Chapter 6 Where It Starts – Photosynthesis

1. Chapter 6Where It Starts – Photosynthesis

2. Autotrophs and Heterotrophs • Autotrophsharvest energy directly from the environment, and obtain carbon from inorganic molecules • Plants and most other autotrophs make their own food by photosynthesis, a process which uses the energy of sunlight to assemble carbohydrates from carbon dioxide and water • Animals and other heterotrophsget energy and carbon by breaking down organic molecules assembled by other organisms

3. 6.2 Sunlight as an Energy Source • Energy flow through nearly all ecosystems on Earth begins when photosynthesizers intercept energy from the sun • Photosynthetic organisms use pigments to capture the energy of sunlight and convert it to chemical energy – the energy stored in chemical bonds

4. Properties of Light • Visible light is part of an electromagnetic spectrum of energy radiating from the sun • Travels in waves • Organized into photons • Wavelength • The distance between the crests of two successive waves of light (nm) • Shorter wavelength have greater energy

5. Electromagnetic Spectrum of Radiant Energy range of heat escaping from Earth’s surface longest wavelengths (lowest energy) shortest wavelengths (highest energy) range of most radiation reaching Earth’s surface visible light gamma rays radiation infrared near-infrared radiation ultraviolet radiation x-rays microwaves radio waves 400 nm 500 nm 600 nm 700 nm

6. Some Photosynthetic Pigments

7. 6.3 Exploring the Rainbow • Photosynthetic pigments work together to harvest light of different wavelengths • Engelmann identified colors of light that drive photosynthesis (violet and red) by using a prism to divide light into colors – algae using these wavelengths gave off the most oxygen

8. Photosynthesis and Wavelengths of Light bacteria alga 400 nm 500 nm 600 nm 700 nm Wavelength

9. Absorption Spectra chlorophyll b phycoerythrobilin phycocyanobilin β-carotene chlorophyll a Light absorption 400 nm 500 nm 600 nm 700 nm Wavelength

10. Two Stages of Photosynthesis • Light-dependent reactions (noncyclic pathway) • First stage of photosynthesis • Light energy is transferred to ATP and NADPH • Water molecules are split, releasing O2 • Light-independent reactions • Second stage of photosynthesis • Energy in ATP and NADPH drives synthesis of glucose and other carbohydrates from CO2 and water

11. Summary: Photosynthesis 6CO2 + 6H2O → light energy → C6H12O6 + 6O2

12. two outer membranes of chloroplast stroma part of thylakoid membrane system: thylakoid compartment, cutaway view Figure 6-5b p105

13. Take-Home Message: Where do the reactions of photosynthesis take place? • In the first stage of photosynthesis, light energy drives the formation of ATP and NADPH, and oxygen is released; in eukaryotic cells, these light-dependent reactions occur at the thylakoid membrane of chloroplasts • The second stage of photosynthesis, the light-independent reactions, occur in the stroma of chloroplasts; ATP and NADPH drive the synthesis of carbohydrates

14. 6.5 Light-Dependent Reactions • Light-dependent reactions convert light energy to the energy of chemical bonds • Photons boost electrons in pigments to higher energy levels • Light-harvesting complexes absorb the energy • Electrons are released from special pairs of chlorophyll a molecules in photosystems • Electrons may be used in noncyclic or cyclic pathways of ATP formation

15. Figure 6-7 p106

16. The Noncyclic Pathway • Photosystems (type II and type I) contain “special pairs” of chlorophyll a molecules that eject electrons • Electrons lost from photosystem II are replaced by photolysisof water molecules – the process by which light energy breaks down a water molecule into hydrogen and oxygen • Electrons lost from a photosystem enter an electron transfer chain (ETC) in the thylakoid membrane

17. The Noncyclic Pathway • In the ETC, electron energy is used to build up a H+ gradient across the membrane • H+ flows through ATP synthase, which attaches a phosphate group to ADP • ATP is formed in the stroma by chemiosmosis, or electron transfer phosphorylation

18. The Cyclic Pathway • When NADPH accumulates in the stroma, the noncyclic pathway stalls • A cyclic pathway runs in type I photosystems to make ATP; electrons are cycled back to photosystem I and NADPH does not form

19. Take-Home Message: What happens during the light-dependent reactions of photosynthesis? • In light-dependent reactions, chlorophylls and other pigments in thylakoid membrane transfer light energy to photosystems • Photosystems eject electrons that enter electron transfer chains in the membrane; electron flow through ETCs sets up hydrogen ion gradients that drive ATP formation • In the noncyclic pathway, oxygen is released and electrons end up in NADPH • A cyclic pathway involving only photosystem I allows the cell to continue making ATP when the noncyclic pathway is not running; NADPH does not form; O2 is not released

20. Photophosphorylation • Photophosphorylationis a light-driven reaction that attaches a phosphate group to a molecule • In noncyclic photophosphorylation, electrons move from water to photosystem II, to photosystem I, to NADPH • In cyclic photophosphorylation, electrons cycle within photosystem I

