Chapter 8 Light Reactions

Need To Know How photosystems convert light energy into chemical energy. How linear electron flow in the light reactions results in the formation of ATP, NADPH, and O2.

Calvin Cycle Light Reactions Figure 8.UN02 H2O CO2 Light NADP ADP Calvin Cycle Light Reactions ATP NADPH O2 [CH2O] (sugar) 3

(a) Excitation of isolated chlorophyll molecule Energy of electron Figure 8.11 Photon (fluorescence) Ground state (b) Fluorescence Excited Chlorophyll molecule Heat e− (a) Excitation of isolated chlorophyll molecule Energy of electron Photon (fluorescence) Ground state Excited Chlorophyll molecule Heat e− (a) Excitation of isolated chlorophyll molecule Energy of electron Ground state Excited Chlorophyll molecule e− (a) Excitation of isolated chlorophyll molecule Energy of electron Chlorophyll molecule Excited state e− Photon Photon When a pigment absorbs light, it goes from a ground state to an excited state, which is unstable. When excited electrons fall back to the ground state, photons are given off, an afterglow called fluorescence. If illuminated, an isolated solution of chlorophyll will fluoresce, giving off light and heat. 4

Mill makes ATP NADPH Photosystem II Photosystem I Photon Photon Figure 8.14 Mill makes ATP NADPH Photon Photon Photosystem II Photosystem I 5

There are two reaction centers Linear electron flow P680 P700 There are two types of photosystems in the thylakoid membrane Photosystem II (PS II) functions first (the numbers reflect order of discovery) and is best at absorbing a wavelength of 680 nm. The reaction-center chlorophyll a of PS II is called P680. Photosystem I (PS I) is best at absorbing a wavelength of 700 nm The reaction-center chlorophyll a of PS I is called P700. Linear electron flow involves the flow of electrons through both photosystems to produce ATP and NADPH using light energy. P680 P700

How a photosystem harvests light! Figure 8.12a Photosystem STROMA Photon Light- harvesting complexes Reaction- center complex Primary electron acceptor e− Thylakoid membrane Pigment molecules Transfer of energy Special pair of chlorophyll a molecules A photosystem consists of a reaction-center complex (a type of protein complex) surrounded by light-harvesting complexes. The light-harvesting complexes (pigment molecules bound to proteins) transfer the energy of photons to the reaction center. A primary electron acceptor in the reaction center accepts excited electrons and is reduced as a result. Solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions. THYLAKOID SPACE (INTERIOR OF THYLAKOID) How a photosystem harvests light! 7

2 H  Photosystem II (PS II) Light  P680 Electron transport chain Figure 8.13-1 Primary acceptor 2 H O2  ATP Photosystem II (PS II) H2O Light  2 1 P680 Electron transport chain Pigment molecules Photosystem I (PS I) P700 3 4 5 6 e− Primary acceptor 2 H O2  ATP NADPH Photosystem II (PS II) H2O e− Light  2 1 P680 Electron transport chain Pigment molecules Photosystem I (PS I) P700 H NADP reductase 3 4 5 6 7 8 Primary acceptor 2 H O2  Photosystem II (PS II) H2O Light  2 1 P680 Pigment molecules 3 e− Primary acceptor 2 H O2  ATP Photosystem II (PS II) H2O Light  2 1 P680 Pq Electron transport chain Cytochrome complex Pc Pigment molecules 3 4 5 e− Primary acceptor 2 e− P680 1 Light Linear electron flow can be broken down into a series of steps A photon hits a pigment and its energy is passed among pigment molecules until it excites P680. An excited electron from P680 is transferred to the primary electron acceptor (we now call it P680). H2O is split by enzymes, and the electrons are transferred from the hydrogen atoms to P680, thus reducing it to P680; O2 is released as a by-product. Each electron “falls” down an electron transport chain from the primary electron acceptor of PS II to PS I. Energy released by the fall drives the creation of a proton gradient across the thylakoid membrane; diffusion of H (protons) across the membrane drives ATP synthesis. In PS I (like PS II), transferred light energy excites P700, causing it to lose an electron to an electron acceptor (we now call it P700) P700 accepts an electron passed down from PS II via the electron transport chain Excited electrons “fall” down an electron transport chain from the primary electron acceptor of PS I. The electrons are transferred to NADP, reducing it to NADPH, and become available for the reactions of the Calvin cycle. This process also removes an H from the stroma Pigment molecules Photosystem II (PS II) 8

Cytochrome complex NADP reductase To Calvin Cycle ATP synthase Figure 8.16 Cytochrome complex NADP reductase Photosystem II Photosystem I Light 4 H 3 Light NADP  H NADPH e− e− 2 H2O 1  2 1 O2 THYLAKOID SPACE (high H concentration) 2 H 4 H To Calvin Cycle The energy from the electron transport chain is used to pump H+ into the thylakoid space. This creates a proton motive force. The protons diffuse out of the thylakoid space through ATP synthase. The energy from the diffusing protons is used to convert ADP to ATP. ATP and NADPH are produced on the side facing the stroma, where the Calvin cycle takes place. In summary, light reactions generate ATP and increase the potential energy of electrons by moving them from H2O to NADPH Thylakoid membrane ATP synthase STROMA (low H concentration) ADP  ATP P i H 9

Electron transport chain ATP synthase Figure 8.15 MITOCHONDRION STRUCTURE CHLOROPLAST STRUCTURE Inter- membrane space H Diffusion Thylakoid space Electron transport chain Inner membrane Thylakoid membrane Comparison of chemiosmosis in mitochondria and chloroplasts. Although very similar, chemiosmosis in cellular respiration and photosynthesis are not identical. In addition to some spatial differences, the key conceptual difference is that mitochondria use chemiosmosis to transfer chemical energy from food molecules to ATP, whereas chloroplasts transform light energy into chemical energy in ATP. ATP synthase Matrix Stroma Key ADP  P i ATP Higher [H] H Lower [H] 10