AP Syllabus focus:
‘Aerobic prokaryotes and eukaryotes both use electron transport chains, but differ in terminal electron acceptors and membrane locations.’
Cellular respiration is conserved across life, but prokaryotes and eukaryotes organise it differently. These differences affect where electron transport occurs, which molecules accept electrons at the end, and how ATP synthesis is coupled to membranes.
Core idea: same logic, different cellular architecture
Respiration transfers electrons from reduced carriers to an electron transport chain (ETC), using released energy to build a proton gradient that powers ATP synthase. What changes between domains is the membrane location of these proteins and, often, the terminal electron acceptor.
Electron transport chain (ETC): A series of membrane-associated proteins that pass electrons through redox reactions, using released energy to move protons and help generate a proton gradient.
In both cell types, the ETC is embedded in a membrane so that protons can be moved to one side, creating spatial separation required for chemiosmosis.
Where the ETC is located
Eukaryotes: mitochondria compartmentalise respiration
In eukaryotes, the ETC is on the inner mitochondrial membrane (cristae), which provides extensive surface area.
This diagram shows the mitochondrial electron transport chain embedded in the inner mitochondrial membrane, with electrons moving through complexes I–IV. It emphasizes proton pumping from the matrix into the intermembrane space and the reduction of O₂ to H₂O at the end of the chain. The figure helps connect membrane architecture to chemiosmosis and ATP production. Source
Protons are pumped from the matrix to the intermembrane space
ATP synthase faces the matrix, where ATP is produced
Membrane compartmentalisation:
helps maintain steep gradients
localises enzymes and substrates
reduces interference from cytosolic conditions
Because mitochondria are dedicated organelles, eukaryotic respiration is tightly linked to mitochondrial structure and transport of molecules across mitochondrial membranes.
This image shows the overall structure of a mitochondrion, including the outer membrane, the folded inner membrane (cristae), the intermembrane space, and the matrix. Seeing the compartments helps explain why proton pumping in eukaryotes creates a gradient specifically between the intermembrane space and matrix. It provides anatomical context for where oxidative phosphorylation proteins are situated. Source
Prokaryotes: plasma membrane (and sometimes internal membranes)
Prokaryotes lack mitochondria, so the ETC is located in the plasma membrane.
This schematic depicts an aerobic bacterial electron transport chain in the plasma membrane, showing electron flow through membrane complexes and proton extrusion to the outside of the cell. It highlights how the proton motive force builds across the bacterial membrane and then drives ATP synthase as protons flow back into the cytoplasm. The “outside vs. cytoplasm” labeling makes the prokaryote/eukaryote location contrast especially easy to visualize. Source
In many aerobic bacteria, infoldings or internal membrane systems increase effective area for ETC components.
Protons are pumped from the cytoplasm to the outside of the plasma membrane
in Gram-negative bacteria, this is often into the periplasmic space
ATP synthase typically allows protons to flow back into the cytoplasm, where ATP is made
Because the “outside” is part of the gradient, prokaryotic respiration is strongly influenced by environmental conditions (for example, external pH)
This membrane placement means prokaryotes can directly couple respiration to transport processes at the cell surface (for example, using proton gradients to drive uptake of nutrients).
Terminal electron acceptors: key difference across environments
At the end of the ETC, electrons must be transferred to a final acceptor so carriers can be re-oxidised and electron flow can continue.
Terminal electron acceptor: The final molecule that receives electrons from the electron transport chain, allowing continued electron flow and regeneration of oxidised electron carriers.
In aerobic respiration (common in many eukaryotes and many prokaryotes), the terminal electron acceptor is oxygen (O₂), which is reduced to water. However, many prokaryotes can respire without oxygen by using alternative acceptors.
A major consequence is ecological flexibility:
Eukaryotes: overwhelmingly depend on O₂ as the terminal electron acceptor for ETC-based respiration (mitochondrial respiration)
Aerobic prokaryotes: also use O₂, but many species can switch pathways depending on oxygen availability
Anaerobic prokaryotes (respiring): may use alternative acceptors, such as:
nitrate (NO₃⁻), producing nitrite or nitrogen gases
sulfate (SO₄²⁻), producing sulfide compounds
other oxidised inorganic molecules, depending on habitat
Using acceptors other than oxygen usually yields less energy per electron pair, but it allows survival where oxygen is limited.
What students should compare directly
Shared features (both domains)
ETCs are membrane-associated and use redox reactions
Electron flow supports a proton gradient
ATP synthase uses proton flow to synthesise ATP (chemiosmotic coupling)
Key contrasts (high yield)
Membrane location
Eukaryotes: inner mitochondrial membrane
Prokaryotes: plasma membrane (sometimes elaborated internally)
Terminal electron acceptors
Eukaryotes: primarily oxygen
Prokaryotes: oxygen or alternatives (depending on species and conditions)
FAQ
Many bacteria encode multiple terminal reductases and regulate them by oxygen sensing.
Human mitochondria are specialised for $O_2$ reduction and lack pathways for using nitrate or sulfate as terminal acceptors.
Gram-negative bacteria often build the gradient into the periplasm between inner and outer membranes.
Gram-positive bacteria pump protons to the exterior of a thick cell wall, so the gradient is across the plasma membrane to the outside environment.
No. Many bacteria use different dehydrogenases, quinones, and terminal oxidases/reductases.
The overall function is conserved (electron transfer + proton motive force), but the protein “modules” vary widely.
Alternative acceptors tend to have lower redox potential than $O_2$, so less free energy is released per electron transferred.
Less energy available generally means fewer protons pumped and a smaller ATP yield.
They regulate membrane permeability, ion transporters, and proton pumps to stabilise internal pH and membrane potential.
Some adjust lipid composition or use sodium-ion gradients as partial substitutes in extreme conditions.
Practice Questions
State two differences between aerobic respiration in prokaryotes and eukaryotes. (2 marks)
Prokaryotic ETC is on the plasma membrane; eukaryotic ETC is on the inner mitochondrial membrane (1)
Prokaryotes may use terminal electron acceptors other than oxygen; eukaryotes primarily use oxygen (1)
Explain how the location of the electron transport chain affects proton gradient formation in prokaryotes compared with eukaryotes, and relate this to terminal electron acceptors. (5 marks)
ETC is membrane-associated in both, enabling proton pumping and a gradient (1)
In eukaryotes, protons are pumped from matrix to intermembrane space across the inner mitochondrial membrane (1)
In prokaryotes, protons are pumped from cytoplasm to outside/periplasm across the plasma membrane (1)
ATP synthase uses proton flow back across the same membrane to make ATP in both (1)
Oxygen is the terminal acceptor in aerobic respiration; many prokaryotes can instead use alternative acceptors (e.g. nitrate) under low oxygen (1)
