PHOTOSYNTHESIS

1. PHOTOSYNTHESIS • Converts light energy into chemical energy through a complex series of biochemical reactions • 6CO2 + 6H2O -> C6H12O6 + 6O2 • Photosynthesis occurs inside the chloroplasts • White light from the sun is composed of different colors of different wavelengths – visible spectrum

8. CHLOROPLAST STRUCTURE • Outer, double membrane • Inner Membrane system: • THYLAKOIDS – flattened sacs • GRANA – stacks of thylakoids • Light harvesting pigments embedded in thylakoid membrane • Surrounding thylakoids, liquid: STROMA

9. Photosynthesis depends upon green pigment CHLOROPHYLL (absorbs light in the blue-violet and orange-red and reflects light in green region • Accessory pigments help harvest light energy

12. LIGHT DEPENDENT RXNS • Visible light (traveling in “photons”, packets of energy) is changed into chemical energy • H2O is split into O2 and H • PS I and II absorb light energy • This light energy is transferred to reaction center, a Chlorophyll a that donates e- to electron carrier

13. LIGHT DEP. RXNS cont’d • Lost e- from PSII is replaced by e- from H2O • At end of electron flow, electrons combine with NADP+ to form NADPH • As electrons flow along electron transport chain, protons build up inside thylakoids

14. H3C H C1 "/, H C C H CHJ H CH3 H H H H H C CH1 J,,/HJI I I I I I I I II! I /c, /C"", /c""' /C, /C, /C""' /c, /C"", /c~ /c, /c" /CH1 H1C C C C C C C C C C C /C" I I I I I I I I I I H3C CHJ H1C, /C H H H H H CHJ H CH3 H C " H1 CH3 f,'-carotene

16. LIGHT DEP. RXNS cont’d • These built up protons will diffuse down concentration gradient through ATP synthase • TAH-DAH!! ATP is made!!! • PRODUCTS: • O2, ATP and NADPH!!!!

19. CALVIN CYCLE • Pathway that produces organic compounds, using energy stored in ATP and NADPH from the light reactions • Occurs in the stroma • CO2 is “fixed” into organic compounds • RuBP (ribulose bisphosphate) is the 5 carbon sugar that CO2 is bound to by the enzyme rubisco

20. More CALVIN… • The new 6 carbon molecule is immediately split into 2 3-carbon molecules (PGA) • PGA converted to PGAL by addition of the phosphate from ATP and the hydrogen from NADPH • ADP, NADP+ and phosphate are used again in the light reactions to form more ATP and NADPH

21. To make one molecule of G3P, three turns of the cycle + 3 CO2 molecules • CO2 is fixed to RuBP by rubisco – produces unstable intermediate -> 3-phosphoglycerate • RuBP is regenerated – one per each trip through the cycle • Uses 9 ATP and 6 NADPH

24. ALTERNATIVE PATHWAYS • Calvin cycle plants = C3 because of PGAL that is formed (3 carbon) • Water loss through stomates is big problem • When air is hot and dry, stomates close to prevent water loss • BAD THING – CO2 levels fall and O2 levels rise, resulting in carbon fixation inhibition

25. C4 PATHWAY • During hottest part of day, C4 plants partially close stomates • A special enzyme fixes CO2 into 4-carbon compounds that is stored in bundle sheath cells and can then enter the Calvin cycle • Corn, sugar cane and crabgrass

26. CAM Photosynthesis • Adaptation to hot, dry climates • Open stomates at night and close them during the day (minimizes water loss) • CO2 that enters at night is fixed into a variety of organic compounds and stored in vacuoles; in morning, stomates close and CO2 is then released during the day and enters Calvin cycle • These plants grow very slowly – cactuses, pineapples

27. Rates of Photosynthesis • 4 limiting factors: • Light intensity, temperature, [CO2], [O2] Active site of Rubisco can bind to O2 or CO2: Photorespiration – results in release of previously fixed CO2 that would otherwise remain in organic form

28. RATES OF PHOTOSYNTHESIS • As light intensity increases, so does rate of photosynthesis • Levels off at a max rate, when all electrons are excited • Same thing for CO2 levels • Temperature increase, rate increases to a point; then, enzymes denature and stomates close to prevent water loss, thus decreasing rate at high temperatures