att.X4xok1JE7kVtHZ6hrQZq8fqLlIz3APzeQIvVecADvNk.jpeg

# Chapter 10: Photosynthesis ## 10.1 Photosynthesis: The Basis of life **Photosynthesis** nourishes almost the entire living world directly or indirectly. * All organisms use **organic compounds** for energy & carbon skeletons. * **Photoautotrophs** use light energy to synthesize organic comp...

# Chapter 10: Photosynthesis ## 10.1 Photosynthesis: The Basis of life **Photosynthesis** nourishes almost the entire living world directly or indirectly. * All organisms use **organic compounds** for energy & carbon skeletons. * **Photoautotrophs** use light energy to synthesize organic compounds. * Plants, algae, cyanobacteria & some protists * **Heterotrophs** live on organic compounds produced by other organisms. * Most bacteria, fungi, & animals * These are the **consumers**. * Photosynthesis converts **light energy** to **chemical energy**. * Energy enters an ecosystem as sunlight and leaves as heat. * Photosynthesis produces oxygen and organic molecules used by heterotrophs for fuel. ### Photosynthesis: Redox Reaction * Water is split and electrons are transferred with $\text{H}^+$ from water to carbon dioxide, reducing it to sugar. * **Oxidation**: $\text{H}_2\text{O} \rightarrow 2\text{H}^+ + 2e^- + \frac{1}{2}\text{O}_2$ * **Reduction**: $\text{CO}_2 + 4\text{H}^+ + 4e^- \rightarrow [\text{CH}_2\text{O}] + \text{H}_2\text{O}$ **Photosynthesis equation**: $\text{Carbon dioxide} + \text{Water} + \text{Light energy} \rightarrow \text{Sugar} + \text{Oxygen}$ $6\text{CO}_2 + 12\text{H}_2\text{O} + \text{Light energy} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 + 6\text{H}_2\text{O}$ * Oxygen is from $\text{H}_2\text{O}$, not $\text{CO}_2$ ### Sites of Photosynthesis * **Chloroplasts**: Site of photosynthesis in plants * **Thylakoids**: Membranous sacs, contain chlorophyll * **Grana**: Stacks of thylakoids * **Stroma**: Internal fluid ## 10.2 The Two Stages of Photosynthesis Photosynthesis consists of the **light reactions** (the photo part) and **Calvin cycle** (the synthesis part). ### The Light Reactions * Occur in the **thylakoids** * Split $\text{H}_2\text{O}$ * Release $\text{O}_2$ * Reduce $\text{NADP}^+$ to $\text{NADPH}$ * Generate ATP from ADP by **photophosphorylation** ### The Calvin Cycle * Occurs in the **stroma** * Uses ATP and NADPH to convert $\text{CO}_2$ to sugar **Carbon fixation**: The initial incorporation of $\text{CO}_2$ into organic molecules. ### Summary The image shows a diagram that summarizes the process of photosynthesis. * **Left**: Light reaction in the thylakoid membrane involves light, $\text{H}_2\text{O}$, $\text{NADP}^+$ and ADP. It gives off $\text{O}_2$, ATP, and NADPH. * **Right**: Calvin cycle in the stroma involves $\text{CO}_2$, ATP, and NADPH. It results in sugar, ADP, and $\text{NADP}^+$. ## 10.3 The Light Reactions: Converting Solar Energy to Chemical Energy ### The Nature of Sunlight * **Electromagnetic spectrum**: Entire range of electromagnetic radiation, or energy. * **Visible light**: Drives photosynthesis * **Photons**: Discrete particles of light ### Photosynthetic Pigments: The Light Receptors * **Pigments**: Substances that absorb visible light * Different pigments absorb different wavelengths * Wavelengths that are not absorbed are reflected or transmitted * **Chlorophyll a**: Main photosynthetic pigment * **Chlorophyll b**: Accessory pigment * **Carotenoids**: Accessory pigment; photoprotection ### Excitation of Chlorophyll by Light * 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, excess energy is released as heat and fluorescence. ### A Photosystem: A Reaction-Center Complex Associated with Light-Harvesting Complexes * **Photosystem**: Consists of a reaction-center complex surrounded by light-harvesting complexes. * **Reaction-center complex**: Protein complex containing a special pair of chlorophyll a molecules and a primary electron acceptor. * **Light-harvesting complex**: Pigment molecules bound to proteins that funnel the energy of photons to the reaction center. * **Primary electron acceptor**: Accepts excited electrons from chlorophyll a in the reaction center. ### Two Types of Photosystems in the Thylakoid Membrane * **Photosystem II (PS II)**: Functions first (best at absorbing a wavelength of 680 nm). The reaction-center chlorophyll a of PS II is called **P680**. * **Photosystem I (PS I)**: Best at absorbing a wavelength of 700 nm. The reaction-center chlorophyll a of PS I is called **P700**. ### Linear Electron Flow Linear electron flow involves both photosystems and produces ATP and NADPH using light energy. 1. A photon hits a pigment in a light-harvesting complex of PS II, and its energy is passed among pigment molecules until it excites **P680**. 2. An electron in P680 is excited to a higher energy level and transferred to the primary electron acceptor. P680 becomes **P680$^+$**. 