Photosynthesis

1. Photosynthesis

2. Photosynthesis • Photosynthesis is the process by which organisms use the energy of the sun to synthesize organic compounds (sugars) from inorganic compounds (CO2 and water) • Photosynthesis transforms the energy of the sun into chemical energy (glucose) • Provides the oxygen we breath, removes CO2, and provides food, energy and shelter!

3. Photosynthesis • Many organisms participate in photosynthesis: • Nearly all plants • Some Protists (Euglena, kelp) • Some bacteria = cyanobacteria

4. Photosynthesis • Carbon dioxide and water are waste products of cellular respiration! Photosynthesis takes these products and converts them to the glucose and O2 necessary for cellular respiration Light energy 6 + 6 H2O CO2 C6H12O6 O2 + 6 Oxygen gas Carbon dioxide Water Glucose Photosynthesis

5. Photosynthesis • Photosynthesis occurs on a cellular level • Chloroplasts are organelles which carry out photosynthesis • Chloroplasts contain chlorophyll, a light-absorbing pigment which give autotrophs their distinctive color

6. Photosynthesis • Chloroplasts are concentrated in mesophyll, the green tissue in the interior of a leaf • Membranes in the chloroplast form the framework where many of the reactions of photosynthesis occur

7. The Chloroplast • A chloroplast contains two membranes (as do mitochondria) • A thick fluid called the stroma fills the inner compartment of the chloroplast • Suspended in the stroma are the thylakoids, a system of interconnected membranous sacs, which enclose another compartment known as the thylakoid space

8. Chloroplast Outer and inner membranes Intermembrane space Thylakoid Stroma Granum Thylakoid space

9. The Chloroplast • Built into the thylakoid membranes are the chlorophyll molecules that capture light energy • Photosynthesis occurs throughout a plant (all the green parts), but is concentrated in the leaves • A plant will invest much of its energy into the production of its leaves in order to capture as much light energy as possible

10. Make like a tree and… • Leaves are designed to capture light and increase the absorption of carbon dioxide • Carbon dioxide enters the leaf (and oxygen exits the leaf) via the stomata, tiny pores protected by guard cells • Water is supplied to the tree via its roots, but may exit the leaves when the stomata are open (a catch 22!); why stomata open at night in many plants

11. Pigments • Pigments are light-absorbing molecules built into the thylakoid membranes • Pigments absorb some wavelengths of light, but reflect others • We do not see the absorbed wavelengths (because their energy has been absorbed by the pigment molecules); we see the wavelengths that the pigment reflects!

12. Pigments • A leaf is green because chlorophyll absorbs colors other than green; absorbs light most strongly in the blue and red, but poorly in the green • Different pigments absorb different wavelengths • Chloroplasts contain different types of pigments

13. Increasing energy 1 m 10–3 nm 103 m 10–5 nm 1 nm 103 nm 106 nm Micro- waves Radio waves Gamma rays UV Infrared X-rays Visible light 400 750 380 600 700 500 Wavelength (nm) 650 nm

14. Pigments Light Reflected light • Chlorophyll a (a type of chlorophyll pigment) absorbs light mainly in the blue-violet (high energy) and red (low energy) wavelengths Chloroplast Absorbed light Thylakoid Transmitted light

15. Autumn color change • Autumn leaf color is a phenomenon that affects the normally green leaves of deciduous trees and shrubs, changing to reds and yellows (and various shades in between) • In late summer, the veins that carry fluids into and out of the leaf are gradually closed off, and chlorophyll decreases

16. Autumn color change • In addition to chlorophyll, other pigments, known as accessory pigments are present in plants; these include carotene, and cyanins • When chlorophyll concentrations decrease at the end of summer, some of these other pigments – which are usually masked by chlorophyll – reveal their colors • Carotene, for example, is especially long-lasting

17. http://ohad.me/?showimage=53

18. Photosynthesis • Plants produce CO2 by splitting water • Stable isotopes (18O and 16O) of oxygen revealed that the O2 produced by plants originates from water (H2O), and not from CO2 Experiment 1 C6H12O6 + 6 H2O + 6 O2 6 CO2 + 12 H2O Not labeled Experiment 2 C6H12O6 + 6 H2O + 6 O2 6 CO2 + 12 H2O Labeled

19. Photosynthesis • Photosynthesis occurs in 2 stages, each with multiple steps 1. Light reactions convert light energy into chemical energy, and produce O2. 2. Light-independent reactions (the Calvin Cycle) assembles glucose molecules using CO2 (carbon fixation) and the energy-rich products of the light reactions

20. 1. Light (dependent) Reactions • Light reactions occur in the thylakoid membranes of the chloroplast • Water is split, providing a source of electrons and giving off O2 as a by-product H2O  2H+ + 1/2 O2 + 2e- • Light energy absorbed by chlorophyll molecules is used to drive the transfer of electrons and H+ from water to NADP+ and generate ATP

21. 1. Light (dependent) Reactions • Light reactions absorb solar energy and convert it to chemical energy (NADPH) • Produces O2 as a by-product • No sugar is produced in light reactions

