1. Essential Idea: Light energy is converted into chemical energy
Topic 8.3 Photosynthesis
Essential Idea: Light energy is converted into chemical energy

2. 8.3 Photosynthesis
Nature of science: Developments in scientific research follow improvements in apparatus—sources of 14C and autoradiography enabled Calvin to elucidate the pathways of carbon fixation. (1.8)
Understandings:
Light-dependent reactions take place in the thylakoid membranes and the space inside them
Light-independent reactions take place in the stroma
Reduced NADP and ATP are produced in the light-dependent reactions
Absorption of light by photosystems generates excited electrons
Photolysis of water generates electrons for use in the light-dependent reactions
Transfer of excited electrons occurs between carriers in thylakoid membranes
Excited electrons from Photosystem II are used to contribute to generate a proton gradient
ATP synthase in thylakoids generate ATP using the proton gradient
Excited electron from Photosystem I are used to reduce NADP
In the light-independent reactions a carboxylase catalyzes the carboxylation of ribulose bisphosphate
Glycerate 3-phosphate is reduced to triose phosphate using reduced NADP and ATP
Triose phosphate is used to regenerate RuBP and produce carbohydrates
Ribulose bisphosphate is reformed using ATP
The structure of the chloroplast is adapted to its function in photosynthesis

3. 8.3 Photosynthesis Applications and Skills:
Application: Calvin’s experiment to elucidate the carboxylation of RuBP
Skill: Annotation of a diagram to indicate the adaptations of a chloroplast to its function

4. The energy of Earth All energy in living systems comes from the sun
Conversion of radiant (light) energy into chemical energy (carbohydrates) = Photosynthesis
Uses carbon dioxide and water to make sugar and oxygen

5. Sunlight
White light is composed of a range of wavelengths (colors)

6. Types of Organisms Autotrophs (producers) Heterotrophs
- auto = self, trophos = feed
- autotrophs make their own organic compounds from inorganic materials
- chemoautotrophs (oxidation of chemicals), photoautotrophs (sunlight)
Heterotrophs
hetero = other, different
Eating or feeding on other organisms, dead, alive or organic material

7. Photoautotrophs

8. Photosynthesis in plants
Chloroplasts are the location of photosynthesis in plants
In all green parts of plants – leaves, stems, etc.
Green color from chlorophyll (photosynthetic pigment)
Found in cells of mesophyll – interior tissue of leaves
Gases exchanges through the stomata
Water enters through xylem of roots

9. Review of Chloroplast
Chloroplasts divided into 3 functional compartments by a system of membranes.
1. Intermembrane space: space between the double chloroplast membrane.
2. Stroma: viscous fluid outside the grana (stacks of thylakoids); light- independent chemical reactions take place here. Carbon dioxide converted to sugar.
3. Thylakoids: flattened membranous sacs inside chloroplast; chlorophyll is found in membranes; function in the light-dependent chemical reactions.
4. Thylakoid space: space inside the thylakoids.

11. Chloroplast Structure Dictates Function
Thylakoids provide a large surface area to maximize light absorption
Small space inside thylakoids allow accumulation of protons and development of electrochemical gradients across the membrane
Fluid space of the stroma contains enzymes responsible for the Light Independent Reactions (Calvin cycle)

12. Photosynthesis reaction
Conversion of radiant (light) energy into chemical energy (carbohydrates) = Photosynthesis
Process known since the 1800s
6CO H2O + light ---> C6H12O6 + 6O2 + 6H2O
Reverse of respiration
C6H12O6 = glucose = carbohydrate

14. Where does the oxygen come from?
Simplified equation = CO2+ H2O  CH2O + O2
Thought O2 came from CO2
Then C and water form the sugar
1. CO2 --> C + O2
2. C + H2O ---> CH2O
Van Niels proved otherwise in 1930s
Working with sulfur bacteria which do not produce O2

15. Van Niels work These bacteria use H2S instead of H2O
CO2+ H2S  CH2O + H2O + 2S
He concluded…
CO2 + 2H2X  CH2O + H2O + 2X
water was source of O2
20 years later proven true in plants as well
radioactive 18O used
CO2 + H2O-->CH2O + H2O + O2

