Photosynthesis: Light-Dependent Reactions

Chlorophyll pigments associated with thylakoid membranes absorb light. Energy can excite electrons that leave chlorophyll, a process described as photoionisation. In the light-dependent pathway, these electrons enter electron-transfer systems rather than becoming part of a sugar molecule directly.

Photolysis splits water and provides electrons to replace those lost from the initial light-activated chlorophyll system. It also produces protons and oxygen. The oxygen released during photosynthesis therefore comes from water, not from splitting carbon dioxide. Accessory pigments broaden the range of wavelengths that can contribute to light absorption.

As electrons pass through membrane-associated carriers, some transferred energy supports proton movement from the stroma into the thylakoid lumen. Photolysis also contributes protons within the lumen. The thylakoid membrane restricts free proton movement, helping maintain an electrochemical gradient.

Protons return towards the stroma through ATP synthase, coupling their movement down the gradient to ATP synthesis from ADP and inorganic phosphate. This is chemiosmosis and the light-driven ATP formation is photophosphorylation. ATP is formed on the stromal side for use in reactions such as the Calvin cycle, not by electrons flowing through a DNA molecule.

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Further light excitation enables electrons to contribute to reduction of NADP, producing reduced NADP, often written NADPH. It carries reducing power needed to reduce carbon compounds in the light-independent reactions. ATP and reduced NADP have different roles and should not be treated as interchangeable products.

The membrane system requires suitable light, electron supply and available acceptors. A redox dye in RP8 can report electron-transfer activity under controlled conditions, but a dye’s colour change is not a direct measurement of total sugar production. Distinguish the specific process measured from the whole photosynthetic pathway.

Try each question before opening the worked response. Use precise biological terms, connect each cause to its effect, and check units and assumptions when interpreting data.