LEAF STRUCTURE & PHOTOSYNTHETIC PIGMENTS

Inside the Chloroplast Thylakoids- Saclike membranes that store chlorophyll and accessory pigments. Grana (Granum-singular)- stacks of thylakoids Stroma is the fluid filled space surrounding the grana.

FIGURE 7-8 (part 1) Events of the light-dependent reactions occur in and near the thylakoid membranes

PHOTOSYNTHESIS

Photosynthesis- converts water and carbon dioxide into chemical energy stored in glucose Chlorophyll C6H12O6 6 H2O 6 O2

Pigments in plants Pigments: proteins that trap light energy from the sun EXAMPLES: Chlorophyll : Appears green A: Traps Red Light B: Traps Blue Light Accessory Pigments: Trap green/yellow Carotene: Appears orange Xanthophyll: Appears yellow The least important color for photosynthesis? GREEN!!

Why Autumn Leaves Turn Color Both chlorophylls and carotenoids are present in leaves Chlorophyll breaks down before carotenoids in dying autumn leaves revealing yellow colors Red fall colors (anthocyanin pigments) are synthesized by some autumn leaves, producing red colors

FIGURE 7-6 Loss of chlorophyll reveals yellow carotenoids

FIGURE 7-2 An overview of photosynthetic structures (a) Photosynthesis occurs primarily in the leaves of land plants. (b) A section of a leaf, showing mesophyll cells where chloroplasts are concentrated and the waterproof cuticle that coats the leaf on both surfaces. (c) A mesophyll cell packed with green chloroplasts. (d) A single chloroplast, showing the stroma and thylakoids where photosynthesis occurs.

FIGURE 7-2b An overview of photosynthetic structures (b) A section of a leaf, showing mesophyll cells where chloroplasts are concentrated and the waterproof cuticle that coats the leaf on both surfaces.

FIGURE 7-2c An overview of photosynthetic structures (c) A mesophyll cell packed with green chloroplasts.

Two Stages of Photosynthesis 1st stage: Light Reactions (Light dependent)- occurs in the thylakoids. 2nd stage: Calvin Cycle (Light Independent)- occurs in the stroma

Two Sets of Chemical Reactions Photosynthesis involves two primary sets of reactions: light-dependent reactions and Calvin Cycle (light-independent reactions). Calvin cycle  Remind students that photosynthesis involves both light-dependent and light-independent reactions. Ask: What do the light-dependent reactions produce? Answer: Oxygen. Click to highlight. Ask: What is converted in the process? Answer: ADP and NADP+ are converted into the energy carriers ATP and NADPH.

LIGHT REACTIONS Photosystem II Light is absorbed by chlorophyll. Electrons in the chlorophyll become energized Energy from the light splits water molecules: 2H + O2 i Electrons from H replace chlorophyll’s electrons that were energized. ATP is made from ETC Photosystem I Light re-energizes chlorophyll electrons Energy from ETC attaches the H+ ions to NADP+ (e- acceptor) to form NADPH. PRODUCTS: ATP, NADPH move to next step (calvin cycle) 6 O2 released via stomata

FIGURE 7-7 The light-dependent reactions of photosynthesis (1) Light is absorbed by photosystem II, and the energy is passed to electrons in the reaction-center chlorophyll molecules. (2) Energized electrons leave the reaction center. (3) The electrons move into the adjacent electron transport chain. (4) The chain passes the electrons along, and some of their energy is used to drive ATP synthesis by chemiosmosis. Energy-depleted electrons replace those lost by photosystem I. (5) Light strikes photosystem I, and the energy is passed to electrons in the reaction-center chlorophyll molecules. (6) Energized electrons leave the reaction center. (7) The electrons move into the electron transport chain. (8) The energetic electrons from photosystem I are captured in molecules of NADPH. (9) The electrons lost from the reaction center of photosystem II are replaced by electrons obtained from splitting water, a reaction that also releases oxygen, and H+ used to form NADPH.

Light Reactions Overview: 1. Light energy is absorbed in the THYLAKOID. 2. Water is split apart 2H and O2 3. CO2 is NOT involved 4. Reactants: chlorophyll a & b, accessory pigments, light energy, water, and NADP 5. Products: O2 , ATP, NADPH

Products of Light Reactions 6 O2 maybe used by plant for cellular respiration or released into the air via stoma NADPH and ATP are created. These move to the second step of photosynthesis- Calvin Cycle

Let’s Review Again 1. Where do the Light Reactions occur? Why do they occur here? What is the main event of the L.R.? What are the final products of L.R.?

FIGURE 7-7 The light-dependent reactions of photosynthesis (1) Light is absorbed by photosystem II, and the energy is passed to electrons in the reaction-center chlorophyll molecules. (2) Energized electrons leave the reaction center. (3) The electrons move into the adjacent electron transport chain. (4) The chain passes the electrons along, and some of their energy is used to drive ATP synthesis by chemiosmosis. Energy-depleted electrons replace those lost by photosystem I. (5) Light strikes photosystem I, and the energy is passed to electrons in the reaction-center chlorophyll molecules. (6) Energized electrons leave the reaction center. (7) The electrons move into the electron transport chain. (8) The energetic electrons from photosystem I are captured in molecules of NADPH. (9) The electrons lost from the reaction center of photosystem II are replaced by electrons obtained from splitting water, a reaction that also releases oxygen, and H+ used to form NADPH.

The Calvin Cycle (C3) Cycle NO LIGHT IS USED Occurs in the STROMA. 6 CO2 used to synthesize 1 glucose (C6H12O6) Carbon dioxide is captured and linked to ribulose bisphosphate (RuBP) ATP and NADPH from light dependent reactions used to power this step.

CALVIN CYCLE C6H12O6

Products of the Calvin Cycle C6H12O6 6 RUBP ADP + Pi NADP+

Summary of Photosynthesis

FIGURE 7-4 Relationship between the light-dependent and light-independent reactions

6 H2O 6 CO2 C6H12O6 6 O2

Factors that Affect Photosynthesis 1. Amount of water 2. Temperatures 3. Light Intensity 4. Amount of CO2