3.5 Cellular Respiration

Cellular respiration is how cells break down biological macromolecules to make ATP, using glycolysis, the Krebs cycle, and the electron transport chain (ETC). The ETC builds a proton gradient across the inner mitochondrial membrane, and ATP synthase uses that gradient to power ATP synthesis through chemiosmosis. For AP Biology, keep the electron flow to proton gradient to ATP synthesis sequence clear.

Cellular Respiration Summary

Cellular respiration is the process cells use to capture energy from biological macromolecules and synthesize ATP. In eukaryotes, glycolysis happens in the cytoplasm, the Krebs cycle happens in the mitochondrial matrix, and the electron transport chain plus oxidative phosphorylation happen across the inner mitochondrial membrane.

For AP Biology, the most important logic is electron flow to proton gradient to ATP synthesis. NADH and FADH2 deliver electrons to the ETC, the ETC builds a proton gradient, and ATP synthase uses chemiosmosis to make ATP. Without oxygen, fermentation regenerates NAD+ so glycolysis can continue.

Why This Matters for the AP Biology Exam

This topic is part of Unit 3 (Cellular Energetics), one of the more heavily weighted parts of the AP Biology exam. You will see cellular respiration in both multiple-choice questions and free-response questions, often paired with photosynthesis so you can compare inputs, outputs, and the role of electron transport chains.

The strongest answers connect structure to function and trace energy and matter through each stage. Expect questions that ask you to predict what happens when something changes, such as removing oxygen, blocking part of the ETC, or shifting pH. You may also need to analyze data or diagrams and support a claim with evidence. You do not need to memorize the individual enzymes, intermediates, or named electron carriers, so focus on inputs, outputs, locations, and cause-and-effect logic.

Key Takeaways

• Cellular respiration and fermentation occur in all forms of life, and the core pathways are conserved across bacteria, archaea, and eukaryotes.
• The three stages are glycolysis (cytoplasm), the Krebs cycle (mitochondrial matrix), and the ETC with oxidative phosphorylation (inner mitochondrial membrane).
• The ETC moves electrons through redox reactions toward a terminal electron acceptor, building a proton gradient; oxygen is the terminal acceptor in aerobic respiration.
• ATP synthase makes ATP when protons flow back across the membrane by chemiosmosis, which is oxidative phosphorylation.
• Folded cristae increase surface area so more ATP can be synthesized; uncoupling the ETC from ATP synthesis releases energy as heat.
• Fermentation regenerates NAD+ so glycolysis can continue without oxygen, producing lactic acid or alcohol.

What Is Cellular Respiration?

Cellular respiration uses energy from biological macromolecules to synthesize ATP, the energy currency cells use to power their work. This process is characteristic of all forms of life, from the simplest bacteria to complex multicellular organisms.

Aerobic cellular respiration in eukaryotes involves a series of coordinated enzyme-catalyzed reactions through three main stages:

• Glycolysis: breaks glucose into pyruvate in the cytoplasm
• Krebs cycle (citric acid cycle): oxidizes the carbons from pyruvate and releases CO₂ in the mitochondrial matrix
• Electron transport chain (ETC): transfers electrons to generate ATP through oxidative phosphorylation

Mitochondrial Structure and Function

In eukaryotes, most of cellular respiration happens in the mitochondria. The structure of the mitochondrion directly supports its job.

• Inner mitochondrial membrane: holds the ETC proteins and ATP synthase. It folds into cristae, and that extra surface area allows more ATP to be synthesized.
• Mitochondrial matrix: the space enclosed by the inner membrane; site of the Krebs cycle.
• Intermembrane space: the region outside the inner membrane where protons build up during electron transport.

In prokaryotes, which lack mitochondria, the same proton-pumping process happens across the plasma membrane instead.

The Stages of Cellular Respiration

Stage 1: Glycolysis (Cytoplasm)

Glycolysis is a biochemical pathway that releases energy from glucose to form ATP (from ADP and inorganic phosphate), NADH (from NAD+), and pyruvate. It happens in the cytoplasm and does not require oxygen, which is why it is found across all domains of life.

• One glucose is split into two pyruvate molecules.
• NAD+ is reduced to NADH as it accepts electrons.
• The ATP made here is produced directly, without the ETC.

Stage 2: Pyruvate Transport and Oxidation

Pyruvate is transported from the cytosol into the mitochondrion, where oxidation occurs. This step:

• Releases CO₂
• Reduces NAD+ to NADH
• Connects glycolysis to the Krebs cycle

Stage 3: The Krebs Cycle (Citric Acid Cycle)

The Krebs cycle takes place in the mitochondrial matrix. During the cycle:

• Carbon dioxide is released from organic intermediates
• ATP is synthesized from ADP and inorganic phosphate
• Electrons are transferred to the coenzymes NAD+ and FAD, reducing them to NADH and FADH₂

Because each glucose produces two pyruvates, the Krebs cycle runs twice per glucose.

Stage 4: Electron Transport Chain and Oxidative Phosphorylation

Electrons extracted during glycolysis and the Krebs cycle are carried by NADH and FADH₂ to the ETC in the inner mitochondrial membrane.

