1. Chapter 3. Photosynthesis

2. Focus of this chapter
The structure and function of chloroplast and its pigments
Mechanisms of photosynthesis including light reaction and dark reaction
Photorespiration
Factors affecting photosynthesis
Allocation, translocation, and partitioning of photoassimilates

3. §1 Photosynthesis discovery and its importance
Definition: the process by which green plants and green algae ( eukarytes), and certain other prokaryotic organisms (e.g. canobacteria) transform light energy into chemical energy.
In green plants, light energy is captured and used to convert water and carbon dioxide into oxygen and energy-rich organic compounds.

4. Discovery of photosynthesis
1771, Joseph Priestley ( ; England, English Minister, scientist) discovered that plant can purify air, which can sustain the burning of a candle or the survival of a little mouse in a sealed environment.
Emigration to USA due to his nonconformist view on religion and politics in 1794.

6. Milestones of Photosynthesis Research
1779, Jan Ingen-Housz ( , the Netherland, physician. Heard the Prestley’s experiment and deeply impressed him during a visit London, he did and published 500 experiments during a single summer!), finally he demonstrated that the plant purification in Priestley’s experiment is dependent on light and its green part.
1782, Jean Senebier ( , Swiss, pastor and librarian) pointed out that purification of the air by green plants in light was dependent on “the presence of fixed air” CO2 participated in photosynthesis
1785, , A. Lavoisier, France, discovered the law of mass conservation.
1804, De Saussure ( , Swiss) suggested H2O took part in photosynthesis because the weight of organic matter and O2always much larger than that the used CO2 while only H2O, light and CO2 were added in his experimental system!
1842, Julius Mayer, Germany, discovered the law of energy conservation and transformation, pointed out solar energy was stored in organic matter.

7. Up to 1804, photosynthesis could be expressed as following
light
Green plant

8. 1930, C. Van Niel, USA, proposed photosynthesis was based on oxidation-reduction reactions and that the primary reaction is a photolysis of water followed by oxygen release.
1932. R.Emerson, W.Arnon, USA, concluded several hundred chls cooperate in photosynthesis
1939. R.Hill, UK, Hill reaction.
1940s. S.Ruben and M Kamen used 18O to label either CO2 or H2O to prove that O2 released in photosynthesis comes from H2O

9. 1954. M.Calvin, USA, Calvin cycle (Nobel Prize in 1961)
1954. D.Arnon, USA, discovered photophosphorylation
, PSⅠ and PSⅡ cooperate in plants.
1984，J.Deisenhofer,R.Huber, H.Michel, The Federal Republic of Germany, solve the structure of a photosynthetic reaction center from a bacterium(Nobel Prize in 1988)
1997, P. Boyer(US), J.Walker(UK), binding change mechanism of ATP synthesis (Nobel Prize in 1997)
Up to now 10 Nobel Prizes have been awarded to the scientists in Photosynthesis!

10. Importance of Photosynthesis
Inorganic substances → Organic substances.
conversing Carbon 2×1011 tons /year　
Light energy → Chemical energy
storing 3× 1021 J/year
Balancing O2 and CO2 concentrations

11. Ancient Atmosphere:
H2O + N2 +NH3 + CO2 + CO + CH4 + H2 + H2S

12. Photosynthesis
- the most important chemical reaction on earth
——J. DEISENHOFER

13. §2 Chloroplast and chloroplast pigments
Factory of photosynthesis
Shape: ellipse
Size: diameter: 37m
thickness: 23m
Number: several to hundreds
in a mesophyll cell depending
on plant species, tissue and
environments

14. I Chloroplast Structure
Envelope(被膜）: outer membrane and inner membrane.
Inner membrane controls solute transport
2. Stroma（基质）: material within the chloroplast, containing DNA, ribosomes and
enzymes (RUBISCO,50%).
Stroma is the site for dark reaction.
3. Thylakoids（类囊体） or lamella（片层）, granum thylakoids and stroma thylakoids.
4. Intrathylakoid space or lumen（类囊体腔）
thylakoid is the site for light reaction, therefore is also called photosynthetic membrane.
I Chloroplast Structure

15. (10～20 nm)
(5～7.5 nm)

17. （二）叶绿体的基本结构

18. II Chloroplast pigments
Other pigments such as anthocyanin, betalains are
also visible but not participate phtosynthesis
1. Category:
chlorophyll
carotenoid
Phycobilin
(in algea)
Chla
Chlb
Chlc,d
Bacteriochl
Higher plants contain chla, chlb, carotene and xanthophyll.
Of all the pigments, only some special chla take part in the conversion of light energy to electric energy, the other pigments, including chlb, carotene, xanthophyll and most chla only attract and transfer light energy.
carotene
xanthophyll
phycoerythrobilin
phycocyanobilin

