8.3 Photosynthesis AHL

1. 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 https://bioknowledgy.weebly.com/ http://commons.wikimedia.org/wiki/File:Photosystem_I.jpg http://en.wikipedia.org/wiki/File:PhotosystemII.PNG

2. Understandings, Applications and Skills * ‘across the thylakoid membrane’ is a more accurate statement as reactions occur on either side of the integral proteins.

3. Some vocab: • Phosphorylation: addition of a phosphate ion • Ie…turning ADP into ATP • Chemiosmosis: Flow of ions through a membrane • After creating a concentration gradient, flow of ions back to equilibrium can be harnessed to phosphorylate ADP into ATP (like water behind a dam) • Electron carriers: Used to transfer electrons from one protein to another. As electrons move they are used to pump ions and create a concentration gradient (like a slinky going down steps) • NADP+ and NADPH: High energy molecule used to help make ATP (When NADPH is turned into NADP+ energy is released) • Lysis:splitting/breaking: water is split during photolysis

4. (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 http://i-biology.net/ahl/08-cell-respiration-photosynthesis/8-2-photosynthesis/

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.U1 Light-dependent reactions take place in the intermembrane space of the thylakoids. Splits water, to generate ATP and NADPH for use in light-independent reaction.

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

8. 8.3.U2 Absorption of light by photosystems generates excited electrons. Chlorophyll in photosytem II absorbs light, raising the energy level of electrons, “photoactivation”

9. 8.3.U3 Photolysis of water generates electrons for use in the light-dependent reactions. Energy from photoactivated electrons is used to pump protons (H+) across the thylakoid membrane and create a concentration gradient. The oxygen is released as a waste product.

10. 8.3.U4 Transfer of excited electrons occurs between membrane protein carriers in thylakoid membranes. Electron carriers are proteins in the membrane

11. 8.3.U5/U6 Excited electrons from Photosystem II and are used to generate a proton gradient. Protons move through ATP Synthase through chemiosomosis, and conduct photophosphorylation (ADP  ATP using light)

12. 8.3.U6 ATP synthase in thylakoids generates ATP using the proton gradient. Animation from Sigma Aldrich: http://tinyurl.com/5k99sc

13. 8.3.U6 ATP synthase in thylakoids generates ATP using the proton gradient.

14. 8.3.U7 Excited electrons from Photosystem I are used to reduce NADP+ to NADPH for use in light independent reactions.

15. 8.3.S1 Annotation of a diagram to indicate the adaptations of a chloroplast to its function.

16. Remember: 2.9.S1 Drawing an absorption spectrum for chlorophyll and an action spectrum for photosynthesis. The absorption spectrumshows the absorbance of light by photosynthetic pigments (here chlorophyll) for all the wavelengths of light. The action spectrumshows the rate of photosynthesis for all the wavelengths of light as a % of the maximum possible rate. % 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. http://i-biology.net/ahl/08-cell-respiration-photosynthesis/8-2-photosynthesis/

17. 8.3.U8 Light-independent reactions (Calvin Cycle) take place in the stroma. Glucose is produced, CO2, ATP and NADPH are used

18. Where does the light reaction take place? • Where does the Calvin Cycle take place? • What is the input/output of the light reaction? • What is the input/output of the Calvin cycle?

19. 8.3.U9 Reduced NADP and ATP are produced in the light-dependent reactions. ATP and NADPH (reduced NADP) are produced by the light dependent reactions

20. 8.3.U10 In the light-independent reactions a carboxylase (Rubisco) catalyses the carboxylation of ribulosebisphosphate. (RuBP)

21. 8.3.U11 Glycerate 3-phosphate is reduced to triose phosphate (usable form that has one less phosphate) using reduced NADPH and ATP

22. 8.3.U12 Triose phosphate is used to regenerate RuBP and produce carbohydrates. 8.3.U13 Ribulosebisphosphate is reformed using ATP.

23. 8.3.U12 Light-independent reactions take place in the stroma.

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

25. 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 RuBPfor use in the light dependent reactions;
The photoactivated electrons are passed to NADP / NADP+ reducing it (to NADPH); Light-independent reaction fixes CO2to make glycerate 3-phosphate;
glycerate 3-phosphate becomes reducedto triose phosphate; The reduction uses both NADPH and ATP; why the colourcoding? 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].

26. 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.

27. 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 synthasechannels (embedded in thylakoids membrane) to produce ATP The Stroma Contains rubiscofor carboxylation of RuBPalong with all the other enzymes required for the Calvincycle.

28. 8.3.U14 The structure of the chloroplast is adapted to its function in photosynthesis.

29. 8.3.U14 The structure of the chloroplast is adapted to its function in photosynthesis.

30. 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 keystructures. Use the previous slides [8.3.U14] to add in annotations to show how the structures are adapted to the chloroplast’s function. http://www.ib.bioninja.com.au/_Media/chloroplast_med.jpeg

31. 8.3.S1 Annotation of a diagram to indicate the adaptations of a chloroplast to its function.

32. 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 keystructures 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. http://www.ib.bioninja.com.au/_Media/chloroplast_med.jpeg

33. 8.3.A1 Calvin’s experiment to elucidate the carboxylation of RuBP. Calvin’s experiment used Chlorellaalgaewhich 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). Samplesof algae were taken at different time intervals. The carbon compounds were separated by chromatography and the compounds containing 14C identified by autoradiography. http://bancroft.berkeley.edu/Exhibits/Biotech/Images/3-9lg.jpg

34. 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 labelledglycerate 3-phosphate than any other compound. Samples were taken at different time intervals after exposure to 14C This indicates that glycerate3-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 http://5e.plantphys.net/images/ch08/wt0802a.png http://bancroft.berkeley.edu/Exhibits/Biotech/Images/3-9lg.jpg

35. 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 Kamenand Ruben • The use of Autoradiography to produce patterns of radioactive decay emissions (autoradiograms) http://5e.plantphys.net/images/ch08/wt0802a.png http://bancroft.berkeley.edu/Exhibits/Biotech/Images/3-9lg.jpg

36. 8.3.U2 Light-independent reactions take place in the stroma. Animations detailing the light-independent reactions: http://www.science.smith.edu/departments/Biology/Bio231/calvin.html http://highered.mheducation.com/sites/9834092339/student_view0/chapter39/calvin_cycle.html https://youtu.be/0UzMaoaXKaM