Photosynthesis part 2

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  • The key point is how carbon dioxide is grabbed out of the air -- carbon fixation -- and then handed off to the Calvin cycle. C4 plants separate the 2 steps of carbon fixation anatomically. They use 2 different cells to complete the process. CAM plants separate the 2 steps of carbon fixation temporally. They do them at 2 different times. The key problem they are trying to overcome is that Rubisco is a very inefficient enzyme in the presence of high O2. In high O2, Rubisco bonds oxygen to RuBP rather than carbon, so the plants have to keep O2 away from Rubsico. C4 & CAM should be seen as variations on *carbon fixation*, because plants had to evolve alternative systems given the limitations of their enzymes and their need to conserve water.
  • Crassulacean Acid Metabolism ( CAM ) Succulents are in the family Crassulaceae the name Crassula is derived from the Latin "crassus" meaning thick and refers to the leaves of these succulent plants CAM plants solve the photorespiration problem by fixing carbon at night (when stomates are open), and put it in "storage" compounds (organic acids like malic acid, isocitric acid) and then in the day (when stomates are closed), they release the CO2 from the "storage" compounds to the Calvin cycle (thereby increasing CO2 in the cells, improving Rubisco's efficiency)
  • C3, C4, and CAM truly refer to the alternative method of carbon fixation -- grabbing carbon out of the air -- and not the Calvin Cycle itself. They *all* use the Calvin Cycle for sugar generation, but they differ in how they turn carbon from thin air into solid stuff. In C4, CO2 is fixed into 4-carbon "storage" compounds like oxaloacetate & malate (hence C4) In CAM, CO2 is fixed into organic acids like malic acid & isocitric acid (hence Crassulacean Acid Metabolism) In C3, while CO2 is initially fixed into a 6-carbon molecule, it is unstable & quickly breaks down to 3-carbon phosphoglycerate (PGA) (hence C3) C4 & CAM should be seen as variations on *carbon fixation*, because plants had to evolve alternative systems given the limitations of their enzymes and their need to conserve water.

Transcript

  • 1. Photosynthesis Reactions
  • 2. Electron carriers
    • Electrons in chlorophyll
    • Sun excites them
    • Electrons gain E
    • These high E electrons require special carrier
    • High E e- are similar to hot coals that need to be transferred
    • Electron carriers needed to transport high-E e-
    • NADP+
      • Nicotinamide adenine dinucleotide phosphate ( Just remember NADP )
      • Accepts 2 high-E e- to become NADPH
      • This is how energy (e-) from sun can be trapped in chemical form
      • NADPH carries high-E e- from chlorophyll to other parts of the chloroplast
      • Help build molecules, such as glucose
  • 3.  
  • 4. Photosystems
    • Clusters of chlorophyll and other pigments in the thylakoid membrane (organized by a set of proteins in the plant cell)
    • Contain few hundred pigment molecules
      • Chlorophyll a (absorbs 680 nm wavelengths)
      • Chlorophyll b (absorbs 700 nm wavelengths)
      • Carotene/carotenoids
    • Light-collecting unit of the cell
    • Solar panel
    • Photosystem II (P680) and
    • Photosystem I (P700)
  • 5.  
  • 6.  
  • 7. Light Dependent Reactions
    • Produce oxygen gas and convert ADP and NADP+ into the energy carriers ATP and NADPH
    • Take place in the THYLAKOID membrane of chloroplast
    • Begins with photosystem II
      • (this was discovered after photosystem I but actually occurs before it)
      • Photosystem II traps light E and transfers excited e- to an ETC
      • “ water-splitting” photosystem
        • Light absorbed by photosystem II is used to break-up water molecules into high-E electrons, oxygen, and H+ ions
        • 2 electrons  replace lost e- in chlorophyll
        • 2 H+ ions  released into the inside of the thylakoid membrane
        • 1 oxygen atom  oxygen released into atmosphere
    • Electrons in chlorophyll are excited  passed along ETC  do electrons in chlorophyll run out?
      • No: the high-E electrons lost by the chlorophyll are replaced by the electrons from “water splitting”
  • 8.
