Light Reaction & Calvin Cycle Objectives: How do pigments like chlorophyll work to capture light energy? What happens to water in the light reaction? What are photosystems? What is their role in photosynthesis? How are ATP and NADPH generated in the light reaction? What are the reactants and products of the light reaction and the calvin cycle? Where does each reaction take place?

Photosynthesis H2OH2OH2OH2O CO 2 O2O2O2O2 C 6 H 12 O 6 Light Reaction “Dark” Reaction Light is Adsorbed ByChlorophyll Which splits water Chloroplast ATP and NADPH 2 ADPNADP Calvin Cycle Energy Used Energy and is recycled. + + ATP = chemical energy NADPH – carries Hydrogen (NADP + H)

Visible Light and the Electromagnetic Spectrum Electromagnetic Energy = Energy that travels as a WAVE. Electromagnetic Spectrum= menu of energy that travels as a wave The shorter the wavelength, the greater the energy….and vice- versa. VISIBLE LIGHT = Wavelength absorbed by plants to make food.

Light Used in Photosynthesis Chlorophyll pigments can absorb only some wavelengths of the visible light from the electromagnetic spectrum. Visible light consists of the following colors or wavelengths in order of increasing wavelengths / decreasing energy: – Violet  Blue  Green  Yellow  Orange  Red. Chloroplasts absorb mainly blue-violet and red-orange lights. Green light is reflected and transmitted by green plants – hence, they appear green.

Chlorophyll absorbs blue-violets and red-orange for photosynthesis. Green light is reflected; That’s why plants appear green.

Redox Rxns (Reducing / Oxidation Reactions) Being “reduced” = accepting electrons (usually accompanied by H ions); electrons are negative; accepting them makes recipient “more” negative (hence reduced) Example: NADP+ is reduced when it accepts electrons. NADP + + 2e + H +  NADPH Being Oxidized: Lose electrons (usually when reacting with oxygen); become more + Example: NADH is oxidized when it donates electrons. NADH  NAD e +H +

Oxidation – Reduction Reactions (Redox) occur together. 6 CO H 2 O  C 6 H 12 O O 2 CO2 is reduced as it accepts electrons and hydrogen ions. H2O is oxidized. It donates electrons and loses hydrogen ions. Reduction Oxidiation

Light Reaction Where: Thylakoid membrane of chloroplast. What: Light energy is absorbed by chlorophyll. Energy is used to SPLIT H 2 O molecules into: 1. H+ ions and electrons (carried by NADPH) 2. O atoms (released as O 2 gas) 3. electrons (go through electron transport chain to make ATP and NADPH)

Light Reaction: 1. Making ATP & NADPH (used in Calvin cycle to make sugar) 2. Release O 2 (from H 2 O) Photosystems: Cluster of hundreds of chlorophyll molecules in thylakoid membrane. Where Light Reaction starts.

Light-Dependent Reactions Photon of light absorbed by pigments in photosystem II (P680); excited electrons pass to primary electron acceptor; Electrons of PS680 replaced by electrons from water (H 2 O  H+,1/2 O 2, and electrons) Photo excited electrons from P680 passed thru. Series of ETC proteins (exergonic; releases energy); energy fr. “falling” electrons used to PUMP H+ ions into thylakoid space (against gradient); Build up of H+ ions used to make ATP in ATP synthase; photo-excited electrons from photosystem II (P680) replaces excited electrons at photosystem I (P700); Excited electrons from photosystem I are used to reduce NADP to NADPH. NADPH is a high energy molecule that will bring electrons and hydrogens to CO2 to make sugar.

Chemiosmosis: Use H+ gradient across membrane to phosphorylate ADP  ATP High [H+] in thylakoid space. Active transport/ Used energy released from “excited” electrons “falling” to ground state in ETC to pump H+ ions against gradient. H+ ions move down gradient to stroma through ATP synthase (transport membrane coupled w/ phosphorylation of ADP

Light-Dependent Reaction Summary 1.Light is absorbed by chlorophyll in PS II (P680) 2.An electron is excited and captured by the primary electron acceptor 3.Water is split to supply electrons to PSII 4.The leftover oxygen atoms pair up to form a molecule of O 2 5.The electrons released from PSII are passed to PS I by an electron transport chain 6.This “fall” of electrons down the chain provides energy to pump H + ions across the membrane which will then power the production of ATP via chemiosmosis. (photophosphorylation) 7.Meanwhile, light excites an electron of chlorophyll at PS I/ The primary electron acceptor captures this and an electron from the bottom of the ETC replaces the lost electron. 8.The excited electron of PS I is passed to NADP+, reducing it to NADPH

8.3: Calvin Cycle Where: Stroma of chloroplast. What: Use ATP and NADPH from light rxns to MAKE GLUCOSE from CO 2. Inputs: 1. CO 2 (Stomate); 2. ATP fr. Light Rxn ; 3. NADPH fr. Light Rxn Outputs 1.Glucose is made (used for energy; stored as starch; used to make cellulose) 2.ADP + P (fr. ATP)  back to light rxn 3.NADP+ (fr. NADPH)  back to light rxn

Calvin Cycle: Carbon fixation = making organic carbon molecules from inorganic carbon.

Light Reaction In Thylakoid Membrane Use light to split water into H+, O, and electrons. Make ATP and NADPH to be used in Calvin Cycle. Calvin Cycle Stroma Use ATP and NADPH to make GLUCOSE from CO 2 Light Independent.