Why do the light-dependent reactions of photosynthesis take place in the thylakoid?
a. Photosystem I is anchored to the membrane, but not photosystem II.
b. The cytochrome complex requires a membrane for chemiosmosis to occur.
c. The light-dependent reactions depend on the presence of carbon dioxide.
d. Light energy is absorbed by the thylakoid membrane.
The correct answer and explanation is:
Correct Answer: d. Light energy is absorbed by the thylakoid membrane.
The light-dependent reactions of photosynthesis take place in the thylakoid membrane because it is specifically structured to capture and convert light energy into chemical energy. The thylakoid membrane contains pigment molecules such as chlorophyll, which are responsible for absorbing sunlight. These pigments are organized into protein complexes called photosystems, which are essential for initiating the photosynthetic process.
When light strikes the pigments in the thylakoid membrane, the energy excites electrons, raising them to higher energy levels. These high-energy electrons are then transferred through a series of proteins embedded in the thylakoid membrane known as the electron transport chain. As the electrons move through this chain, their energy is used to pump protons (hydrogen ions) from the stroma into the thylakoid lumen, creating a proton gradient.
This proton gradient is essential for chemiosmosis, which powers ATP synthase, an enzyme also located in the thylakoid membrane. As protons flow back into the stroma through ATP synthase, the energy released is used to produce ATP from ADP and inorganic phosphate. In addition to ATP, the light-dependent reactions also produce NADPH by transferring electrons to NADP+.
These energy-rich molecules, ATP and NADPH, are then used in the light-independent reactions (Calvin cycle) that occur in the stroma to fix carbon dioxide into glucose.
The thylakoid membrane’s structural organization and pigment arrangement make it an efficient site for capturing light energy and converting it into usable chemical energy. This is why the light-dependent reactions occur in the thylakoid and not elsewhere in the chloroplast. None of the other options accurately describe the key role of the thylakoid in absorbing light energy, which is why option d is correct.