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Structures Photosynthesis occurs only in plants and a small
number of single-celled organisms (like algae). To be able to photosynthesize, you must have a
specific organelle: the Chloroplast.
Prof.DR. Samih Tamimi Bio304101
Chloroplasts
chloroplastsin plant cell
cross sectionof leafleaves
chloroplast
absorbsunlight & CO2
makeenergy & sugar
chloroplastscontain
chlorophyll
CO2
Prof.DR. Samih Tamimi Bio304101
• Chloroplasts– double membrane– stroma
• fluid-filled interior– thylakoid sacs– grana stacks
• Thylakoid membrane contains– chlorophyll molecules– electron transport chain– ATP synthase
• H+ gradient built up within thylakoid sac
Plant structure H+ H+
H+
H+
H+H+
H+ H+
H+H+
H+
outer membrane inner membrane
thylakoidgranum
stroma
thylakoid
chloroplast
ATP
Prof.DR. Samih Tamimi Bio304101
Photosynthesis
• Light reactions– light-dependent reactions– energy conversion reactions
• convert solar energy to chemical energy• ATP & NADPH
• Calvin cycle– light-independent reactions– sugar building reactions
• uses chemical energy (ATP & NADPH) to reduce CO2 & synthesize C6H12O6Prof.DR. Samih Tamimi Bio304101
Pigments of photosynthesis
• Chlorophylls & other pigments– embedded in thylakoid membrane– arranged in a “photosystem”
• collection of molecules– structure-function relationship
Prof.DR. Samih Tamimi Bio304101
• pigments:
• chlorophyll a
• accessory pigments
-energy-absorbing ring
-hydrocarbon tail
- carotenoids
- photoprotective
Photosynthesis – light absorption
- chlorophyll b
Light: absorption spectra• Photosynthesis gets energy by absorbing wavelengths of light
– chlorophyll a • absorbs best in red & blue wavelengths & least in green
– accessory pigments with different structures absorb light of different wavelengths
• chlorophyll b, carotenoids, xanthophylls
Prof.DR. Samih Tamimi Bio304101
Photosystems of photosynthesis
• 2 photosystems in thylakoid membrane– collections of chlorophyll molecules – act as light-gathering molecules– Photosystem II
• chlorophyll a• P680 = absorbs 680nm
wavelength red light – Photosystem I
• chlorophyll b• P700 = absorbs 700nm
wavelength red light
reactioncenter
antennapigmentsProf.DR. Samih Tamimi Bio304101
Photosynthesis – light absorption
light harvesting complex
• energy absorbed from light - to pigments
• to reaction center
- two special chlorophyll a
- 1° electron acceptor
• light harvesting complex & reaction center = photosystem (PS)
- proteins
Pigments are held by proteins in the thylakoid membranes
Photosynthesis – energy transfer
• PSII: absorbs 680 nm,
• PS I: absorbs 700 nm,
(less energy)
splits water, powerful ETC, ATP made
e- from PSII, short ETC, NADPH made
Photosynthesis – energy transfer
• e- from PS II electron transport chain (ETC) PS I
NADPH• e- from PS I 2nd ETC e- carrier: NADP+
• e- in PS II, from split H20
ETC of Photosynthesis
Photosystem II
Photosystem I
chlorophyll a
chlorophyll b
Prof.DR. Samih Tamimi Bio304101
12
ETC of Photosynthesis
Photosystem IIP680
chlorophyll a
OH H
H
H
e e
ee
e-
e-
H+H+
H+H+
H+H+
H+H+
H+H+
H+
H+H+
H+H+
H+H+
H+H+
H+H+
H+ATP
thylakoidchloroplast
H+
+H
OO
Plants SPLIT water!
fill the –e vacancy
Prof.DR. Samih Tamimi Bio304101
12
H+
H+
3
4
H+
ADP + Pi
H+H+
H+
H+ H+
H+H+H+
ee
e e
ATP
to Calvin Cycle
energy to buildcarbohydrates
H+H+
H+H+
H+H+
H+H+
H+H+
H+
H+H+
H+H+
H+H+
H+H+
H+H+
H+ATP
thylakoidchloroplast
Photosystem IIP680
chlorophyll a
ETC of Photosynthesis
ATP
Prof.DR. Samih Tamimi Bio304101
ee
e e
sun
5
Photosystem IIP680
chlorophyll a
Photosystem IP700
chlorophyll b
e e
ETC of Photosynthesis
e e fill th
e e– vacancy
Prof.DR. Samih Tamimi Bio304101
6
electron carrier
ee
e e
5sun
NADPH toCalvin Cycle
Photosystem IIP680
chlorophyll a
Photosystem IP700
chlorophyll b$$ in the bank…reducing power!
ETC of Photosynthesis
Prof.DR. Samih Tamimi Bio304101
split H2O
ETC of Photosynthesis
O
ATP
to Calvin CycleH+
H+
H+
H+
H+
H+
H+
H+
H+H+
H+
e e
e e
sun sun
Prof.DR. Samih Tamimi Bio304101
ETC of Photosynthesis• ETC uses light energy to produce
– ATP & NADPH• go to Calvin cycle
• PS II absorbs light– excited electron passes from chlorophyll to “primary
electron acceptor”– need to replace electron in chlorophyll– enzyme extracts electrons from H2O & supplies them
to chlorophyll• splits H2O• O combines with another O to form O2
• O2 released to atmosphere• and we breathe easier!
Prof.DR. Samih Tamimi Bio304101
Photophosphorylation
NONcyclicphotophosphorylatio
n
cyclicphotophosphorylatio
n
ATP
NADP
Prof.DR. Samih Tamimi Bio304101
The Calvin cycle uses ATP and NADPH to convert CO2 to sugar
The Calvin cycleIs similar to the citric acid
cycle in mitochondriaOccurs in the stroma
Prof.DR. Samih Tamimi Bio304101
The Calvin cycle has three phases
1. Carbon fixation2. Reduction3. Regeneration of the CO2
acceptor (RuBP)
Prof.DR. Samih Tamimi Bio304101
The Calvin Cycle Steps
CARBON FIXATION1. CO2 enters cycle and attached to a 5-
carbon sugar called ribulose biphosphate (RuBP) forming 6-C molecule (unstable)
Enzyme RUBISCO catalyzes reaction
2. Unstable 6-C molecule immediately breaks down to 3 3-C molecules called 3-phosphoglycerate (3-PGA)
Prof.DR. Samih Tamimi Bio304101
REDUCTION3. Each 3-phosphoglycerate (3-PGA) gets an
additional phosphate from ATP (from LIGHT RXN) becomes 1,3 phosphoglycerate
4. NADPH reduces 1,3 phosphoglycerate to Glyceraldehyde-3-phosphate (G3P)
– G3P = a sugar that stores potential energy
– Every 3 CO2 yields 6 G3P’s BUT only 1 can be counted in net gain for carbohydrate (GLUCOSE) production
Prof.DR. Samih Tamimi Bio304101
REGENERATION OF CO2 ACCEPTOR (RuBP)
5. The C- skeletons of 5 G3P molecules are rearranged into 3 RuBP molecules
– ATP is used !!!!
Prof.DR. Samih Tamimi Bio304101
The Calvin cycle
Phase 1: Carbon fixation
Phase 2:Reduction
Phase 3:Regeneration ofthe CO2 acceptor(RuBP)
Also known as PGAL
NOTE:
MORE ATP is
needed than
NADPH!!
Prof.DR. Samih Tamimi Bio304101