    • High-E electrons move through the ETC from photosystem II to photosystem I
    • Energy from electrons is used by molecules in the ETC to transport H+ ions from the stroma into the inner thylakoid space
  • 9.
    • Pigments in photosystem I use Energy from light to re-energize the electrons
    • NADP+ then picks up these high-E electrons and H+ ions at the outer surface of the thylakoid membrane
    • NADP+ becomes NADPH
  • 10.
    • As electrons are passed from chlorophyll to NADP+, H+ ions are pumped across membrane
    • Inside of thylakoid membrane fills up with positive H+ ions, outside in negative
    • Chemosmosis occurs
    • ATP synthase turns making ADP  ATP
  • 11. ETC proteins
    • Photosystem II (recieves light & splits water)
      • Oxygen-evolving complex
      • Plastoquinone
      • Cytochrome
      • Plastocyanin
    • Photosystem I (receives more light)
      • Ferredoxin
      • Ferrdoxin-NADP reductase
        • NADP  NADPH
      • ATP synthase
        • ADP  ATP
  • 12.  
  • 13.  
  • 14.  
  • 15.  
  • 16.  
  • 17. Overview of Light Dependent Rxn
    • Use:
      • ADP
      • NADP+
      • Water
    • Produce:
      • Oxygen
      • ATP
      • NADPH
    • Why are these products important?
      • Provide energy to build energy-containing sugars from low-energy compounds in Calvin cycle
  • 18. Calvin Cycle/ Light-Independent Reactions
    • So what do we have from pour light-dependent rxns?
      • High-E electrons stored in ATP and NADPH
      • “ chemical energy”
      • But plants cannot store this chemical energy for more than a few minutes…must change this chemical energy into something that can be stored for long periods of time
    • Calvin cycle
      • Uses ATP and NADPH from light-dependent rxn to produce high-E sugars
  • 19.  
  • 20. Step 1
    • Six carbon dioxide molecules enter cycle from atmosphere
      • Enzyme adds each CO2 molecule to a Ribulose biphosphate, RuBP molecule (a 5-carbon molecule) making six unstable 6-carbon molecules
    • The six unstable 6-carbon molecules immediately break off into 12 3-carbon molecules called 3-phosphoglycerate, 3-PGA
  • 21.  
  • 22. Step 2
    • Twelve 3-carbon molecules are converted into higher energy forms using energy from ATP and high-E electrons from NADPH
      • Twelve 3-PGAs are converted into twelve energized Glyceraldehyde 3-phosphates (G3P).
        • 3-PGAs + ATP  1, 3-biphosphoglycerate
        • 1, 3-biphosphoglycerate + NADPH  Glyceraldehyde 3-phosphate
  • 23.  
  • 24. Step 3
    • Two of the G3Ps (twelve 3-carbon molecules) are removed from the cycle
    • Plant uses these two G3Ps (3-carbon molecules) to make sugars, lipids, amino acids, and other compounds plant needs for metabolism and growth
  • 25.  
  • 26. Step 4
    • Remaining ten G3Ps (3-carbon molecules) use ATP and rearrange themselves
      • ADP and NADP+ go back to light rxns
    • Converted back into RuBP molecules (six 5-carbon molecules)
    • Calvin cycle begins again with six new CO2 molecules
  • 27.  
  • 28. Calvin cycle overview
    • Uses:
      • Six molecules of CO2
      • NADPH
      • ATP
    • Produces:
      • One 6-carbon sugar “glucose”
  • 29.  
  • 30.  
  • 31.  
  • 32. Photosynthesis overview
    • Two sets of reactions work together
      • Light dependent
        • Trap energy of sunlight into chemical form
      • Calvin cycle/light independent
        • Use chemical energy to produce stable-high energy sugars from carbon dioxide and water
  • 33. Factors that effect rate of photosyntehsis
    • Water availability
      • Shortage of water can slow or stop photosyn.
