1. Photosynthesis Light Dependent Reactions Part Two Photosystem I & Photosystem II

2. Photosynthesis is a two stage process The first process is the Light Dependent Process (Light Reactions), requires the direct energy of light to make energy carrier molecules that are used in the second process. The Light Independent Process (or Dark Reactions) occurs when the products of the Light Reaction are used to form C-C covalent bonds of carbohydrates. The Light Reactions occur in the grana and the Dark Reactions take place in the stroma of the chloroplasts.

4. Photosynthesis includes of take place in takes place in uses to produce use Light- dependent reactions Calvin cycle Thylakoid membranes StromaNADPH ATP Energy from sunlight ATPNADPHO2O2 Chloroplasts High-energy sugars Section 8-3 Concept Map

5. Chloroplast Light O2O2 Sugars CO 2 Light- Dependent Reactions Calvin Cycle NADPH ATP ADP + P NADP + Chloroplast Section 8-3 Figure 8-7 Photosynthesis: An Overview

6. ADPATP Energy Adenosine diphosphate (ADP) + PhosphateAdenosine triphosphate (ATP) Partially charged battery Fully charged battery Section 8-1 Figure 8-3 Comparison of ADP and ATP to a Battery

7. In the Light Dependent Processes (Light Reactions) light strikes chlorophyll a in such a way as to excite electrons to a higher energy state. In a series of reactions the energy is converted (along an electron transport chain) into ATP and NADPH. Water is split in the process, releasing oxygen as a by-product of the reaction. The ATP and NADPH are used to make C-C bonds in the Light Independent Process (Dark Reactions).

8. Photolysis: Oxidizing Water Removing Hydrogen This part of photosynthesis occurs in the granum of a chloroplast where light is absorbed by chlorophyll; a type of photosynthetic pigment that converts the light to chemical energy. This reacts with water (H 2 O) and splits the oxygen and hydrogen molecules apart. From this dissection of water, the oxygen is released as a by-product while the reduced hydrogen acceptor makes its way to the second stage of photosynthesis, the Calvin cycle or Light Independent Reaction.

9. The general reaction of photosynthetic photolysis can be given as: H 2 A + 2 photons (light) 2e - + 2H + + A The chemical nature of "A" depends on the type of organism. In purple sulfur bacteria, hydrogen sulfide (H 2 S) is oxidized to sulfur (S). In oxygenic photosynthesis, water (H 2 O) serves as a substrate for photolysis resulting in the generation of diatomic oxygen (O 2 ) from carbon dioxide (CO 2 ). This is the process which returns oxygen to earth's atmosphere.

10. Wavelength & Photolysis The effectiveness of photons of different wavelengths depends on the absorption spectra of the photosynthetic pigments in the organism. Chloroohylls absorb light in the violet-blue and red parts of the spectrum, while accessory pigments capture other wavelengths as well.

11. Each absorbed photon causes the formation of an exciton (an electron excited to a higher energy state) in the pigment molecule. The energy of the exciton is transferred to a chlorophyll molecule (P680, where P stands for pigment and 680 for its absorption maximum at 680 nm) in the reaction center of photosystem II via resonance energy transfer. P680 can also directly absorb a photon at a suitable wavelength.

12. Photolysis during photosynthesis occurs in a series of light-driven oxidation events. The energized electron (exciton) of P680 is captured by a primary electron acceptor of the photosynthetic electron transfer chain and thus exits photosystem II.

13. In order to repeat the reaction, the electron in the reaction center needs to be replenished. This occurs by oxidation of water in the case of oxygenic photosynthesis. The electron-deficient reaction center of photosystem II (P680*) is the strongest biological oxidizing agent yet discovered, which allows it to break apart molecules as stable as water.

14. The water-splitting reaction is catalyzed by the oxygen evolving complex of photosystem II. This protein-bound inorganic complex contains four manganese ions, plus calcium and chloride ions as cofactors. Two water molecules are complexed by the manganese cluster, which then undergoes a series of four electron removals (oxidations) to replenish the reaction center of photosystem II. At the end of this cycle, free oxygen (O 2 ) is generated and the hydrogen of the water molecules has been converted to four protons released into the thylakoid lumen.

15. These protons, as well as additional protons pumped across the thylakoid membrane coupled with the electron transfer chain, form a proton gradient across the membrane that drives photophosphorylation and the generation of chemical energy in the form of ATP.

16. The electrons reach thep700 reaction center of photosystem I where they are energized again by light. They are passed down another electron transfer chain and finally combine with the coenzyme NADP + and protons outside the thylakoids to NADPH. The net oxidation reaction of water photolysis can be written as: 2H 2 O + 2NADP + + 8 photons (light) 2NADPH + 2H + + O 2

17. The free energy change (ΔG) for this reaction is 102 kilocalories per mole. Since the energy of light at 700 nm is about 40 kilocalories per mole of photons, approximately 320 kilocalories of light energy are available for the reaction. Approximately one-third of the available light energy is captured as NADPH during photolysis and electron transfer. An equal amount of ATP is generated by the resulting proton gradient. Oxygen as a byproduct is of no further use to the reaction and thus released into the atmosphere.