16. Photosynthesis is a Redox Process
Water is split
Electrons are transferred along with hydrogen ions from water to CO2 reducing it to a sugar
Electrons must increase potential energy when moved from water to sugar
Increase in energy provided by light

17. An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle (Layer 1)

18. An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle (Layer 2)

19. An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle (Layer 3)

20. 2 step process Light Dependent Reactions = photo
Light Independent Reactions (Calvin cycle) = synthesis
Light dependent reactions are in the thylakoid, light independent reactions in the stroma
Transforming light energy into chemical energy of ATP and NADPH
Creation of sugars from inorganic compounds

21. Nature of light Electromagnetic energy or radiation
Different wavelengths – distance between wave crests
From nM (gamma rays) to KM(radio waves)
Visible light nm to 750nm
Different wavelengths are detected as different colors by the human eye
Photon - light behaving as a particle
Amount of energy inversely related to wavelength
Violet has 2x the energy of red

23. When energy meets matter
It is reflected, transmitted, or absorbed
Pigment - substance that absorbs certain wavelengths of light
We see reflected light
Photosynthetic energy absorption in visible range
Blue & Red are most used by photosynthesis

25. Light Reactions Photo part of photosynthesis
Chlorophyll in the thylakoid membranes transform light energy into chemical energy of ATP & NADPH
Absorbed light boosts electrons to an excited state
NADP+ accepts excited electrons and stores their energy
Water is split - O2 is given off

26. Photosynthetic Pigments
Chlorophyll A only pigment that directly participates in light reactions
Chlorophyll B absorbs in yellow green then transfers energy to chlorophyll A
Accessory Pigments
Caratenoids - yellow orange
Broadens light available for photosynthesis
May function as photoprotectors

28. Photoexcitation of Chlorophyll
Chlorophyll absorbs photons of light
Photon energy excites electrons to higher orbitals
A photons energy must match exactly the energy required to boost electron to its excited state – different for each pigment
Excited electrons fall back releasing energy as light and heat (afterglow = fluorescence)
Conversion of light to heat – Cars on hot day

30. Photosystems
In thylakoid membrane chlorophyll organized with other molecules in photosystems
“Antenna array” of chlorophyll A, B, & carotenoid pigments, clustered around a reaction center
Reaction center is a single chlorophyll A associated with the “primary electron acceptor” (PEA)
Chlorophyll A passes electrons to the PEA
PEA traps excited electrons before they fall back to ground state

32. Photosystems
2 types of photosystems in thylakoid membrane cooperate in light reactions
Called Photosystem I & II – different PEA
Photosystem I – P700 best absorbance at 700 nm wavelength (red)
Photosystem II – P680 also in red
Identical chlorophyll but different protein association

33. Electron Flow Light drives synthesis of ATP and NADPH
Energy transformation based on flow of electrons
Two routes of electron flow
Noncyclic electron flow = predominates during light reactions – ejected electrons don’t cycle back to ground state
Cyclic electron Flow = uses Photosystem I not PS II – no production of NADPH or O2

35. Noncyclic Electron Flow: Step 1
Photosystem II absorbs light energy
Excited electron in reaction center – P680 is captured by the PEA
Oxidized chlorophyll now strong oxidizing agent
Electron hole must be filled

37. Noncyclic Electron Flow: Step 2
Enzyme extracts electrons from water and supplies them to P680
Reaction splits water into 2 hydrogen ions and an oxygen atom
Oxygen atom immediately combines with another to form O2
Water split to produce O2 = Photolysis

39. Noncyclic Electron Flow: Step 3
Photoexcited electron passes from PEA of PS II to Photosystem I via electron transport chain
Similar to the one in cellular respiration
Chloroplast version has electron carrier called plastoquinone (Pq)
cytochrome containing molecule

41. Noncyclic Electron Flow: Step 4
Cascading electrons produce ATP in thylakoid membrane in exergonic reaction
Called Noncyclic Phosphorylation
Method of phosphorylation is chemiosmosis
ATP generated in light reaction provides energy chemical energy for the synthesis of sugar in Calvin cycle

43. Noncyclic Electron Flow: Step 5
Electron reaching the bottom of the transport chain fills a hole in the P700 chlorophyll in photosystem I
Replaces electron that light energy drove from chlorophyll PEA of PS I

