The Dual Nature of Light

1. The Dual Nature of Light Wave and Particle

2. Light as a particle • Particles or packets of light are known as photons • Brightness or intensity depends on the number of photons absorbed/unit area/unit time • Photon carries fixed amount of energy • Determines how fast it vibrates • high energy = fast • low energy = slow

3. Light as a wave • The distance moved by a photon during one of it vibrations is referred to as its wavelength • abbreviations used in class: •  = lambda = wavelength • unit = nanometers (nm) • One nanometer = 10-9 meter •  = nu = my shorthand for light

4. Long wavelength Low energy Short wavelength High energy

5. Effective wavelengths for photosynthesis • A prism separates visible light into a color spectrum • The visible spectrum • ROYGBIV • Between 380 and 750 nm

6. Why only visible light? Sunlight has three components • 4% UV (ultraviolet radiation) TOO STRONG! • ionizing radiation • breaks weak chemical bonds • causes DNA damage and sunburn • absorbed by O2 and O3 (ozone), glass, plastic • 52% IR (infrared radiation) TOO WEAK! • low energy • most energy of IR converted to heat • 44% visible light JUST RIGHT! • suitable energy for life - photosynthesis • absorbed by pigments

7. What Happens to Light? • Reflected • Chlorophyll is green because it reflects the green wavelengths. • Sky is blue because water molecules reflect blue wavelengths. • Transmitted • Glass permits most visible wavelengths to pass through. • Absorbed • Chlorophyll absorbs all colors except green, especially red and blue regions.

8. Pigments • Molecules that absorb light • Black absorbs all wavelengths • White absorbs no wavelengths • Color that we see is the reflected wavelengths

10. Quality of Light • Light quality (availability of different wave-lengths) can limit rate of photo-synthesis • Blue and red wavelengths are absorbed by chlorophyll • Green wavelengths are reflected or transmitted • Therefore most plants look green to us • Usable wavelengths are called PAR – photosynthetically active radiation

11. Chlorophyll • Structure • tetrapyrrolic rings • Mg+2 atom • phytol chain (tail) • Forms • chl a • grass green • absorbs in red and violet blue regions • chl b, c, and d • bluish green

13. Chlorophyll • Absorption spectrum: absorbs all wavelengths except green • Absorbs most strongly in red and blue region of spectrum • Action spectrum: doesn’t match absorption spectrum • Chl a is primary photosynthetic pigment • But there must be others

16. Accessory Pigments • Functions • Extend range of photosynthesis by absorbing wavelengths not picked up by chl a • Carotenoids also protect against photo-oxidation • Oxygen radicals formed by excited electrons kill cells (basis for some herbicides)

17. Accessory Pigment Types • Chlorophylls • Chl a is the main photosynthetic pigment • Grassy green • absorbs around 420 and 660 nm • Chl b • bluish green • absorbs 453 and 642 nm • about half as abundant as chl a • Chl c and d

19. Accessory Pigment Types • Carotenoids • fat soluble • absorb 460 and 550 nm • Blue, bluegreen, violet • Structure - see plant chem. Notes • Example: -carotene • reddish yellow, orange • precursor to vitamin A (split to get 2 vit A) • needed to produce retinal pigment for vision • carrots, tomatoes, bananas, squash, fall leaves

20. Accessory Pigment Types • Carotenoids are also found in animals • egg yolks • flamingo feathers • squid ink • corals • fish • amphibians • lots of colors • proteins attached to carotenoids • remove protein => see red or orange • this is what happens when you cook shrimp or lobster

21. Absorption spectra

22. Accessory Pigment Types • Xanthophylls and fucoxanthin (brown algae) • yellowish or red • absorb blue light (some plants grow towards blue light) • not as efficient as -carotene • Phycoerythrin and phycocyanin found in cyanobacteria and red algae • Remember: these bacteria as well as the algae contribute to production in aquatic systems

23. Pigments • Light reaching the surface of the earth • Peak irradiation occurs at approx 500 nm • Light intensity drops off at higher & lower wavelengths

24. Pigments • If we combine all of these, including chlorophyll and carotenoids, there is good light absorp-tion across the entire visible spectrum!

25. Absorption of Light by Water

26. Absorption of Light by Water • Red & blue wavelengths • preferentially absorbed by water • Wavelengths 500 – 600 nm • absorbed least by water • correspond with absorption spectra of phycobilins.

27. Ecological Significance of Phycobilins • Organisms with Phycobilin pigments are better adapted for life in an aquatic environment and can exploit greater depths in lakes and oceans! • Therefore, red algae and the cyanobacteria can survive at greater depths than any other algae or vascular plants.

28. Why Do Leaves Change Color in the Fall? • Chlorophyll synthesis shuts down • Chlorophyll molecules break down • “True” colors of leaves show through • Accessory pigments • Carotenoids - orange • Xanthophylls - yellow • Anthocyanins - red, purple • In central vacuole In chromoplasts

29. Quantity of Light • Amount of light reaching thyllakoid membranes is limited by • Location of chloroplast in leaf • Incident angle of sunlight as it strikes leaf • Self shading by other leaves • Shading by competitors • See discussion of Chazdon 1985 and Tanaka et al. 2001.

30. Global Light Availability • Tropical latitudes - day and night equal • Polar latitudes - continuously light at midsummer, continuously dark at midwinter • Maximum sunlight energy greater in tropics than polar regions

31. Global Light Availability • Maximum sunlight energy greater at high altitudes than at sea level • Damaging UV-B radiation greater in tropics than polar regions, high elevations vs. low elevations • Biochemical protection: • Flavonoids to absorb UV-B • Increased levels of antioxidant enzymes and • DNA repair enzymes