19. Hydrogen Ion Movement Photosystem II Inner Thylakoid Space Thylakoid Membrane Stroma ATP synthase Electron Transport Chain Photosystem IATP Formation Chloroplast Section 8-3 Photosystem I & II Free H+ ions from photolysis produce ATP through ATP Synthase protein The free e- produce ATP & NADPH through the electron transport chain

20. Electron Transport Chain A series of coupled oxidation / reduction reactions where electrons are passed like hot potatoes from one membrane-bound protein/enzyme to another before being finally attached to a terminal electron acceptor (usually oxygen or NADPH). ATP is formed by this process. Coupled series of oxidation/reduction reactions during which ATP is generated by energy transfer as electrons move from high reducing state to lower reducing state

21. Electron Transport Chain in the Thylakoid Membrane

23. In the Light Independent Process, carbon dioxide from the atmosphere (or from water for aquatic/marine organisms) is captured and modified by the addition of Hydrogen to form carbohydrates (general formula of carbohydrates is [CH 2 O] n ).

24. Carbon Fixation The incorporation of carbon dioxide into organic compounds is known as carbon fixation. The energy for this comes from the first phase of the photosynthetic process. Living systems cannot directly utilize light energy, but can, through a complicated series of reactions, convert it into C-C bond energy that can be released by glycolysis and other metabolic processes.

25. What are Photosystems Photosystems are clusters of several hundred molecules of chlorophyll in a thylakoid where photosynthesis takes place. Eukaryotes have two types of photosystems: I and II. The series of green photoreceptive pigments involved in the light reactions, which occur in the thylakoids of the chloroplast (in eukaryotes). Energy from light is passed to the electrons as they move through the photosystem pigments

26. Eukaryotes have Photosystem II and Photosystem I. Photosystem I uses chlorophyll a, in the form referred to as P700. Photosystem II uses a form of chlorophyll a known as P680. Both "active" forms of chlorophyll a function in photosynthesis due to their association with proteins in the thylakoid membrane.

28. Hydrogen Ion Movement Photosystem II Inner Thylakoid Space Thylakoid Membrane Stroma ATP synthase Electron Transport Chain Photosystem IATP Formation Chloroplast Section 8-3 Figure 8-10 Light-Dependent Reactions Photosystem I & II Electrons (e-) move along proteins, the thylakoid membrane releasing energy at each step (similar to a marble going down a marble game) and produces ATP from photosystem II and NADPH from photosystem I

29. Photophosphorylation Photophosphorylation is the process of converting energy from a light-excited electron into the pyrophosphate bond of an ADP molecule. This occurs when the electrons from water are excited by the light in the presence of P680. The energy transfer is similar to the chemiosmotic electron transport occurring in the mitochondria.

30. Photolysis & Photophosphorlation Light energy causes the removal of an electron from a molecule of P680 that is part of Photosystem II. The P680 requires an electron, which is taken from a water molecule, breaking the water into H + ions and O -2 ions. These O -2 ions combine to form the diatomic O 2 that is released.

31. The electron is "boosted" to a higher energy state and attached to a primary electron acceptor, which begins a series of redox reactions, passing the electron through a series of electron carriers (proteins), eventually attaching it to a molecule in Photosystem I.

32. Light acts on a molecule of P700 in Photosystem I, causing an electron to be "boosted" to a still higher potential. The electron is attached to a different primary electron acceptor (that is a different molecule from the one associated with Photosystem II). The electron is passed again through a series of redox reactions, eventually being attached to NADP + and H + to form NADPH, an energy carrier needed in the Light Independent Reaction (Calvin Cycle)

33. The electron from Photosystem II replaces the excited electron in the P700 molecule. There is thus a continuous flow of electrons from water to NADPH. Electrons may not enter photosystem I if NADPH is not needed for the Calvin Cycle or when cells may require additional ATP, or when there is no NADP + to reduce to NADPH. In Photosystem II, the pumping to H ions into the thylakoid and the conversion of ADP + P into ATP is driven by electron gradients established in the thylakoid membrane and the ATP synthase protein

34. Noncyclic photophosphorylation Produces ATP & NADPH

35. Cyclic Photophosphorylation Produce ATP

36. Chemiosmosis as it operates in photophosphorylation within a chloroplast

37. Credits http://staff.jccc.net/pdecell/photosyn/lightreact. html http://staff.jccc.net/pdecell/photosyn/lightreact. html http://www.emc.maricopa.edu/faculty/farabee/b iobk/biobookps.html http://www.emc.maricopa.edu/faculty/farabee/b iobk/biobookps.html http://www.biology-online.org http://en.wikipedia.org/wiki/Photodissociation Damon, Alan, Randy McGonegal, and Patricia Tosto. Higher Level Biology Developed Specifically for the IB Diploma. Edinburg England: Heinemann, 2007. Print.