19. Euglenoids: 裸藻（眼虫藻）；Diatoms(硅藻）；Dinoflagellates: 沟鞭藻；Prochlorophytes:原绿生物；
Heliobacteria: 日光杆菌。

20. 2. Chemical Composition of Photosynthetic Pigments 1). Chlorophylls
Head: Tetrapyrrole ring system (porphyrin ring) is a hydrophilic portion, it chelates a magnesium ion.
The tetrapyrrole ring shows a system of conjugated double bonds
Tail: long phytol tail is hydrophobic.
Difference in chla and chlb lies in ring II

21. Chlorophyll is an Diester
COOCH3
chla C32H30ON4Mg
COOC20H39
chlb C32H28O2N4Mg
So :saponification reaction
COOCH COOK
C32H30ON4Mg KOH → C32H30ON4Mg CH3OH
COOC20H COOK
+C20H39OH
and substitution reaction (Mg replaced by H or Cu)
COOCH COOCH3
C32H30ON4Mg HCl→ C32H30ON4H MgCl2
COOC20H COOC20H39
去镁叶绿素,褐色

22. Chla is cyan, chlb is olivine in color
COOCH COOCH3
C32H30ON4H (CH3COO)2Cu→ C32H30ON4Cu
COOC20H COOC20H39
+ 2CH3COOH
Chlorophyll is not soluble in water, but it can be dissolved in organic solvents such as ethanol, acetone, aether
Chla is cyan, chlb is olivine in color

23. 2). Carotenoids Carotene: C40H56 xanthophyll:C40H56O2
Carotenoids also have conjugated double bonds and participate in light absorption and transmission.
Carotene is very effective in protecting chl
Carotene is orange and xanthophyll is yellow in color.
Carotenoids are organic solvent-soluble, not water-soluble
Carotenoids are antioxidants

24. OH OH
xanthophyll

25. 3). Phycobilin
Open tetrapyrrol.
soluble in hot water and diluted acid

26. §3 Photochemical Features of Chlorophylls
1） The nature of sun light（4H+→ He + sun light)
Reaching earth: 300~2600nm, only visible light is available for photosynthesis

27. (1) Light is Part of Electromagnetic Spectrum
Wavelength is the distance in space between wave crests

28. (2) LIGHT IS ALSO A PARTICLE WHICH WE CALL A PHOTON
Each photon contain an amount of energy which we call quantum.
Quantum energy is frequency-related.
E = Nh =Nhc/,
where N is Avogdaro constant (6.02×1023), h is Planck’s constant (6.63×10-34J·S), c is light speed (2.9979×108 m·s-1),  is light wavelength.
Therefore, E is negatively related to .

30. 2） Selective absorption of photosynthetic pigments to light
(A) Chl
Characteristics of chlorophyll absorption spectrum:
Blue light: 430～450 nm; Red light: 640～660 nm

31. Absorption Peak of chl Red light Blue light Chla 660nm 430nm chlb
Chla is more effective in Red zone while chlb is more effective in Blue zone

32. Chlorophyll is the main pigment involved in photosynthesis

34. Why is leaf or chlorophyll solution green?

35. (C ) Fluorescence（荧光现象) and Phosphorescence (磷光现象）

36. Left: A transparent-green chlorophyll solution of ground up spinach leaves and acetone. Right: Beam of light directed at the chlorophyll solution producing a reddish glow called fluorescence.

37. Fluorescence and phosphorescence are emitted by excited electrons

38. Due to the loss of energy during relaxation, the re-emitted fluorescence is of low energy, thus long wavelength.

39. e Singlet exited state Intersystem crossing Triplet excited state
fluorescence
phosphorescence
light
Ground state

40. Note:
Fluorescence and Phosphorescence suggest that chlorophyll can be excited by light, which is the first important step in conversing light energy to chemical energy.
It is easy to see fluorescence emitted from isolated chlorophyll solution, but it is hard to see it from a leaf of intact plants, because in intact leaves the absorbed energy is not reemitted, rather, it is transmitted, converted to NADPH and ATP, and at last used to fix CO2.

41. 3 Synthesis of Chlorophyll
Chlorophyll is first artificially synthesized by Woodward in (Nobel prize in 1965)
Precursor: Glu orα-KG

43. Factors influencing the synthesis of chlorophyll
Light
Without light, chlorophyll can’t be synthesized
Etiolation: a phenomenon caused by NO LIGHT.
Mineral nutrition
N、Mg (component of chl)；
Fe、Cu、Zn (activator of chl synthesis)；
Mn (maintain grana structure)
Chlorosis: a phenomenon caused by MINERAL DEFICIENCY
See in chapter 2

44. Temperature Water Minimum: 2~4℃ Optimum: <30℃ Maximum:40℃
Lack of water accelerates the decomposition of chl and reduces its synthesis.
Think about the reason for formation of autumn color