The Light Reactions Chapter 3.3 Photosynthesis The Light Reactions Chapter 3.3

Photosynthesis Review light CO2 + H2O C6H12O6 + O2 photosynthesis – creating sugar using light Only chloroplast organelles and special bacteria have the proteins necessary to carry out photosynthesis.

Photosynthesis: Two Major Processes The Light Reactions Calvin cycle harvests light energy to split water, creating O2 , ATP and NADPH process of producing C6H12O6 , using ATP for energy and producing NADP+

Photosynthesis: Two Major Processes

Chloroplast Structure: A Review (thylakoid space) - compare and contrast mitochondria and chloroplast structure

The Light Reactions Absorption of light photons Photoexcitation Electron transport Photophosphorylation (chemiosmosis) Absorption of light photons Similar to ETC in mitochondria ATP synthesis due to electrochemical gradient

Photoexcitation e- gain energy when atoms absorb energy. e- fall back to lowest energy level (ground state) if it isn’t transferred to another molecule Q: What happens to electrons when light strikes them? A: Excited, gain energy, higher potential energy level - think of glow in the dark objects (the light we see is electrons falling back to ground state)

Isolated Chlorphyll If an isolated solution of chlorophyll is illuminated It will fluoresce, giving off heat and light

Photosystems Made up of a variety of proteins reaction center surrounded by a number of light-harvesting complexes Contain chlorophyll and other light absorbing pigments Located in the thylakoid membrane

Reaction Centre Contains a primary electron acceptor contains chlorophyll a molecule which the light energy is focused in a photosystem

Two Types of Photosystems Photosystem I (PS I) Has P700 chlorophyll a within reaction centre Best at absorbing 700 nm wavelength (far red part of spectrum) Phosystem II (PS II) Has P680 chlorophyll a within reaction centre Best at absorbing 680 nm wavelength (red)

Purposes of Photosystems Two purposes: to collect as much light energy as possible excite chlorophyll a and transfer its electrons to an electron acceptor and through a series of proteins (electron transport)

Electron Transport Electron transport occurs in the thylakoid membrane. Two mechanisms of electron transport: Non-cyclic electron flow (the primary pathway) Cyclic electron flow

Non-Cyclic Electron Flow: An Overview LIGHT REACTOR NADP+ ADP ATP NADPH CALVIN CYCLE [CH2O] (sugar) STROMA (Low H+ concentration) Photosystem II H2O CO2 Cytochrome complex O2 1 1⁄2 2 Photosystem I Light THYLAKOID SPACE (High H+ concentration) Thylakoid membrane synthase Pq Pc Fd reductase + H+ NADP+ + 2H+ To Calvin cycle P 3 H+ 2 H+ +2 H+

Step 1: PS II A photon of light strikes a pigment molecule in a light harvesting complex and is relayed to other pigment molecules until it reaches one of the P680 chlorophyll a pigments within the reaction centre. This excites one of the P680 electrons and is captured by the primary electron acceptor. PSII splits a water molecule into 2 electrons, 2 hydrogen ions (2 H+) and 1 oxygen atom. These electrons replace (one by one) the electrons lost to the primary electron acceptor.

PS II: The Details STROMA THYLAKOID MEMBRANE THYLAKOID SPACE (LUMEN) (Low H+ Concentration) THYLAKOID MEMBRANE THYLAKOID SPACE (LUMEN) (High H+ Concentration)

Step 2: Pq, Cytochrome Complex, Pc Each photoexcited electron passes from the primary electron acceptor of PSII to PSI via an ETC (similar to the ETC in cellular respiration) The ETC between PSII and PSI is made up of: Pq (plastiquinone) - mobile Cytochrome Complex Pc (Plastocyanin) - mobile

Pq, CC, Pc: The Details STROMA (Low H+ Concentration) As Plastoquinone (Pq) transfers electrons to the Cytochrome Complex, protons are pumped across the membrane into the thylakoid space (lumen) This exergonic “fall” of electrons to a lower energy level provides energy for the active transport of H+ ions against its concentration gradient. Electrons are then transferred to Plastocyanin (Pc), also a moveable component on thylakoid surface in lumen THYLAKOID MEMBRANE THYLAKOID SPACE (LUMEN) (High H+ Concentration)

Step 3: PS I A photon of light strikes a pigment molecule in a light harvesting complex and is relayed to other pigment molecules until it reaches one of the P700 chlorophyll a pigments within the reaction centre. This excites one of the P700 electrons and is captured by the primary electron acceptor, creating an electron “hole” in the P700. This hole is filled by an electron that reaches the bottom of the ETC from PS II.

PS I: The Details

Step 4: Fd and NADP+ Reductase Electrons are transferred to ferrodoxin (Fd) – moveable component on thylakoid surface in stroma Electrons are transferred to NADP+ reductase final electron acceptor is NADP+ that is reduced to NADPH

NADH vs. NADPH: A Review

Step 5: ATP Synthase protons pumped into the lumen (from Step 1 and Step 2)pass through ATP synthase by facilitated diffusion ATP produced in stroma photophosphorylation – light-dependent formation of ATP by chemiosmosis

The spatial organization of chemiosmosis Differs in chloroplasts and mitochondria Key Higher [H+] Lower [H+] Mitochondrion Chloroplast MITOCHONDRION STRUCTURE Intermembrance space Membrance Matrix Electron transport chain H+ Diffusion Thylakoid Stroma ATP P ADP+ Synthase CHLOROPLAST

Non-Cyclic Electron Flow LIGHT REACTOR NADP+ ADP ATP NADPH CALVIN CYCLE [CH2O] (sugar) STROMA (Low H+ concentration) Photosystem II H2O CO2 Cytochrome complex O2 1 1⁄2 2 Photosystem I Light THYLAKOID SPACE (High H+ concentration) Thylakoid membrane synthase Pq Pc Fd reductase + H+ NADP+ + 2H+ To Calvin cycle P 3 H+ 2 H+ +2 H+

Electron transport chain Photosystem II (PS II) Photosystem-I (PS I) ATP NADPH NADP+ ADP CALVIN CYCLE CO2 H2O O2 [CH2O] (sugar) LIGHT REACTIONS Light Primary acceptor Pq Cytochrome complex PC e P680 e– + 2 H+ Fd reductase Electron Transport chain Electron transport chain P700 + 2 H+ + H+ Energy Diagram (Z scheme) Energy of Electrons Pq -

Non-Cyclic Electron Flow Summary H2O is split to produce O2 (released from cell) and H+ ions (released into lumen) enzyme complexes pump protons from stroma to lumen NADP+ is final electron acceptor and produces NADPH chemiosmosis to synthesize ATP

Light Reaction Animation http://www.youtube.com/ watch?v=hj_WKgnL6MI

Cyclic Electron Flow Non-cyclic electron flow produces roughly equal amounts of ATP and NADPH However, Calvin Cycle uses more ATP than NADPH Cyclic electron flow makes up the difference in ATP (without producing more NADPH). H2O CO2 Light LIGHT REACTIONS CALVIN CYCLE Chloroplast [CH2O] (sugar) NADPH NADP  ADP + P O2 ATP

Cyclic Electron Flow Primary Fd acceptor Pq NADP+ reductase Cytochrome complex Pc NADP+ reductase NADPH ATP Photosystem II Photosystem I

Cyclic Electron Flow Summary only involves photosystem I (P700) ferrodoxin returns electrons back to cytochrome complex Only ATP produced, no NADPH

To Do: Section 3.2 Questions (pg. 154-155) # 1-3, 11 Section 3.3 Questions (pg. 166-167) #1-4, 6, 8a(i-iii), 8b