1. Biology HL Mrs. Ragsdale
Photosynthesis
Biology HL
Mrs. Ragsdale

2. Light
Photosynthesis – process in which autotrophs convert sunlight into chemical energy
Light –dependant
Require sunlight
Light-independent
Able to carry on in darkness
Wavelengths of light – action spectrum
400 – 525 violet-blue
525 – 625 green-yellow
625 – 700 orange-red

3. Action Spectrum

4. Translation -
Greatest absorption of light is in the violet-blue and red range
Least amount of light absorbed is yellow-green
Green is the color of the pigment chlorophyll located in chloroplasts
Green color reflected

5. Chloroplast Anatomy Inner and outer membrane
Stroma – the fluid filled interior
Location of light-independent reactions
Thylakoid membrane – membrane that is folded up into stacked layers
Sight of energy capture and transfer
Location of light-dependant reactions
Sunlight is driving ATP and NADPH formation

6. Electron Micrograph Inner and Outer Membranes Stroma
Thylakoid membrane
70s ribosomes
Starch Granules
Lipid Droplet

7. Light-dependant reactions
Takes place in Thylakoid membrane
Chlorophyll absorbs light
Water molecules are split into O₂ and energy
Electrons to get excited and raise up in energy levels
Photosystem I and II
Pigments of the thylakoid membrane located in clusters of about 200 – 300 individual pigment molecules

8. Reaction Centre
Photosystems pass around photon’s energy, until the energy reaches a specific wavelength
Most of the energy is lost as light and heat
Wavelength corresponds to the reaction centre which traps the remaining energy
Excited electrons – chlorophyll a is “photoactivated”
Chlorophyll a passes the excited electrons to an electron transport chain
Energy primarily used to make ATP

9. Photolysis and Photophosphorylation
Photolysis – process by which water is split into hydrogen and oxygen atoms
Photophosphorylation – process by which ATP is made during the light reaction

10. Photosystem I and II Photosystem I Photosystem II
Cyclic pathway reaction centre (P700)
Makes ATP only
ATP production via chemiosmosis, similar to the mitochondria
Takes place in-between the inner and outer thylakoid membranes so that a proton gradient can build up
Photosystem II
Noncyclic pathway reaction centre (P680)
Involves ATP production AND high energy NADP⁺ reduced into NADPH

11. Photosystem II – Occurs first, believe it or not!
Plastoquinone – electron acceptor and ultimate product of photosystem II
Carries a pair of electrons plus most of the light energy
Plastoquinone accepts two excited electrons and then moves to a different position in the plasma membrane (remember the fluid mosaic model?)
Plastoquinone gets reduced once it accepts the two electrons
Reduced plastoquinone is really good at stealing hydrogens so it is what we call a strong “oxidizing agent”
Causes hydrolysis of water molecules
2H2O -> O2 + 4H+ +4e-

12. Photosystem II – non cyclic photophosphorylation
Reduced plastoquinone brings its two electrons to the beginning of the electron transport chain where it is passed along by different carriers
As the electron passes, it fuels H+ pumps that create a concentration gradient in the intermembrane space of the thylakoid
Chemiosmosis fuels photophosphorylation
ADP -> ATP
The electron transport chain ends at plastocyanin.
Reduced plastocyanin needed in photosystem I

13. Photosystem I – which actually happens second… yay science!
Ultimate product = NADPH (electron carrier)
Light + chlorophyll leads again to photoactivation in the reaction centre
Again the excited electron is passed along an electron carrier chain
Final carrier is ferredoxin, a protein in the fluid outside the thylakoid. When reduced, it is able to provide the H+ to convert NADP+ into NADPH + H+
At the end, the electron that photosystem I donates is replaced by an electron from plastocyanin from photosystem II

14. Photosystem I – Cyclic photophosphorylation
Photosystem II and I are linked together. Ultimately photosystem II’s end product – plastocyanin donates electrons that re-excite electrons
This provides the energy to reduce NADP+ into NADPH
Because there is a flow of electronic energy between the two photosystems, we call Photosystem I cyclic while Photosystem II is noncyclic
Recap: photosystem II – non cyclic photophosphorylation while photosytem I – cyclic photophosphorylation

15. Light-dependant reactions in the thylakoid

16. Photosynthesis Video
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17. Light Independent Reactions

18. Light Independent Phase:
Occurs in stroma
Involves CO2 gas as the ultimate carbon source
Begins with a series of carbon fixation reactions:
Ribulose bisphosphate or RuBP is combined with carbon dioxide and catalyzed by Rubisco
The catalyzation of rubisco creates two molecules of glycerate 3-phosphate.
In order to convert glycerate 3-phosphate into a carbon compound hydrogen must be added. This process requires energy!
2 ATP are used up -> ADP + 2 Phosphate
2 NADPH are oxidized into NADP+
End result so far are 2 triose phosphate molecules

19. Regeneration of RuBP
Two triose phosphate molecules combine to make hexose phosphate
Hexose phosphate can be combined using condensation reactions to form starch.
Not all triose phosphate is stored, however!! The cycle must be renewed.
For every RuBP molecule that is consumed, one must be renewed.
For every 3 RuBP molecules, 6 triose phosphate are created. 5 of these must be used, however to regenerate the 3 RuBP molecules
For every turn of the Calvin cycle, only 1 net triose phosphate is created
For one single molecule of glucose, six turns of the Calvin cycle are necessary!!

20. CO₂ enters the system
Ribulose bisphosphate (RuBP)
CO₂ is released
2 Glycerate 3-Phosphate (GP)
Regeneration of RuBP with 5 triose phosphate molecules
ATP and NADPH provide energy
6 Triose Phosphate Molecules (TP)
ADP and NADP⁺ return to light dependant cycle
After 6 Revolutions: Glucose!

22. Rate of Photosynthesis

23. Effects of light intensity
Low light intensity slows the rate of creating energy to fuel the light-independent calvin cycle hence slowing rate of photosynthesis
High light intensity increases this rate, however, its efficacy plateaus

24. Carbon dioxide concentration
High rates of carbon dioxide increases rate of photosynthesis
Similar to light, however, it reaches a plateau after raising to a certain rate

25. Temperature
At extreme low AND high temperatures, photosynthesis can slow down and even stop
Optimum temperatures are a narrow range. Varying from the ideal temperature can drastically change the rate