1. carbon dioxide water glucose water oxygen
Photosynthesis
6CO2 + 12H2O C6H12O6 + 6H2O + 6O2
carbon dioxide water glucose water oxygen
Reactants: 6CO2 + 12H2O
Products: C6H12O6 + 6H2O + 6O2
Photosynthesis vs. Cellular Respiration:
6CO2 + 12H2O C6H12O6 + 6H2O + 6O2
C6H12O6 + 6O CO2 + 12H2O

2. Discovery of Photosynthesis
The work of many scientists led to the discovery of how photosynthesis works:
Jan Baptista van Helmont ( )
Joseph Priestly ( )
Jan Ingen-Housz ( )
F. F. Blackman ( )

3. Early discoveries
Joseph Priestly: Candle with bell jar and mouse experiment – He concluded that air is necessary for the growth of a plant. He discovered the fact that plants restore oxygen in the air.
Jan Ingenhousz: Experiment with aquatic plant in light and dark – He concluded that sunlight is essential for plant processes that purify the air.
Julius Von Sachs: Green parts of plant make glucose and store as starch.
T.W. Engelmann: Spilt light using prism into 7 colours (VIBGYOR) - Green algae Cladophora placed in a suspension of aerobic bacteria - Bacteria were used to detect the sites of O2 evolutions.
Cornelius van Niel: He did experiment with purple and green bacteria and demonstrated photosynthesis is a light dependent process with hydrogen from H2O reduces CO2 to carbohydrates. He concluded that oxygen comes from H2O, and not from CO2.  Finally, the correct equation for photosynthesis was discovered.                       
  6CO2 + 12H2O C6H12O6 + 6H2O + 6O2

4. Discovery of Photosynthesis
C. B. van Niel, 1930‘s
proposed a general formula:
CO2+H2A + light energy CH2O + H2O + 2A
H2A is the electron donor
van Niel identified water as the source of the O2 released from photosynthesis
Robin Hill confirmed van Niel’s proposal that energy from the light reactions fuels carbon fixation

5. Photosynthesis Photosynthesis is divided into two reactions:
1. Light-dependent reactions
capture energy from sunlight
reduce NADP+ to NADPH
produce ATP
occur in chloroplasts
2. Carbon fixation reactions
use ATP and NADPH to synthesize organic molecules from CO2
Occurs in stroma

6. AN OVERVIEW OF PHOTOSYNTHESIS
The light reactions convert solar energy to chemical energy
Produce ATP & NADPH
Light
Chloroplast
NADP
ADP
+ P
The Calvin cycle makes sugar from carbon dioxide
ATP generated by the light reactions provides the energy for sugar synthesis
The NADPH produced by the light reactions provides the electrons for the reduction of carbon dioxide to glucose
Calvin
cycle
Light
reactions

7. Photosynthesis Overview
Photosynthesis takes place in chloroplasts
Thylakoid membrane – internal membrane arranged in flattened sacs
contain chlorophyll and other pigments
Grana – stacks of thylakoid membranes
Stroma – semiliquid substance surrounding thylakoid membranes

8. Fig

10. Energy from Light Reactions Fuels Carbon Fixation
Photon: a particle of light
acts as a discrete bundle of energy
energy content of a photon is inversely proportional to the wavelength of the light
Photoelectric effect: removal of an electron from a molecule by light
occurs when photons transfer energy to electrons
Light energy is absorbed by pigments: molecules that absorb visible light

11. Energy from Light Reactions
Fuels Carbon Fixation

12. Pigments Pigments: molecules that absorb visible light
chlorophyll a
chlorophyll b
carotenoids
Each pigment has a characteristic absorption spectrum: the range and efficiency of photons it is capable of absorbing.

13. Different pigments absorb light differently

14. Pigments Chlorophyll a – primary pigment in plants and cyanobacteria
absorbs violet-blue and red light
Chlorophyll b – secondary pigment absorbing light wavelengths that chlorophyll a does not absorb

15. Pigments Structure of pigments:
Porphyrin ring: complex ring structure with alternating double and single bonds
magnesium ion at the center of the ring
photons excite electrons in the ring
electrons are shuttled away from the ring

17. Pigments
Accessory pigments: secondary pigments absorbing light wavelengths other than those absorbed by chlorophyll a
increase the range of light wavelengths that can be used in photosynthesis
include: chlorophyll b, carotenoids, phycobiloproteins
carotenoids also act as antioxidants

18. Photosynthesis: Photosystems
Photosystem: Enzyme complexes for photosynthesis
enzymes use light to oxidize H2O and reduce CO2 to carbohydrates
Located in the thylakoid membrane of plants, algae and cyanobacteria or the cytoplasmic membrane of photosynthetic bacteria

19. Photosystem Organization
A photosystem consists of:
1. an antenna complex of hundreds of accessory pigment molecules
2. a reaction center of one or more chlorophyll a molecules
Energy of electrons is transferred through the antenna complex to the reaction center

21. Photosystem Organization
At the reaction center, the energy from the antenna complex is transferred to chlorophyll a
This energy causes an electron from chlorophyll to become excited
chlorophyll absorbs a photon, looses an electron
The excited electron is transferred from chlorophyll a to an electron acceptor, quinone
Water donates an electron to chlorophyll a to replace the excited electron

23. Light-Dependent Reactions
In chloroplasts, two linked photosystems are used in noncyclic photophosphorylation
1. Photosystem I
reaction center pigment (P700) with a peak absorption at 700nm
2. Photosystem II
reaction center pigment (P680) has a peak absorption at 680nm
This system produces energy, O2, and NADPH for biosynthesis of carbohydrates

