Topic 8.2

1. Topic 8.2 Photosynthesis

2. Electron micrograph of chloroplast • Double outer membrane • Internal membranes called thylakoids which is the location of the light dependent reaction • Internal space of the thylakoid called lumen. • Stroma surrounding the thylakoids and inside the double membrane of the chloroplast. This is the location of the light independent reaction that includes the Calvin cycle. • The stroma often contains starch grains and oil droplets both products of photosynthesis Book page 170

3. 2 parts of photosynthesis Light Dependent Reaction in which • Must have sun • Energy of sun is trapped by chlorophyll molecules (oxidation) • ADP is reduced to ATP and NADP+ reduced to NADPH. • This reaction takes place in the thylakoid membranes. The Light Independent Reaction which • uses the chemical energy from the LDR to fix atmospheric carbon into organic molecules such as glucose. • The process does not require light and can occur in both the light and dark periods. • This reaction takes place in the stroma

4. The Light Dependant Reactions- Chlorophyll molecules are embedded in the thylakoid membrane - Hydrocarbon tail anchors them

5. Photosystems • A photosystem is a group of several hundred pigment molecules (chlorophyll a,chlorophyll b and carotenoids-orange & yellow) • Photon-packet of light • When a photon strikes one pigment molecule of a photosystem, the energy is transferred from molecule to molecule until it reaches a special chlorophyll a molecule

6. This special chlorophyll a is in the reaction center of the photosystem • At the reaction center, an excited electron will be released to start a series of other reactions • This whole process is called Photoactivation

7. Photosystem II and Photosystem I(Non-cyclic electron flow)

8. Non-cyclic electron flow is the predominant route. • When light is absorbed by Photosystem II, an electron is excited to a higher energy level in the reaction center and is captured by the primary acceptor in thylakoid membrane. This creates a “hole” that must be filled in Photosystem II. • An enzyme extracts electrons from H2O in order to replace the electron in Photosystem II. This splits the water molecule (photolysis) into 2 H+ atoms and a oxygen molecule, (which immediately combines with another oxygen molecule and released as O2.)

9. Each photoexcited electron passes from the primary electron acceptor of Photosystem II to Photosystem I via an electron transport chain located in the thylakoid membrane. This is a system of carrier molecules. • As the electrons cascade down the chain, their “fall” to a lower energy level is harnessed to produce ATP. This ATP synthesis is called photophosphorylation because it is adding a high energy phosphate bond to an ADP by using light. ATP is used as energy to fuel the Calvin Cycle of the light independent reactions

10. When an electron reaches the bottom of the electron transport chain, it fills a “hole” in Photosystem I that exists because an excited electron had left it in the same manner as Photosystem II. • Primary electron acceptor of Photosystem I passes electrons to 2nd electron chain. The electron, eventually, is used to reduce a molecule of NADP+ into NADPH. This molecule provides the reducing power for the synthesis of sugar in the Calvin cycle.

11. Photosystem I(Cyclic Photophosphorylation)

12. Under certain conditions, excited electrons take an alternate path which uses only Photosystem I. • When the primary electron acceptor of Photosystem I delivers the excited electron to one of the carrier molecules in the 2nd electron chain (Fd), it has two choices. • It can send it down the 2nd electron chain to produce NADP-H. (as in non-cyclic PP) • It can place back in the 1st electron chain to work its way back to Photosystem I. (cyclic PP)

13. A greater amount of ATP than NADP-H is needed to run the light independent reactions. • Non-cyclic PP produces even amounts of both, so cyclic PP kicks in to increase the amount of ATP. • It may be that high amounts of NADP-H actually dictate when cyclic PP takes place. Cyclic and Non-cyclic PP animation

14. Chemiosmosis(How ATP is synthesized)

15. Chemiosmosis cont. • Water molecules are photolysed on the inside surface of the thylakoid membrane (position 1) • The Oxygen molecules are released to leave, H+ molecules build up in the thylakoid space • The electron transport chains of photophosphorylation also pump in H+ molecules (position 2) • Both of these steps build up a high H+ gradient inside the thylakoid space • By simple diffusion the H+ ions travel across the concentration gradient

16. Chemiosmosis cont. • This is harnessed by ATP synthase (enzyme) which phosphorylates ADP (adds a phosphate bond) to make an ATP • H+ atoms are now available in the stroma for use in reducing NADP+ into NADP-H Chemiosmosis animation

17. Light Independent Reactions • The light independent reactions (referred to as the Calvin cycle) take place in the stroma of the chloroplast • The first reaction involves ribulose biphosphate (RuBP, 5C) which is a product of the Calvin Cycle 1. Ribulose biphosphate carb-oxylase (rubisco) attaches C from CO2 to RuBP making it a 6C product. Called carbon fixation

18. The 6C product of rubisco is immediately split into 2, 3C molecules called glycerate 3-phospate. This process is relatively slow and large amounts of rubisco are needed • Glycerate 3 phosphate is reduced to a different 3C, sugar, triose phosphate (TP) - NADPH provides the reducing power by donating its H+ - ATP also provides the energy needed for this reaction

19. 4. 1/6 of all triose phosphate molecules made combine to become glucose for storage in plant as starch. The other 5/6 go through another series of reactions to regenerate RuBP to restart the Calvin cycle 5. RuBP regeneration C3 + C3 = C6 C6 + C3 = C4 + C5 C4 + C3 = C7 C7 + C3 = C5 + C5 Calvin Cycle animation

20. Structure Large thylakoid surface area Small space inside thylakoids Fluid filled stroma Function Increases area for light absorption Accumulation and concentration of Hydrogen Ions Concentration of Calvin cycle enzymes Structure and Function of Chloroplast

21. Action Spectrum • Shows the rate of photosynthesis at different wavelengths of light • Maximum rates of photosynthesis are at the blue ends and red ends of the visible light spectrum • The lowest rates are in the green and yellow wavelengths • Thus, red and blue light is absorbed by pigments to do photosynthesis, while green and yellow are reflected (that’s why plants look green)

22. Action spectrum vs. Absorption spectrum • Absorbed light represented by action spectrum is a combination of light absorbed by several different pigments • Red and blue light are the peaks and correspond to the rate of photosynthesis

23. Limiting Factors of Photosynthesis • The rate of photosynthesis is affected by light intensity, CO2 conc. and temperature. • Under a specific set of conditions one of these factors will slow down the overall process of photosynthesis. This factor would be called the limiting factor

24. (c) is the normal amount of CO2 in the atmosphere • Because of this, the amount of CO2 is often the limiting factor

25. (c) is the amount of sun in the summer • Light intensity is not a limiting factor for much of the year

26. Temperature could be a limiting factor in many ways (too hot for enzymes, too cool for enzymes, cause evaporation of too much H2O)