21. 6.7 Light-Independent Reactions • The cyclic, light-independent reactions of the Calvin-Benson cycle are the “synthesis” part of photosynthesis • Calvin-Benson cycle • Enzyme-mediated reactions that build sugars in the stroma of chloroplasts

22. Carbon Fixation • Carbon fixation • Extraction of carbon atoms from inorganic sources (atmosphere) and incorporating them into an organic molecule • Builds glucose from CO2 • Uses bond energy of molecules formed in light-dependent reactions (ATP, NADPH)

23. The Calvin-Benson Cycle • The enzyme rubisco attaches CO2 to RuBP • Forms two 3-carbon PGA molecules • PGAL is formed • PGAs receive a phosphate group from ATP, and hydrogen and electrons from NADPH • Two PGAL combine to form a 6-carbon sugar • Rubisco is regenerated

24. 4 2 other molecules 3 glucose 1 Calvin– Benson Cycle Stepped Art Figure 6-10 p109

25. Take-Home Message: What happens in light-independent reactions of photosynthesis? • Light-independent reactions of photosynthesis run on the bond energy of ATP and energy of electrons donated by NADPH; both formed in the light-dependent reactions • Collectively called the Calvin–Benson cycle, these carbon-fixing reaction use hydrogen (from NADPH), and carbon and oxygen (from CO2) to build sugars

26. 6.8 Adaptations: Different Carbon-Fixing Pathways • Environments differ, and so do details of photosynthesis: • C3 plants • C4 plants • CAM plants

27. Stomata • Stomata • Small openings through the waxy cuticle covering epidermal surfaces of leaves and green stems • Allow CO2 in and O2 out • Close on dry days to minimize water loss

28. C3 Plants • C3 plants • Plants that use only the Calvin–Benson cycle to fix carbon • Forms 3-carbon PGA in mesophyll cells • Used by most plants, but inefficient in dry weather when stomata are closed • Example: barley

29. Photorespiration • When stomata are closed, CO2 needed for light-independent reactions can’t enter, O2 produced by light-dependent reactions can’t leave • Photorespiration • At high O2 levels, rubisco attaches to oxygen instead of carbon • CO2 is produced rather than fixed

30. A C3 Plant: Barley palisade mesophyll cell spongy mesophyll cell

31. C4 Plants • C4 plants • Plants that have an additional set of reactions for sugar production on dry days when stomata are closed; compensates for inefficiency of rubisco • Forms 4-carbon oxaloacetate in mesophyll cells, then bundle-sheath cells make sugar • Examples: Corn, switchgrass, bamboo

32. A C4 Plant: Millet mesophyll cell bundle-sheath cell

33. CAM Plants • CAM plants(Crassulacean Acid Metabolism) • Plants with an alternative carbon-fixing pathway that allows them to conserve water in climates where days are hot • Forms 4-carbon oxaloacetate at night, which is later broken down to CO2 for sugar production • Example: succulents, cactuses

34. A CAM Plant: Jade Plant

35. Take-Home Message:How do carbon-fixing reactions vary? • When stomata are closed, oxygen builds up inside leaves of C3 plants; rubisco then can attach oxygen (instead of carbon dioxide) to RuBP; photorespiration reduces the efficiency of sugar production, so it can limit the plant’s growth • Plants adapted to dry conditions limit photorespiration by fixing carbon twice: C4 plants separate the two sets of reactions in space; CAM plants separate them in time

36. Biofuels Revisited • The first cells on Earth were chemoautotrophs that extracted energy and carbon from inorganic molecules in the environment, such as hydrogen sulfide and methane • The evolution of photosynthesis dramatically and permanently changed Earth’s atmosphere • Photoautotrophs use photosynthesis to make food from CO2 and water, releasing O2 into the atmosphere

37. Effects of Atmospheric Oxygen • Selection pressure on evolution of life • Oxygen radicals • Development of ATP-forming reactions • Aerobic respiration • Formation of ozone (O3) layer • Protection from UV radiation

38. The Atmospheric Carbon Cycle • Photosynthesis removes carbon dioxide from the atmosphere, and locks carbon atoms in organic compounds • Aerobic organisms break down organic compounds for energy, and release CO2 into the atmosphere • Since photosynthesis evolved, these two processes have constituted a more or less balanced cycle of the biosphere • Today, Earth’s atmosphere is out of balance – the level of CO2 is increasing, mainly as a result of human activity

39. Fossil Fuels • When we burn fossil fuels, carbon that has been locked for hundreds of millions of years is released back into the atmosphere, mainly as carbon dioxide • Today, we release about 28 billion tons of carbon dioxide into the atmosphere each year, more than ten times the amount we released in the year 1900 • Increased atmospheric CO2 contributes to global warming and disrupts natural biological systems