1. Photosynthesis Dr.Samih Tamimi
Biology

2. Prof.DR. Samih Tamimi Bio304101
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

3. chloroplasts in plant cell chloroplasts contain chlorophyll
absorb sunlight & CO2
cross section of leaf
leaves
CO2
chloroplasts in plant cell
chloroplasts contain chlorophyll
chloroplast
make energy & sugar
Prof.DR. Samih Tamimi Bio304101

4. Prof.DR. Samih Tamimi Bio304101
Plant structure
chloroplast H+
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
ATP
thylakoid
outer membrane
inner membrane
granum
stroma
thylakoid
A typical mesophyll cell has chloroplasts, each about 2-4 microns by 4-7 microns long.
Each chloroplast has two membranes around a central aqueous space, the stroma.
In the stroma are membranous sacs, the thylakoids.
These have an internal aqueous space, the thylakoid lumen or thylakoid space.
Thylakoids may be stacked into columns called grana.
Prof.DR. Samih Tamimi Bio304101

5. 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 C6H12O6
Prof.DR. Samih Tamimi Bio304101

6. Prof.DR. Samih Tamimi Bio304101
A Look at Light
The spectrum of color V I B G Y O R
Prof.DR. Samih Tamimi Bio304101

7. 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

8. Photosynthesis – light absorption
• pigments:
• chlorophyll a
-energy-absorbing ring
-hydrocarbon tail
• accessory pigments
- chlorophyll b
- carotenoids
- photoprotective

9. 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

10. 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
reaction center
Photons are absorbed by clusters of pigment molecules (antenna molecules) in the thylakoid membrane.
When any antenna molecule absorbs a photon, it is transmitted from molecule to molecule until it reaches a particular chlorophyll a molecule = the reaction center.
At the reaction center is a primary electron acceptor which removes an excited electron from the reaction center chlorophyll a.
This starts the light reactions.
Don’t compete with each other, work synergistically using different wavelengths.
antenna pigments
Prof.DR. Samih Tamimi Bio304101

11. Photosynthesis – light absorption
Pigments are held by proteins in the thylakoid membranes
light harvesting complex
• energy absorbed from light - to pigments
• to reaction center
- two special chlorophyll a
- proteins
- 1° electron acceptor
• light harvesting complex & reaction center = photosystem (PS)

12. Photosynthesis – energy transfer
• PSII: absorbs 680 nm,
splits water, powerful ETC, ATP made
• PS I: absorbs 700 nm, (less energy)
e- from PSII, short ETC, NADPH made

13. Photosynthesis – energy transfer
• e- in PS II, from split H20
• e- from PS II electron transport chain (ETC) PS I
• e- from PS I nd ETC e- carrier: NADP+
 NADPH

14. Prof.DR. Samih Tamimi Bio304101
ETC of Photosynthesis
chlorophyll a
Photosystem II
chlorophyll b
Photosystem I
Two places where light comes in.
Remember photosynthesis is endergonic -- the electron transport chain is driven by light energy.
Need to look at that in more detail on next slide
Prof.DR. Samih Tamimi Bio304101

15. Photosystem II P680 chlorophyll a
ETC of Photosynthesis
sun 1 e
PS II absorbs light
Excited electron passes from chlorophyll to the primary electron acceptor
Need to replace electron in chlorophyll
An enzyme extracts electrons from H2O & supplies them to the chlorophyll
This reaction splits H2O into 2 H+ & O- which combines with another O- to form O2
O2 released to atmosphere
Chlorophyll absorbs light energy (photon) and this moves an electron to a higher energy state
Electron is handed off down chain from electron acceptor to electron acceptor
In process has collected H+ ions from H2O & also pumped by Plastoquinone within thylakoid sac.
Flow back through ATP synthase to generate ATP.
Photosystem II P680 chlorophyll a
Prof.DR. Samih Tamimi Bio304101

16. Photosystem II P680 chlorophyll a
ETC of Photosynthesis
thylakoid
chloroplast H+ H+
ATP
Plants SPLIT water! 1 O H 2 e O O H e- H+ +H e e
fill the –e vacancy
Photosystem II P680 chlorophyll a
Prof.DR. Samih Tamimi Bio304101

17. energy to build carbohydrates Photosystem II P680 chlorophyll a
ETC of Photosynthesis
thylakoid
chloroplast H+ H+
ATP e H+ 3 1 2 e H+
ATP 4
to Calvin Cycle H+
ADP + Pi
energy to build carbohydrates
Photosystem II P680 chlorophyll a
ATP
Prof.DR. Samih Tamimi Bio304101

18. Photosystem I P700 chlorophyll b Photosystem II P680 chlorophyll a
ETC of Photosynthesis e e
sun
fill the e– vacancy e 5
Need a 2nd photon -- shot of light energy to excite electron back up to high energy state.
2nd ETC drives reduction of NADP to NADPH.
Light comes in at 2 points.
Produce ATP & NADPH e e
Photosystem I P700 chlorophyll b
Photosystem II P680 chlorophyll a
Prof.DR. Samih Tamimi Bio304101

19. ETC of Photosynthesis e e electron carrier 6 NADPH to Calvin Cycle 5
sun
NADPH to Calvin Cycle
Need a 2nd photon -- shot of light energy to excite electron back up to high energy state.
2nd ETC drives reduction of NADP to NADPH.
Light comes in at 2 points.
Produce ATP & NADPH
Photosystem I P700 chlorophyll b
Photosystem II P680 chlorophyll a
$$ in the bank… reducing power!
Prof.DR. Samih Tamimi Bio304101

20. Prof.DR. Samih Tamimi Bio304101
ETC of Photosynthesis
sun
sun e e H+
Two places where light comes in.
Remember photosynthesis is endergonic -- the electron transport chain is driven by light energy.
Need to look at that in more detail on next slide O
to Calvin Cycle
split H2O
ATP
Prof.DR. Samih Tamimi Bio304101

21. 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

22. Photophosphorylation
cyclic
photophosphorylation
NADP
NONcyclic
photophosphorylation
ATP
Prof.DR. Samih Tamimi Bio304101

23. Prof.DR. Samih Tamimi Bio304101

24. The Calvin cycle uses ATP and NADPH to convert CO2 to sugar
Is similar to the citric acid cycle in mitochondria
Occurs in the stroma
Prof.DR. Samih Tamimi Bio304101

25. Prof.DR. Samih Tamimi Bio304101
The Calvin cycle has three phases 1. Carbon fixation 2. Reduction 3. Regeneration of the CO2 acceptor (RuBP)
Prof.DR. Samih Tamimi Bio304101

26. Enzyme RUBISCO catalyzes reaction
The Calvin Cycle Steps
CARBON FIXATION
1. 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

27. Prof.DR. Samih Tamimi Bio304101
REDUCTION
3. 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

28. 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

29. MORE ATP is needed than NADPH!!
The Calvin cycle
NOTE:
MORE ATP is needed than NADPH!!
Phase 1: Carbon fixation
Phase 3: Regeneration of the CO2 acceptor (RuBP)
Phase 2: Reduction
Also known as PGAL
Prof.DR. Samih Tamimi Bio304101

30. Prof.DR. Samih Tamimi Bio304101
Calvin Cycle Overview
For 1 G3P molecule made
9 ATP molecules are used
6 NADPH molecules are used
G3P (aka PGAL)= starting material to make other organic molecules (glucose, starch, etc.)
Prof.DR. Samih Tamimi Bio304101