Introduction to Photosynthesis (181-200)

1. Introduction to Photosynthesis (181-200) Life on Earth is SOLAR powered Photosynthesis (Ps) nourishes almost all living organisms Autotrophs - mainly Ps organisms (photoautotrophs) that make their own food (using sun E, CO2, and H2O) Also called producers of the biosphere Exs = green plants and Ps protist groups (fig 10.2) Heterotrophs - get E from organic compounds produced by other organisms Also called consumers of the biosphere Exs = fungi, animals, & many protist groups Photosynthesis converts light E to chemical E of food

2. The Process That Feeds the Biosphere • Photosynthesis • Is the process that converts solar (light) energy into chemical energy • Plants and other autotrophs • Are the producers of the biosphere

3. Figure 10.1 Plants are photoautotrophs • They use the energy of sunlight to make organic molecules from water and carbon dioxide

4. These organisms use light energy to drive the synthesis of organic molecules from carbon dioxide and (in most cases) water. They feed not only themselves, but the entire living world. (a) On land, plants are the predominant producers of food. In aquatic environments, photosynthetic organisms include (b) multicellular algae, such as this kelp; (c) some unicellular protists, such as Euglena; (d) the prokaryotes called cyanobacteria; and (e) other photosynthetic prokaryotes, such as these purple sulfur bacteria, which produce sulfur (spherical globules) (c, d, e: LMs). (a) Plants (c) Unicellular protist 10 m (e) Purple sulfur bacteria 1.5 m Figure 10.2 (d) Cyanobacteria (b) Multicellular algae 40 m Photosynthesis • Occurs in plants, algae, certain other protists, and some prokaryotes

5. Heterotrophs • Heterotrophs • Obtain their organic material from other organisms • Are the consumers of the biosphere • Includes fungi, animals, many protist groups and many bacteria

6. Chloroplasts – Sites of Ps within the cell • Primarily found in leaves (mesophyll = main part of a leaf) • Stomata = regulated holes in leaves where gas exchange occurs (what gases does a plant need to exchange for Ps?) • Organelles enclosed by a double-membrane system (endosymbiosis) • Stroma = internal fluid-filled cavity • Thylakoids = system of interconnected membrane sacs (separates the stroma from the thylakoid space) • Grana = stacks of thylakoids • Chlorophyll = green pigment that absorbs light E = molecular bridge between sunlight and Ps activity • Molecules are embedded in the thylakoid membrane system

7. Leaf cross section Vein Mesophyll CO2 O2 Stomata Figure 10.3 Chloroplasts: The Sites of Photosynthesis in Plants • The leaves of plants • Are the major sites of photosynthesis

8. Mesophyll Chloroplast 5 µm Outer membrane Thylakoid Intermembrane space Thylakoid space Granum Stroma Inner membrane 1 µm Chloroplasts • Chloroplasts • Are the organelles in which photosynthesis occurs • Contain thylakoids and grana • Stroma is the fluid in the internal cavity • Chlorophyll is imbedded in the thylakoid membranes

9. Tracking Atoms Through Photosynthesis: • Photosynthesis is summarized as 6 CO2 + 12 H2O + Light energy  C6H12O6 + 6 O2 + 6 H2 O OR CO2 + H2O  [CH2O] + O2 Overall Ps equation has been known since the 1800s The equation for Ps (fig 10.4) = reverse of respiration But carbohydrates are not made by simply reversing what happens in respiration BOTH processes occur in plant cells!

10. Reactants: 12 H2O 6 CO2 6 H2O 6 O2 C6H12O6 Products: Figure 10.4 The Splitting of Water • Chloroplasts split water into • Hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules

11. Photosynthesis as a Redox Process • Photosynthesis is a redox process • Water is oxidized, carbon dioxide is reduced

12. Two Stages of Photosynthesis Two stages of Ps (fig 10.5): 1.Light rxns: depend on light  make ATP & NADPH and give off O2 NADPH = very similar in structure to NADH (just add a phosphate group to NADH) = the e- carrier Photophosphorylation = how ATP is generated (using chemiosmosis again) 2.Calvin cycle: use ATP and NADPH to fix C from the atmosphere into organic compounds Carbon fixation = initial incorporation of C into organic compounds

13. The Two Stages of Photosynthesis • Photosynthesis consists of two processes • The light reactions • The Calvin cycle

14. The Light Reactions • The light reactions • Occur in the grana • Split water, release oxygen, produce ATP, and form NADPH

15. The Calvin Cycle • The Calvin cycle • Occurs in the stroma • Forms sugar from carbon dioxide, using ATP for energy and NADPH for reducing power

16. H2O CO2 Light NADP  ADP + P LIGHT REACTIONS CALVIN CYCLE ATP NADPH Chloroplast [CH2O] (sugar) O2 Figure 10.5 An overview of photosynthesis

17. Light Reactions • The light reactions convert solar energy to the chemical energy of ATP and NADPH

18. Light • Light = electromagnetic energy, which travels in waves • Wavelength = distance between crests/troughs of waves (nm - km) • Smaller wavelengths = stronger light waves • Electromagnetic spectrum (fig 10.6) = entire range of light • Visible light (380-750 nm) important to biological systems • Different pigments absorb different wavelengths and reflect others (what we see that makes them colored) • What wavelength of light do plants reflect?

