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### **Chapter 8: Photosynthesis** #### **8.1 Photosynthesis: Life from Light** * Plants and other autotrophs are the producers of the biosphere. * Photosynthesis nourishes almost the entire living world directly or indirectly. **Concept 8.1 Photosynthesis converts light energy to the chemical...

### **Chapter 8: Photosynthesis** #### **8.1 Photosynthesis: Life from Light** * Plants and other autotrophs are the producers of the biosphere. * Photosynthesis nourishes almost the entire living world directly or indirectly. **Concept 8.1 Photosynthesis converts light energy to the chemical energy of food** * **Autotrophs** sustain themselves without eating anything derived from other organisms. * Autotrophs are the producers of the biosphere, producing organic molecules from $CO_2$ and other inorganic molecules. * Almost all plants are **photoautotrophs,** using the energy of sunlight to make organic molecules from $H_2O$ and $CO_2$. * Photosynthesis occurs in plants, algae, certain other protists, and some prokaryotes. * These organisms feed not only themselves but also most of the living world. **(a) Plants (b) Multicellular alga (c) Unicellular protists (d) Cyanobacteria (e) Purple sulfur bacteria** **Figure 8.1 Photoautotrophs** * **Heterotrophs** obtain their organic material from other organisms. * Heterotrophs are the consumers of the biosphere. * Some heterotrophs feed on other living organisms. * Other heterotrophs, called **decomposers,** feed on dead organisms or organic litter. * Almost all heterotrophs, including humans depend on photoautotrophs for food and $O_2$. #### **The Structure of Chloroplasts** * Photosynthesis occurs in **chloroplasts**. * These organelles are present in the cells of **mesophyll**, the tissue in the interior of the leaf. * $CO_2$ enters and $O_2$ exits the leaf through microscopic pores called **stomata**. * A chloroplast has two membranes surrounding a central space called the **stroma.** * In the stroma are interconnected membranous sacs called **thylakoids**. * The thylakoids are stacked in columns called **grana.** * **Chlorophyll**, the pigment that gives leaves their green color, resides in the thylakoid membranes. **Figure 8.2 The location of photosynthesis in a plant** #### **Tracking Atoms Through Photosynthesis: Scientific Inquiry** * Photosynthesis is summarized as: $6CO_2 + 12H_2O + Light energy \rightarrow C_6H_{12}O_6 + 6O_2 + 6H_2O$ * Reactants: Carbon dioxide, Water * Products: Glucose, Oxygen, Water * Chloroplasts split $H_2O$ into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules. **Experiment Question: Do plants incorporate carbon atoms from carbon dioxide into carbohydrates?** * Approach: * In the 1930s, C. B. van Niel of Stanford University challenged the previously accepted hypothesis that photosynthesis splits carbon dioxide and then adds water to the carbon. * Van Niel proposed that plants split water as a source of electrons from hydrogen ions, releasing oxygen as a byproduct. $CO_2 \rightarrow [CH_2O] + H_2O$ (Older hypothesis) $CO_2 + 2H_2O \rightarrow [CH_2O] + H_2O + O_2$ (Van Niel's hypothesis) * Tracer experiments confirmed that $O_2$ released by photosynthesis comes from $H_2O$ and not from $CO_2$. #### **The Two Stages of Photosynthesis: A Preview** * Photosynthesis consists of the **light reactions** (the photo part) and **Calvin cycle** (the synthesis part) * The light reactions (in the thylakoids): * Split $H_2O$ * Release $O_2$ * Reduce $NADP^+$ to NADPH * Generate ATP from ADP by photophosphorylation * The Calvin cycle (in the stroma) forms sugar from $CO_2$, using ATP and NADPH * The Calvin cycle begins with carbon fixation, incorporating $CO_2$ into organic molecules. **Figure 8.3 An overview of photosynthesis** #### **8.2 The Light Reactions Convert Solar Energy to the Chemical Energy of ATP and NADPH** **Concept 8.2 The light reactions convert solar energy to the chemical energy of ATP and NADPH** * Chloroplasts are solar-powered chemical factories. * Their thylakoids transform light energy into the chemical energy of ATP and NADPH. #### **The Nature of Sunlight** * **Light** is a form of electromagnetic energy, also called electromagnetic radiation. * Like other electromagnetic energy, light travels in rhythmic waves. * **Wavelength** is the distance between crests of electromagnetic waves. * Wavelength determines the type of electromagnetic energy. * The **electromagnetic spectrum** is the entire range of electromagnetic energy, or radiation. * **Visible light** consists of wavelengths (including