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Published bySharon Brooks Modified over 9 years ago
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The Dark Reaction - - light-independent - - energy stored in ATP and NADPH (from light reaction) is used to reduce CO 2 to sugar
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Three independent ways to reduce CO 2 to make sugar: 1. the Calvin cycle (C3), 2. C4 photosynthesis, 3. crassulacean acid metabolism (CAM).
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The Calvin Cycle 1. CO 2 is fixed by rubisco 1.CO 2 + RuBP unstable C6 2 PGA 2. Reduction of CO 2 to make G3P - Uses ATP and NADPH - G3P is exported to cytoplasm to make starch, sucrose, oils 3. Regenerating RuBP 1.- for enery 12 molecules of G3P made in the Calvin cycle two are “released” 2.- the Calvin cycle needs to “turn” 6 times to make one glucose!!!
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G3P - - one-third forms starch - - two-thirds are converted to sucrose and then hydrolyzed in other parts of plant into glucose and fructose - - Ultimately used as a source of C for nucleic acids, amino acids, fats…
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Rubisco ● Ribulose bisphosphate carboxylase oxygenase ● fixes CO 2 & O 2 ● Enzyme in Calvin Cycle (1 st step) ● Most abundant protein on Earth – About 50% total plant protein!
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Transpiration: water loss Leaf transpiration occurs through stomata Stomata open to allow the diffusion of carbon dioxide gas from the air for photosynthesis Transpiration also cools plants and enables mass flow of mineral nutrients from roots to shoots
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Stomata - lungs openings on the surface of the leaf that allow the exchange of gases between air spaces in the leaf and the atmosphere Guard cells –control the size of the stoma in response to environmental conditions
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● The size of the guard cell changes when water moves into or out of the cell ● K + ions are actively pumped into the guard cell and water follows by osmosis ● Light, and CO 2 concentration affect the movement of K + ions into the cells ● Generally stomata are open during the day and closed at night
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Photorespiration ● the reaction of RuBP with oxygen, reduces the efficiency of photosynthesis ● rubisco is inefficient: “fixes” O 2, as well as CO 2 ● C3 plants lose 20% of their energy to fix one CO 2 ● this gets worse with heat!
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- Under hot and dry conditions (daytime) plants will close their stomata to prevent water loss - This causes a build up of oxygen since CO 2 can’t enter…so MORE photorespiration
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Avoiding Photorespiration ● C3 – The majority of plants ● C4 – CO 2 temporarily stored as 4-C organic acids resulting in more more efficient C exchange rate – Advantage in high light, high temperature, low CO 2 – Many grasses and crops (e.g., corn, sorghum, millet, sugar cane) ● CAM – Stomata open during night – Advantage in arid climates – Many succulents (e.g. cacti)
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Fig. 10.21
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Comparison of Photosynthesis in C 3 and C 4 Plants VARIABLEC 3 PLANTSC 4 PLANTS Photorespirati on ExtensiveMinimal Perform Calvin cycle? Yes Primary CO 2 acceptor RuBPPEP CO 2 -fixing enzyme Rubisco (RuBP carboxylase/oxyge nase) PEP carboxylase and rubisco First product of CO 2 fixation 3PG (3-carbon compound) Oxaloacetate (4- carbon compound) Affinity of carboxylase for CO 2 ModerateHigh Photosynthetic cells of leaf MesophyllMesophyll + bundle sheath Classes of chloroplasts OneTwo
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- C3 photosynthesis: about 3.5 billion years ago, - C4 plants appeared about 12 million years ago. - A possible factor in the emergence of the C4 pathway is the decline in atmospheric CO2
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CAM PLANTS - - CAM is similar to C4 - - CO2 is fixed to a 4-carbon compound - - Separated by time rather than space: At night cooler and water loss is minimized stomata open and CO2 is fixed in mesophyll cells to form the 4- carbon oxaloacetate, which is converted into malic acid. During the day when the stomata close to reduce water loss, the accumulated malic acid is shipped to the chloroplasts to form sugars
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Fig. 10.22
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CAM Plants
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Global Environmental Change & Photosynthesis: C3 vs. C4 vs. CAM ● Increasing CO2 ● Increasing chronic and acute temperatures ● Changes in water
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*At high CO2, C3 more efficient than C4 at all temps. (photosynthesis only, not other processes)
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