Question

What is the processes that creates proton gradient across thylakoid membrane? (a) Splitting of water molecule on inner side of membrane (towards lumen). (b) \(\mathrm{H}^{+}\) carrier transport \(\mathrm{H}^{+}\) ion from stroma to lumen. (c) NADH reductase removes \(\mathrm{H}^{+}\) ion from stroma for reduction of NADP \(^{+}\). (d) All the above

Short answer

The correct answer is (d), all of the options create a proton gradient across the thylakoid membrane, with the exception that option (c) does not actively transport H+ ions into the lumen. Splitting of water molecules ((a) option) and H+ carrier transport ((b) option) result in the influx of H+ into the lumen, increasing the proton concentration. NADH reductase ((c) option) lowers the stroma's H+ concentration indirectly but does not directly affect the proton concentration inside the thylakoid.

Step-by-step solution

Analyze The Role of Water Splitting

Option (a) states that the splitting of water molecules on the inner side of the membrane adds H+ ions to the lumen. This process indeed takes place during photosynthesis: when light is absorbed, water molecules are split into hydrogen (H+) ions, electrons, and oxygen. The electrons fuel other steps in photosynthesis while H+ ions increase the proton concentration inside the membrane.

Examine The Role of Carrier Transport

Option (b) refers to the carrier-mediated transport of H+ ions from the stroma into the lumen. In the light-dependent reactions, certain proteins embedded in the thylakoid membrane work as proton pumps, using energy from electrons to transport H+ ions across the membrane against their concentration gradient. This further enhances the H+ amount inside the membrane.

Investigate The Role of Reductase Enzymes

Option (c) refers to the role of NADH reductase. This enzyme does indeed remove H+ ions from the stroma during the light-dependent photosynthesis reactions, reducing NADP+ to NADPH. This process, however, does not directly contribute to the proton gradient across the thylakoid membrane, as these removed H+ ions are not transported into the lumen.

Key concepts

Thylakoid Membrane

The thylakoid membrane is a vital component of chloroplasts, the sites of photosynthesis in plant cells. Its structure is complex, containing various embedded proteins that facilitate light-dependent reactions. To understand its role, imagine the thylakoid membrane as an energy conversion center, where sunlight is transformed into chemical energy.

Within the thylakoid membrane, several structures coexist which are essential for photosynthesis. First, there are photosystems – clusters of chlorophyll and other pigments that capture light energy. Additionally, the membrane houses electron transport chains and ATP synthase, both crucial for energy transfer and synthesis.

One key function of the thylakoid membrane is to provide a space for the accumulation of hydrogen ions. By housing the light-dependent reactions, it maintains an environment separate from the surrounding stroma, which helps establish the proton gradient critical for ATP production. Thus, the thylakoid membrane is not just a physical barrier; it's a hub for the intricate dance of particles energized by sunlight.

Light-dependent Reactions

The light-dependent reactions are the initial phase of photosynthesis, where light energy is harnessed to produce ATP and NADPH, two molecules that store energy for the cell's use. These reactions occur within the thylakoid membranes of the chloroplasts.

During these reactions, photolysis of water takes place, photosystems become excited by light energy, and electron transport chains generate ATP and NADPH. Here's how it happens step by step:

• Photons of light hit the photosystems, exciting electrons.
• These high-energy electrons are transferred along electron transport chains, which pump protons into the thylakoid lumen, creating a high concentration of H+ ions.
• The flow of protons back into the stroma through ATP synthase generates ATP from ADP and inorganic phosphate.
• Electrons ultimately reduce NADP+ to NADPH, which carries energy for use in the Calvin cycle.

Through these intricate processes, the light-dependent reactions convert solar energy into chemical energy while establishing a proton gradient essential for ATP synthesis.

Water Splitting

Water splitting is a critical process within the light-dependent reactions of photosynthesis and it is central to establishing the proton gradient across the thylakoid membrane. It occurs on the inner side of the membrane, towards the lumen.

When water molecules (H2O) are split, they undergo a reaction called photolysis, which is catalyzed by the enzyme complex known as the oxygen-evolving complex. This process results in three crucial products:

• Oxygen (O2), which is a byproduct released into the atmosphere.
• Electrons, which replenish the electrons lost by chlorophyll in photosystems I and II.
• Protons (H+), which contribute to the proton gradient across the thylakoid membrane.

The protons released into the lumen increase the proton concentration, intensifying the proton motive force. This force then drives the synthesis of ATP as protons flow through the ATP synthase enzyme. Thus, water splitting isn't just about oxygen production; it is pivotal to the energy transformation in photosynthesis through the creation of a proton gradient.