1. Essential idea: Light energy is converted into chemical energy.
8.3 Photosynthesis AHL
Essential idea: Light energy is converted into chemical energy.
The images show Photosystem II (right) and Photosystem I (left). These protein complexes contain a number of chlorophyll and other pigments which allow them to absorb light energy. The photosystems use light energy to excite electrons and split water molecules freeing hydrogen ions (Photosystem II only). These two process provide the energy and some of the key ingredients required to produce glucose.
By Chris Paine

2. Understandings, Applications and Skills
Statement
Guidance
8.3.U1
Light-dependent reactions take place in the intermembrane space of the thylakoids.*
8.3.U2
Light-independent reactions take place in the stroma.
8.3.U3
Reduced NADP and ATP are produced in the light-dependent reactions.
8.3.U4
Absorption of light by photosystems generates excited electrons.
8.3.U5
Photolysis of water generates electrons for use in the light-dependent reactions.
8.3.U6
Transfer of excited electrons occurs between carriers in thylakoid membranes.
8.3.U7
Excited electrons from Photosystem II are used to contribute to generate a proton gradient.
8.3.U8
ATP synthase in thylakoids generates ATP using the proton gradient.
8.3.U9
Excited electrons from Photosystem I are used to reduce NADP.
8.3.U10
In the light-independent reactions a carboxylase catalyses the carboxylation of ribulose bisphosphate.
8.3.U11
Glycerate 3-phosphate is reduced to triose phosphate using reduced NADP and ATP.
8.3.U12
Triose phosphate is used to regenerate RuBP and produce carbohydrates.
8.3.U13
Ribulose bisphosphate is reformed using ATP.
8.3.U14
The structure of the chloroplast is adapted to its function in photosynthesis.
8.3.A1
Calvin’s experiment to elucidate the carboxylation of RuBP.
8.3.S1
Annotation of a diagram to indicate the adaptations of a chloroplast to its function.
* ‘across the thylakoid membrane’ is a more accurate statement as reactions occur on either side of the integral proteins.

3. Carbon is ‘fixed’ from carbon dioxide and used to produce to glucose.
(From SL) 2.9.U1 Photosynthesis is the production of carbon compounds in cells using light energy.
Photosynthesis is a metabolic pathway. Carbon dioxide and along with water is used to produce carbohydrates. Oxygen is released as a waste gas.
Light energy is transferred to chemical energy stored in the glucose molecule
Water is split: the hydrogen is used to help in the production of glucose, but the oxygen is excreted as a waste gas.
Carbon is ‘fixed’ from carbon dioxide and used to produce to glucose.
n.b. metabolic pathways are controlled by enzymes

4. A great narrated animation that covers photosynthesis in detail:
Watch, take notes and use the rest of the presentation to clarify your understanding and map your notes against objectives.

5. 8.3.S1 Annotation of a diagram to indicate the adaptations of a chloroplast to its function.
As the presentation progresses take note of how the structure of a chloroplast is adapted to its function.

6. 8.3.S1 Annotation of a diagram to indicate the adaptations of a chloroplast to its function.

7. 8.3.U1 Light-dependent reactions take place in the intermembrane space of the thylakoids.

8. 8.3.U1 Light-dependent reactions take place in the intermembrane space of the thylakoids.

9. Animations detailing the light-dependent reactions:
8.3.U1 Light-dependent reactions take place in the intermembrane space of the thylakoids.
Animations detailing the light-dependent reactions:

10. 8.3.U4 Absorption of light by photosystems generates excited electrons.

11. 8.3.U5 Photolysis of water generates electrons for use in the light-dependent reactions.

12. Electron carriers are proteins in the membrane
8.3.U6 Transfer of excited electrons occurs between carriers in thylakoid membranes.
Electron carriers are proteins in the membrane

13. 8.3.U7 Excited electrons from Photosystem II are used to contribute to generate a proton gradient.

14. Animation from Sigma Aldrich: http://tinyurl.com/5k99sc
8.3.U8 ATP synthase in thylakoids generates ATP using the proton gradient.
Animation from Sigma Aldrich:

15. 8.3.U8 ATP synthase in thylakoids generates ATP using the proton gradient.

16. 8.3.U9 Excited electrons from Photosystem I are used to reduce NADP.

17. Remember: 2.9.S1 Drawing an absorption spectrum for chlorophyll and an action spectrum for photosynthesis.
The action spectrum shows the rate of photosynthesis for all the wavelengths of light as a % of the maximum possible rate.
The absorption spectrum shows the absorbance of light by photosynthetic pigments (here chlorophyll) for all the wavelengths of light.
% of the maximum rate of photosynthesis
The presence photosystems I & II and the different proportions of of pigments explains the mismatch between the spectrums, e.g. the double peaks in the red wavelengths of light.

18. 8.3.U2 Light-independent reactions take place in the stroma.

19. Animations detailing the light-independent reactions:
8.3.U2 Light-independent reactions take place in the stroma.
Animations detailing the light-independent reactions:

20. 8.3.U3 Reduced NADP and ATP are produced in the light-dependent reactions.
ATP and NADPH (reduced NADP) are produced by the light dependent reactions

21. 8.3.U10 In the light-independent reactions a carboxylase catalyses the carboxylation of ribulose bisphosphate.

22. 8.3.U11 Glycerate 3-phosphate is reduced to triose phosphate using reduced NADP and ATP.

