Light Dependent / Light Independent Reactions Biology Module 1 Light Dependent / Light Independent Reactions Presentation by Dr. Han Ong and Gayle Ross Arkansas Science Specialist

Where do the reactions happen inside the chloroplast? Biology Module 1 Where do the reactions happen inside the chloroplast? H2O CO2 Light Independent/ Calvin Cycle Light dependent The light dependent reaction supplies the energy for the light independent reaction ATP NADPH Light dependent supplies energy for Calvin Cycle O2 Carbohydrates Arkansas Science Specialist 2

Biology Module 1 Light Dependent Arkansas Science Specialist 3

Capturing light through light absorbing pigments Biology Module 1 Capturing light through light absorbing pigments Photosynthetic organisms posses specialized pigments to harvest visible and infrared light These pigments are called chromophores Chromophores exist in many different forms, depending on what types of light are available to be harvested Look at the size and complexity of these molecules. In living systems, complex molecules require lots of energy to produce, so they must be very important.  - carotene Phycoerythrin Chlorophyll Arkansas Science Specialist 4

Absorption spectra of chromophores Biology Module 1 Absorption spectra of chromophores A peak is where the specific light quality is absorbed Arkansas Science Specialist 5

Biology Module 1 Why are plants green? Arkansas Science Specialist 6

Biology Module 1 Why are plants green? Arkansas Science Specialist 7

Biology Module 1 Arkansas Science Specialist 8

Absorption spectra of chromophores Biology Module 1 Absorption spectra of chromophores A peak is where the light is absorbed Arkansas Science Specialist 9

How do chromophores harvest light energy? Biology Module 1 How do chromophores harvest light energy? Arkansas Science Specialist 10

Photoexcitation: harvesting energy from excited molecules Biology Module 1 Photoexcitation: harvesting energy from excited molecules I. Resonance transfer Excited state An electron is at rest in a ground state electron Ground state Molecule I Molecule II 1 Arkansas Science Specialist 11

Photoexcitation: harvesting energy from excited molecules Biology Module 1 Photoexcitation: harvesting energy from excited molecules I. Resonance transfer Excited state Radiation or photon energy (usually from light) irradiates the ground state electron Energy (light) Ground state Molecule I Molecule II 2 Arkansas Science Specialist 12

Photoexcitation: harvesting energy from excited molecules Biology Module 1 Photoexcitation: harvesting energy from excited molecules I. Resonance transfer Excited state The photon energy shifts the electron to an excited state Ground state Molecule I Molecule II 3 Arkansas Science Specialist 13

Photoexcitation: harvesting energy from excited molecules Biology Module 1 Photoexcitation: harvesting energy from excited molecules I. Resonance transfer Excited state Because an electron cannot remain at an excited state for long, it starts to lose its energy as it returns to the ground state Ground state Molecule I Molecule II 4 Arkansas Science Specialist 14

Photoexcitation: harvesting energy from excited molecules Biology Module 1 Photoexcitation: harvesting energy from excited molecules I. Resonance transfer Excited state The energy released from the first electron excites a neighboring electron from a ground state to an excited state. This process is called resonance transfer. Ground state Molecule I Molecule II 5 Arkansas Science Specialist 15

Photoexcitation: harvesting energy from excited molecules Biology Module 1 Photoexcitation: harvesting energy from excited molecules I. Resonance transfer Excited state The process continues as the energy is relayed to excite other neighboring electrons. Ground state Molecule I Molecule II 6 Arkansas Science Specialist 16

Photoexcitation: harvesting energy from excited molecules Biology Module 1 Photoexcitation: harvesting energy from excited molecules II. Electron transfer Excited state The process of electron transfer begins the same way as resonance transfer Energy (light) Ground state Molecule I Molecule II 7 Arkansas Science Specialist 17

Photoexcitation: harvesting energy from excited molecules Biology Module 1 Photoexcitation: harvesting energy from excited molecules II. Electron transfer Excited state However, instead of returning to the ground state, the electron of Molecule I is transferred directly to Molecule II, where it loses a little bit of energy Ground state Molecule I Molecule II 8 Arkansas Science Specialist 18

Photoexcitation: harvesting energy from excited molecules Biology Module 1 Photoexcitation: harvesting energy from excited molecules II. Electron transfer Excited state However, instead of returning to the ground state, the electron of Molecule I is transferred directly to Molecule II, where it loses a little bit of energy Ground state Molecule I Molecule II 9 Arkansas Science Specialist 19

Photoexcitation: harvesting energy from excited molecules Biology Module 1 Photoexcitation: harvesting energy from excited molecules II. Electron transfer Excited state The lost of the electron makes Molecule I positively charged, and Molecule II negatively charged. The electron continues to be transferred to other molecules, until all energy has been lost. Ground state Molecule I Molecule II 10 Arkansas Science Specialist 20

Resonance & electron transfer in chlorophyll Biology Module 1 Resonance & electron transfer in chlorophyll Arkansas Science Specialist 21

Biology Module 1 Harvested electron is channeled to the thylakoid membrane to start the light dependent reaction The electron is funneled by chlorophyll pigments to activate the first photosystem (Photosystem II or P680) It is the second photosystem discovered, and absorbs light at 680nm light Photosystem II (P680) Cytochrome bf Photosystem I (P700) Remember, photosystem I is named I because it was discovered first! A A Arkansas Science Specialist 22

