1. Photosynthesis
Chapter 8

2. Photosynthesis Overview
Energy for all life on Earth ultimately comes from photosynthesis
6CO2 + 12H2O C6H12O6 + 6H2O + 6O2
Oxygenic photosynthesis is carried out by
Cyanobacteria
7 groups of algae
All land plants – chloroplasts

3. Chloroplast Thylakoid membrane – internal membrane
Contains chlorophyll and other photosynthetic pigments
Pigments clustered into photosystems
Grana – stacks of flattened sacs of thylakoid membrane
Stroma lamella – connect grana
Stroma – semiliquid surrounding thylakoid membranes

4. Stages Light-dependent reactions
Require light
Capture energy from sunlight
Make ATP and reduce NADP+ to NADPH
Carbon fixation reactions or light-independent reactions
Does not require light
Use ATP and NADPH to synthesize organic molecules from CO2
Animation

5. Discovery of Photosynthesis
Jan Baptista van Helmont (1580–1644)
Demonstrated that the substance of the plant was not produced only from the soil
Joseph Priestly (1733–1804)
Living vegetation adds something to the air
Jan Ingen-Housz (1730–1799)
Proposed plants carry out a process that uses sunlight to split carbon dioxide into carbon and oxygen (O2 gas)

6. F.F. Blackman (1866–1947)
Came to the startling conclusion that photosynthesis is in fact a multistage process, only one portion of which uses light directly
Light versus dark reactions
Enzymes involved

7. C. B. van Niel (1897–1985) Robin Hill (1899–1991)
Found purple sulfur bacteria do not release O2 but accumulate sulfur
Proposed general formula for photosynthesis
CO2 + 2 H2A + light energy → (CH2O) + H2O + 2 A
Later researchers found O2 produced comes from water
Robin Hill (1899–1991)
Demonstrated Niel was right that light energy could be harvested and used in a reduction reaction

8. Pigments Molecules that absorb light energy in the visible range
Light is a form of energy
Photon – 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

9. Absorption spectrum
When a photon strikes a molecule, its energy is either
Lost as heat
Absorbed by the electrons of the molecule
Boosts electrons into higher energy level
Absorption spectrum – range and efficiency of photons molecule is capable of absorbing

10. Many pigments Organisms have evolved a variety of different pigments
Only two general types are used in green plant photosynthesis
Chlorophylls
Carotenoids
In some organisms, other molecules also absorb light energy

11. Chlorophylls Chlorophyll a Chlorophyll b
Main pigment in plants and cyanobacteria
Only pigment that can act directly to convert light energy to chemical energy
Absorbs violet-blue and red light
Chlorophyll b
Accessory pigment or secondary pigment absorbing light wavelengths that chlorophyll a does not absorb

12. Structure of chlorophyll
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

13. Action spectrum
Relative effectiveness of different wavelengths of light in promoting photosynthesis
Corresponds to the absorption spectrum for chlorophylls

14. Other Pigments Carotenoids Phycobiloproteins
Carbon rings linked to chains with alternating single and double bonds
Can absorb photons with a wide range of energies
Also scavenge free radicals – antioxidant
Protective role
Phycobiloproteins
Important in low-light ocean areas

15. Photosystem Organization
Antenna complex
Hundreds of accessory pigment molecules
Gather photons and feed the captured light energy to the reaction center
Reaction center
1 or more chlorophyll a molecules
Passes excited electrons out of the photosystem

16. Antenna complex Also called light-harvesting complex
Captures photons from sunlight and channels them to the reaction center chlorophylls
In chloroplasts, light-harvesting complexes consist of a web of chlorophyll molecules linked together and held tightly in the thylakoid membrane by a matrix of proteins

17. Reaction center Transmembrane protein– pigment complex
When a chlorophyll in the reaction center absorbs a photon of light, an electron is excited to a higher energy level
Light-energized electron can be transferred to the primary electron acceptor, reducing it
Oxidized chlorophyll then fills its electron “hole” by oxidizing a donor molecule

18. Light-Dependent Reactions
Primary photoevent
Photon of light is captured by a pigment molecule
Charge separation
Energy is transferred to the reaction center; an excited electron is transferred to an acceptor molecule
Electron transport
Electrons move through carriers to reduce NADP+
Chemiosmosis
Produces ATP
Capture of light energy

19. Cyclic photophosphorylation
In sulfur bacteria, only one photosystem is used
Generates ATP via electron transport
Anoxygenic photosynthesis
Excited electron passed to electron transport chain
Generates a proton gradient for ATP synthesis

20. Chloroplasts have two connected photosystems
Oxygenic photosynthesis
Photosystem I (P700)
Functions like sulfur bacteria
Photosystem II (P680)
Can generate an oxidation potential high enough to oxidize water
Working together, the two photosystems carry out a noncyclic transfer of electrons that is used to generate both ATP and NADPH

21. The connection
Photosystem I transfers electrons ultimately to NADP+, producing NADPH
Electrons lost from photosystem I are replaced by electrons from photosystem II
Photosystem II oxidizes water to replace the electrons transferred to photosystem I
2 photosystems connected by cytochrome/ b6- f complex

