Stage 1: Capturing Light Energy
In the first step of the light reactions, solar energy is absorbed by the chlorophyll (and acces-
sory pigments) within the chloroplast’s thylakoid membranes. This absorbed energy excites
electrons in the thylakoid membranes. The electrons are then transferred from the thylakoid
membranes by a series of electron carrier molecules. The series of electron carrier molecules
that transfers electrons is called the electron transport chain. During this process water
molecules in the thylakoid are split to replace the electrons that left the pigment, releasing
oxygen and adding hydrogen ions (H+) to the thylakoid space. As the thylakoid becomes a
reservoir for hydrogen ions, achemiosmotic gradientforms as there are more hydrogen
ions in the thylakoid than in the stroma. As H+ions flows from the high concentration in the
thylakoid to the low concentration in the stroma, they provide energy as they pass through
an enzyme called ATP synthase. ATP synthase uses the energy of the movement of H+
ions to make ATP. Meanwhile, highly energized electrons from the electron transport chain
combine with the electron carrier NADP+to become NADPH (Figure4.10). NADPH will
carry this energy in the electrons to the next phase of photosynthesis, the Calvin Cycle.
Figure 4.10: The light reactions includes the movement of electrons down the electric trans-
port chain, splitting water and releasing hydrogen ions into the thylakoid space. ( 5 )
Stage 2: Producing Food
DuringtheCalvinCycle,whichoccursinthestromaofthechloroplast,glucoseisformedfrom
carbon dioxide and the products of the light reactions. During the first step CO 2 is attached
to a 5-carbon molecule (called Ribulose-5-Phosphate, RuBP), forming a 6-carbon molecule.
ThisreactioniscatalyzedbyanenzymenamedRuBisCo, whichisthemostabundantprotein
in plants and maybe on Earth! The 6-carbon molecule formed by this reaction immediately
splits into two 3-carbon molecules, and the 3-carbon molecule is rearranged to a 3-carbon
carbohydrate. The energy and electrons needed for this process are provided by the ATP and