules, and to capture that energy as ATP built from ADP and inorganic phosphate. College Board adds a detail that students skip and questions do not: respiration and fermentation are characteristic of every form of life, not of animals alone.

If you want the short version, the cellular respiration AP Biology definition is this. It is the enzyme-catalyzed breakdown of glucose to make ATP, with oxygen as the terminal electron acceptor whenever oxygen is available.

The pathway is also very old, and College Board treats that as evidence rather than as trivia. It files glycolysis and oxidative phosphorylation under a learning objective about common ancestry, because those core pathways survive in all three domains of life: Archaea, Bacteria, Eukarya. College Board treats that common biochemistry as one more line of support, which is part of why the same course later asks you to work from a careful natural selection definition.

Cellular respiration is one of the AP Biology topics collected in our concepts hub.

What College Board does and does not ask you to know

This is the part most study guides leave out, and it changes how you should study the topic. The Course and Exam Description carries three separate exclusion statements on Topic 3.5 alone, and between them they rule out most of what a textbook chapter on respiration contains.

• You are not expected to name the individual carriers that make up the chain itself. CED p.84.
• The step-by-step detail of these pathways is out of scope, and so is every enzyme name and every intermediate compound along the way. CED p.84.
• Nobody is asking you to commit glycolysis or the Krebs cycle to memory step by step, with the molecular structures and enzyme names attached. College Board prints that exclusion twice, at CED pp.84 and 85.

Put it plainly: the cellular respiration steps themselves, reaction by reaction, are the part you are not being asked to hold in your head.

The same point turns up in plainer language in the Unit 3 teaching notes at CED p.72, where College Board draws a line between memorizing molecules and showing that you understand how molecular events connect to the function of a whole organism and to carbon moving through an ecosystem. The second thing is what gets tested.

So what is in scope? The same page is specific about it. Know the inputs and the outputs of each pathway. Be able to predict how a change in the reactants changes what the pathway does. Be able to explain what happens to an organism, and to an ecosystem, when one of these processes is disrupted. And expect to graph data from an experiment and calculate a reaction rate, with the units included in the answer.

The stages of cellular respiration, and where each one happens

Four stage names are in common use, and they are a reasonable way to hold the sequence in your head. They are not College Board’s structure. The CED splits the topic into two learning objectives instead, and it folds the oxidation of pyruvate into its description of the Krebs cycle rather than giving it a stage of its own. The sequence below is the textbook one, described the way College Board describes it.

The proton gradient, and what it does

As electrons move along the chain, protons are pushed across the inner mitochondrial membrane, so the proton concentration ends up high outside the membrane and low inside the matrix. One consequence is easy to state and easy to get backwards, which is exactly why it is worth learning: the pH inside the matrix is higher than the pH in the intermembrane space. The inner membrane is heavily folded, and that folding increases its surface area so more ATP can be made. Prokaryotes do the same thing across the plasma membrane instead.

Two more details come straight from College Board and almost nobody publishes them. Prokaryotes that live with oxygen use it at the end of the chain the way we do, and ones that live without it put a different molecule there instead. And when oxidative phosphorylation is decoupled from electron transport, the energy comes out as heat rather than as ATP, which is something endothermic organisms use to regulate their body temperature.

What happens when there is no oxygen

With no oxygen there is nothing to accept the electrons at the end of the chain, so the chain backs up and the Krebs cycle stops with it. Fermentation keeps glycolysis running by regenerating NAD⁺, and it produces organic molecules as it goes: alcohol in some organisms, lactic acid in others.

ATP: how much, and why College Board publishes no number

Textbooks converge on roughly 30 to 32 ATP per glucose from aerobic respiration, and older ones give 36 to 38. Both figures circulate widely and there is nothing wrong with carrying one of them.

What is wrong is calling either one an AP Biology fact. Search all 240 pages of the current Course and Exam Description and 30 to 32 is not there, 36 to 38 is not there, and the phrase ATP yield does not appear at all. College Board publishes no ATP total for cellular respiration anywhere in the document that defines the course.

