The Sun's Energy and Photosynthesis: A Renewable Source of Life

2. Nearly All Life On Earth—including You—can Trace its Source Of Energy Back To The Sun

3. Biology and Society: A Solar Revolution • The electricity that photovoltaic (PV) solar panels produce from sunlight can be used directly, stored in batteries, or transferred to the national electric grid. • Living organisms that capture sunlight using photosynthesis can also serve as energy sources. • Plant matter can be burned. • Plant materials can be used to produce biofuels. • All of these methods of capturing the energy of sunlight are renewable.

4. Chapter Thread

5. The Basics of Photosynthesis • Photosynthesis • is used by plants, algae (protists), and certain bacteria, • transforms light energy into chemical energy, • uses carbon dioxide (CO2) and water (H2O) as starting materials, and • releases oxygen gas as a by-product. • The chemical energy produced via photosynthesis is stored in the bonds of sugar molecules.

6. The Basics of Photosynthesis (Cont.) • Organisms that generate their own organic matter from inorganic ingredients are called autotrophs. • Plants and other organisms that do this by photosynthesis—photoautotrophs—are the producers for most ecosystems. • The fact that nearly all organisms can trace their source of energy back to the sun clearly illustrates that the ability to transform energy and matter is vital to the existence of life on Earth.

7. A diversity of photoautotrophs

8. Identifying Major Themes • Most ecosystems depend on the process of photosynthesis to convert the energy in sunlight into high-energy organic molecules, which then provide fuel for other organisms. Which major theme is illustrated by this action? • The relationship of structure to function • Information flow • Pathways that transform energy and matter • Interactions within biological systems • Evolution

9. Chloroplasts: Sites of Photosynthesis • Chloroplasts are • light-absorbing organelles, • the site of photosynthesis, and • found mostly in the interior cells of leaves. • Their green color is from chlorophyll, a pigment (light-absorbing molecule) in the chloroplasts that plays a central role in converting solar energy to chemical energy.

10. Chloroplasts: Sites of Photosynthesis (Cont.) • Stomata are tiny pores in leaves where carbon dioxide enters and oxygen exits. • Membranes within the chloroplast form the framework where many of the reactions of photosynthesis occur.

11. Journey into a leaf

12. Journey into a leaf (Cont.) • Like a mitochondrion, a chloroplast has a double-membrane envelope. • The inner membrane encloses a compartment filled with stroma, a thick fluid. • Suspended in the stroma are interconnected membranous sacs called thylakoids. • The thylakoids are concentrated in stacks called grana (singular, granum). • The chlorophyll molecules that capture light energy are built into the thylakoid membranes.

13. An Overview of Photosynthesis • In the overall equation for photosynthesis, notice • that the reactants of photosynthesis, carbon dioxide (CO2) and water (H2O), are the same as the waste products of cellular respiration, and • that photosynthesis produces what respiration uses—glucose (C6H12O6) and oxygen (O2).

14. Figure 7.UN03

15. An Overview a of Photosynthesis (Cont.) • The overall process of photosynthesis can be divided into two stages connected by energy- carrying and electron-carrying molecules: • the light reactions in which chlorophyll in the thylakoid membranes absorbs solar energy, which is then converted to the chemical energy of ATP and NADPH, and • the Calvin cycle, which uses the products of the light reactions to power the production of sugar from carbon dioxide.

16. A road map for photosynthesis

17. An Overview of Photosynthesis (Cont.) • The initial incorporation of carbon from the atmosphere into organic compounds is called carbon fixation. This can help reduce the concentration of carbon dioxide in the atmosphere. • The relationship between photosynthesis, carbon fixation, and global climate is a good example of how interactions between biological components at many different levels affect all life on Earth. Checkpoint: What molecules are the inputs of photosynthesis? What molecules are the outputs?

18. BioFlix Animation: Photosynthesis

19. Identifying Major Themes • Deforestation in Asia can alter the climate in North America. Which major theme is illustrated by this action? • The relationship of structure to function • Information flow • Pathways that transform energy and matter • Interactions within biological systems • Evolution

20. The Light Reactions: Converting Solar Energy to Chemical Energy: The Nature of Sunlight • Sunlight, like other forms of radiation, travels through space as rhythmic waves. The distance between the crests of two adjacent waves is called a wavelength. • The full range of radiation is called the electromagnetic spectrum.

21. The electromagnetic spectrum

22. The Process of Science: What Colors of Light Drive Photosynthesis? • Background: In 1883, German biologist Theodor Engelmann saw that certain bacteria living in water cluster in areas with higher oxygen concentrations. He formed a hypothesis that oxygen-seeking bacteria will congregate near regions of algae performing the most photosynthesis. • Method: Engelmann placed freshwater algal cells within a drop of water on a microscope slide. • Results: Most bacteria congregated around algae exposed to red-orange and blue-violet light.

