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Splash. Chapter Introduction Autotrophy and Photosynthesis 4.1 What Are Autotrophs? 4.2 Overview of Photosynthesis 4.3 The Light Reactions 4.4 The Calvin Cycle Photosynthesis and the Environment 4.5 Rate of Photosynthesis 4.6 Photorespiration and Special Adaptations

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  1. Splash

  2. Chapter Introduction Autotrophy and Photosynthesis 4.1 What Are Autotrophs? 4.2 Overview of Photosynthesis 4.3 The Light Reactions 4.4 The Calvin Cycle Photosynthesis and the Environment 4.5 Rate of Photosynthesis 4.6 Photorespiration and Special Adaptations 4.7 Photosynthesis and the Atmosphere Chemoautotrophy 4.8 Varieties of Chemoautotrophs 4.9 Chemoautotrophs and the Environment Chapter Highlights Chapter Animations Chapter Menu Contents

  3. Learning Outcomes By the end of this chapter you will be able to: A State the importance of photosynthesis and identify the plant structures that are involved in photosynthesis. BIdentify the steps by which light energy is converted to chemical energy during the light reactions. C Discuss the importance of the Calvin cycle. D Describe how environmental factors affect the rate of photosynthesis and photorespiration. E Describe the effects of photosynthesis on the atmosphere. F Discuss varieties of chemoautotrophs and their distribution in the environment. Learning Outcomes

  4. This photo shows a group of fungi. Autotrophy: Collecting Energy from the Nonliving Environment • How do organisms obtain energy from their environment? • How could animals survive without plants? Chapter Introduction 1

  5. This photo shows a group of fungi. Autotrophy: Collecting Energy from the Nonliving Environment • Photosynthesis can be compared to a “living bridge”—connecting the Sun with the organisms on Earth by providing the energy needed for life. • Some bacteria obtain energy from minerals such as iron or sulfur instead of sunlight. Chapter Introduction 2

  6. End of the Introduction

  7. Autotrophy and Photosynthesis 4.1 What are Autotrophs? • Photosynthesis, performed by plants, some bacteria, and other small organisms called algae, uses the energy of sunlight to convert carbon dioxide to sugars. Autotrophs such as plants that depend on photosynthesis for both energy and carbon compounds are known as photoautotrophs. We depend on photosynthesis for agricultural production and the products of ancient photosynthesis—petroleum and coal. 4.1 What are Autotrophs? 1

  8. In the upper geyser basin at Yellowstone National Park, note the steam rising from the water. The water is too hot to support photoautotrophs. Autotrophy and Photosynthesis 4.1 What are Autotrophs? (cont.) • In environments where photoautotrophs cannot survive, bacteria called chemoautotrophs obtain energy by oxidizing inorganic substances such as iron, sulfur, or other minerals. 4.1 What are Autotrophs? 2

  9. Autotrophy and Photosynthesis 4.1 What are Autotrophs? (cont.) 4.1 What are Autotrophs? 3

  10. Autotrophy and Photosynthesis 4.2 Overview of Photosynthesis • Photoautotrophs have adapted to take advantage of sunlight. Light consists of a vibrating electric and magnetic field like a wave. The length of the waves determines the light’s color and energy; the shorter the wave, the greater its energy. 4.2 Overview of Photosynthesis 1

  11. Autotrophy and Photosynthesis 4.2 Overview of Photosynthesis (cont.) Energy that radiates from the Sun forms a continuous series of waves called a spectrum. The range of wavelengths that animals can detect with their eyes—visible light—is roughly the same range plants use in photosynthesis. Shorter wavelengths (blue light) have more energy than longer wavelengths (red light). 4.2 Overview of Photosynthesis 2

  12. Autotrophy and Photosynthesis 4.2 Overview of Photosynthesis (cont.) • Photoautotrophic cells contain light-absorbing substances, or pigments, that absorb visible light. The light-absorbing pigments are embedded in membranes within cells, called thylakoids, that form closed sacs. In the cells of plants and algae, each thylakoid sac is part of an organized structure called a chloroplast. 4.2 Overview of Photosynthesis 3

  13. Autotrophy and Photosynthesis 4.2 Overview of Photosynthesis (cont.) Electron micrograph of a chloroplast in a leaf of corn, Zea mays, x24,000. The darker areas are stacks of thylakoids called grana; the drawing shows the structure of one enlarged granum. Photosynthetic pigments are embedded in the thylakoid membranes; DNA, RNA, and Calvin-cycle enzymes are in the stroma. 4.2 Overview of Photosynthesis 4