Why the number is not a College Board figure

The exclusion statements above explain it. A precise ATP total is arithmetic carried out over the individual steps, and the individual steps are the thing the exam explicitly does not ask you to memorize. One widely read study site tells students that the current CED has settled on 30 to 32. It has not. There is no figure in the CED to settle on.

What the exam does expect is the direction and the reason behind it. Most of the ATP comes from oxidative phosphorylation rather than from glycolysis or the Krebs cycle. Take the oxygen away and you are left with what glycolysis alone can produce. That comparison is something a free-response question can genuinely ask you to make, and it does not need a total.

How cellular respiration is tested in AP Biology

Cellular respiration is Topic 3.5, the last of the five topics in Unit 3, Cellular Energetics. College Board weights the unit at 12 to 16 percent, and the qualifier matters. It prints the number twice, at CED p.24 and again at p.205, and both times it scopes it to the multiple-choice section. At 12 to 16 percent Unit 3 ties Unit 6 and sits behind Unit 7, so it is one of the heavier units rather than the heaviest.

The more useful fact is which skill College Board attaches to this particular topic. Each of Unit 3’s five topics carries one suggested science-practice skill, and Topic 3.5 is the only one that carries skill 4.A, construct a graph. Enzymes take 1.B and 3.C, environmental effects on enzymes take 6.E, cellular energy takes 6.C, photosynthesis takes 6.B. Respiration takes the graph.

Progress Check 3, and the two free-response questions behind it

The AP Classroom Progress Check for this unit is about 19 multiple-choice questions plus two free-response questions, a composition College Board gives on two agreeing pages of the CED. Both free-response questions are marked partial, and College Board explains the label itself: a partial version is a shortened form of a question, used to prepare students for the longer version they will meet on the exam.

Those two questions are the exam’s second and third free-response questions, and College Board publishes exactly how their points divide.

Part B of the second question is worth 4 points. Line up all 24 part values across the six free-response questions and no other part matches it. One graph, 4 points.

There is a wrinkle here worth stating rather than smoothing over. The exam-information pages put 4 points on Part B. A different page of the same document, in the teaching-approaches section, says that up to three points are available for building the graph correctly. Both are current and neither is obviously out of date, so the statement both support is this: Part B carries four points, and College Board’s own guidance on scoring graphs treats up to three of those points as riding on whether the graph itself is built right.

What earns the points on the graph

College Board publishes the list its readers work from. It sits in the teaching-approaches section rather than the exam section, which is probably why almost nobody quotes it.

• The type of graph has to suit the data being plotted.
• The x-axis carries the independent variable with its units and the y-axis carries the dependent variable with its units. A legend is needed whenever more than one condition appears on the same graph.
• Scaled intervals have to be uniform. A break in the scale gets marked in the axis and in the data line. If the readings do not begin at zero, the line should not be dragged back to the origin, and it should not be extended beyond the final reading unless a dashed line flags that stretch as a prediction.
• Data points have to be identifiable, and error bars are required whenever the data table gives the standard error of the mean.
• Trend lines belong on scatter plots and on any graph carrying more than one set of data.

One more thing from the same page. A graph title earns no point, and College Board still tells students to write one.

For how all six free-response questions are structured and scored, see our guide to AP Biology free response questions.

The three mistakes College Board names

Most guides list the mix-ups their author happens to have seen. College Board names three for this unit, in writing, at CED p.72, and they are not the ones you would guess.

• Thinking only animals carry out cellular respiration. The required content contradicts this directly: respiration and fermentation are characteristic of every form of life, and the core pathways are conserved across Archaea, Bacteria, and Eukarya. Plants respire. So do bacteria.
• Thinking of oxygen as something photosynthesis creates from carbon dioxide. College Board’s own required content says where the oxygen comes from: water splits, supplying electrons to replace the ones photosystem II has lost. The oxygen is a product of that split.
• Thinking only plants carry out photosynthesis. Algae and cyanobacteria do it too, and College Board credits cyanobacterial photosynthesis with producing the oxygenated atmosphere in the first place.

One more is worth adding, and it is ours rather than College Board’s. Oxygen is not the fuel. Glucose is the fuel. Oxygen is the terminal electron acceptor, the thing waiting at the end of the chain to take the electrons so that the chain can keep moving.