Question

A larger proton gradient is required to form a single ATP in chloroplasts than in mitochondria. Suggest a reason why. Hint: Ions can move across the thylakoid membrane more easily than across the inner mitochondrial membrane.

Short answer

Due to greater ion permeability of the thylakoid membrane, a larger proton gradient is needed in chloroplasts to produce ATP efficiently.

Step-by-step solution

Identify the Role of the Proton Gradient

Both the chloroplasts (in plants) and mitochondria (in plants and animals) generate ATP using a proton gradient. This gradient is necessary to drive ATP synthase, the enzyme that synthesizes ATP from ADP and inorganic phosphate.

Compare Membrane Permeabilities

The thylakoid membrane in chloroplasts is more permeable to ions compared to the inner mitochondrial membrane. This implies that protons can leak across the thylakoid membrane more easily than the inner mitochondrial membrane.

Consider the Implications of Membrane Permeability

Due to this increased permeability, a larger proton gradient is required across the thylakoid membrane to achieve the same driving force for ATP synthesis as compared to the mitochondria. This compensates for the loss of protons due to the leakage.

Conclusion

A larger proton gradient in chloroplasts is necessary to maintain the same efficiency of ATP production as the mitochondria, given the differences in membrane permeability.

Key concepts

Chloroplasts ATP synthesis

In chloroplasts, ATP synthesis occurs during the process of photosynthesis. Chloroplasts use light energy to drive the production of ATP, which is essential for carbon fixation in the Calvin cycle.
ATP synthesis in chloroplasts involves the creation of a proton gradient across the thylakoid membrane. Light energy is absorbed by chlorophyll and other pigments, which leads to the splitting of water molecules.
This process releases oxygen, electrons, and protons. The protons are pumped into the thylakoid lumen, creating a high concentration inside and a low concentration in the stroma.
ATP synthase, located in the thylakoid membrane, uses this proton gradient to convert ADP and inorganic phosphate (Pi) into ATP. This enzyme operates like a molecular turbine, driven by the flow of protons from the thylakoid lumen back into the stroma.

Mitochondria proton gradient

Mitochondria are the powerhouses of the cell, generating ATP through oxidative phosphorylation. This process is part of cellular respiration and takes place in the inner mitochondrial membrane.
Electrons are transferred through a series of protein complexes in the electron transport chain (ETC). As electrons move through the ETC, protons are pumped from the mitochondrial matrix to the intermembrane space, creating a proton gradient.
This gradient is a form of potential energy, with a high concentration of protons in the intermembrane space and a low concentration in the matrix.
ATP synthase, embedded in the inner mitochondrial membrane, harnesses the energy of the returning protons to synthesize ATP from ADP and Pi. The tight inner membrane prevents protons from leaking, maintaining a strong gradient for efficient ATP production.

Membrane permeability

Membrane permeability is a crucial factor in the efficiency of ATP synthesis. The thylakoid membrane in chloroplasts is more permeable to ions, especially protons, compared to the inner mitochondrial membrane.
This permeability affects the sustainability of the proton gradient. In chloroplasts, protons can easily diffuse back across the thylakoid membrane, reducing the gradient.
This means chloroplasts need a larger proton gradient to compensate for the proton leakage and to ensure sufficient driving force for ATP synthase operation.
In contrast, the inner mitochondrial membrane is less permeable, allowing a more stable and efficient proton gradient. This low permeability ensures that most protons contribute to the electrochemical gradient, enhancing ATP synthesis.

Thylakoid membrane

The thylakoid membrane, found in chloroplasts, plays a vital role in photosynthesis and ATP production. It forms flattened sac-like structures, stacked in grana, where light-dependent reactions occur.
The membrane is embedded with proteins and pigments, including photosystem II, photosystem I, and ATP synthase. Photosystems capture light energy to drive the production of ATP and NADPH.
Protons are pumped into the thylakoid lumen during these light reactions, creating a proton gradient. However, due to the higher permeability of the thylakoid membrane, this gradient is less stable.
This means chloroplasts must generate a larger proton gradient compared to mitochondria to maintain effective ATP synthesis. This compensates for the ease with which protons can leak back across the membrane.