  14. Autotrophy and Photosynthesis 4.2 Overview of Photosynthesis (cont.) • Most photosynthesis depends on the green pigment chlorophyll, found in the thylakoids. Plants contain two forms of chlorophyll, a and b. The structure of chlorophyll a. Chlorophyll b differs only in having a CHO—group in place of the circled CH3—. The part of the molecule shown in green absorbs light; the hydrophobic tail helps to keep the molecule anchored in the lipid-rich thylakoid membrane. 4.2 Overview of Photosynthesis 5

  15. Autotrophy and Photosynthesis 4.2 Overview of Photosynthesis (cont.) • Chlorophylls a and b absorb light in the violet/blue and orange/red ranges, but not in the green range. 4.2 Overview of Photosynthesis 6

  16. Autotrophy and Photosynthesis 4.2 Overview of Photosynthesis (cont.) • The green light that is not absorbed gives leaves their color. Other accessory pigments absorb additional wavelengths of light. As the chlorophyll content of leaves declines in the fall, the accessory pigments become more visible. 4.2 Overview of Photosynthesis 7

  17. Autotrophy and Photosynthesis 4.2 Overview of Photosynthesis (cont.) • The process of photosynthesis involves three energy conversions: 1. absorption of light energy 2. conversion of light energy into chemical energy 3. storage of chemical energy in the form of sugars 4.2 Overview of Photosynthesis 8

  18. Autotrophy and Photosynthesis 4.2 Overview of Photosynthesis (cont.) • In the light reactions, pigment molecules in the thylakoids absorb light and convert it to chemical energy. The energy produced by light reactions is used to make 3-carbon sugars from carbon dioxide in a series of reactions known as the Calvin cycle. 4.2 Overview of Photosynthesis 9

  19. Autotrophy and Photosynthesis 4.2 Overview of Photosynthesis (cont.) Solar energy is converted to chemical energy in the thylakoid membranes. Enzymes of the Calvin cycle use this energy to reduce carbon dioxide, forming sugars. 4.2 Overview of Photosynthesis 10

  20. Autotrophy and Photosynthesis 4.2 Overview of Photosynthesis (cont.) • The following equation summarizes the overall reactions of photosynthesis: 4.2 Overview of Photosynthesis 11

  21. Autotrophy and Photosynthesis 4.3 The Light Reactions • During light reactions, chlorophyll and other pigments in the thylakoid: • absorb light energy • water molecules are split into hydrogen and oxygen • light energy is converted to chemical energy which powers sugar production in the Calvin cycle 4.3 The Light Reactions 1

  22. The light reactions of photosynthesis 4.3 The Light Reactions 2

  23. Autotrophy and Photosynthesis 4.3 The Light Reactions (cont.) • The light-absorbing pigments form two types of clusters, called photosystems (PS) I and II. The chlorophyll and other pigments in each photosystem absorb light energy and transfer it from one molecule to the next. All this energy is funneled to a specific chlorophyll a molecule called the reaction center. 4.3 The Light Reactions 3

  24. Autotrophy and Photosynthesis 4.3 The Light Reactions (cont.) • Some of the electrons from the reaction-center molecules jump to other molecules, known as electron carriers, forming an electron transport system between the two photosystems. Electrons from PSII move through the system to replace electrons lost from PSI. PSII receives replacements for these electrons from an enzyme near its reaction center that oxidizes water. 4.3 The Light Reactions 4

  25. The photosynthetic bacterium Chromatium oxidizes hydrogen sulfide gas instead of water, producing the yellow sulfur globules visible in its cells. Autotrophy and Photosynthesis 4.3 The Light Reactions (cont.) • Some photosynthetic bacteria obtain electrons from hydrogen sulfide gas (H2S) instead of water, resulting in solid sulfur as a by-product. 4.3 The Light Reactions 5

  26. Autotrophy and Photosynthesis 4.3 The Light Reactions (cont.) • When electrons from water reach PSI, they receive an energy boost giving them enough energy to reduce a molecule known as NADP+ (nicotinamide adenine dinucleotide phosphate). The electrons, along with protons from water, combine with NADP+ to convert it to its reduced form, NADPH, ending the electron flow in the light reactions. NADPH provides the protons and electrons needed to reduce carbon dioxide in the Calvin cycle. 4.3 The Light Reactions 6