      • Adaptations
        • Desert plants and conifers
          • Waxy coating
    • Temperature
      • Photosyn. Depends on enzymes that function between 0*C and 35*C
      • Low temp. may cause photosyn. to stop
    • Intensity of light
      • Increase light intensity=increase rate of photosynthesis
      • After a certain level of intensity, plant reaches its max rate of photosynthesis
  • 34.  
  • 35. Problems
    • Calvin cycle- RuBP binds with CO2 to make 6-C compound that changes immediately into 3- Carbon compounds that eventually make G3P then sugar
    • Enzyme that catalyzes this reaction is called Rubisco
    • Problem with Rubisco is that its not good at grabbing CO2…gets easily confused by oxygen
    • When levels of CO2 inside cell are low, Rubisco starts grabbing oxygen
    • This cause photorespiration
      • When plant uses light and consumes oxygen, producing carbon dioxide
      • This uses up energy…NOT good
      • Big problem on hot, dry days when stomata close and can’t get CO2
    • So how do plants deal?
  • 36. Reducing photorespiration
    • Separate carbon fixation from Calvin cycle
      • C4 plants
        • PHYSICALLY separate carbon fixation from Calvin cycle
          • different cells to fix carbon vs. where Calvin cycle occurs
          • store carbon in 4C compounds
        • different enzyme to capture CO 2 (fix carbon)
          • PEP carboxylase
        • different leaf structure
      • CAM plants
        • separate carbon fixation from Calvin cycle by TIME OF DAY
        • fix carbon during night
          • store carbon in 4C compounds
        • perform Calvin cycle during day
  • 37. Special Plants
    • C4 plants
      • "four-carbon”
      • plants initially attach CO 2 to PEP (phosphoenolpyruvate) to form the four-carbon compound oxaloacetate using the enzyme PEP carboxylase.
      • This takes place in mesophyll cells.
      • Oxaloacetate is then pumped to another set of cells, the bundle sheath cells,
      • In the bundle sheath cells it releases the CO 2 for use by Rubisco.
      • Now plant can use Calvin Cycle
      • By concentrating CO 2 in the bundle sheath cells, C4 plants promote the efficient operation of the Calvin cycle and minimize photorespiration.
      • C4 plants include corn, sugar cane, and many other tropical grasses
  • 38. C4 plants
    • A better way to capture CO 2
      • 1st step before Calvin cycle, fix carbon with enzyme PEP carboxylase
        • store as 4C compound
      • adaptation to hot, dry climates
        • have to close stomates a lot
        • different leaf anatomy
      • sugar cane, corn, other grasses…
    sugar cane corn
  • 39. Comparative anatomy C3 C4 Location, location,location ! PHYSICALLY separate C fixation from Calvin cycle
  • 40. Special Plants
      • CAM (crassulacean acid metabolism) plants
      • plants initially attach CO 2 to PEP (phosphoenolpyruvate) to form the four-carbon compound oxaloacetate using the enzyme PEP carboxylase.
      • CAM plants fix carbon at night and store the oxaloacetete (organic acid) in large vacuoles within the cell.
        • During the day, CO2 is released from these organic acids and used in the Calvin cycle
      • Then they can open their stomatas at night (cool weather) and lets CO2
      • The avoid water loss and to use the CO 2 for the Calvin cycle during the day when it can be driven by the sun's energy.
      • CAM plants are more common than C4 plants
      • Ex. Are cacti, pineapples and other succulent plants.
  • 41. CAM ( Crassulacean Acid Metabolism ) plants
    • Adaptation to hot, dry climates
      • separate carbon fixation from Calvin cycle by TIME
        • close stomates during day
        • open stomates during night
      • at night : open stomates & fix carbon in 4C “storage” compounds
      • in day : release CO 2 from 4C acids to Calvin cycle
        • increases concentration of CO 2 in cells
      • succulents, some cacti, pineapple
    It’s all in the timing !
  • 42. CAM plants succulents cacti pine apple
  • 43. C4 vs CAM Summary C4 plants separate 2 steps of C fixation anatomically in 2 different cells CAM plants separate 2 steps of C fixation temporally = 2 different times night vs. day solves CO 2 / O 2 gas exchange vs. H 2 O loss challenge