45. Noncyclic Electron Flow: Step 6
PEA of PS I passes photoexcited electrons to ferredoxin (Fd) – iron containing protein
Electrons transferred from Fd to NADP+
Redox reaction which stores high energy electrons in NADPH
NADPH provides reducing power for sugar synthesis in Calvin cycle

46. Light dependent reactions use solar power to produce ATP and NADPH to fuel sugar production in the Calvin cycle

47. Cyclic electron flow
Cyclic electron flow = uses Photosystem I not PS II
Thus no production of NADPH or O2
Short circuit back into electron transport chain
Does produce ATP  cyclic phosphorylation
Calvin cycle consumes more ATP than NADPH
Cyclic electron flow makes up the difference in ATP required

49. Review of chemiosmosis
Mechanism of generating ATP
Electron transport chain pumps protons across the membrane while electrons are shuttled through different carriers
Protons pumped to the inside of thylakoids
Protons accumulate, pH and charge increase  move out to stroma through channels
Movement through channels in ATP synthase drives production of ATP

51. Chloroplast chemiosmosis
Light energy captured by photosystems drives electrons through electron transport chain
ATP formation in the stroma
Thylakoid pH in process drops to ~5

53. Light dependent reaction summary
Noncyclic electron flow removes electrons from water
Electrons stored in NADPH at high potential energy
Light driven electron current creates ATP
OVERALL  Light energy converted to chemical energy as NADPH and ATP

54. The Independent Reactions: synthesis of sugars
Takes place in the stroma
Carbon enters as CO2 and leaves as sugar
ATP is the energy source
NADPH provides reducing power for adding high energy electrons
Direct product of Calvin cycle is glyceraldehyde-3-phosphate (G3P)
3 cycles to make this product
Actually fixing three molecules of CO2

55. Calvin Cycle Phase 1: Fixation
Ribulose bisphosphate (RuBP) receives CO2
Rubisco is the enzyme that catalyzes this step
most abundant protein in chloroplast – probably most abundant on earth
Product of reaction is unstable 6 carbon intermediate  splits into two 3-phosphoglycerates

57. Calvin cycle Phase 2: Reduction
Each 3-phosphoglycerate receives another phosphate from ATP (6 used total)
Pair of electrons donated from NADPH reduces to G3P (sugar) (6 used total)
For every 3 CO2 there are 6 G3P
Only 1 exits the cycle, the other 5 must be recycled to regenerate the three molecules of RuBP

59. Calvin Cycle Phase 3: Regeneration
Complex series of reactions
Carbon skeletons of the 5 G3P molecules are rearranged into 3 molecules of RuBP
3 more ATP molecules consumed here
RuBP ready to accept another CO2 and the cycle continues

61. Calvin Cycle Summary For the synthesis of 1 molecule of sugar
Require the input of 9 ATP and 6 NADPH
Light reactions regenerate ATP, NADPH
G3P can be used to produce other sugars

63. A 8.3.1 Calvin’s experiment to elucidate the carboxylation of RuBP
Melvin Calvin and Andrew Benson used the lollipop apparatus
They replaced the carbon in CO2 with radioactive isotope 14C  14CO2
They supplied this to algae (Chlorella) and took samples at short time intervals, killed them and fixed them to hot methanol, and found carbon compounds in the algae contained 14C using double-way paper chromatography and autoradiography

64. A 8.3.1 Calvin’s experiment to elucidate the carboxylation of RuBP
The autoradiogram results showed that were more labeled glycerate 3-phosphate in the first 5 seconds
This supported this was the first product of carbon fixation
The autoradiogram for 30 seconds showed other labeled carbon compounds
Glycerate 3-phosphate

65. S 8.3.1 Annotation of a diagram to indicate the adaptations of a chloroplast to its function
There is a relationship between the structure of a chloroplast and its function (similar to what we saw with the mitochondria)
Chloroplasts absorb light- they have special pigments for this arranged in photosystems, they have large thylakoid membranes and thylakoids are arranged in stacks of grana
Chloroplasts produce ATP by photophosphorylation- a proton gradient is needed to produce ATP and since the thylakoids are so small this gradient develops quickly allowing for quick synthesis of ATP
Chloroplasts carry out the many chemical reactions of the Calvin cycle- the Calvin cycle occurs in the stroma, ATP and NADP are passed from the thylakoids to the stroma for the Calvin Cycle