those that drive photosynthesis) that produce colors we can see. * Light also behaves as though it consists of discrete particles, called **photons**. **Figure 8.4 The electromagnetic spectrum** #### **Photosynthetic Pigments: The Light Receptors** * **Pigments** are substances that absorb visible light. * Different pigments absorb different wavelengths. * Wavelengths that are not absorbed are reflected or transmitted. * Leaves appear green because chlorophyll reflects and transmits green light. * A **spectrophotometer** measures a pigment's ability to absorb various wavelengths. * An **absorption spectrum** is a graph plotting a pigment's light absorption versus wavelength. * The absorption spectrum of chlorophyll *a* suggests that violet-blue and red light work best for photosynthesis. * An **action spectrum** profiles the relative effectiveness of different wavelengths of radiation in driving a process. * The action spectrum for photosynthesis was first demonstrated by Theodor W. Engelmann. * In his experiment, he exposed different segments of a filamentous alga to different wavelengths. * Areas receiving wavelengths favorable to photosynthesis produced excess $O_2$. * He used the growth of aerobic bacteria clustered along the alga as a measure of $O_2$ production. **Figure 8.5 Why are plants green?** **Figure 8.6 Inquiry: Which wavelengths of light are most effective in driving photosynthesis?** * Chlorophyll *a* is the main photosynthetic pigment. * **Chlorophyll b** is an accessory pigment. * **Carotenoids** are separate accessory pigments. * Accessory pigments such as Chlorophyll $b$ broaden the spectrum of light that can be used in photosynthesis. * **Carotenoids** absorb excessive light that would damage chlorophyll. **Figure 8.7 Absorption spectra for chlorophyll *a*, chlorophyll *b*, and carotenoids** #### **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, photons are given off, an afterglow called **fluorescence**. * If a solution of chlorophyll is illuminated, it will fluoresce and give off heat. **Figure 8.8 Excitation of isolated chlorophyll by light** #### **A Photosystem: A Reaction-Center Complex Associated with Light-Harvesting Complexes** * A **photosystem** consists of a **reaction-center complex** surrounded by **light-harvesting complexes**. * The **light-harvesting complexes** (pigment molecules bound to proteins) transfer the energy of photons to the reaction center. * A **primary electron acceptor** in the reaction center accepts excited electrons and is reduced as a result. * Solar-powered transfer of an electron from a chlorophyll *a* molecule to the primary electron acceptor is the the first step of the light reactions. **Figure 8.9 A photosystem** #### **Two Types of Photosystems in the Thylakoid Membrane** * There are two types of photosystems in the thylakoid membrane: **Photosystem II (PS II)** and **Photosystem I (PS I)**. * Photosystem II (PS II) functions first (the numbers reflect order of discovery). * The reaction-center chlorophyll *a* of PS II is called **P680**, because it is best at absorbing light having a wavelength of 680 nm. * Photosystem I (PS I) is 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. * There are several steps in linear electron flow: 1. A photon hits a pigment in a light-harvesting complex of PS II, and its energy is passed among other pigment molecules until it excites P680. 2. An excited electron from P680 is transferred to the primary electron acceptor (we now call it $P680^+$). 3. $H_2O$ is split by enzymes, and the electrons are transferred from the hydrogen atoms to $P680^+$, thus reducing it to P680; $O_2$ is released as a byproduct. 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. Light energy excites PS I, which loses an electron to its primary electron acceptor (we now call it $P700^+$). 8. An electron "falls" down an electron transport chain from the primary electron acceptor of PS I to the protein ferredoxin (Fd). 