Photosynthesis
I. Overview
A. Background
1. The conversion of light energy (from the sun)
into chemical energy (stored in sugar & organic
molecules.
2. Plants, algae (protists), cyanobacteria,
phytoplankton
3. Plants are primary producers – produce
organic molecules from CO2 & H2O. They are the
bottom of the terrestrial food chain. All complex
organisms on land depend on plants (ultimately) for
food & O2.
B. Photosynthetic Structures
Fig 10.4
1. Cyanobacteria
2. Protists
3. Plants
a. Chloroplast
i. Location
ii. Structure
2 membranes (inner & outer)
Thylakoid membranes – site of the light reactions
Stroma – site of the Calvin cycle, contains rubisco
C. PhotoSyNthesis (PSN) - overview
1. Reaction
a. 6CO2 + 6H20
C6H12O6 + 6O2
O2 is released as a by-product from the splitting of
water
2. 2 interdependent pathways:
a. Light reactions – chlorophyll absorbs light
energy to create high energy molecules: ATP &
NADPH + H+. These are used to drive:
b. Calvin cycle – carbon fixation
Figure 10.4 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle (Layer 3)
Fig 10.6
D. Characteristics of Light
1. What is light?? = electromagnetic energy
(radiation)
2. Consists of groups of particles called photons
3. Travels in waves
4. Wavelength = distance between peaks in wave
5. Only visible light drives PSN: 380 to 750nm
Figure 10.5 The electromagnetic spectrum
Fig 10.7
E. Plant Pigments
1. Molecular substances that absorb visible light.
2. Chlorophyll a – main PSN pigment, absorbs
red & blue light (reflects green) to initiate the
light reactions
3. Accessory pigments – absorb light energy &
transfer it to Chlorophyll a, Chlorophyll b,
Carotenoids, Xanthophylls
Fig 10.10
II. Photosystems
A. Light Dependent Reactions
1. Background
a. The functional units of chlorophyll &
accessory pigments that work together to absorb a
photon of light. “Kicks” electrons of a Mg+ atom to an
excited state (high energy)
b. 2 main components:
i. Light-harvesting complex – accessory
pigments absorb light, pass their excited electrons to
ii. Reaction center complex – a pair of
chlorophyll molecules. Passes the excited electrons to a
primary electron acceptor.
Fig 10.13
Fig 10.12
c. Types of photosystems in the thylakoid
membranes:
i. Photosystem I (P700) – absorbs 700nm
wavelengths best (far-red)
ii. Photosystem II (P680) – absorbs 680nm
wavelengths best (red)
Fig 10.14
2. Non-cyclic Photophosphorylation
a. Structure and Location
b. Process or Steps
i. PS II absorbs light, splitting water into H+, electrons, and O2.
ii. Further light absorption by PS II kicks the electrons up to an
excited state.
iii. Excited electrons are passed from PS II to PS I along an
electron transport chain (PSII  Pq  cytochrome complex  Pc
 PS I). Energy released at every step used to drive ATP
synthesis
iv. When electron reaches PS I, light absorption kicks it up to
excited state again.
v. Excited electron is passed to Fd to NADP+ - the terminal
electron acceptor. Reduced to NADPH
Figure 10.18
c. ATP Production
i. Pathway
Non-cyclic electron flow produces ATP & NADPH + H+.
Energy released during electron transfer drives proton pump at
cytochrome complex.
H+ pumped from stroma into thylakoid lumen, creating an
electrochemical gradient
H+ diffusion back out into stroma drives ATP synthesis in Stroma.
d. Electron Carrier (NADPH + H+)
i. Pathway
3. Cyclic Photophosphorylation
a. Structure and Location
b. Process or Steps
i. Produces ATP only
ii. electrons transferred from Fd back to the
cytochrome complex (instead of to NADP+)
iii. thus more H+ pumped into lumen thus more ATP
produced
Fig 10.16
c. Why cyclic flow?
i. Non-cyclic flow produces equal amounts of ATP &
NADPH + H+.
ii. But… Calvin cycle requires more ATP than NADPH +
H+. So…. NADPH + H+ builds up in the stroma, triggering
The shift to cyclic phosphorylation
Fig 10.6
B. Light Independent Reactions
1. The Calvin Cycle
a. Location
The use of ATP & NADPH + H+ to convert CO2 to
carbohydrates
2. Process and Steps:
a. Carbon fixation: RuBP + CO2 Rubisco 3PGA
b. Reduction:
3PGA + 2ATP + NADPH + H+
G3P
c. Regeneration of RuBP from G3P
d. G3P leaves the chloroplast to become ?!?!
Fig 10.19
III. Problems and Limitations
A. Overall efficiency < 35%
B. Wavelength (λ) of light – shorter λ
have greater energy. PSN use of longer λ
means more photons needed
C. Cyclic phosphorylation – need extra
photons just for extra ATP production
D. Photorespiration
1. Steps
a. light-dependent inhibition of C fixation
b. Cause: increased O2 concentration in leaf –
How? Stomata are closed
c. Rubisco adds O2 to RuBP to make a compound
(phosphoglycolate) that is converted back to CO2 in the
mitochondria.
d. No carb’s produced, but energy required.
e. Wastes about 50% of C compounds in the
chloroplast
IV. Plant Adaptations
A. C3 Plants senescent
1. Strategy
2. Examples
B. C4 plants
1. Strategy
a. 4-C compound instead of a 3-C compound (3PGA)
as first product of Calvin cycle
b. CO2 + PEP PEP carboxylase Oxaloacetate
Malate
c. Malate exported to bundle sheath cells
d. CO2 released from malate to enter Calvin cycle
2. Examples
Fig 10.20
3. Why is the C4 pathway more efficient?
a. PEP carboxylase has much higher affinity for CO2
than Rubisco
b. CO2 is stored as malate in the bundle-sheath cells
a. thus plenty of CO2 even when stomata closed
c. Benefits?
i. Increased WUE (?)
ii. Little photorespiration
C. Crassulacean Acid Metabolism (CAM)
1. Strategy
a. Stomata open only at night
b. CO2 is stored in organic acids in mesophyll cells.
c. During day, CO2 released from organic acids and
enters Calvin cycle in same mesophyll cells
d. Same benefits as C4 pathway, but even greater
WUE.
2. Examples
Fig 10.21