AP Syllabus focus:
‘Cellular respiration uses energy in biological macromolecules to synthesize ATP; respiration and fermentation occur in all life forms.’
Cellular respiration is the core set of energy-harvesting reactions that convert chemical energy stored in macromolecules into usable cellular work. AP Biology emphasizes what it produces (ATP) and how broadly it is shared across life.
Purpose and big-picture outcomes
What cellular respiration does
Cellular respiration is a collection of enzyme-mediated reactions that extract energy from biological macromolecules (especially carbohydrates and fats) and transfer that energy into ATP while releasing lower-energy products.
Cellular respiration: A set of metabolic pathways that oxidise organic molecules and capture released energy to synthesise ATP, often using an electron transport system.
Cells use respiration to:
Maintain ATP supply for cellular work (transport, motion, biosynthesis)
Convert energy into forms that can be coupled to endergonic processes
Manage electrons by transferring them to electron acceptors (aerobic or anaerobic contexts)
ATP as the immediate energy currency
ATP stores transferable energy in phosphate bonds; cells continuously recycle ATP from ADP + Pi because ATP demand is constant and local.
ATP (adenosine triphosphate): A nucleotide that provides immediate energy for cellular processes via phosphate group transfer and hydrolysis.
Core principles behind ATP production
Energy extraction is largely redox chemistry
Respiration captures energy through oxidation–reduction (redox) reactions:
Organic molecules become oxidised (lose electrons; often lose H as well)
Electron carriers (commonly NAD⁺ and FAD) become reduced, temporarily storing high-energy electrons
Electrons ultimately flow to a final electron acceptor, allowing pathways to continue
This electron flow releases energy in controllable steps, enabling ATP synthesis instead of losing most energy as heat at once.
Two general ways cells make ATP during energy harvesting
Cells make ATP by:
The electron transport chain transfers electrons through membrane complexes, using released free energy to pump protons (H+) and build an electrochemical gradient. ATP synthase then harnesses proton flow down that gradient to convert ADP + Pi into ATP—this is chemiosmotic (oxidative) phosphorylation. Source
Substrate-level phosphorylation: an enzyme transfers a phosphate from a metabolite to ADP
Chemiosmotic phosphorylation (via an electron transport system): energy from electron transfer helps power ATP formation
At the overview level, AP Biology expects you to connect macromolecule oxidation → electron transfer → ATP synthesis, not memorise every intermediate.
Respiration and fermentation across life
Universality and flexibility
The syllabus stresses that respiration and fermentation occur in all life forms, reflecting shared metabolic solutions:
Many organisms can respire when suitable electron acceptors are available
When a usable external electron acceptor is unavailable, cells may rely on fermentation to keep ATP production going at a lower yield
Fermentation as a survival strategy (when oxygen is absent)
Fermentation is an anaerobic strategy that supports continued ATP production by maintaining electron carrier balance:
Glycolysis reduces NAD+ to NADH while producing a small amount of ATP; fermentation restores NAD+ by transferring electrons from NADH to an organic acceptor. In lactic acid fermentation, pyruvate is reduced to lactate, regenerating NAD+ and allowing glycolysis (and limited ATP production) to continue without oxygen. Source
Regenerates oxidised electron carriers (e.g., NAD⁺) so energy-harvesting reactions can continue
This schematic summarizes lactic acid fermentation as an electron-carrier recycling pathway: NADH donates electrons to reduce pyruvate, forming lactate while regenerating NAD+. By restoring NAD+, the cell keeps glycolysis running to make ATP via substrate-level phosphorylation when the electron transport chain cannot operate. Source
Produces organic end products (the specific products vary by organism)
Makes less ATP per fuel molecule than respiration, but sustains life in low-oxygen conditions
FAQ
Fats are more reduced (more C–H bonds), so their oxidation releases more high-energy electrons for carriers, typically increasing potential ATP yield per molecule.
A strong acceptor has high electron affinity, allowing a large drop in free energy as electrons flow to it, which can be harnessed to drive ATP synthesis.
ATP is chemically reactive and not efficient for long-term storage. Cells store energy mainly as glycogen or fats and regenerate ATP rapidly as needed.
Fermentation regenerates NAD$^+$ but captures less energy per fuel molecule, so cells must consume substrate faster to meet ATP demands, often constraining biosynthesis.
Key control points respond to ADP/ATP and NADH/NAD$^+$ ratios, shifting pathway flux so ATP synthesis increases when energy demand rises and slows when demand falls.
Practice Questions
State two key outcomes of cellular respiration and name one type of biological macromolecule that can supply energy for ATP synthesis. (3 marks)
Any two outcomes (1 mark each): ATP is synthesised; energy is released from organic molecules; electrons are transferred to acceptors; lower-energy products are formed.
One macromolecule named (1 mark): carbohydrate (e.g., glucose) or lipid/fat (protein accept if justified).
Explain how cells capture energy from organic molecules to synthesise ATP, referring to redox reactions, electron carriers, and why fermentation may be used. (6 marks)
Organic molecules are oxidised to release energy/electrons (1).
Redox reactions transfer electrons to carriers such as NAD / FAD (1).
Carriers are reduced and later re-oxidised, enabling continued electron flow (1).
Energy from electron transfer is used to drive ATP synthesis (accept substrate-level or chemiosmotic description) (1).
Respiration typically uses an external terminal electron acceptor (oxygen may be named) (1).
Fermentation is used when no suitable external acceptor is available to regenerate NAD and allow ATP production to continue (1).