How the ETC works:

• The ETC transfers electrons through a series of oxidation-reduction reactions.
• Electrons move toward the terminal electron acceptor, oxygen, in aerobic respiration. Aerobic prokaryotes also use oxygen, while anaerobic prokaryotes use other molecules as terminal acceptors.
• As electrons pass along the chain, protons (H+) are moved across the inner mitochondrial membrane.

Proton gradient:

• A region of high proton concentration forms in the intermembrane space, and a region of low proton concentration stays in the matrix.
• The pH inside the matrix is higher than in the intermembrane space.
• In prokaryotes, this proton movement happens across the plasma membrane.

ATP synthesis through chemiosmosis:

• Protons flow back through membrane-bound ATP synthase, and this flow drives the formation of ATP from ADP and inorganic phosphate.
• This is called oxidative phosphorylation in aerobic respiration.
• The folded cristae increase surface area, allowing more ATP to be made.

Heat generation:

• Decoupling oxidative phosphorylation from electron transport releases energy as heat.
• Endothermic organisms can use this heat to help regulate body temperature.

Fermentation: Energy Without Oxygen

Fermentation allows glycolysis to proceed in the absence of oxygen and produces organic molecules such as alcohol and lactic acid. Its key role is to regenerate NAD+ from NADH so glycolysis can keep running and keep producing ATP.

Lactic Acid Fermentation

• Pyruvate is converted using NADH, regenerating NAD+
• Occurs in some cells when oxygen is limited

Alcoholic Fermentation

• Used by yeast and some bacteria
• Produces ethanol and CO₂ while regenerating NAD+

Respiration and fermentation are fundamental energy-harvesting processes found across all forms of life. Different organisms rely on different pathways depending on their structures and environmental conditions.

Comparing Energy Pathways

Aerobic cellular respiration releases far more usable energy per glucose than fermentation because the ETC and oxidative phosphorylation extract energy from the electrons carried by NADH and FADH₂.

The exact ATP yield can vary by organism and conditions, so focus on the idea that aerobic respiration produces much more ATP than fermentation rather than on a single fixed number.

Connections Worth Knowing

These points are useful context, not extra facts you need to memorize.

• Core metabolic pathways like glycolysis and oxidative phosphorylation are conserved across all domains of life, which supports the idea of common ancestry.
• Photosynthesis and cellular respiration work together in ecosystems: photosynthesis stores energy in carbohydrates and releases O₂, while respiration uses O₂ and releases CO₂. Both rely on electron transport chains and ATP synthase.

How to Use This on the AP Biology Exam

Multiple Choice

• Match each stage to its location: glycolysis in the cytoplasm, Krebs cycle in the matrix, ETC in the inner membrane.
• Track inputs and outputs. Know which steps make ATP, which reduce NAD+ or FAD, and where CO₂ is released.
• Identify oxygen as the terminal electron acceptor in aerobic respiration and recognize that other molecules fill that role in anaerobic prokaryotes.

Written Responses

• Use precise cause-and-effect language. For example, electrons moving through the ETC pump protons, the protons build a gradient, and proton flow through ATP synthase drives ATP synthesis.
• When a question changes a condition, predict the downstream effect. If oxygen is removed, the ETC backs up, NADH cannot be reoxidized, and the cell shifts toward fermentation to regenerate NAD+.
• Support claims with evidence and reasoning that connects structure to function, such as cristae increasing surface area for ATP synthesis.

Data and Diagrams

• Be ready to read mitochondrial diagrams and label the matrix, inner membrane, intermembrane space, and where the proton gradient forms.
• If you calculate a rate from experimental data, show your work and include units in your final answer.

Common Trap

• Do not waste time memorizing enzyme names, intermediates, or specific electron carriers; those are outside what the exam expects. Focus on inputs, outputs, locations, and the logic of the proton gradient.

Common Misconceptions

• Only animals carry out cellular respiration. Plants, fungi, and many microbes all respire. Plants do both photosynthesis and respiration.
• Fermentation produces a lot of ATP. Fermentation does not make ATP by itself; its job is to regenerate NAD+ so glycolysis can keep producing a small amount of ATP without oxygen.
• Oxygen is used throughout respiration. Oxygen acts at the end of the ETC as the terminal electron acceptor. Glycolysis and the Krebs cycle do not use oxygen directly.
• Protons build up in the matrix. Protons accumulate in the intermembrane space, making the matrix less acidic, so matrix pH is higher than in the intermembrane space.
• The ETC directly makes ATP. The ETC builds the proton gradient; ATP synthase makes the ATP as protons flow back across the membrane.
• Mitochondria and chloroplasts do the same job. Respiration releases stored energy as ATP, while photosynthesis captures and stores energy in carbohydrates.

Related AP Biology Guides

• Unit 3 Overview: Cellular Energetics
• 3.1 Enzymes
• 3.2 Environmental Impacts on Enzyme Function
• Cellular Respiration Review
• 3.4 Photosynthesis
• 3.3 Cellular Energy