Question 36 — Biochemistry (25 marks) Answer parts (a), (b) and (c) of the question on pages 2–4 of the Section II Writing Booklet - HSC - SSCE Biology - Question 36 - 2015 - Paper 1

Photosystem I and photosystem II operate collaboratively to enhance the efficiency of photosynthesis. Photosystem II absorbs light energy first, exciting electrons and initiating the process of photolysis, where water is split to release oxygen and provide electrons. These high-energy electrons are then transferred to Photosystem I, which re-excites them using additional photons of light. This sequence allows for a more efficient transfer of energy, maximizing the capture of light across different wavelengths and minimizing the energy lost during the electron transport chain.

Blackman and Matgel observed that the rate of photosynthesis depended on light intensity, which led them to conclude that light is necessary for the initial steps of photosynthesis. Their experiments showed that at low light levels, the light-dependent reactions were the limiting factor, indicating that these reactions must occur first before the subsequent temperature-dependent reactions can take place. Thus, they hypothesised that photosynthesis must commence with light-dependent processes that capture and convert light energy.

The light reactions of photosynthesis occur in the thylakoid membranes of the chloroplast. These membranes contain chlorophyll, which captures light energy. In contrast, the dark reactions (Calvin cycle) take place in the stroma of the chloroplast, the fluid-filled space surrounding the thylakoids. The stroma is where ATP and NADPH produced in the light reactions are utilized to fix carbon dioxide into glucose. This separation allows for optimized environments for each part of the photosynthetic process.