3. $\text{H}_2\text{O}$ is split to replace the electron. This reaction releases $\text{O}_2$ and protons. * $2\text{H}_2\text{O} \rightarrow 4\text{H}^+ + \text{O}_2$ 4. Each electron falls down an electron transport chain from the primary electron acceptor of PS II to PS I. 5. Energy released by the fall drives the creation of a proton gradient across the thylakoid membrane. 6. Potential energy stored in the proton gradient drives production of ATP by **chemiosmosis**. 7. In PS I, light energy excites **P700**, which loses an electron to the primary electron acceptor. P700 becomes **P700$^+$**. 8. Electrons are passed in a series of redox reactions from the primary electron acceptor of PS I down a second electron transport chain to **ferredoxin (Fd)**. 9. The enzyme **NADP$^+$ reductase** catalyzes the transfer of electrons from Fd to NADP$^+$. Two electrons are required for reduction to NADPH. **Products**: NADPH and ATP ### Cyclic Electron Flow * Uses only PS I * Produces ATP, but no NADPH or $\text{O}_2$ * Generates surplus ATP, satisfying the higher demand in the Calvin cycle ### Chemiosmosis: The Energy-Coupling Mechanism * Chloroplasts and mitochondria generate ATP by chemiosmosis. * **Chemiosmosis**: The use of energy in a $\text{H}^+$ gradient to drive cellular work. * **ATP synthase**: The enzyme that makes ATP. The image shows a diagram of chemiosmosis in both mitochondria and chloroplasts. * **Mitochondria**: Transfer chemical energy from food to ATP. * **Chloroplasts**: Transform light energy into chemical energy (ATP). ### Organization of the Thylakoid Membrane The image illustrates the spatial organization of chemiosmosis within the thylakoid membrane. * PS II and PS I harvest light energy to energize electrons. * Electron transport chains shuttle electrons and pump protons across the membrane. * ATP synthase uses the proton gradient to make ATP. ## 10.4 The Calvin Cycle: Using ATP and NADPH to Make Sugar ### The Calvin Cycle The Calvin cycle has three phases: 1. **Carbon fixation**: $\text{CO}_2$ is incorporated into RuBP by **rubisco** 2. **Reduction**: ATP and NADPH are used to reduce 3-PGA to G3P 3. **Regeneration**: RuBP is regenerated from G3P * **G3P**: glyceraldehyde-3-phosphate ### Carbon Fixation (Phase 1) * Each $\text{CO}_2$ molecule is attached to **RuBP** (ribulose 1,5-bisphosphate) * Catalyzed by **rubisco** * Most abundant protein in chloroplasts and maybe on Earth * The product is 3-phosphoglycerate ### Reduction (Phase 2) * ATP phosphorylates each 3-phosphoglycerate to 1,3-bisphosphoglycerate * NADPH reduces 1,3-bisphosphoglycerate to G3P ### Regeneration (Phase 3) * Carbon skeletons of five G3P molecules are rearranged into three RuBP molecules * 3 more ATP are used * RuBP is ready to receive $\text{CO}_2$ again ### Output * For net synthesis of 1 G3P molecule, the cycle must take place three times * Three $\text{CO}_2$ molecules are fixed * The Calvin cycle spends 9 ATP and 6 NADPH ## 10.5 Alternative Mechanisms of Carbon Fixation ### Photorespiration * On hot, dry days, plants close **stomata**, which conserves $\text{H}_2\text{O}$ but also prevents $\text{CO}_2$ from entering the leaf. * $\text{O}_2$ builds up * **Photorespiration**: Rubisco adds $\text{O}_2$ instead of $\text{CO}_2$ to RuBP * Consumes $\text{O}_2$ and organic fuel and releases $\text{CO}_2$ without producing ATP or sugar ### C4 Plants * **C4 plants**: Minimize the cost of photorespiration by incorporating $\text{CO}_2$ into four-carbon compounds in mesophyll cells * These four-carbon compounds are exported to bundle-sheath cells, where they release $\text{CO}_2$ that is then used in the Calvin cycle The image displays a diagram that shows the two different locations of carbon fixation in C4 plants. * **Mesophyll cell**: $\text{CO}_2$ is converted to a 4-carbon compound. * **Bundle-sheath cell**: The 4-carbon compound releases $\text{CO}_2$ which enters the Calvin cycle. ### CAM Plants * **CAM plants**: Open their stomata at night, incorporating $\text{CO}_2$ into organic acids * Stomata close during the day, and $\text{CO}_2$ is released from organic acids and used in the Calvin cycle The image explains how CAM plants undergo carbon fixation. * **Night**: Stomata open and $\text{CO}_2$ is fixed into organic acids. * **Day**: Stomata close and $\text{CO}_2$ is released from organic acids to the Calvin cycle. ### Comparison of C4 and CAM Plants * **C4 plants**: Carbon fixation and the Calvin cycle occur in different cells * **CAM plants**: Carbon fixation and the Calvin cycle occur in the same cell at different times ## 10.6 The Importance of Photosynthesis: A Review ### Photosynthesis * The energy entering chloroplasts as sunlight gets stored as chemical energy in organic compounds * Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize all the organic molecules of cells * Plants store excess sugar as starch in structures such as roots, tubers, seeds, and fruits * Photosynthesis is the source of all the food and $\text{O}_2$ on Earth