22. 2. Light-independent Reactions • Light-independent reactions occur in the stroma of chloroplasts • Consist of the Calvin Cycle, which assembles sugar molecules using CO2 and NADPH • The incorporation of Carbon from CO2 into organic compounds is called carbon fixation

23. 2. Light-independent Reactions • The Calvin Cycle does not require light, but occurs during daylight hours when the light reactions power the cycle by supplying NADPH and ATP • Often called “dark reactions” • NADPH provides the electrons to reduce Carbon from CO2 into glucose, and ATP powers the cycle

24. CO2 H2O Chloroplast Light NADP+ ADP  P LIGHT REACTIONS CALVIN CYCLE (in stroma) (in thylakoids) ATP Electrons NADPH Sugar O2

25. How do photosystems capture solar energy? • In the thylakoid membrane, chlorophyll molecules are organized into clusters (with other pigments and proteins) called photosystems • A photosystems consists of a number of light-harvesting pigments bound to proteins surrounded by a reaction center complex

26. How do photosystems capture solar energy? • The pigments absorb light energy and pass the electrons (energy) from molecule to molecule until it reaches the reaction center • The reaction center complex contains a pair of chlorophyll a molecules and an electron acceptor • There are 2 photosystems: Photosystem II and Photosystem I

27. Photosystem Light-harvesting complexes Reaction center complex Photon Primary electron acceptor e– Thylakoid membrane Transfer of energy Pigment molecules Pair of Chlorophyll a molecules

28. Photosystems • So how does this work? • When light is absorbed by the pigments, energy passes from pigment to pigment molecules until it reaches the reaction center of the photosystem where it excites an electron of chlorophyll to a higher energy state that is captured by the primary electron acceptor

29. Photosystems • Water is split, supplying its electrons to the chlorophyll molecule which lost its electrons to the primary electron acceptor, releasing O2 as a by-product • Photosystems I and II were named in order of their discovery, but Photosystem II functions first in the sequence of steps that make up the light reactions

30. A mechanical analogy of the light reactions e– ATP e– e– NADPH e– e– e– Mill makes ATP Photon e– Photon Photosystem II Photosystem I

31. Photosynthesis: a review • Light reactions occur in the thylakoid membrane • 2 photosystems capture solar energy which energizes electrons • Photosystems transfer these excited electrons through electron transport chains which produces ATP and NADPH • Water is split and O2 is released

32. Photosynthesis: a review • In the stroma of the chloroplast, sugars are produced via the Calvin Cycle (light-independent reactions) • CO2 is reduced to form glucose

33. Photosynthesis: a review • The sugar produced by plants during photosynthesis provides the starting materials to make structural components such as cellulose • 50% of this sugar goes toward cellular respiration (plants respire!)

34. Photosynthesis: a review • Most plants make considerably more food each day than they need • They stockpile this sugar as _____? storing it in roots, tubers, and fruits (sound familiar?) • Plants not only produce fuel for themselves, but ultimately provide food for virtually all other organisms (heterotrophs)

35. Global warming and the greenhouse effect

36. Global warming and the greenhouse effect • CO2 is an important greenhouse gas • Greenhouse gases are gases in the atmosphere that absorb heat radiation • When solar radiation reaches the atmosphere, visible light passes is absorbed by the planet’s surface warming it

37. Global warming and the greenhouse effect • This heat is radiated back as longer, infrared wavelengths, which are absorbed by gases in the atmosphere reflecting some of the heat back to earth • Very important! Without these greenhouse gases, the Earth’s surface temperature would be ~33 ̊C (59 ̊F) cooler than at present • This process is called the greenhouse effect

39. Global warming and the greenhouse effect • CO2, water vapor, and methane are naturally-occurring greenhouse gases • Photosynthetic organisms absorb billions of tons of CO2 every year! • Most of this carbon returns to the atmosphere via cellular respiration, but much of it remains locked in large tracts of forests and in long-term storage as fossil fuels buried deep under the Earth’s surface

40. Global warming • Since 1850 (the start of the Industrial Revolution), the concentration of CO2 in the atmosphere has increased ~40% • Increasing concentrations of CO has been linked to global warming, a steady rise in Earth’s surface temperature • Earth’s average surface temperature has increased by 1.2-1.4 ̊F in the last 100 years!

41. Annual Average Global Surface Temperature Anomalies 1880-2006

42. 400 2000 1800 Carbon Dioxide (CO2) Methane (CH4) 1600 350 Nitrous Oxide (N2O) 1400 CH4 (ppb) CO2 (ppm), N2O (ppb) 1200 300 1000 800 250 600 1000 2000 500 1500 0 Year

43. Concentration of CO2 1958-2006

44. Global warming • Rapid warming is changing the global climate • Sea level rise, melting ice & permafrost, extreme weather events, changing weather patterns

45. Global climate change • Global climate change affects biomes, ecosystems, communities and populations • The earlier arrival of warm temperatures in the spring is disrupting ecological communities • What would happen if plants bloomed before their pollinators arrived? • What will happen to coral under increasing temperature?