1. 8.2 Photosynthesis: An Overview
Photosynthesis – process by which plants make food using energy from the sun
Plants are autotrophs that make their own source of chemical energy.
Chemical equation of photosynthesis:
Occurs in chloroplasts
6CO2 + 6H2O  C6H12O6 + 6O2
sun
CO2 O2
ATP
sugars
H2O
enzymes

2. Chloroplast structure:
8.2 Photosynthesis: An Overview
Chloroplast structure:
Double membrane structure
Light-dependent reactions take place
in the membrane of the thylakoids
(stacks of thylakoids = grana)
Light-independent reactions take
place in stroma (fluid interior of
chloroplast)
chloroplast
stroma
grana (thylakoids)
chloroplast
leaf cell
leaf

3. Plant pigments in chloroplasts absorb sunlight.
8.2 Photosynthesis: An Overview
Plant pigments in chloroplasts absorb sunlight.
Absorb light at different wavelengths (what you don’t see)
Reflect what is not absorbed (what you see).

4. Plant pigments absorb sunlight.
8.2 Photosynthesis: An Overview
Plant pigments absorb sunlight.
Main pigment
– chlorophyll a
– absorb at blue and red region
and reflect green
Accessory pigments
– chlorophyll b, carotenoid,
xanthophyll, carotene
– absorb closer to green region
allowing plant to use light
energy across spectrum

5. Photosynthesis occurs in two stages.
8.2 Photosynthesis: An Overview
Photosynthesis occurs in two stages.
sun
ENERGY building reactions
(Light Rxn)
ADP
NADP
ATP
NADPH
SUGAR building reactions
(Dark Rxn)
used immediately to synthesize sugars
sugar
H2O
CO2

6. Photosynthesis occurs in two stages.
8.2 Photosynthesis: An Overview
Photosynthesis occurs in two stages.
Light-dependent reaction
- requires sunlight (aka “light reaction”)
- only runs during daytime
- energy-building reaction (making ATP & NADPH)
- occurs in thylakoid of chloroplast
Light-independent reaction
- does not require sunlight (aka “dark reaction” or
“Calvin Cycle”)
- runs all day
- sugar-building reaction (making glucose)
- occurs in stroma of chloroplast

7. Light-Dependent Reactions
8.3 The Reactions of Photosynthesis
Light-Dependent Reactions
First set of reactions – depend on sun for energy
Occur in membrane of thylakoids
Produces ATP and NADPH – energy carrier molecules which will be used to run Light Independent Reactions

8. Light-Dependent Reactions
8.3 The Reactions of Photosynthesis
Light-Dependent Reactions
The light-dependent reactions include groups of molecules called photosystems.
Photosystem II absorbs sunlight at 680 nm (P680).
Photosystem I absorbs sunlight at 700 nm (P700).

9. 8.3 The Reactions of Photosynthesis
Photosystem II captures and transfers energy.
PS II absorbs sunlight at 680 nm.
H2O molecules are split; O2 is released; energized e- enter ETC.
H+ ions are transported down ETC and pumped across thylakoid membrane
PS II

10. 8.3 The Reactions of Photosynthesis
Photosystem I captures energy and produces energy-carrying molecules.
PS I absorbs sunlight at nm
energized electrons are used to make NADPH
hydrogen ions flow through ATP synthase in the thylakoid membrane
ATP synthase makes ATP from ADP
NADPH and ATP are sent to stroma to run Light-Independent Reaction
PS I

11. Summary of Light-Dependent Reaction
8.3 The Reactions of Photosynthesis
Summary of Light-Dependent Reaction

12. Light-Independent Reactions
8.3 The Reactions of Photosynthesis
Light-Independent Reactions
Second set of reactions
Do not depend on sunlight
Occur in stroma
Use CO2 as source of carbon to build glucose via
Calvin Cycle

13. Light-Independent Reactions
8.3 The Reactions of Photosynthesis
Light-Independent Reactions
The Calvin Cycle (aka “Carbon Fixation”)
Carbon from CO2 enter the cycle and add to 5-C molecule to form 6-C molecules.
ATP and NADPH from Light-Dependent Reactions are used to split the 6-C molecules to 3-C molecules.
3-C molecules are rearranged to form high-energy 3-C molecules. Two 3-C
molecules form
one 6-C glucose.
3-C molecules are
changed back to
5-C molecules to
continue the cycle. 1 2 3 4
SIX CO2 make
ONE glucose!!