24. Light-Dependent Reactions
Photosystem II acts first:
accessory pigments shuttle energy to the P680 reaction center
excited electrons from P680 are transferred to the cytochrome/b6-f complex - connects photosystems II and I
electron lost from P680 is replaced by an electron released from hydrolysis: the splitting of water

25. Light-Dependent Reactions
The b6-f complex is a series of electron carriers
an electron transport chain
electron carrier molecules are embedded in the thylakoid membrane
protons are pumped into the thylakoid space to form a proton gradient

26. Light-Dependent Reactions
Photosystem I:
receives energy from an antenna complex
energy is shuttled to P700 reaction center
excited electron is transferred to a membrane-bound electron carrier
electrons are used to reduce NADP+ to NADPH
electrons lost from P700 are replaced from the b6-f complex

27. Z Diagram of Photosystems I and II
Fig. 8.13
Z Diagram of Photosystems I and II

28. Fig

29. Fig

30. Fig

31. Light-Dependent Reactions
ATP is produced via chemiosmosis:
ATP synthase is embedded in the thylakoid membrane
protons have accumulated in the thylakoid space
protons move into the stroma only through ATP synthase
ATP is produced from ADP + Pi

34. Carbon Fixation Reactions
Calvin cycle
biochemical pathway that allows for carbon fixation
incorporates CO2 into organic molecules
occurs in the stroma
uses ATP and NADPH as energy sources

35. Carbon Fixation Reactions
To build carbohydrates, cells need:
1. Energy
provided by ATP from light-dependent reactions
2. Reduction Potential: hydrogen atoms
provided by NADPH from photosystem I

36. Carbon Fixation Reactions
The Calvin cycle has 3 phases:
1. Carbon Fixation
RuBP + CO molecules PGA 3 phosphoglycerate
2. Reduction
PGA is reduced to G3P glyceraldehyde-3-phosphate
3. Regeneration of RuBP
G3P is used to regenerate RuBP

37. Carbon Fixation Reactions
Carbon fixation – the incorporation of CO2 into organic molecules
occurs in the first step of the Calvin cycle:
ribulose-bis-phosphate + CO (PGA)
5 carbon molecule carbon carbons
Reaction is catalyzed by rubisco

39. Carbon Fixation Reactions
Glucose is not a direct product of the Calvin cycle
2 molecules of G3P leave the cycle
each G3P contains 3 carbons
2 G3P are used to produce 1 glucose in reactions in the cytoplasm

40. Carbon Fixation Reactions
During the Calvin cycle, energy is needed. The energy is supplied from:
18 ATP molecules
12 NADPH molecules

41. Photorespiration Rubisco has 2 enzymatic activities:
1. Carboxylation – the addition of CO2 to RuBP
favored under normal conditions
2. Photorespiration – the oxidation of RuBP by the addition of O2
favored in hot conditions
CO2 and O2 compete for the active site on RuBP.

42. Photorespiration When Rubisco reacts with O2 instead of CO2
Occurs under the following conditions:
Intense Light (high O2 concentrations)
High heat
Photorespiration is estimated to reduce photosynthetic efficiency by 25%

43. Why high heat?
When it is hot, plants close their stomata to conserve water
They continue to do photosynthesis  use up CO2 and produce O2  creates high O2 concentrations inside the plant  photorespiration occurs

44. C2 oxidative carbon cycle:
Input 4C
Output 3C
75% C recovery rate

45. C4 Photosynthesis
Certain plants have developed ways to limit the amount of photorespiration
C4 Pathway*
CAM Pathway*
* Both convert CO2 into a 4 carbon intermediate  C4 Photosynthesis

46. Fig

47. Fig

48. Photorespiration
Some plants can avoid photorespiration by using an enzyme other than rubisco.
PEP carboxylase
adds CO2 to phosphoenolpyruvate (PEP)
a 4 carbon compound is produced
CO2 is later released from this 4-carbon compound and used by rubisco in the Calvin cycle

50. Photorespiration C4 plants use PEP carboxylase to capture CO2
CO2 is added to PEP in mesophyll cell
the resulting 4-carbon compound is moved into a bundle sheath cell where the CO2 is released and used in the Calvin cycle

53. Photorespiration CAM plants
CO2 is captured at night when stomata are open
PEP carboxylase adds CO2 to PEP to produce a 4 carbon compound
this compound releases CO2 during the day
CO2 is then used by rubisco in the Calvin cycle

54. C4 Pathway CO2 is fixed into a 4-carbon intermediate
Has an extra enzyme– PEP Carboxylase that initially traps CO2 instead of Rubisco– makes a 4 carbon intermediate

55. C4 Pathway
The 4 carbon intermediate is “smuggled” into the bundle sheath cell
The bundle sheath cell is not very permeable to CO2
CO2 is released from the 4C malate  goes through the Calvin Cycle
C3 Pathway

56. How does the C4 Pathway limit photorespiration?
Bundle sheath cells are far from the surface– less O2 access
PEP Carboxylase doesn’t have an affinity for O2  allows plant to collect a lot of CO2 and concentrate it in the bundle sheath cells (where Rubisco is)

57. CAM Pathway Fix CO2 at night and store as a 4 carbon molecule
Keep stomates closed during day to prevent water loss
Same general process as C4 Pathway

58. How does the CAM Pathway limit photorespiration?
Collects CO2 at night so that it can be more concentrated during the day
Plant can still do the calvin cycle during the day without losing water

60. Factors affecting photosynthesis
Light
Water
Temperature
Wind speed
CO2 concentration
Blackman proposed the law of limiting factors in According to this law, when a process depends on a number of factors, its rate is limited by the pace of the slowest factor.