Biology Module 1 Harvested electron is channeled to the thylakoid membrane to start the light dependent reaction Energy transferred to PSII allows it to draw an electron from a water molecule The electron is transferred to a series of electron acceptor molecules Oxygen and ATP molecules are produced light Photosystem II (P680) Cytochrome bf Photosystem I (P700) A A H2O O2 ATP production Arkansas Science Specialist 23

Biology Module 1 Harvested electron is channeled to the thylakoid membrane to start the light dependent reaction The electron is passed on to cytochrome bf, an intermediate acceptor molecule More ATP is produced The electron is passed on to more acceptor molecules light Photosystem II (P680) Cytochrome bf Photosystem I (P700) A A H2O O2 ATP ATP production production Arkansas Science Specialist 24

Biology Module 1 Harvested electron is channeled to the thylakoid membrane to start the light dependent reaction The electron is passed on to Photosystem I (or P700), and it is excited again by harvested light energy The electron is passed through more aceptor molecules Energy transferred from this process is used to produce a NADPH molecule light light NADPH production A Photosystem II (P680) Cytochrome bf Photosystem I (P700) A A H2O O2 ATP ATP production production Arkansas Science Specialist 25

A different view of the light dependent reaction Biology Module 1 A different view of the light dependent reaction A A A A A A light A Energy potential A A A light Photosystem I (P700) Photosystem II (P680) Products: ATP ATP H2O O2 NADPH Arkansas Science Specialist 26

Where do the reactions happen inside the chloroplast? Biology Module 1 Where do the reactions happen inside the chloroplast? H2O CO2 NADP+ ADP Light dependent Light Independent/ Calvin Cycle ATP NADPH Light independent uses energy supplied by light reaction to reduce atmospheric CO2 into carbohydrates O2 Carbohydrates Arkansas Science Specialist 27

Light Independent/ Calvin Cycle Biology Module 1 Arkansas Science Specialist 28

Light Independent is a cyclic process Biology Module 1 Light Independent is a cyclic process Several acceptor molecules are used in the process to produce a carbohydrate molecule These acceptor molecules must then be re-cycled to produce more carbohydrates This cycle is called the Calvin cycle Light Independent = Calvin Cycle Arkansas Science Specialist 29

Biology Module 1 Melvin Calvin (1911-1997) University of California, Berkeley Tracked radioactive-labeled carbon (in CO2) as it travels through the plant during photosynthesis Found that the radioactive-labeled carbon is incorporated into carbohydrates formed by the plant The process was named the Calvin-Benson-Bassham cycle Nobel Prize in Chemistry, 1961 Arkansas Science Specialist 30

The Calvin Cycle- Light Independent CO2 Rubisco Biology Module 1 The Calvin Cycle- Light Independent CO2 Three carbon dioxide molecules are fixed in the stroma of chloroplasts by an enzyme known as ribulose-1,5-biphosphate carboxylase….or simply called Rubisco Rubisco There are 3 parts of the calvin cycle Blue-fix Carbon from the air, Green-carbon is reduced or assembled into carbohydrates, Yellow regenerates the molecules involved Rubisco is the most abundant enzyme on Earth: about 40 million tons, or about 20 pounds per every living person but it is only found in photosynthetic organisms Arkansas Science Specialist 31

The Calvin Cycle- Light Independent CO2 Carbohydrates ATP ADP NADPH Biology Module 1 The Calvin Cycle- Light Independent CO2 Fixation One ATP and one NADPH molecule (supplied by the light reaction) is used to further reduce the fixed carbons Reduction ATP ADP NADPH NADP+ Carbohydrates Arkansas Science Specialist 32

The Calvin Cycle Light Independent CO2 Carbohydrates ADP ATP Biology Module 1 ATP ADP Another ATP molecule is used to recycle the acceptor molecule in order to receive more fixed carbon dioxide, and the cycle continues The Calvin Cycle Light Independent CO2 Fixation Reduction Carbohydrates Regeneration Arkansas Science Specialist 33

The Calvin Cycle- Light Independent (a more complex view) Biology Module 1 The Calvin Cycle- Light Independent (a more complex view) Arkansas Science Specialist 34

How efficient is the photosynthetic process? Biology Module 1 How efficient is the photosynthetic process? The efficiency to fix and store one molecule of CO2 = ~28% Only certain wavelength of light can be absorbed (400 - 700nm), which equals ~43% of total solar radiation Canopy limits light absorption to 80% Respiration and other processes use 33% of energy stored in leaves, leaving 67% of energy Overall efficiency = 0.28 X 0.43 X 0.8 X 0.67 = 0.065 Therefore, as a biochemical process, photosynthesis has a 6.5% efficiency! Arkansas Science Specialist 35

Frameworks Strand: Molecules and Cells Biology Module 1 Frameworks Strand: Molecules and Cells Standard 3: Students shall demonstrate an understanding of how cells obtain and use energy (energetics). MC.3.B.1 Compare and contrast the structure and function of mitochondria and chloroplasts MC.3.B.2 Describe and model the conversion of stored energy in organic molecules into usable cellular energy (ATP): glycolysis citric acid cycle electron transport chain MC.3.B.3 Compare and contrast aerobic and anaerobic respiration: lactic acid fermentation alcoholic fermentation MC.3.B.4 Describe and model the conversion of light energy to chemical energy by photosynthetic organisms: light dependent reactions light independent reactions MC.3.B.5 Compare and contrast cellular respiration and photosynthesis as energy conversion pathways Arkansas Science Specialist