Overview of the entire process of photosynthesis with a focus on the light reactions

1. PHOTOSYNTHESISPHOTOSYNTHESIS
The Process That Feeds the Biosphere
•All organisms either directly or indirectly depend on
photosynthesis
•Autotrophs-
Produce organic molecules from
CO2 & other inorganic materials
•Heterotrophs-
*Plants, algae, & some protists are
photoautotrophs, they use light to
synthesize glucose
Depend on organic
compounds produced by
other organisms
“self-feeders” Consumers

2. All heterotrophs depend on autotrophs for food
and the oxygen that’s a biproduct of
photosynthesis

3. Photosynthsis Converts Light
Energy to Chemical Energy of Food
Which organelle is
responsible for
photosynthesis??
Chloroplasts
What is the chemical energy that autotrophs produce??
Glucose
What are the raw materials that go into photosynthesis? What are
the products? 6CO2 + 6H2O + Light Energy C6H12O6 + 6O2
Which plant organ is the primary site of photosynthesis?
Leaves

4. Anatomy of the Leaf & Its Chloroplasts

5. How do we know what we
know about photosynthesis??
Tracking Atoms:
The OVERALL chemical reaction of photosynthesis is the reverse
of cell respiration, but the process of photosynthesis is NOT the
reverse of cell respiration:
What’s the equation for cell respiration?
C6H12O6 + 6O2  6CO2 + 6H2O + ATP energy
What’s the equation for photosynthesis?
6CO2 + 6H2O + light energy  C6H12O6 + 6O2

6. Sunlight
Photosynthesis
Cellular
respiration
Heat
Relationship Between Cell
Respiration & Photosynthesis

7. Photosynthesis Equation in its
Simplest Form:
CO2 + H2O  [CH2O] + O2
General formula for a carbohydrate

8. The Splitting of WaterThe Splitting of Water
Before the 1930’s, it was widely believed that the O2 produced by
plants is a result of the splitting of CO2 during photosynthesis
Thanks to C.B. van Niel & others, we now know that the O2
produced is a result of the splitting of a water molecule during
photosynthesis:
Van Niel’s Experimental Results
Sulfur bacteria: CO2 + 2H2S  [CH2O] + H2O + 2S
Plants: CO2 + 2H2O  [CH2O] + H2O +
O2
General: CO2 + 2H2X  [CH2O] + H2O + 2X
Conclusion: photosynthetic organisms require a hydrogen
source which is split during photosynthesis

9. Later Tests Using Oxygen-18 Isotope
Markers Confirmed the Same….
Exp #1: When the oxygen in water is labeled…
CO2 + 2H2O  [CH2O] + H2O + O2
Exp #2: When the oxygen in carbon dioxide is labeled…
CO2 + 2H2O  [CH2O] + H2O + O2

10. Tracking Atoms Through
Photosynthesis….

11. Photosynthesis as a Redox ProcessPhotosynthesis as a Redox Process
Cell Resp Photosynthesis
Employs redox
reactions
Employs redox
reactions
e-
lose potential
energy as they move
down ETC
e-
gain potential
energyrequires
energy from light
By-product = H2O By-product = O2
Gain or
Lose?
Gain or
Lose?
? ?

12. Spinach leaf chromatography
Mash a bit of spinach lead on the paper
– the more pigment you get on the paper the better
•Place the paper in a beaker with alcohol.
•Be careful not to let the spinach line touch the
alcohol
•Watch carefully, without disturbing.

13. Two Stages of Photosynthesis:
Light Reactions & Calvin Cycle
Light Reactions:
•Light is absorbed by
chlorophyll
•Energy from the light
splits H2O
•H from water reduces
NADP+
(an e- acceptor)
to NADPH
•Chemiosmosis makes
ATP from ADP. This
process is known as
photophosphorylation
•Products of light reactions:
Energy = ATP & NADPH
By-product = O2
Occurs in thylakoid

14. Two Stages of Photosynthesis:
Light Reactions & Calvin Cycle
The Calvin Cycle:
•CO2 from atmosphere
enters chloroplast & is
incorporated into
organic molecules =
carbon fixation
•NADPH from the light
rxns reduces the fixed
carbon (by donating H)
to carbohydrate; ATP
from light rxns is
required for this step
•Product of Calvin Cycle:
glucose
Occurs in stroma

15. The Light Reactions Convert Solar
Energy to the Chemical Energy of
ATP & NADPH
The Nature of Sunlight (electromagnetic energy):
•Light has properties of both waves & particles (photons)
•Visible light is the
part of the EMS that
drives photosynthesis
•The shorter the
wavelength, the
higher the energy

16. Photosynthetic Pigments are Light Receptors
•Pigments, like
chlorophyll either
absorb, transmit,
or reflect light
•The wavelength of
light that is reflected
is the color that we
detect with our eyes
•Which wavelength
(color) of
Light is least
effective for
Photosynthesis??
Green

17. These accessory pigments allow
chloroplasts to take advantage
of a broader spectrum

18. Excitation of Chlorophyll by Light
•Energy from a photon of light raises a pigment molecule’s
e- from ground state to an unstable excited state
•When the e- drops back down to its ground state, energy is
emitted in the form of heat & light
•Chloroplasts in plants don’t fluoresce because the excited e-
is passed to an e- acceptor rather than dropping back down to
ground state

19. A closer look at
The Light Reactions

20. Light Reactions
• Occurs in the
thylakoid
membranes at
regions called
photosystems
• Reactants: light
and water
• Products:
oxygen, ATP,
NADPH

21. A photosystem is a bundle of
pigment molecules and proteins
•Photosystems act as an antenna for light
Made of 2
chlorophyll “a” molecules
& a primary e- acceptor
Special because they
can use light to boost
an e- to the next
energy level…light
energy has now been
converted to chemical
energy

22. • The light reactions are carried out by:
– Photosystem II
• contains chlorophyll
• Strips electrons from water
– An electron transport chain
• makes ATP
– Photosystem I
• contains chlorophyll
• Gives electrons to NADPH

23. In between PS II and PS I is an
ETC that works just like the one
in Cellular Respiration

24. •This is called: Non-Cyclic Flow of
Electrons
– Electrons move in a Z pattern from PS II to PS I, not a cycle
– This process makes the ATP and NADPH for the Calvin Cycle

25. Photosystem I also carries out
Cyclic Electron Flow
•Only ATP is
produced, not NADPH
Cyclic e- Flow
•This is because the
Calvin Cycle needs
more ATP than can
be made by Non-
Cyclic Electron Flow

26. Processing Time
• Label the diagram
Include:
Water
Light
PS II
ETC
PS I
Oxygen
H+
e-
ATP
NADPH
ATP synthase
ADP + Pi