AP Bio Study Guide

Big Idea 2.A

How does photosynthesis work?

It may help to look at this diagram of a chloroplast to visually make sense of the processes below.

Light-dependent reactions:

Electrons in Photosystem II (located in the thylakoid membrane) are energized by light. Two energized electrons are passed to the primary electron acceptor. These electrons then travel through the electron transport chain, losing energy that is used to phosphorylate an average of 1.5 ATP molecules. The electron transport chain ends with Photosystem I, where electrons are again energized and passed to a different primary electron acceptor. The two electrons pass through a short electron transport chain, and at the end combine with NADP+ and H+ (NADP+ is reduced) to form NADPH (which carries H+ to the Calvin cycle).

The energy lost during the electron transport chain is used to pump protons (H+) into the thylakoid space. These protons then flow out to the stroma with the gradient, through the channel protein ATP synthase. Energy generated by the passage of protons provides the energy for the ATP synthase to phosphorylate ADP to ATP.

Calvin Cycle:

The energy captured in the light reactions as ATP and NADPH powers the production of glucose, which takes place in the stroma. The Calvin cycle uses CO2, ATP, and NADPH to produce ADP, NADP+, and glucose.

You do not need to memorize the steps of the Calvin cycle, the structure of the molecules, or the names of enzymes (except ATP synthase).