3.4 Photosynthesis

What else do you need to know about Photosynthesis?

The depth includes the light-dependent reactions (photosystems I and II, chlorophylls absorbing light, water splitting, ETC, proton gradient, and ATP synthase/chemiosmosis) and the Calvin cycle (using ATP and NADPH to fix carbon). Evolutionary context (prokaryotic origins of photosynthesis and oxygenation of the atmosphere) is also required. Crucially, you do NOT need to memorize the steps in the Calvin cycle, the structures of molecules, the names of enzymes (except ATP synthase), the full names of specific electron carriers in the ETC, or specific intermediates of the pathways.

Can you do these?

• I can explain that photosynthesis uses carbon dioxide, water, and light energy to make carbohydrates and oxygen, and describe its evolutionary origins in prokaryotes.
• I can identify the stroma and thylakoids within the chloroplast and describe their roles in the Calvin cycle and light reactions.
• I can describe how the light reactions capture energy to yield ATP and NADPH, which power the Calvin cycle.
• I can describe the role of the electron transport chain in chloroplasts and how electrons are transferred to NADP+ to form NADPH.
• I can explain how chlorophylls absorb light energy to boost electrons in photosystems I and II, and how water is split to replace electrons.
• I can describe how photosystems I and II are embedded in the thylakoid membrane and connected by an ETC.
• I can explain how the transfer of electrons through the ETC establishes an electrochemical gradient of protons across the thylakoid membrane.
• I can explain how the proton gradient drives the synthesis of ATP via ATP synthase through chemiosmosis (photophosphorylation).
• I can describe how the energy from ATP and NADPH is used in the Calvin cycle to produce carbohydrates from carbon dioxide in the stroma.

How is Photosynthesis tested on the exam?

This topic is paired with the science practice of Argumentation (Skill 6.B: Support a claim with evidence from biological principles, concepts, processes, and data). You may be asked to evaluate data or scenarios related to photosynthesis and construct arguments. Mark schemes will not reward or require the recall of specific Calvin cycle intermediates, enzyme names (other than ATP synthase), or full names of electron carriers. Focus should be on the conceptual flow of energy and matter, the establishment of the proton gradient, and the coupling of the light reactions to the Calvin cycle.