AP Bio Cellular Respiration: Explained Simply

Cellular respiration is how cells break down organic molecules like glucose to make ATP, the energy currency that powers cellular work. In AP Biology, you need to know its stages — glycolysis, pyruvate oxidation, the Krebs cycle, and oxidative phosphorylation — where each happens, and how energy moves from glucose to electron carriers to ATP. Most exam points come from explaining what happens when one step is disrupted, so focus on the connections, not just the names.

What is the overall equation for cellular respiration?

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (ATP and heat)

Notice it is essentially photosynthesis in reverse. Glucose is oxidized (loses electrons) and oxygen is reduced (gains electrons), ending up in water. The CO₂ you exhale comes from the carbon atoms of the food you broke down.

What are the stages of cellular respiration?

In prokaryotes, which lack mitochondria, glycolysis and the Krebs cycle occur in the cytoplasm and the electron transport chain is in the plasma membrane.

What happens in glycolysis?

Glycolysis splits one six-carbon glucose into two three-carbon pyruvate molecules.

• It occurs in the cytosol and does not require oxygen.
• It uses 2 ATP up front and makes 4, for a net gain of 2 ATP.
• It reduces 2 NAD⁺ to 2 NADH.

Because glycolysis is found in nearly all organisms and does not need oxygen or membrane-bound organelles, it is considered evidence of common ancestry and likely evolved early in life's history.

What happens in pyruvate oxidation and the Krebs cycle?

When oxygen is available, pyruvate enters the mitochondrial matrix.

Pyruvate oxidation: each pyruvate loses a carbon as CO₂ and becomes acetyl-CoA, producing NADH.

Krebs cycle: acetyl-CoA enters a cycle of reactions that fully oxidizes the remaining carbons to CO₂. Along the way, the cycle produces NADH, FADH₂, and a small amount of ATP.

The big idea: by the end of the Krebs cycle, the glucose carbons have all been released as CO₂, and most of the energy from glucose is now stored in the electron carriers NADH and FADH₂, not yet in ATP.

What happens in the electron transport chain?

This is where most ATP is made, and it is the most heavily tested part of the topic.

1. Electron delivery. NADH and FADH₂ drop off high-energy electrons at protein complexes embedded in the inner mitochondrial membrane.
2. Electron transport and proton pumping. As electrons move from complex to complex, they release energy that is used to pump H⁺ from the matrix into the intermembrane space, building a concentration gradient.
3. Oxygen as the final electron acceptor. At the end of the chain, electrons combine with O₂ and H⁺ to form water. Without oxygen, the chain backs up and stops.
4. Chemiosmosis. H⁺ ions flow back into the matrix through ATP synthase, which uses that energy to phosphorylate ADP into ATP.

The combination of electron transport and chemiosmosis is called oxidative phosphorylation.

Why the inner membrane is folded

The inner mitochondrial membrane folds into cristae, which increase surface area for electron transport chain proteins and ATP synthase. That is a classic structure-function answer.

What happens when there is no oxygen? Fermentation

Without oxygen, the electron transport chain stops, NADH cannot drop off its electrons, and the cell runs low on NAD⁺. Since glycolysis needs NAD⁺, it would stop too.

Fermentation solves this by regenerating NAD⁺ from NADH, allowing glycolysis to keep producing its 2 ATP per glucose.

The key AP point: the purpose of fermentation is regenerating NAD⁺, not making extra ATP. It yields far less ATP per glucose than aerobic respiration.

How do you predict what happens when respiration is disrupted?

Prediction questions are where strong students separate themselves. Trace the effect through the system.

The uncoupling example has a real biological use: in brown adipose tissue, certain proteins let H⁺ leak across the membrane, so energy is released as heat to help regulate body temperature. AP questions sometimes describe this context and ask you to explain it.

How do photosynthesis and cellular respiration connect?

The two processes form a cycle of matter and energy flow:

• Photosynthesis uses CO₂ and H₂O to make sugar and O₂.
• Cellular respiration uses sugar and O₂ to make CO₂, H₂O, and ATP.

Both rely on an electron transport chain that pumps H⁺ across a membrane and on ATP synthase powered by chemiosmosis. If you can explain one, you can explain the other. Read our guide to AP Bio photosynthesis to see the side-by-side comparison.

Tips for cellular respiration questions on the AP exam

1. Follow the electrons. Glucose to NADH and FADH₂ to the ETC to oxygen.
2. Follow the protons. Pumped into the intermembrane space, flow back through ATP synthase.
3. Name locations precisely — cytosol, matrix, inner membrane, intermembrane space.
4. Answer the "why." "Oxygen accepts electrons at the end of the chain, so without it, electron flow stops" beats "oxygen is needed."

For more on how to structure these answers, see our AP Bio FRQ guide, and if you are weighing the course, read is AP Bio hard?.

How Ivystar can help

Cellular respiration is dense, but it becomes manageable once you can trace electrons and protons through each stage. Ivystar tutors from schools like Harvard, Penn, and Stanford teach exactly that in our AP Exam Prep program, backed by a "Get an A or we work for free" class-grade guarantee. Estimate your score with the AP Biology score calculator, then book a free strategy call.