4.1 Capturing Solar Energy lesson

4.1 Capturing Solar Energy: The Light-Depende nt Reactions Learning goals: Explain the chemical changes and energy conversions associated with the photosynthesis Write out the overall balanced chemical equation for photosynthesis Identify all the components that are involved in the light-dependent reactions. How many ATP, electron carriers, reactants and products Explain the role of photosystem I & II in the light-dependent reactions What is Photosynthesis? “Photo” meaning light, refers to the reactions that capture light energy “synthesis” refers to the reactions that produce a carbohydrate Overview of Photosynthesis: Photosynthesis transforms light energy into the chemical energy of high-energy compounds (ie. glucose) Enables plants to produce structural and metabolic substances that aid in their survival such as cellulose and starch Chlorophyll - green coloured pigment, absorbs energy and begins photosynthesis The above reaction is the result of more than 100 distinct chemical reactions. Two main sets of reactions: Light-dependent reactions: light energy is trapped and used to produce ATP & NADPH Light-independent reactions: energy of ATP and reducing power of NADPH are used to produce high-energy organic molecules. Overview of Photosynthesis: Water enters the plant through the roots and is transported to the leaves through the veins Carbon dioxide enters through opening in the leaves called stomata The CO2 and H2O diffuse into the cells and enter the chloroplasts, where photosynthesis occurs Most photosynthetic cells contain anywhere from 40-200 chloroplasts A typical leaf may have 500 000 chloroplasts per square mm Structure of the Leaf and Chloroplast: Chloroplasts: A membrane system within the chloroplast forms interconnected disks called thylakoids, that look like flattened sacs Often stacked to form structures called grana (singular, granum) Thylakoids absorb energy from the sun Surrounding the grana in the chloroplasts is a fluid-filled interior called the stroma The stroma contains enzymes that catalyze the conversion of the CO2 and H2O into carbohydrates Structure of the Leaf and Chloroplast: The Absorption of Light Energy: Light is absorbed in the form of packets of energy called photons that contain specific amounts of energy Each wavelength (colour) of visible light is associated with photons of one distinct amount of energy Longer-wavelength photons have smaller amounts of energy and shorter-wavelength photons have larger amounts of energy Pigments: compounds that absorb certain wavelengths of visible light while reflecting others In a leaf, photosynthetic pigment will trap light energy and pass it along to other compounds Photosynthetic pigments are embedded throughout the thylakoid membrane There are a variety of pigments, but the main types are chlorophyll a & b Leaves appear green because chlorophyll molecules in leaf cells reflect green and yellow wavelengths of light and absorb other wavelengths (red and blue) Chlorophyll a 🡪 only pigment that can transfer the energy of light to the carbon-fixation reactions of photosynthesis. Chlorophyll b 🡪 acts as accessory pigment, absorbing photons that chlorophyll a absorbs poorly or not at all. Figure 4.4 (pg. 158) Photosystems Capture Energy: Photosystems are protein-based complexes composed of clusters of pigments that absorb light energy When a pigment molecule absorbs a photon, the molecule passes the energy to chlorophyll a The combination of these pigments and chlorophyll a is called the reaction center The antenna complex gathers energy from light so that the energy can be directed to the reaction center The Light-Dependent Reactions: The Light-Dependent Reactions: The purpose of the Light-Dependent reactions of photosynthesis is to 🡪 make ATP and NADPH, to fuel the Calvin Cycle 🡪 Make Glucose!! 3 parts: 1. Photoexcitation: absorption of photon by an electron of chlorophyll. 2. Electron transport: transport excited electron through electron carriers 🡪 causes H+ pumping through photosynthetic membrane, creation of H+ reservoir, and eventual electron acceptor reduction. 3. Chemiosmosis: ATPase complexes causes phosphorylation of ADP 🡪 to ATP “PHOTOPHOSPHORYLATION” Photoexcitation: An electron must become “excited” for photosynthesis to start It uses Photosystems (complex of chlorophyll molecules, accessory pigments and proteins embedded in thylakoid membrane) that absorb photons of a particular wavelength and through the LIGHT REACTIONS, transfer their energy ADP + Pi to make ATP NADP+ to form NADPH (electrons and H supplied by H2O that enter thylakoids from stroma) Photoexcitation: How the antenna complex works: Two Different Photosystems: PSI: chlorophyll a (chl.a) in reaction center is called P700 because absorption peaks at 700 λ [wavelength (nm)]. PSII: contains chl.a in reaction center called P680. chl.a in PSII & PSI (same), but different absorption due to associated proteins in reaction center Electron Transport: Non-cyclic Flow Photosynthesis starts when: a photon strikes PS II & excites a chl. P680 electron (e-) The excited e- is captured by primary e- acceptor (PHEOPHYTIN) & through a series of redox reactions is transferred to PLASTOQUINONE PQ (electron carrier) A “water splitting enzyme” associated with PS II, splits water into O2, H+, e-. One of the e- is used to replace the missing e- in chl. P680. O2 leaves the cell & H+ remain in the thylakoid space Non-Cyclic Electron Flow: From PQ, the energized electrons are transferred, one by one, along the electron transport system (similar to ETC in cellular respiration). With each transfer of electrons, a small amount of energy is released, which is used by b6-f complex to pump H+ from stroma into thylakoid space. H+ concentration gradient across thylakoid membrane e- then transferred to plastocyanin Pc (electron carrier), and eventually replacing an e- lost by PS I when a photon hit it. e- from PS I is transferred to FERREDOXIN (Fd), then to the enzyme NADP reductase, which uses the e- and H+ ions from stroma to reduce NADP+ to NADPH. 🡪Non-Cyclic: once an e- is lost by a reaction centre chl. molecule within a PS, it does not return to that system, but instead it ends up in NADPH. Non-Cyclic electron flow will eventually generate 1 NADPH and ~1 ATP. However, the light-independent reactions require 3 ATP to 2 NADPH. Therefore: 🡪Cyclic Electron Flow: uses PSI only. Photon ejects electron from PSI (P700). Electron passed to Fd🡪 goes through b6-f complex and back to PS1 (P700). Therefore, generating a proton gradient for ATP synthesis, but NOT NADPH. Cyclic Electron Flow: 3. Chemiosmosis Embedded in the thylakoid membrane is the enzyme ATP synthase. H+ can only go through this enzyme to move down the concentration gradient. Protons in the thylakoid lumen form an electrochemical gradient that drives phosphorylation of ADP to ATP as H+ move through ATP synthase. This is known as PHOTOPHOSPHORYLATION 2 e- are required to reduce NADP+ to NADPH.