23. Investigating how light wavelength affects photosynthesis

24. Chloroplast Pigments • The selective absorption of light by leaves explains why they appear green to us. • Light of that color is poorly absorbed by chloroplasts and is thus reflected or transmitted toward the observer. • Energy cannot be destroyed, so the absorbed energy must be converted to other forms.

25. Why are leaves green?

26. Animation: Light and Pigments

27. Chloroplast Pigments • Chloroplasts contain several different pigments that absorb light of different wavelengths. • Chlorophyll a participates directly in light reactions and absorbs mainly blue-violet and red light. • Chlorophyll b does not participate directly in the light reactions, but it conveys absorbed energy to chlorophyll a. • Chloroplasts also contain a family of yellow-orange pigments called carotenoids, which absorb mainly blue-green light.

28. Photosynthetic pigments

29. How Photosystems Harvest Light Energy • Light behaves as waves and discrete packets of energy called photons, fixed quantities of light energy. The shorter the wavelength of light, the greater the energy of a photon. • When a pigment molecule absorbs a photon, one of the pigment’s electrons gains energy and is now said to be “excited.” • An excited electron usually loses its excess energy and falls back to its ground state. • Most pigments release heat energy as their light-excited electrons fall back to their ground state.

30. How Photosystems Harvest Light Energy (Cont.) • Some pigments emit light as well as heat after absorbing photons. • The fluorescent light emitted by a glow stick is caused by a chemical reaction that excites electrons of a fluorescent dye. • The excited electrons quickly fall back down to their ground state, releasing energy in the form of fluorescent light.

31. Excited electrons in pigments

32. How Photosystems Harvest Light Energy • In the thylakoid membrane, chlorophyll molecules are organized with other molecules into photosystems. • Each photosystem has a cluster of a few hundred pigment molecules, including chlorophylls a and b and some carotenoids. • This cluster of pigment molecules functions as a light-gathering antenna.

33. A photosystem: light-gathering molecules that focus light energy onto a reaction center

34. Identifying Major Themes • The reactions of photosynthesis are driven by membrane-bound enzymes, and the folded shape of the thylakoid greatly increases the area on which these enzymes can operate. Which major theme is illustrated by this action? • The relationship of structure to function • Information flow • Pathways that transform energy and matter • Interactions within biological systems • Evolution

35. How the Light Reactions Generate ATP and NADPH • Two photosystems cooperate in the light reactions: • Photons excite electrons in the chlorophyll of the first photosystem. • Energized electrons from the first photosystem pass down an electron transport chain to the second photosystem. The chloroplast uses the energy released by this electron “fall” to make ATP. • The second photosystem transfers its light-excited electrons to NADP+, reducing it to NADPH.

36. The light reactions of photosynthesis

37. How the Light Reactions Generate ATP and NADPH (Cont.) • The light reactions are located in the thylakoid membrane. • The two photosystems and the electron transport chain that connects them transfer electrons from H2O to NADP+, producing NADPH.

38. How the thylakoid membrane converts light energy to the chemical energy of NADPH and ATP. Checkpoint: Why is water required as a reactant in photosynthesis?

39. The light reactions illustrated using a hard-hat analogy

40. How the Light Reactions Generate ATP and NADPH (Cont.) • The mechanism of ATP production during the light reactions is very similar to the mechanism we saw in cellular respiration. In both cases, • an electron transport chain pumps hydrogen ions across a membrane and • ATP synthases use the energy stored by the H+ gradient to make ATP. • The main difference is that food provides the high-energy electrons in cellular respiration, whereas light-excited electrons flow down the transport chain during photosynthesis.

41. The Calvin Cycle: Making Sugar from Carbon Dioxide • The Calvin cycle functions like a sugar factory within a chloroplast. • The Calvin cycle constructs an energy-rich sugar molecule called glyceraldehyde 3-phosphate (G3P) using carbon from CO2, energy from ATP, and high-energy electrons from NADPH. • The plant cell can then use G3P as the raw material to make glucose and other organic compounds.

42. The Calvin cycle

43. Evolution Connection: Creating a Better Biofuel Factory https://www.youtube.com/watch?v=sQK3Yr4Sc_kPhotosynthesis (13.00)

44. Evolution Connection: Creating a Better Biofuel Factory • The production of biofuels is highly inefficient. • Biomechanical engineers are working to solve this dilemma by turning to an obvious example: evolution by natural selection. • Scientists can use a process called directed evolution, in which scientists in the laboratory determine which organisms are the fittest. • Directed evolution of biofuel production often involves microscopic algae rather than plants because algae are easier to manipulate and maintain within the laboratory.

45. Microscopic biofuel factories

46. Evolution Connection: Creating a Better Biofuel Factory (Cont.) • In a typical directed evolution experiment, the researcher starts with a large collection of algae. • The algae are exposed to mutation-promoting chemicals. • The algae are screened for their ability to produce the most useful biofuel in the largest quantity. • Algae that can best perform this task are grown and subjected to more rounds of mutation and selection. • Repeated many times, the algae may slowly improve their ability to efficiently produce biofuels.