The Chemiosmotic Hypothesis: Proton Gradient and ATP Synthesis
How does a chloroplast actually make ATP? The answer is not a direct gift of light, but a quiet displacement of protons across the thylakoid membrane. The chemiosmotic hypothesis, advanced by Peter Mitchell in 1961, proposes that light merely builds a proton gradient, and the breakdown of that gradient does the work of synthesising ATP.
In this quiz you will test your grasp of:
- Where protons accumulate in photosynthesis (the lumen) versus respiration (the intermembrane space of mitochondria).
- The three processes that create the gradient: water splitting on the inner side, proton transport by an H carrier, and removal of protons from the stroma for NADP+ reduction.
- The two portions of ATP synthase: CF0 embedded in the membrane and CF1 protruding towards the stroma.
- The measurable pH decrease in the lumen and the conformational change in CF1 that drives ATP synthesis.
Work carefully through each question — the geography of the membrane matters as much as the chemistry.
Q1. The chemiosmotic hypothesis was put forward mainly to explain which process?
The chemiosmotic hypothesis explains the mechanism by which ATP is actually synthesised in the chloroplast, linking it to a proton gradient across the thylakoid membrane.
Q2. In photosynthesis, ATP synthesis is linked to the development of a proton gradient across which membrane?
In photosynthesis the relevant membranes are those of the thylakoid, across which the proton gradient develops.
Q3. In photosynthesis, protons accumulate towards the inside of the membrane, that is, in the:
Proton accumulation in photosynthesis is towards the inside of the membrane, i.e., in the lumen of the thylakoids.
Q4. In respiration, protons accumulate in which region when electrons move through the ETS?
In respiration, protons accumulate in the intermembrane space of the mitochondria when electrons move through the electron transport system.
Q5. Splitting of the water molecule takes place on which side of the thylakoid membrane?
Water splitting occurs on the inner side of the membrane, so the protons produced accumulate within the lumen of the thylakoids.
Q6. The primary acceptor of electrons is located towards which side of the membrane?
The primary acceptor of electrons is located towards the outer side of the membrane, where it transfers its electron to an H carrier.
Q7. What does the H carrier do while transporting an electron?
The H carrier removes a proton from the stroma while transporting an electron, and releases that proton into the lumen when it passes the electron on.
Q8. When the H carrier passes its electron to the electron carrier on the inner side of the membrane, the proton is released into the:
When the H carrier passes on its electron to the electron carrier on the inner side, the proton is released into the inner side, i.e., the lumen side of the membrane.
Q9. The NADP reductase enzyme is located on which side of the thylakoid membrane?
The NADP reductase enzyme is located on the stroma side of the membrane, where protons are removed from the stroma for the reduction of NADP+.
Q10. Which of the following is NOT one of the processes that creates the proton gradient across the thylakoid membrane?
Movement of protons through the ##\ce{CF0}## channel breaks the gradient down; it does not create it. The gradient is created by water splitting, the H carrier, and proton removal for NADP+ reduction.
Q11. The accumulation of protons in the lumen produces which measurable change there?
The proton gradient across the thylakoid membrane is accompanied by a measurable decrease in pH in the lumen.
Q12. The proton gradient is broken down by the movement of protons across the membrane to the stroma through the:
The gradient is broken down due to the movement of protons across the membrane to the stroma through the transmembrane channel of the ##\ce{CF0}## of the ATP synthase.
Q13. Which statement correctly describes the ##\ce{CF0}## portion of ATP synthase?
##\ce{CF0}## is embedded in the thylakoid membrane and forms a transmembrane channel that carries out facilitated diffusion of protons across the membrane.
Q14. The ##\ce{CF1}## portion of ATP synthase protrudes on the outer surface of the thylakoid membrane on the side that faces the:
##\ce{CF1}## protrudes on the outer surface of the thylakoid membrane on the side that faces the stroma.
Q15. What directly causes the ##\ce{CF1}## particle to synthesise ATP?
The breakdown of the gradient provides enough energy to cause a conformational change in the ##\ce{CF1}## particle, which makes the enzyme synthesise several molecules of ATP.