Photosynthesis: Light
Reaction
PAGES 116-124
I. Light and Pigments
A. Light: Visible light includes the
colors of the rainbow
B. Pigments: Compounds that absorb
specific colors of light. Colors that
are absorbed are NOT visible.
ex. Markers and dyes have
pigments
I. Light and Pigments
C. Chloroplast Pigments
1. Chlorophyll a –
Pigment that absorbs
the blue and red light
energy of sunlight.
Located in thylakoid
membrane
Chloroplast
are green
because
they don’t
absorb
green!
I. Light and Pigments
C. Chloroplast Pigments
2. Chlorophyll b
Absorbs lots of blues and some reds
3. Carotenoids
Three pigments (Orange, Brown, and Yellow) that focus on absorbing blues and greens
These
colors are
seen in the
fall and in
vegetables
II. Light Reaction
1. First stage of photosynthesis
2. Main points:
• Absorb light energy
• Convert light energy to chemical energy: ATP and NADPH
• Splits H2O to Produce H+ and electrons (e-)
• Oxygen is released
3. GOAL: make ATP to power the Calvin Cycle
II. Light Reaction
4. Reaction: 6CO2 + 6H2O + light →C6H12O6 (sugar)+ 6O2
III. Steps of the Light Reaction
Step 1: Absorb Light
a. Clusters of pigments called photosystems
absorb light energy
b. Photosystems are located in the thylakoid
membrane
◦ Thylakoid Membrane: Flattened sacs within the
chloroplast. Stacks of thylakoids are called grana.
III. Steps of the Light Reaction
Step 2: Move down Electron Transport Chain
a. Light excites electrons from the photosystems and sends the
electrons down the electron transport chain (ETC)
b. ETC: made up of multiple proteins embedded in the thylakoid
membrane
III. Steps of the Light Reaction
Step 2: Move down Electron Transport
Chain
c. Water splits, which donates the
electrons needed to refresh the
photosystem.
d. This creates oxygen gas which diffuses
out of cell
e. Electrons moving down chain attach H+
to NADP+ to make NADPH, which will be
used later on in photosynthesis
III. Steps of the Light Reaction
Step 3: Chemiosmosis
a. A. Makes ATP, will be used later in photosynthesis
b. B. H+ moves through ATP synthase to make ATP
Light Reaction Summary
Reaction
Light
Reaction
Location
Thylakoid
Membrane
Reactants
Sunlight
H2O (Water)
Products
O2 (oxygen)
ATP
NADPH