Light Dependent Reactions
Recall – Chloroplast Structure
• Step 1: Photosystem II absorbs light,
and the energy is given to electrons.
– The electrons come from water – H20 is
decomposed by enzymes into electrons,
hydrogen ions, and oxygen. The e- are
energized by light , the H+ ions are
released into the thylakoid membrane,
and the O2 is released as waste.
Reactions of Photosynthesis
Refer to Text Section 8.3
(particularly Figures 8-10 & 8-11)
• Step 2: High-energy e- are passed
along a number of carriers to
photosystem I. The energy release
from these e- is used to pump H+ from
the stroma into the thylakoid space.
Light Dependent Reactions, cont’d
Light Dependent Reactions, cont’d
• Step 3: Photosystem I pigments reenergize the e- and attaches these
along with H+ ions from inside the
thylakoid to NADP+ to make NADPH.
• Step 5: H+ ions cannot cross the
membrane directly, but they can
pass through a membrane-bound
enzyme called ATP synthase. The
enzyme spins like a turbine and in
the process adds a phosphate to
ADP to create ATP.
• Step 4: The inside of the thylakoid
continues to fill with H+ ions, creating
a high concentration and a positive
charge. There is a low concentration
and a negative charge outside the
thylakoid in the stroma.
Light Independent Reactions
• Step 1: Six CO2 molecules react
with six 5-carbon molecules to
form twelve 3-carbon molecules.
• Step 2: These twelve molecules
are converted to higher energy
forms using ATP and high energy
e- from NADPH.
Photosynthesis Overview
Light Independent Reactions, cont’d
• Step 3: Two of the 3-carbon
molecules are removed from the
cycle and form a 6-carbon
molecule – a sugar.
• Step 4: The remaining ten 3carbon molecules are formed
into six 5-carbon molecules and
reenter the cycle in step 1.
• converts light energy into chemical energy
• carbon dioxide and water combine to form sugars and
oxygen
• sugars (sometimes then made into starch) are used as energy
storage for the plant, oxygen is released as a waste product
6 H2O + 6 CO2
C6H12O6 + 6 O2
• In photosynthesis, TWO major sets of chemical reactions
occur:
a) Light Dependent Reactions
– water molecules are split and the oxygen is released
– energy from the sun is trapped and channeled into
forming high energy compounds: ATP and NADPH
b) Light Independent Reactions (AKA Calvin Cycle)
- carbon dioxide is assembled into sugar using high
energy compounds from the light dependent reactions
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