9. The enzyme $NADP^+$ reductase catalyzes the transfer of electrons from Fd to $NADP^+$, reducing it to NADPH. **Figure 8.10 Linear electron flow** #### **Cyclic Electron Flow** * **Cyclic electron flow** uses only photosystem I and produces ATP, but not NADPH. * No oxygen is released in cyclic electron flow. * Some organisms such as purple sulfur bacteria have PS I but not PS II. * Cyclic electron flow may be photoprotective. **Figure 8.11 Cyclic electron flow** #### **A Comparison of Chemiosmosis in Chloroplasts and Mitochondria** * Chloroplasts and mitochondria generate ATP by chemiosmosis, but use different sources of energy. * Mitochondria transfer chemical energy from food to ATP. * Chloroplasts transform light energy into chemical energy of ATP. * ATP and NADPH are produced on the side facing the stroma, where the Calvin cycle takes place. * In summary, the light reactions use solar power to generate ATP and NADPH, which provide chemical energy and reducing power, respectively, to the Calvin cycle. **Figure 8.12 Chemiosmosis powers ATP synthesis in the light reactions** #### **8.3 The Calvin Cycle Uses ATP and NADPH to Convert $CO_2$ to Sugar** **Concept 8.3 The Calvin cycle uses ATP and NADPH to convert $CO_2$ to sugar** * The Calvin cycle is similar to the citric acid cycle in that the starting material is regenerated after each cycle. * Unlike the citric acid cycle, the Calvin cycle is anabolic rather than catabolic. * It builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH. #### **The Calvin Cycle** * The Calvin cycle has three phases: 1. **Carbon fixation** ($CO_2$ enters the cycle and is attached to RuBP by the enzyme RuBisCO). 2. **Reduction** (ATP and NADPH are used to reduce 3PG to G3P). 3. **Regeneration of the $CO_2$ acceptor (RuBP)** (G3P is used to regenerate RuBP so that the cycle can continue). **Figure 8.13 The Calvin cycle** #### **8.4 Alternative Mechanisms of Carbon Fixation Have Evolved in Hot, Arid Climates** **Concept 8.4 Alternative mechanisms of carbon fixation have evolved in hot, arid climates** * Dehydration is a problem for plants, sometimes requiring trade-offs with other metabolic processes, especially photosynthesis. * On hot, dry days, plants close **stomata**, which conserves $H_2O$ but also limits photosynthesis. * The closing of stomata reduces access to $C0_2$ and causes $O_2$ to build up. * These conditions favor an apparently wasteful process called **photorespiration**. #### **Photorespiration: An Evolutionary Relic?** * In most plants (called $C_3$ plants), initial fixation of $CO_2$, via rubisco, forms a three-carbon compound (3-phosphoglycerate). * In **photorespiration**, rubisco adds $O_2$ instead of $CO_2$ in the Calvin cycle. * Photorespiration consumes $O_2$ and organic fuel and releases $CO_2$ without producing ATP or sugar. * Photorespiration limits damaging buildup of $O_2$. #### **$C_4$ Plants** * $C_4$ plants minimize the cost of photorespiration by incorporating $CO_2$ into four-carbon compounds in mesophyll cells * This step requires the enzyme PEP carboxylase. * $C_4$ plants can fix $CO_2$ even when $CO_2$ concentrations are low. * These four-carbon compounds are exported to bundle-sheath cells, where they release $CO_2$ that is then used in the Calvin cycle. **Figure 8.14 $C_4$ photosynthesis** #### **CAM Plants** * **CAM (crassulacean acid metabolism)** plants open their stomata at night, incorporating $CO_2$ into organic acids. * Stomata close during the day, and $CO_2$ is released from organic acids and used in the Calvin cycle. **Figure 8.15 CAM photosynthesis** #### **$C_4$ and CAM Photosynthesis Compared** * $C_4$ photosynthesis spatially separates the initial carbon fixation and the Calvin cycle * CAM photosynthesis temporally separates carbon fixation and the Calvin cycle. **Table 8.1 A comparison of $C_3$, $C_4$, and CAM plants** | Feature | $C_3$ | $C_4$ | CAM | | :---------------------------- | :----------------------------------------- | :------------------------------------------------------------------------------- | :--------------------------------------------------------------------------------------- | | Initial $CO_2$ fixation | Calvin cycle | PEP carboxylase | PEP carboxylase | | Product | 3-PGA | Oxaloacetate | Malate | | Location | Mesophyll cells | Mesophyll cells | Mesophyll cells | | Calvin cycle | Mesophyll cells | Bundle-sheath cells | Mesophyll cells | | Photorespiration | High | Low | Low | | Stomata open | Day | Day | Night | | Examples | Rice, wheat, soybeans | Sugarcane, corn | Cacti, pineapples | | Adaptation | Moderate temperatures and plenty of water | Hot temperatures and sunny weather where water may be limited; high light intensity | Hot and dry climates with intense sunlight and very little water; low soil-nutrient levels | #### **8.5 The Significance of Photosynthesis: A Review** **Concept 8.5 The significance of photosynthesis: a review** * In addition to food production, photosynthesis produces the oxygen in our atmosphere **Figure 8.16 A review of the reactions of photosynthesis** #### **The Importance of 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 the organic molecules of cells. * Photosynthesis is the foundation of the biosphere.