3.5 Cellular Respiration
- Syllabus
- 2025
- Topic
- 3.5
- Level
- —
Learning objectives
3.5A—Describe the processes and structural features of mitochondria that allow organisms to use energy stored in…Describe the processes and structural features of mitochondria that allow organisms to use energy stored in biological macromolecules.• Cellular respiration uses energy from biological macromolecules to synthesize ATP. Respiration and fermentation are characteristic of all forms of life.• Aerobic cellular respiration in eukaryotes involves a series of coordinated enzymecatalyzed reactions that capture energy from biological macromolecules.• The ETC transfers electrons in a series of oxidation-reduction reactions that establish an electrochemical gradient across membranes.- i. In cellular respiration, electrons delivered by NADH and FADH2 are passed to a series of electron acceptors as they move t oward the terminal electron acceptor, oxygen. Aerobic prokaryotes use oxygen as a terminal electron acceptor, while anaerobic prokaryotes use other molecules.- Exclusion: The full names of the specific electron carriers in the electron transport chain are beyond the scope of the AP Exam. Specific steps, names of enzymes, and intermediates of the pathways for these processes are beyond the scope of this course and the AP Exam.3.5B—Explain how cells obtain energy from biological macromolecules in order to power cellular functionsExplain how cells obtain energy from biological macromolecules in order to power cellular functions.• Glycolysis is a biochemical pathway that releases the energy in glucose molecules to form ATP (from ADP and inorganic phosphate), NADH (from NAD+), and pyruvate.• Pyruvate is transported from the cytosol to the mitochondrion where oxidation occurs. This process releases electrons during the Krebs (citric acid) cycle, reducing NAD+ to NADH and FAD to FADH2, and releasing CO2 . 77 Cellular Energetics UNIT 3• The Krebs cycle takes place in the mitochondrial matrix. During the Krebs cycle, carbon dioxide is released from organic intermediates, ATP is synthesized from ADP and inorganic phosphate, and electrons are transferred by the coenzymes NAD+ and FAD.• Electrons extracted in glycolysis and Krebs cycle reactions are transferred by NADH and FADH2 to the ETC in the inner mitochondrial membrane.• When electrons are transferred between molecules in a sequence of reactions as they pass through the ETC, an electrochemical gradient of protons (hydrogen ions) across the inner mitochondrial membrane is established. The pH inside the mitochondrial matrix is higher than in the intermembrane space.• Fermentation allows glycolysis to proceed in the absence of oxygen and produces organic molecules such as alcohol and lactic acid.• Exclusion: Memorization of the steps in glycolysis and the Krebs cycle, and of the structures of the molecules and the names of the enzymes involved, is beyond the scope of this course and the AP Exam.
Mitochondrial Structure Enables ATP Synthesis
Cellular respiration transfers energy from biological macromolecules into ATP through coordinated enzyme-catalyzed reactions. In eukaryotes, the mitochondrion's compartmentalized inner membrane makes electron transport and chemiosmosis possible.
| Mitochondrial feature | Functional contribution |
|---|---|
| Inner membrane and its folds | Holds the ETC and ATP synthase; folds increase surface area for ATP synthesis |
| Intermembrane space | Accumulates protons, creating higher proton concentration than in the matrix |
| Matrix | Lower proton concentration provides the other side of the electrochemical gradient |
NADH and FADH2 deliver electrons to the ETC. Redox transfers move electrons toward a terminal electron acceptor and are coupled to proton movement across the inner membrane. Protons then flow back through ATP synthase, driving ADP + inorganic phosphate → ATP. In aerobic respiration oxygen is the terminal acceptor; aerobic prokaryotes run this membrane process across the plasma membrane, while anaerobic prokaryotes may use other acceptors.
Electron flow does not drive ATP formation by direct contact with ATP synthase; it establishes the proton gradient that powers chemiosmosis. If oxidative phosphorylation is decoupled from electron transport, gradient energy can be released as heat. Specific ETC carrier names and pathway intermediates are outside AP scope.
How Cells Extract Energy from Glucose
Cells obtain usable energy from glucose in stages. Early reactions make some ATP directly and transfer high-energy electrons to NADH and FADH2; these carriers then supply electrons to the membrane ETC that supports most ATP synthesis in aerobic respiration.
| Stage and location | Main outcome |
|---|---|
| Glycolysis, cytosol | Glucose energy yields ATP, NADH, and pyruvate |
| Pyruvate oxidation and Krebs cycle, mitochondrial matrix | Carbon dioxide is released; ATP, NADH, and FADH2 are produced |
| ETC and oxidative phosphorylation, inner mitochondrial membrane | NADH/FADH2 electrons establish a proton gradient; ATP synthase uses chemiosmosis to make ATP |
Electron transfer through the ETC creates higher proton concentration in the intermembrane space and lower concentration in the matrix, so matrix pH is higher. Proton flow down this electrochemical gradient through ATP synthase couples electron-energy transfer to ATP production.
When oxygen is absent, fermentation allows glycolysis to continue and produces organic products such as alcohol or lactic acid; it does not replace the ETC with another high-yield ATP pathway. AP Biology requires the purposes, locations, and major products—not memorization of every glycolysis or Krebs-cycle step, intermediate, or enzyme.