23. 8.3.U12 Triose phosphate is used to regenerate RuBP and produce carbohydrates.
8.3.U13 Ribulose bisphosphate is reformed using ATP.

24. 8.3.U2 Light-independent reactions take place in the stroma.

25. Explain how the light-independent reactions of photosynthesis rely on the light-dependent reactions. (6 marks)

26. Light causes photoactivation / excitation of electrons;
Explain how the light-independent reactions of photosynthesis rely on the light-dependent reactions. (6 marks)
Light causes photoactivation / excitation of electrons;
This leads to the generation of both ATP and NADPH in the light dependent reactions;;
The flow of electrons causes pumping of protons into thylakoid;
ATP formation when protons pass back across thylakoid membrane;
ATP needed to regenerate RuBP for use in the light dependent reactions;
The photoactivated electrons are passed to NADP / NADP+ reducing it (to NADPH);
Light-independent reaction fixes CO2 to make glycerate 3-phosphate;
glycerate 3-phosphate becomes reduced to triose phosphate;
The reduction uses both NADPH and ATP;
why the colour coding?
A good understanding of photosynthesis should enable you to, not only explain the reactions, but to connect the reactions (above), compare the process with other similar processes (e.g. respiration [8.2]) and to relate it to your understanding of other topics, e.g. plant biology [9].

27. 8.3.U14 The structure of the chloroplast is adapted to its function in photosynthesis.
Palisade cells are found close to the top surface of leaves. They contain a high density of chloroplasts to enable efficient absorption of light.

28. Thylakoid membrane & stacked discs (grana)
8.3.U14 The structure of the chloroplast is adapted to its function in photosynthesis.
Thylakoid membrane & stacked discs (grana)
Thylakoids provide a large surface area for light absorption and light dependent reactions
Chlorophyll (and other pigments) molecules are grouped together to form the photosystems which are embedded in the membrane along with the electron carriers.
folds in thylakoid allow photosystems and electron carriers to be close together
Thylakoid spaces
The spaces collect H+ for chemiosmosis, the low volume enables a the H+ gradient to generated rapidly.
H+ flows back to the stroma, down the H+ gradient, through ATP synthase channels (embedded in thylakoids membrane) to produce ATP
The Stroma
Contains rubisco for carboxylation of RuBP along with all the other enzymes required for the Calvin cycle.

29. Compare and contrast chloroplasts and mitochondria
8.3.U14 The structure of the chloroplast is adapted to its function in photosynthesis.
Compare and contrast chloroplasts and mitochondria

30. Compare and contrast chloroplasts and mitochondria
8.3.U14 The structure of the chloroplast is adapted to its function in photosynthesis.
Compare and contrast chloroplasts and mitochondria

31. 8.3.S1 Annotation of a diagram to indicate the adaptations of a chloroplast to its function.
The three diagrams of a chloroplast show a 2D (left) and (bottom left) 3D diagrams plus a coloured electron micrograph (bottom right). Each diagram is labelled to show how to identify the key structures.
Use the previous slides [8.3.U14] to add in annotations to show how the structures are adapted to the chloroplast’s function.

32. 8.3.S1 Annotation of a diagram to indicate the adaptations of a chloroplast to its function.

33. 8.3.S1 Annotation of a diagram to indicate the adaptations of a chloroplast to its function.

34. 8.3.S1 Annotation of a diagram to indicate the adaptations of a chloroplast to its function.
The three different diagrams of a chloroplast show a 2D (left) and 3D diagrams (bottom left) plus a coloured electron micrograph (bottom right) and how to identify the key structures on each. Use the previous slides [8.3.U14] to add annotations to show how the different structures dictate its function.
Extension questions:
Explain why chloroplasts possess DNA.
State the function of the ribosomes present in the stroma
Explain how ribosomes are linked to photosynthesis.

35. Use the animations to learn about Calvin’s experiments
8.3.A1 Calvin’s experiment to elucidate the carboxylation of RuBP.
Use the animations to learn about Calvin’s experiments

36. 8.3.A1 Calvin’s experiment to elucidate the carboxylation of RuBP.
Calvin’s experiment used Chlorella algae which was placed in a thin glass vessel (called the lollipop vessel).
The Algae was given plenty of light, carbon dioxide (CO2) and hydrogen carbonate (HCO3-) containing normal carbon (12C).
At the start of the experiment the carbon compounds were replaced with compounds containing radioactive carbon (14C).
Samples of algae were taken at different time intervals.
The carbon compounds were separated by chromatography and the compounds containing 14C identified by autoradiography.

37. Calvin’s experiment analysed the results using autoradiograms
8.3.A1 Calvin’s experiment to elucidate the carboxylation of RuBP.
Calvin’s experiment analysed the results using autoradiograms
After only 5 seconds there is more labelled glycerate 3-phosphate than any other compound.
Samples were taken at different time intervals
after exposure to 14C
This indicates that glycerate 3-phosphate is the first product of carbon fixation
After 30 seconds a range of different labelled compounds occur showing the intermediate and final products of the light-independent reactions

38. 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)
Calvin’s experiment and his subsequent discoveries were only possible due to improvements in technology. Key developments in that process include:
The discovery of 14C in 1945 by Kamen and Ruben
The use of Autoradiography to produce patterns of radioactive decay emissions (autoradiograms)

39. Bibliography / Acknowledgments
Jason de Nys