  27. Autotrophy and Photosynthesis 4.3 The Light Reactions (cont.) • The solar energy that the electrons receive from PSII powers the active transport of protons across the thylakoid membrane. The concentrated protons inside the thylakoid then diffuse out, transferring energy to the enzyme complex ATP synthetase. The enzyme uses this energy to synthesize ATP from ADP and phosphate. 4.3 The Light Reactions 7

  28. Autotrophy and Photosynthesis 4.3 The Light Reactions (cont.) • There are two reasons that photosynthesis does not stop with the synthesis of ATP and NADPH: 1. ATP and NADPH are not particularly stable compounds. 2. The light reactions do not produce any new carbon compounds that the organism can use to grow. 4.3 The Light Reactions 8

  29. Autotrophy and Photosynthesis 4.4 The Calvin Cycle • The Calvin cycle conserves the chemical energy produced in the light reactions in the form of sugars that the organism can use for growth. • The Calvin cycle occurs in the stroma of a chloroplast. • The Calvin cycle completes the process of photosynthesis. 4.4 The Calvin Cycle 1

  30. Reduction of carbon dioxide to sugars in the Calvin cycle 4.4 The Calvin Cycle 2

  31. Autotrophy and Photosynthesis 4.4 The Calvin Cycle (cont.) • The Calvin cycle includes the following steps: 1. A molecule of carbon dioxide combines with the 5-carbon sugar-phosphate, ribulose bisphosphate (RuBP), producing an unstable 6-carbon molecule that immediately splits into two molecules of the 3-carbon acid, phosphoglyceric acid (PGA). 2. Two enzymatic steps reduce each molecule of PGA to the 3-carbon sugar-phosphate, phosphoglyceraldehyde (PGAL), requiring one molecule each of ATP and NADPH. 4.4 The Calvin Cycle 3

  32. Autotrophy and Photosynthesis 4.4 The Calvin Cycle (cont.) 3. A series of enzymatic reactions, combines and rearranges molecules of PGAL, eventually producing a 5-carbon sugar-phosphate. 4. Using an ATP molecule to add a second phosphate group to the 5-carbon sugar-phosphate, a molecule of the starting material, RuBP, is created, completing the cycle. 4.4 The Calvin Cycle 4

  33. Autotrophy and Photosynthesis 4.4 The Calvin Cycle (cont.) • Three turns of the Calvin cycle, results in the formation of six molecules of PGAL, one of which is available for the organism to use for maintenance and growth. Sugar-phosphates such as PGAL are removed from the Calvin cycle for use in other cellular functions. Plants that use only the Calvin cycle to fix carbon dioxide are called C3 plants. 4.4 The Calvin Cycle 5

  34. Autotrophy and Photosynthesis 4.4 The Calvin Cycle (cont.) Plants use the sugars produced in photosynthesis to supply energy and carbon skeletons for growth and other cell work. Much of this sugar is converted to sucrose or starch. 4.4 The Calvin Cycle 6

  35. End of Section 1

  36. Photosynthesis and the Environment 4.5 Rate of Photosynthesis • Environmental conditions such as light intensity, temperature, and the concentrations of carbon dioxide and oxygen all affect the rate of photosynthesis. Environmental effects on organisms are usually described in terms of how they affect the rate, or activity per unit of time, of a biological process. 4.5 Rate of Photosynthesis 1

  37. Photosynthesis and the Environment 4.5 Rate of Photosynthesis (cont.) • The rate of photosynthesis levels off before the light reaches the intensity of full sunlight. As light intensity increases, the rate of photosynthesis increases and then reaches a maximum rate. Data are generalized to show trends in C3 plants. 4.5 Rate of Photosynthesis 2

  38. Photosynthesis and the Environment 4.5 Rate of Photosynthesis (cont.) • In very bright light, chlorophyll accumulates energy faster than it can transfer that energy to the electron transport system. As extra energy passes to oxygen molecules, the oxygen may react with water to form hydroxyl ions (OH–) or hydrogen peroxide (H2O2) and a decline in photosynthesis, called photoinhibition, may occur. 4.5 Rate of Photosynthesis 3

  39. Photosynthesis and the Environment 4.5 Rate of Photosynthesis (cont.) • Temperature affects photosynthesis differently from light intensity. As temperature increases, the rate of photosynthesis also increases, and then declines. Data are generalized to show trends in C3 plants that grow best between 20˚C and 30˚C. 4.5 Rate of Photosynthesis 4

