The depth of immersion of key components in the Photosystem 2 of pea plants was measured using a technique called ESR at very low temperatures. The researchers found that P680, pheophytin, and a secondary electron donor are located at different depths within the photosynthetic membrane, with values ranging from 2-4 A to 14-20 A. This information helps us understand how these components are arranged in the membrane during the process of photosynthesis.