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1. Chapter 10
Photosynthesis

2. Photosynthesis
“Synthesis from light”
6CO2 + 6 H 2O → C6 H12O6 + 6O2
Plants take in CO2, produce
carbohydrates, and release water and O 2
Light is required

3. Photosynthesis
Stomata

4. Two parts to Photosynthesis
Light reactions:
Convert light energy
to chemical energy
as ATP and NADPH
Light-independent
reactions: Use ATP
and NADPH (from
the light reactions)
plus CO2 to produce
carbohydrates

5. Light Chemistry
Light is a form of electromagnetic
radiation (travels as a wave and a
particle)
Light is propagated as waves:
- the energy of light is inversely
proportional to its wavelength.
Light also behaves as particles:
- called photons.

6. Examples of Wavelengths

7. Light Chemistry
Pigments: Molecules that absorb specific
wavelengths in the visible range of the
spectrum are called pigments.
When a photon meets a molecule (pigment) it can be:
Scattered: photon bounces off the molecule
Transmitted: photon is passed through the molecule
Absorbed: molecule acquires the energy of the photon.
The molecule goes from ground state to excited
state

8. Light Chemistry

9. Pigments
Absorption spectrum:
Plot of wavelengths
absorbed by a
pigment
Action spectrum: Plot
of biological activity
as a function of
exposure to varied
wavelengths of light

10. Types of Pigments
Chlorophylls a and b
Accessory pigments: Absorb in red and
blue regions, transfer the energy to
chlorophylls.
Examples: carotenoids and phycobilins

11. Antennae Complex

12. Antennae Systems
Pigments are arranged in antenna systems, or
light-harvesting complexes.
A photosystem consists of multiple antenna
systems and their pigments and surrounds a
reaction center.
Pigments are packed together on thylakoid
membrane proteins.
Excitation energy passes from pigments that
absorb short wavelengths to those that
absorb longer wavelengths, and ends up in
the reaction center pigment.

13. Antennae Systems

14. Reaction Centers
The reaction center converts light energy
into chemical energy.
The excited chlorophyll a molecule (Chl*)
is a reducing agent (electron donor).
A is an acceptor molecule (oxidizing
agent).
+ −
Chl + A → Chl + A
*
* + −

15. Electron Transport
Two systems of electron transport:
Noncyclic electron transport:
- produces NADPH and ATP
Cyclic electron transport:
- produces ATP only

16. Noncyclic Electron Transport
Light energy is used to oxidize water →
O2, H+, and electrons.
Two photosystems required
- Photosystem I (P700)
- Photosystem II (P680)

17. Noncyclic Electron Transport
Photosystem II Photosystem I
• Light energy • Light energy
oxidizes water → O2, reduces NADP+ to
H+, and electrons. NADPH
• Reaction center has • Reaction center has
chlorophyll a chlorophyll a
molecules P680— molecules: P700—
absorb at 680nm. absorb in the 700nm
range

18. Noncyclic Electron Transport
The “Z scheme” model of noncyclic
electron transport:
• Extracts electrons from water and
transfers them to NADPH, using energy
from photosystems I and II and
resulting in ATP synthesis
• Yields NADPH, ATP and O2

19. Noncyclic Electron Transport

20. How Do We Make ATP

21. Photophosphorylation
Light-driven production of ATP
Type of Chemiosmosis:
H+ is transported via electron carriers
across the thylakoid membrane into the
lumen (creating an electrochemical
gradient.)

22. Cyclic Electron Transport
Only makes ATP
An electron from an excited chlorophyll
molecule cycles back to the same
chlorophyll molecule.
Cyclic electron transport begins and
ends in photosystem I.
Released energy is stored and can be
used to form ATP.

23. Cyclic Electron Transport

24. Light-Independent Rections
(Stroma)
CO2 fixation: CO2 is reduced to
carbohydrates.
Enzymes in the stroma use the energy in
ATP and NADPH to reduce CO2.
Production of ATP and NADPH is light-
dependent; therefore CO2 fixation must
also take place in the light.

25. Calvin Cycle
The enzyme catalyzing the intermediate
formation is rubisco—ribulose
bisphoshate carboxylase/oxygenase—
the most abundant protein in the world.
CO2 is first added to an acceptor
molecule—5-C RuBP; the 6-C
compound immediately breaks down
into two molecules of 3PG.

26. Calvin Cycle
Consists of 3 processes:
• Fixation of CO2
• Reduction of 3PG to
G3P
• Regeneration of RuBP

27. Carbon Fixation
Rubisco:
ribulose bisphoshate
carboxylase/oxygenase
6RuBP + 6CO2 12 (3PG)
Grabs CO2 gas from the air
“fixation”

28. Reduction and Sugar
Production
ATP and NADPH from
the light reaction are
used to covert
12x(3PG) to 12xG3P
2 G3P used to make
sugar
Rest used to
regenerate RuBP

29. Regenerate RuBP
Uses more ATP to
convert 10xsG3P
back to 6xRuBP
Starts over the cycle..

30. Summary of Calvin Cycle
It take 6 turns to make 1 Light indirectly provides the
glucose molecule substrates needed for the
Requires 18 ATP Calvin Cycle
12 NADPH
2 G3P go towards glucose
10 G3P replenish RuBP
½ glucose used to make starch
2/3 converted to a disaccharide
called sucrose (mobile)

31. Photorespiration
Rubisco is an oxygenase Consumes O2, releases
as well as a CO2, and takes place in
carboxylase. light.
It can add O2 to RuBP Opposite of the Calvin
instead of CO2; Cycle
reducing the amount of
CO2 converted to
carbohydrates may limit
plant growth.
Uses ATP and NADPH