22. Noncyclic photophosphorylation
Plants use photosystems II and I in series to produce both ATP and NADPH
Path of electrons not a circle
Photosystems replenished with electrons obtained by splitting water
Z diagram

23. Photosystem II Resembles the reaction center of purple bacteria
Core of 10 transmembrane protein subunits with electron transfer components and two P680 chlorophyll molecules
Reaction center differs from purple bacteria in that it also contains four manganese atoms
Essential for the oxidation of water
b6-f complex
Proton pump embedded in thylakoid membrane

24. Photosystem I
Reaction center consists of a core transmembrane complex consisting of 12 to 14 protein subunits with two bound P700 chlorophyll molecules
Photosystem I accepts an electron from plastocyanin into the “hole” created by the exit of a light-energized electron
Passes electrons to NADP+ to form NADPH

28. Chemiosmosis Electrochemical gradient can be used to synthesize ATP
Chloroplast has ATP synthase enzymes in the thylakoid membrane
Allows protons back into stroma
Stroma also contains enzymes that catalyze the reactions of carbon fixation – the Calvin cycle reactions

29. Production of additional ATP
Noncyclic photophosphorylation generates
NADPH
ATP
Building organic molecules takes more energy than that alone
Cyclic photophosphorylation used to produce additional ATP
Short-circuit photosystem I to make a larger proton gradient to make more ATP

30. Question 7 Where does the Calvin Cycle take place?
A. stroma of the chloroplast
B. thylakoid membrane
C. cytoplasm surrounding the chloroplast
D. chlorophyll molecule
E. outer membranes of the chloroplast

31. Carbon Fixation – Calvin Cycle
To build carbohydrates cells use
Energy
ATP from light-dependent reactions
Cyclic and noncyclic photophosphorylation
Drives endergonic reaction
Reduction potential
NADPH from photosystem I
Source of protons and energetic electrons

32. Calvin cycle Named after Melvin Calvin (1911–1997)
Also called C3 photosynthesis
Key step is attachment of CO2 to RuBP to form PGA
Uses enzyme ribulose bisphosphate carboxylase /oxygenase or rubisco

33. 3 phases Carbon fixation Reduction Regeneration of RuBP
RuBP + CO2 → PGA
Reduction
PGA is reduced to G3P
Regeneration of RuBP
PGA is used to regenerate RuBP
3 turns incorporate enough carbon to produce a new G3P
6 turns incorporate enough carbon for 1 glucose

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35. Output of Calvin cycle
Glucose is not a direct product of the Calvin cycle
G3P is a 3 carbon sugar
Used to form sucrose
Major transport sugar in plants
Disaccharide made of fructose and glucose
Used to make starch
Insoluble glucose polymer
Stored for later use

36. Energy cycle
Photosynthesis uses the products of respiration as starting substrates
Respiration uses the products of photosynthesis as starting substrates
Production of glucose from G3P even uses part of the ancient glycolytic pathway, run in reverse
Principal proteins involved in electron transport and ATP production in plants are evolutionarily related to those in mitochondria

37. Photorespiration Rubisco has 2 enzymatic activities
Carboxylation
Addition of CO2 to RuBP
Favored under normal conditions
Photorespiration
Oxidation of RuBP by the addition of O2
Favored when stoma are closed in hot conditions
Creates low-CO2 and high-O2
CO2 and O2 compete for the active site on RuBP

38. Types of photosynthesisC3
Plants that fix carbon using only C3 photosynthesis (the Calvin cycle)
C4 and CAM
Add CO2 to PEP to form 4 carbon molecule
Use PEP carboxylase
Greater affinity for CO2, no oxidase activity
C4 – spatial solution
CAM – temporal solution

39. C4 plants Corn, sugarcane, sorghum, and a number of other grasses
Initially fix carbon using PEP carboxylase in mesophyll cells
Produces oxaloacetate, converted to malate, transported to bundle-sheath cells
Within the bundle-sheath cells, malate is decarboxylated to produce pyruvate and CO2
Carbon fixation then by rubisco and the Calvin cycle

40. The cost of doing business this way
C4 pathway, although it overcomes the problems of photorespiration, does have a cost
To produce a single glucose requires 12 additional ATP compared with the Calvin cycle alone
C4 photosynthesis is advantageous in hot dry climates where photorespiration would remove more than half of the carbon fixed by the usual C3 pathway alone

41. CAM plants
Many succulent (water-storing) plants, such as cacti, pineapples, and some members of about two dozen other plant groups
Stomata open during the night and close during the day
Reverse of that in most plants
Fix CO2 using PEP carboxylase during the night and store in vacuole

42. CAM plants in action
When stomata closed during the day, organic acids are decarboxylated to yield high levels of CO2
High levels of CO2 drive the Calvin cycle and minimize photorespiration

43. Compare C4 and CAM Both use both C3 and C4 pathways
C4 – two pathways occur in different cells
CAM – C4 pathway at night and the C3 pathway during the day