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Transport in Plants

Transport in Plants . or Why do they need so much durn water?. To answer this we need a Quick Review of Photosynthesis. The source of all metabolic energy on earth (this is why plants are way cooler than animals!). What does photosynthesis do?. PS is a 2-step process FIRST STEP:

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Transport in Plants

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  1. Transport in Plants orWhy do they need so much durn water?

  2. To answer this we need a Quick Review of Photosynthesis The source of all metabolic energy on earth (this is why plants are way cooler than animals!)

  3. What does photosynthesis do? • PS is a 2-step process • FIRST STEP: • turns light energy into chemical energy • (your book calls these the “light-dependent reactions”) • SECOND STEP: • uses that chemical energy to “fix” atmospheric carbon into sugar • (your book calls these the “light-independent reactions, or the “Calvin Cycle”)

  4. Where does photosynthesis take place? • In chloroplasts • In higher plants these are located primarily in leaf cells

  5. A closer look at thylakoids • These are internal membranes in the chloroplast • They create an “inner thylakoid space” similar to the inter membrane space in mitochondria

  6. Chlorophyll • The main energy receiving molecule in chloroplasts • Light can easily excite its electrons • Electron excitation makes chlorophyll become a good electron donor (i.e. a reducer in a red/ox reaction) • These energetic electrons can now enter an electron transport system similar to that in mitochondria

  7. Other pigments contribute to light absorption: • Carotenes (absorb in the blue: appear orange-yellow) • Xanthophylls (absorb in the blue-green: appear reddish) • These pigments pass their energy to chlorophyll

  8. All higher land plants have the same photosynthetic pigments • The things we call “algae” have different photosynthetic pigments • Red • Brown • “Blue-green” • Green • Some unicellular marine algae can change their photosynthetic pigments rather quickly (why?)

  9. ATP is made by “chemiosmosis” (Remember that?)

  10. OK, that was step #1 of photosynthesis • The second step uses the energy (ATP and NADPH) created in step one to fix atmospheric carbon in a process called the “Light-independent reactions” or “Calvin Cycle” • This occurs in the stroma of the chloroplast • CO2 is “sucked” out of the air by enzymatically attaching it to a 5C sugar called ribulose bisphosphate (RuBP) • The enzyme that does this is called RuBP Carboxylase, aka RuBisCO (the most abundant protein in the world) • The product is… what would you guess? (5+1=?) • … Two 3C sugars called phosphoglycerate • Six turns of the CC ultimately produce one glucose molecule for export • These glucoses are the basis for ALL metabolic life on earth

  11. So plants get ALL their carbon from the atmosphere • That means their cells must be in contact with the atmosphere • That means their cells (which are full of water) lose water through evaporation • This is more or less important for different plants: • Aquatic plants – no problem • Plants in Oneonta – usually no problem • Plants in very dry places – BIG problem!

  12. Cross section of a typical leaf

  13. Stomates Comprised of two opposing “guard cells”

  14. How do plants solve this CO2/H20 conflict? • There are two main ways: • Morphoplogical • Biochemical

  15. Morphological adaptations to dry conditions (xerophytes) • Strategies: • Avoid dry weather • Only grow/flower etc. when it is wet (be dormant the rest of the time) • Get more water • Have really extensive root systems • Be a really good water extracter (actually a biochemical solution) • Lose less water from leaves • Several strategies for this

  16. What are the major environmental factors that cause the loss of water from leaves? • Humidity • Temperature • Lower temperatures = lower humidity • Wind • Which of these can plants alter?

  17. Ways to cut down on transpiration • Have a low surface/area ratio • Thicker – water is further from the surface • Reduce the size of leaves (sometimes to “spines”) • Stems are perpendicular to the sun - cooler • Have a waxy cuticle • Some xerophytes have 10X the wax on their surface • Have fewer stomates • Good and bad as a solution to this problem • Curl leaves • Stomates are more plentiful on the bottoms of leaves - maintains higher local humidity • Have “sunken” stomates • Creates a “still” area – maintains a higher local humidity • Have hairy leaves • Same…

  18. Sunken stomates

  19. Leaf hairs

  20. Here’s another glitch... • PS evolved when there was tons of CO2 and very little O2 in the atmosphere • The modern atmosphere is 21% O2, .035% CO2 • RuBisCO sometimes “accidentally” fixes O2 instead of CO2 (called “photorespiration”) • Very, very energy inefficient • Worse where it is hot and dry • “Tropical grasses” have evolved a “pre-fixation” strategy which minimizes photorespiration • The first product is a 4C compound, so the process is called “C4 photosynthesis”

  21. The End.

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