Top: schematic representation of the sea slug E. timida (left), showing the cells (center) that contain the functional chloroplasts (right) stolen from its food source Acetabularia acetabulum. Bottom: schematic of the photosynthetic transfer chain in the thylakoid (Th) membrane of kleptoplasts in E. timida sea slugs. From the left: the P680 chlorophyll in photosystem II (PSII) absorbs light and transfers an electron to plastoquinone (PQ), which, through a series of steps, passes the electron to the P700 chlorophyll in photosystem I (PSI). P680 regains its electron from the splitting of two molecules of water (H2O) into four protons (H+) and one oxygen (O2) molecule. Photosystem I absorbs light and transfers an electron to ferredoxin (Fd) and its accompanying enzyme (ferredoxin-NADP reductase, FNR), which uses the electrons to convert NADP+ to NADPH. This sequence of reactions creates a build-up of protons inside the thylakoids that can be transported out into the stroma of the chloroplasts by an enzyme called ATP synthase (ATPs), releasing the energy needed to make ATP, a molecule that fuels chemical reactions in cells. If the photosystems become overloaded with photons, this leads to the production of molecules that can harm the sea slug’s cells. Kleptoplasts are protected from this damage by maintaining plastoquinone oxidized in the dark (so it is able to accept more electrons when photosystem II absorbs light), and by donating the excess electrons from PSI to alternative molecules, such as flavodiiron proteins (FLV). The other molecules shown in the figure are cytochrome b6/f (cyt), plastocyanin (PC), adenosine diphosphate (ADP) and inorganic phosphate (Pi).