1. Chapter 5
Photosynthesis

2. Photosynthesis 6CO2 + 6H2O → C6H12O6 + 6O2
Converting light energy into chemical energy to assemble organic molecules
Two stages
Light-dependant reactions
Absorption of photons of light
PI and PII
Light-independent reactions (Calvin Cycle)
Does not require light
6CO2 + 6H2O → C6H12O6 + 6O2

3. Photosynthesis
Photosynthesis takes place in the green portions of plants
Leaf of flowering plant contains mesophyll tissue
Cells containing chloroplasts
Specialized to carry on photosynthesis
CO2 enters leaf through stomata
Diffuses into chloroplasts in mesophyll cells
In stroma, CO2 fixed to C6H12O6 (sugar)
Energy supplied by light

5. Chloroplasts Photosynthesis takes place Consists of Both stages Stroma
Aqueous environment
Houses enzymes used for reactions
Thylakoid membranes
Form stacks of flattened disks called grana
Contains chlorophyll and other pigments

7. Photosynthetic Stages
Light-dependant reactions
Occur in the thylakoid membranes
capture energy from sunlight
make ATP and reduce NADP+ to NADPH
Carbon fixation reactions (light-independent reactions)
Occurs in the stroma
use ATP and NADPH to synthesize organic molecules from CO2

8. Photosynthesis

9. Capturing Light Energy
Pigments
Absorb photon
Excited electron moves to a high energy state
Electron is transferred to an electron accepting molecule – primary electron acceptor

10. Pigments Chlorophyll a Accessory pigments
A pigment molecule does not absorb all wavelengths of light
Chlorophyll a
Donates electron to PEA
Accessory pigments
Chlorophyll b
Carotenoids
Known as antenna complex
Transfers light energy to chlorophyll a

11. Pigments
Photosynthesis depends on the absorption of light by chlorophylls and carotenoids

12. Pigments and Photosystems
Chlorophylls and carotenoids do not float freely within thylakoid
Bound by proteins
Proteins are organized into photosystems
Two types
Photosystem I
Photosystem II

13. Photosystem I and II Composed of Reaction Centre PI - Contains p700
Large antenna complex
pigment molecules surrounding reaction centre
Reaction Centre
Small number of proteins bound to chlorophyll a moelcules and PEA
PI - Contains p700
PII - Contains p680

14. How Photosystem I and II Work
Trap photons of light
Energy trapped used to energize chlorophyll a molecule in reaction centre
Chlorophyll a is oxidized (loses electrons)
Transfers electrons to PEA
Water molecule donates electron chlorophyll a lost
Transported through electron transport chain
High energy electrons are used to drive ATP synthesis and assemble glucose molecules

15. How Photosystem I and II Work

16. Light Dependant Reactions
Photosystem II

17. Linear Electron Transport and ATP Synthesis

18. The Role of Light Energy
Z scheme
Two photons of light needed for production of NADPH

19. Oxygen
How many photons of light are needed to produce a single molecule of oxygen?
2 H₂O → 4 H⁺ + 4 e⁻ + O₂

20. Cyclic Electron Transport
PI can function independently from PII
Produces additional ATP molecules
Reduction of CO₂ requires ATP

21. Light-Independent Reactions
Carbon Fixation
Series of 11 enzyme-catalyzed reactions
NADPH reduces CO₂ into sugars
Overall process is endergonic
ATP is hydrolyzed to supply energy of reactions
Divided into three phases
Fixation
Reduction
Regeneration

22. Calvin Cycle: Fixation: C₃ Metabolism
CO2 is attached to 5-carbon RuBP molecule
Result in a 6-carbon molecule
This splits into two 3-carbon molecules (3PG)
Reaction accelerated by RuBP Carboxylase (Rubisco)
CO2 now “fixed” because it is part of a carbohydrate

23. Calvin Cycle: Reduction
Two 3PG is phosphorylated
ATP is used
Molecule is reduced by NADPH
Two G3P are produced

24. Calvin Cycle: Regeneration
RuBP is regenerated for cycle to continue
Takes 3 cycles
Produces 3 RuBP molecules
Process (3 turns of cycle)
3CO₂ combine with 3 molecules of RuBP
6 molecules of 3PG are formed
6 3PG converted to 6 G3P
5 G3P used to regenerate 3 RuBP molecules
1 G3P left over

25. G3P Ultimate goal of photosynthesis
Raw material used to synthesize all other organic plant compounds (glucose, sucrose, starch, cellulose)
What is required to make 1 molecule of G3P?
9 ATP
6 NADPH
What is required to make 1 molecule of glucose?
18 ATP
12 NADPH
2 G3P

26. Alternate Mechanisms of Carbon Fixation
Problems with photosynthesis
Not enough CO₂ % of atmosphere
Rubisco
can also catalyze O₂
Slows Calvin Cycle, consumes ATP, releases carbon – photorespiration
Decrease carbon fixation up to 50%
Stomata
Hot dry climates
Low levels of CO₂

27. C₄ Metabolism

28. C₄ Plants Minimize photorespiration Calvin Cycle C₄ Cycle
Performed by bundle-sheath cells
Separates exposure of Rubisco to O₂
C₄ Cycle
CO₂ combines with PEP (3 carbon molecule)
Produces oxaloacetate (4 carbon molecule)
Oxaloacetate reduced to malate
Malate diffuses into bundle-sheath cells and enters chloroplast
Malate oxidized to pyruvate releasing CO₂

29. C₄ vs C₃ C₄ Can open stomata less Require 1/3 to 1/6 as much rubisco
Lower nitrogen demand
Run Calvin cycle and C₄ cycles simultaneously

30. CAM Plants Crassulacean Acid Metabolism
Run Calvin Cycle and C₄ at different time of the day
C₄ - night
Calvin Cycle – day