19. The Nature of Sunlight • Light • Is a form of electromagnetic energy, which travels in waves • Wavelength • Is the distance between the crests of waves • Determines the type of electromagnetic energy

20. 1 m 106 nm 10–5 nm 106 nm 1 nm 10–3 nm 103 nm 103 m Micro- waves Radio waves Gamma rays X-rays UV Infrared Visible light 380 450 500 550 600 650 700 750 nm Shorter wavelength Longer wavelength Lower energy Higher energy Figure 10.6 The electromagnetic spectrum • The electromagnetic spectrum • Is the entire range of electromagnetic energy, or radiation

21. The visible light spectrum • The visible light spectrum • Includes the colors of light we can see • Includes the wavelengths that drive photosynthesis

22. Photosynthetic Pigments • Photosynthetic pigments absorb specific wavelenths of light • Absorption spectrum = a pigment’s light absorption vs. wavelength • Spectrophotometer = instrument that measures absorbance of specific wavelengths (fig 10.8) • Beam of light sent through solution  fraction of light transmitted at each wavelength measured

23. Photosynthetic Pigments: Light Receptors • Photosynthetic Pigments • Are substances that absorb specific wavelengths within the visible light spectrum

24. Light Reflected Light Chloroplast Absorbed light Granum Transmitted light Figure 10.7 Pigments • Reflect some light, which include the colors we see

25. The spectrophotometer • The spectrophotometer • Is a machine that sends light through pigments and measures the fraction of light transmitted at each wavelength Transmitted light is NOT absorbed by that particular pigment

26. Refracting prism Chlorophyll solution Photoelectric tube White light Galvanometer 2 3 1 0 100 4 Slit moves to pass light of selected wavelength Green light The high transmittance (low absorption) reading indicates that chlorophyll absorbs very little green light. 0 100 The low transmittance (high absorption) reading chlorophyll absorbs most blue light. Blue light Figure 10.8 An absorption spectrum • An absorption spectrum • Is a graph plotting light absorption versus wavelength

27. Photosynthetic Pigments • Chlorophyll a (fig 10.10) absorption spectrum (fig 10.9a) • Chlorophyll b = accessory pigment similar to chl. a • When chlorophyll pigment absorbs light energy boosts an e- to an orbital of higher energy level (pigment is in its excited state) • If chlorophyll is isolated from chloroplast (fig 10.11) fluoresces (emits light) in red-orange end of spectrum (E given off as heat) • Carotenoids = other accessory pigments (hydrocarbons) reflecting various shades of orange/yellow/red (fig 10.9a) • Most important function = photoprotection (absorb & dissipate excess light E)

28. Pigment Absorption Spectra • The absorption spectra of chloroplast pigments • Provide clues to the relative effectiveness of different wavelengths for driving photosynthesis

29. Three different experiments helped reveal which wavelengths of light are photosynthetically important. The results are shown below. EXPERIMENT RESULTS Chlorophyll a Chlorophyll b Absorption of light by chloroplast pigments Carotenoids Wavelength of light (nm) (a) Absorption spectra. The three curves show the wavelengths of light best absorbed by three types of chloroplast pigments. Figure 10.9 Absorption spectra of three pigments in chloroplasts

30. Rate of photosynthesis (measured by O2 release) (b) Action spectrum. This graph plots the rate of photosynthesis versus wavelength. The resulting action spectrum resembles the absorption spectrum for chlorophyll a but does not match exactly (see part a). This is partly due to the absorption of light by accessory pigments such as chlorophyll b and carotenoids. The action spectrum for photosynthesis • Profiles the relative effectiveness of different wavelengths of radiation in driving photosynthesis

31. Aerobic bacteria Filament of alga 500 600 700 400 (c) Engelmann‘s experiment. In 1883, Theodor W. Engelmann illuminated a filamentous alga with light that had been passed through a prism, exposing different segments of the alga to different wavelengths. He used aerobic bacteria, which concentrate near an oxygen source, to determine which segments of the alga were releasing the most O2 and thus photosynthesizing most. Bacteria congregated in greatest numbers around the parts of the alga illuminated with violet-blue or red light. Notice the close match of the bacterial distribution to the action spectrum in part b. Light in the violet-blue and red portions of the spectrum are most effective in driving photosynthesis. CONCLUSION The action spectrum for photosynthesis • Was first demonstrated by Theodor W. Engelmann

32. CH3 in chlorophyll a in chlorophyll b CHO CH2 CH3 CH H C C C Porphyrin ring: Light-absorbing “head” of molecule note magnesium atom at center C C CH3 C C H3C CH2 C N C N H C C Mg H N C C N H3C C C CH3 C C C C C H H CH2 H C C O CH2 O O C O O CH3 CH2 Hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts: H atoms not shown Figure 10.10 Types of Chlorophyll • Chlorophyll a • Is the main photosynthetic pigment • Chlorophyll b • Is an accessory pigment

33. Other Pigments • Other accessory pigments • Absorb different wavelengths of light and pass the energy to chlorophyll a

34. Excited state e– Heat Energy of election Photon (fluorescence) Ground state Chlorophyll molecule Photon Figure 10.11 A Excitation of Chlorophyll by Light • When a pigment absorbs light • It goes from a ground state to an excited state, which is unstable

35. Figure 10.11 B Chlorophyll absorbs energy • If an isolated solution of chlorophyll is illuminated • It will fluoresce, giving off light and heat • The excited electron drops back to the ground-state orbital.

36. Tomorrow, we will start with the different types of photosynthetic pigments, and which wavelengths of light each absorbs. • We will also discuss the light reaction portion of photosynthesis. The light reaction produces ATP and NADPH which go to power the fixation and reduction of carbon dioxide into sugar by the Calvin Cycle.