2. Photosynthesis 1: Light-Dependent Reactions This may get confusing… try to follow along with the diagram on p160 of your text! Light-Dependent Reactions This may get confusing… try to follow along with the diagram on p160 of your text!

3. Overview  Photosynthesis transforms light energy into chemical energy.  6CO2 + 6H2O + energy -> C6H12O6 + 6O2  Two sets of reactions make up photosynthesis: light dependent and light independent reactions  Water enters the plant through the roots, CO2 enters through the stomata (openings in the leaves)  They then diffuse into the chloroplasts of the cells  Photosynthesis transforms light energy into chemical energy.  6CO2 + 6H2O + energy -> C6H12O6 + 6O2  Two sets of reactions make up photosynthesis: light dependent and light independent reactions  Water enters the plant through the roots, CO2 enters through the stomata (openings in the leaves)  They then diffuse into the chloroplasts of the cells

4. Chloroplasts  The chloroplasts’ inner membranes form interconnected disks called thylakoids that look like flattened sacs and will usually be stacked on top of one another to form grana  Surrounding the grana is a fluid-filled interior called the stroma  The chloroplasts’ inner membranes form interconnected disks called thylakoids that look like flattened sacs and will usually be stacked on top of one another to form grana  Surrounding the grana is a fluid-filled interior called the stroma

5. Absorption of Light Energy  Energy is absorbed from light in packets of energy called photons  Each wavelength (colour) of visible light is associated with photons of one distinct amount of energy  A compound that absorbs that certain amount of light energy is called a pigment  Pigments that absorb energy and then pass it on to another compound are called photosynthetic pigments  Photosynthetic organisms have a variety of photosynthetic pigments, but the main ones are chlorophyll a and chlorophyll b.  Energy is absorbed from light in packets of energy called photons  Each wavelength (colour) of visible light is associated with photons of one distinct amount of energy  A compound that absorbs that certain amount of light energy is called a pigment  Pigments that absorb energy and then pass it on to another compound are called photosynthetic pigments  Photosynthetic organisms have a variety of photosynthetic pigments, but the main ones are chlorophyll a and chlorophyll b.

6.  Chlorophyll a and b absorb most wavelengths other than green ones. They reflect the green.  Carotenoids absorb blue and green wavelengths  Chlorophyll a and b absorb most wavelengths other than green ones. They reflect the green.  Carotenoids absorb blue and green wavelengths

7. Photosystems Capture Energy  Chlorophyll molecules act in clusters embedded in the thylakoid membrane with proteins in the centre of them and make up a photosystem.  Chlorophyll molecules in these photosystems are able to absorb light energy at various wavelengths.  A pigment absorbs a photon, the molecule passes the energy to a unique pair of chlorophyll a molecules called the reaction centre.  An antenna complex includes all the surrounding pigment molecules that gather the light energy, they then transfer that energy to the reaction centre  When the reaction centre gets the energy, it “excites” an electron (raises it to a higher energy level) and is then passed to an electron-accepting molecule  Chlorophyll molecules act in clusters embedded in the thylakoid membrane with proteins in the centre of them and make up a photosystem.  Chlorophyll molecules in these photosystems are able to absorb light energy at various wavelengths.  A pigment absorbs a photon, the molecule passes the energy to a unique pair of chlorophyll a molecules called the reaction centre.  An antenna complex includes all the surrounding pigment molecules that gather the light energy, they then transfer that energy to the reaction centre  When the reaction centre gets the energy, it “excites” an electron (raises it to a higher energy level) and is then passed to an electron-accepting molecule

8.  Chloroplasts of plants have 2 photosystems called photosystem I (P700) and photosystem II (P680) based on their wavelengths of light they absorb.

9. Light Dependent Reactions Step 1:  P680 molecule in the reaction centre of photosystem II absorbs a photon, exciting an electron pair.  Excited electron pair leaves the photosystem II, leaving the P680 positively charged  P680+ attracts electrons from water and splits it to H+ and O2 gas.  The O2 is released by the plant  2 H2O molecules are needed to get 4 electrons and one O2 molecule.  The electrons are absorbed by the P680+ and can now excite more of them with light photon energy.  P680 molecule in the reaction centre of photosystem II absorbs a photon, exciting an electron pair.  Excited electron pair leaves the photosystem II, leaving the P680 positively charged  P680+ attracts electrons from water and splits it to H+ and O2 gas.  The O2 is released by the plant  2 H2O molecules are needed to get 4 electrons and one O2 molecule.  The electrons are absorbed by the P680+ and can now excite more of them with light photon energy.

10. Light Dependent Reactions Step 2:  From the electron acceptor, the electrons are transferred one by one along a series of eletron-carrying molecules (electron transport system)  Each transfer of electrons releases a small amount of energy  Energy is used by a b6-f complex to pump hydrogen ions form the stroma across the thylakoid membrane  This creates an electrochemical gradient  From the electron acceptor, the electrons are transferred one by one along a series of eletron-carrying molecules (electron transport system)  Each transfer of electrons releases a small amount of energy  Energy is used by a b6-f complex to pump hydrogen ions form the stroma across the thylakoid membrane  This creates an electrochemical gradient

11. Light Dependent Reactions Step 3:  While steps 1 and 2 are taking place, light energy is also being absorbed by photosystem I.  Energy is transferred to the reaction centre P700 where electrons are being excited.  Excited electrons are passed to an electron acceptor but this time the electrons are replaced by electrons coming from the end of the electron transport system of photosystem II.  While steps 1 and 2 are taking place, light energy is also being absorbed by photosystem I.  Energy is transferred to the reaction centre P700 where electrons are being excited.  Excited electrons are passed to an electron acceptor but this time the electrons are replaced by electrons coming from the end of the electron transport system of photosystem II.

12. Light Dependent Reactions Step 4:  Electrons that were received by the electron acceptor from photosystem I are used by NADP+ to form NADPH.  The reducing power of NADPH will be used in the light independent reactions.  Electrons that were received by the electron acceptor from photosystem I are used by NADP+ to form NADPH.  The reducing power of NADPH will be used in the light independent reactions.

14. Chemiosmosis  Movement of hydrogen ions through ATP synthase creates ATP  Called photophosphorylation  Movement of hydrogen ions through ATP synthase creates ATP  Called photophosphorylation

16. Noncyclic and Cyclic Photophosphorylation  Non-cyclic is what we just looked at. NADPH accepts the electron and then moves to the Calvin Cycle.  The passage of one electron pair generates 1 NADPH and slightly more than 1 ATP.  This doesn’t produce enough to go through with the light-independent reactions, so chloroplasts will do cyclic photophosphorylation.  Non-cyclic is what we just looked at. NADPH accepts the electron and then moves to the Calvin Cycle.  The passage of one electron pair generates 1 NADPH and slightly more than 1 ATP.  This doesn’t produce enough to go through with the light-independent reactions, so chloroplasts will do cyclic photophosphorylation.

17.  Excited electron pairs leave photosystem I and are passed to an electron acceptor  The pass back to the b6-f complex the proton gradient is generated in the same manner as in noncyclic photophosphorylation and ATP synthesis from the electrochemical gradient can occur.  Neither NADPH nor oxygen is produced  Excited electron pairs leave photosystem I and are passed to an electron acceptor  The pass back to the b6-f complex the proton gradient is generated in the same manner as in noncyclic photophosphorylation and ATP synthesis from the electrochemical gradient can occur.  Neither NADPH nor oxygen is produced