  40. Photosynthesis and the Environment 4.5 Rate of Photosynthesis (cont.) • An increase in carbon dioxide concentration increases the rate of photosynthesis to a maximum point, after which the rate levels off. Above the carbon dioxide saturation point, further increases in carbon dioxide concentration have no effect on photosynthesis. 4.5 Rate of Photosynthesis 5

  41. Photosynthesis and the Environment 4.5 Rate of Photosynthesis (cont.) • The effects of light, temperature, and carbon dioxide all interact with each other. The factors in shortest supply have the most effect on the rate of photosynthesis. Limiting factors are environmental factors such as food, temperature, water, or sunlight that restrict growth, metabolism, or population size. 4.5 Rate of Photosynthesis 6

  42. Photosynthesis and the Environment 4.5 Rate of Photosynthesis (cont.) At high light intensity, the rate of photosynthesis is greater at 25˚C than at 15˚C. Thus, temperature can be a limiting factor when further increases in light intensity no longer stimulate photosynthesis. Data are generalized for typical C3 plants. 4.5 Rate of Photosynthesis 7

  43. Photosynthesis and the Environment 4.6 Photorespiration and Special Adaptations • Normal atmospheric concentrations of oxygen (about 21%) can inhibit photosynthesis by up to 50%. Increasing concentrations of oxygen inhibit the rate of photosynthesis in C3 plants. 4.6 Photorespiration and Special Adaptations 1

  44. Photosynthesis and the Environment 4.6 Photorespiration and Special Adaptations (cont.) • The enzyme rubisco incorporates carbon dioxide into sugars in the Calvin cycle. The molecular structures of oxygen and carbon dioxide are held together by double bonds that keep the atoms about the same distance apart, allowing rubisco to bind to either molecule. 4.6 Photorespiration and Special Adaptations 2

  45. Photosynthesis and the Environment 4.6 Photorespiration and Special Adaptations (cont.) • When carbon dioxide binds to rubisco and combines with RuBP, two molecules of PGA form. When oxygen binds to rubisco and combines with RuBP, one molecule of PGA, and one molecule of the 2-carbon acid glycolate form. In a pathway called photorespiration, glycolate is transported out of the chloroplast and partly broken down to carbon dioxide, resulting in a loss of fixed carbon atoms. 4.6 Photorespiration and Special Adaptations 3

  46. Photosynthesis and the Environment 4.6 Photorespiration and Special Adaptations (cont.) Photorespiration occurs simultaneously with photosynthesis and results in the loss of previously fixed carbon dioxide. Both processes depend on the enzyme rubisco, which can react with either carbon dioxide or oxygen. High carbon dioxide levels favor photosynthesis over photorespiration. High oxygen levels promote photorespiration. 4.6 Photorespiration and Special Adaptations 4

  47. Photosynthesis and the Environment 4.6 Photorespiration and Special Adaptations (cont.) • C4 plants have adapted to hot, dry, conditions by having two systems of carbon dioxide fixation that occur in different parts of the leaves. Surrounding each vein in the leaves is a layer of tightly packed cells, the bundle sheath. 4.6 Photorespiration and Special Adaptations 5

  48. Photosynthesis and the Environment 4.6 Photorespiration and Special Adaptations (cont.) • The mesophyll cells fix carbon dioxide by combining it with a 3-carbon acid. The resulting 4-carbon acid is rearranged and then transported to the bundle-sheath cells There, carbon dioxide is released from the 4-carbon acid and refixed by rubisco, forming PGA by way of the Calvin cycle. Many C4 plants can be about twice as efficient as C3 plants in converting light energy to sugars. 4.6 Photorespiration and Special Adaptations 6

  49. Photosynthesis and the Environment 4.6 Photorespiration and Special Adaptations (cont.) Carbon dioxide first combines with a 3-carbon acid in the outside mesophyll cells. The resulting 4-carbon acid is then transported into the bundle-sheath cells, where carbon dioxide is released to the Calvin cycle and refixed by rubisco. 4.6 Photorespiration and Special Adaptations 7

  50. Photosynthesis and the Environment 4.6 Photorespiration and Special Adaptations (cont.) • Another specialization for photosynthesis found in some desert plants is calledCAM, for crassulacean acid metabolism. CAM plants open their stomates at night and incorporate carbon dioxide into organic acids. During the day, the stomates close, conserving water and enzymes then break down the organic acids, releasing carbon dioxide that enters the Calvin cycle. 4.6 Photorespiration and Special Adaptations 8

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