PHOTOSYNTHESIS
CHAPTER 10
Lecture Objectives
 What Is Photosynthesis?
 How Does Photosynthesis Convert Light Energy into Chemical Energy?
 How Is Chemical Energy Used to Synthesize Carbohydrates?
 How Does Photosynthesis Interact with Other Pathways?
Light reactions:
“Synthesis from Light”
Where does the Oxygen Come From?
TWO PATHWAYS
Light-independent reactions:
OVERVIEW
LIGHT
Electromagnetic Spectrum
Light travels in waves (wavelength)
Photons
PIGMENTS
Molecules able to absorb energy from different wavelengths of the visible spectrum
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
ABSORPTION AND ACTION SPECTRUM
PLANT PIGMENTS
Chlorophyll a and b
Accessory Pigments: Carotenoids and Phycobilins
Energy absorbed from Light can:
• Released as heat and/or light
• Be transferred
• Used for a chemical rxn
• Fluorescence: no work can be done
PHOTOSYSTEM
Two systems of electron transport
Noncyclic electron transport—produces NADPH and ATP
Cyclic electron transport—produces ATP only
Check for Understanding!
Which statement about photosystems is false?
a. They span the thylakoid membrane.
b. The reaction centers convert absorbed light energy into chemical energy.
c. Each consists of a single antenna system.
d. They are multi-protein complexes.
Noncyclic electron transport
Light energy is used to oxidize water → O2, H+, and electrons.
After excitation by light, Chl+ is an unstable molecule and seeks electrons.
Chlorophyll is a strong oxidizing agent and takes electrons from water, splitting the water
molecule.
TWO PHOTOSYSTEMS
Photosystem II
Light energy oxidizes water → O2, H+, and electrons.
Reaction center has chlorophyll a molecules P680—absorb at 680nm.
Photosystem I
Light energy reduces NADP+ to NADPH
Reaction center has chlorophyll a molecules: P700—absorb in the 700nm range
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
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.
CYCLIC ELECTRON TRANSPORT
Which of the following is not a product of the light reactions of photosynthesis?
a. ATP
b. Glucose
c. NADPH + H+
d. Oxygen gas
Photophosphorylation:
Light-driven production of ATP—a chemiosmotic mechanism
H+ is transported via electron carriers across the thylakoid membrane into the lumen—creating
an electrochemical gradient.
CHEMIOSMOTIC PRODUCTION OF ATP
CARBON FIXATION
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.
THE CALVIN CYCLE
CO2 is first added to an acceptor molecule—5-C RuBP; the 6-C compound immediately breaks
down into two molecules of 3PG.
The enzyme catalyzing the intermediate formation is rubisco—ribulose bisphoshate
carboxylase/oxygenase—the most abundant protein in the world.
The Calvin cycle consists of 3 processes:
•
•
•
Fixation of CO2
Reduction of 3PG to G3P
Regeneration of RuBP
Which of the following about the fixation of CO2 (the Calvin cycle) is false?
a. The majority of the G3P produced is used to make sugars.
b. The cycle requires ATP and NADPH produced in the light reactions.
c. The cycle must run multiple times to produce glucose.
d. The cycle regenerates RuBP, the acceptor for CO2.
e. Light is required to initiate the cycle.
The Calvin cycle is stimulated by light:
Protons pumped from stroma into thylakoids increase the pH which favors the activation of
rubisco
Electron flow from photosystem I reduces disulfide bonds to activate Calvin cycle enzymes
Metabolic Interactions in Plants