Transcript PhotosynI
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: – 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”) • Where does photosynthesis take place? • In chloroplasts • In higher plants these are located primarily in leaf cells 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 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 • 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 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?) ATP is made by “chemiosmosis” (Remember that?) 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 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! Cross section of a typical leaf Stomates Comprised of two opposing “guard cells” How do plants solve this CO2/H20 conflict? • There are two main ways: –Morphoplogical –Biochemical 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 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? 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… Sunken stomates Leaf hairs 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” The End.