1. Photosynthesis Two-stage process Light-dependent reaction
(includes light harvesting)
Occurs only during daylight
Light-independent reaction
May continue in the dark

2. Evidence for two stages
Temperature has a clear effect on the rate of photosynthesis
This suggests that although light levels initially limit the rate, there is a second stage controlled by temperature-sensitive enzymes
A plant given alternating periods of dark and light forms more carbohydrate than one in continuous light
Period of darkness ensures that all the product from first stage is converted into carbohydrate before it builds up

3. Excitation of electrons
Photon of light hits chlorophyll molecule
Energy transferred to the electrons in the molecule
Electrons raised to higher energy levels
If they are raised to a high enough level they will dissociate
Electrons picked up by an electron acceptor
ATP formed as the electron is passed along an electron transport chain

4. Electron transport chain
electron acceptor
ATP
ADP + Pi
final electron acceptor

5. Light-dependent stage
Occurs in the thylakoids
Involves splitting of water by light – photolysis
ADP is converted to ATP by light – photophosphorylation
Cyclic photophosphorylation uses only PSI
Non-cyclic photophosphorylation uses both PSI and PSII

7. Cyclic photophosphorylation
electron acceptor e-
ATP
ADP + Pi
Chlorophyll
PS I e-
light

8. Non-cyclic photophosphorylation
electron acceptor e- 2H
ATP
electron acceptor e-
ADP + Pi
2H+ e-
Photosystem I
Photosystem II
H2O e-
light
light
½O2

9. To the light independent reaction
NADP
NADP+ e-
NADPH + H+ 2H
To the light independent reaction
2H+

10. Splitting of water is catalysed by an enzyme – perhaps PSII!
Photolysis of water
4 H2O → 4 H+ + 4 OH-
4 OH- - 4 e- → O2 + 2 H2O
Splitting of water is catalysed by an enzyme – perhaps PSII!
NADP+
Photosystem II

11. Light-dependent stage
Water is converted to protons, electrons and oxygen
Reduced NADP is generated
Occurs in the thylakoid membrane

12. Non-cyclic photophosphorylation
electron acceptor e- 2H
ATP
electron acceptor e-
ADP + Pi
2H+ e-
Photosystem I
Photosystem II
H2O e-
light
light
½O2

13. To the light independent reaction
NADP
NADP+ e-
NADPH + H+ 2H
To the light independent reaction
2H+

15. Melvin Calvin
Member of the Radiation Laboratory at Berkeley, University of California
Studied using radioactively labelled carbon dioxide
14CO2 fed to Chlorella algae and its path tracked

16. Light-independent stage
Carbon dioxide is converted to carbohydrate
Occurs in the stroma of the chloroplast

17. Match structure and function
Double membrane chloroplast envelope
Thylkakoids have large surface area
ATP synthase molecules in thylakoid membrane
Stroma contains enzymes, sugars and organic acids
As much light as possible can be absorbed
Needed for the light-dependent reaction to take place
Reactants kept close to reaction sites
Produce ATP in the light-dependent reaction

18. Three phases Carboxylation Reduction Regeneration
Carbon dioxide fixation with ribulose bisphosphate (RuBP)
Reduction
Reduction of glycerate 3-phosphate (GP) to glyceraldehyde 3-phosphate (GALP)
Regeneration
Re-formation of the CO2
acceptor molecules

19. Chemicals

20. Phase 1: Carboxylation Rubisco CO2 GP (3C) unstable6C molecule RuBP GP

21. RUBISCO

22. Phase 2: Reduction GP (3C) GALP (3C) GP GALP 2NADPH + H+ 2NADP+
From light dependent reaction
or “triose phosphate”
2ATP
2ADP + 2Pi GP
(3C)
GALP
(3C) GP
GALP
2NADPH + H+
2NADP+
From light dependent reaction

23. Phase 3: Regeneration 5 -- 6
ATP
ADP + Pi
GALP
(3C)
RuBP(6C)
GALP

24. Sugars formed GALP (3C) Hexose Sugar (6C) GALP 1 -- 6 ATP ADP + Pi
Polymerised to starch

26. Calvin Cycle

27. Calvin Cycle

28. Glycerate-3 Phosphate (GP)
Calvin Cycle
CO2
(1C)
Glycerate-3 Phosphate (GP)
(3C)
RuBP
(5C)
Triose Phosphate
(GALP)
Hexose Sugar (6C) x3 x2
ADP
ATP x5
ATP
ADP
NADPH
NADP+

29. Synthesis of Organic Substances
GALP
GALP
GALP GP
GALP
fatty acids
glycerol
lipids
amino
acids
hexose

30. The Maths 2 molecules of GALP are made each turn
5 out of 6 molecules of GALP are used to regenerate RuBP
2 molecules of GALP are needed to make a hexose sugar
How many turns are needed to make 1 molecule of hexose sugar?
How many molecules of ATP and reduced NADP are needed?