Notes

### Notes on Capturing Solar Energy: The Light-Dependent Reactions

1. Overview of Photosynthesis:

- Photosynthesis: Conversion of light energy into chemical energy (glucose).

- Enables plants to produce vital substances like cellulose and starch.

- Chlorophyll: Pigment absorbing light energy to initiate photosynthesis.

2. Components Involved:

- Light-dependent reactions: Produce ATP & NADPH.

- Light-independent reactions: Utilize ATP & NADPH to synthesize organic molecules.

3. Photosynthesis Process:

- Water and carbon dioxide are essential inputs.

- Water absorbed through roots; CO2 enters through stomata.

- Occurs in chloroplasts, containing stacks of thylakoids called grana.

4. Chloroplast Structure:

- Thylakoids: Absorb light energy.

- Grana: Stacks of thylakoids.

- Stroma: Fluid-filled interior where CO2 and H2O are converted into carbohydrates.

5. Absorption of Light Energy:

- Light absorbed as photons, varying in energy based on wavelength.

- Pigments trap light energy and transfer it to other compounds.

- Chlorophyll a & b are primary pigments; they reflect green light.

6. Photosystems:

- Protein complexes with pigments that absorb light energy.

- Consist of antenna complex and reaction center.

- Photosystem I (P700) and Photosystem II (P680) absorb light at different wavelengths.

7. Light-Dependent Reactions:

- Goal: Produce ATP and NADPH to fuel Calvin Cycle.

- Three phases: Photoexcitation, Electron transport, Chemiosmosis (Photophosphorylation).

8. Photoexcitation:

- Chlorophyll absorbs photons, energizing electrons.

- Electrons transferred through Photosystems, generating ATP and NADPH.

9. Electron Transport:

- Non-cyclic flow involves transfer of electrons along carriers.

- Water splitting enzyme facilitates O2 release and generates H+ ions.

10. Cyclic Electron Flow:

- Utilizes Photosystem I to generate ATP but not NADPH.

11. Chemiosmosis:

- Proton gradient formed across thylakoid membrane.

- ATP synthase converts this gradient into ATP via phosphorylation.

12. Summary:

- Light-dependent reactions crucial for capturing solar energy.

- Convert light energy into chemical energy (ATP, NADPH).

- Essential for subsequent synthesis of glucose in the Calvin Cycle.