What are the light-harvesting units located in the thylakoid membranes called?

The light-harvesting units located in the thylakoid membranes are known as photosystems. Photosystems are complexes of proteins and pigments, primarily chlorophyll, that play a critical role in capturing light energy during the process of photosynthesis. They comprise two main components: Photosystem I and Photosystem II, each with distinct functions in the light-dependent reactions of photosynthesis.

In these structures, chlorophyll molecules absorb specific wavelengths of light, initiating the process of converting light energy into chemical energy. This energy is then used to generate ATP and NADPH, which are essential for the subsequent stages of photosynthesis, including the Calvin cycle.

Chlorophylls, while essential pigments in the light-harvesting process, do not represent the entire complex necessary for harnessing light energy. Protons are involved in the electron transport chain and ATP synthesis but do not capture light. Ribosomes are cellular structures involved in protein synthesis and are not related to light harvesting in photosynthesis. Thus, photosystems encompass the collective structure and function of the light-harvesting units within the thylakoid membranes, making them the correct choice in this context.