Plant Adaptations to the Environment '06

Plant Adaptations to the Environment '06 - Plant...

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Plant Adaptations to the Environment
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Topics Quick review of photosynthesis in C 3 , C 4 , and CAM plants Photorespiration Components of dark respiration Controls on flow of CO 2 into leaf mesophyll cells Carbon allocation among plant parts Thermal energy balance Temperature effects on photosynthesis and respiration Plant adaptations to variation in nutrient availability
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Photosynthesis in simplified form: 6CO 2 + 12H 2 O C 6 H 12 O 6 + 6O 2 + 6H 2 O Photosynthesis can be separated into light and dark reactions. In the light reaction, light energy is trapped by chlorophyll in the chloroplasts and converted to ATP (“energy packet”) and NADPH (a strong reductant). In the dark reaction, which does not require light, CO 2 is fixed into carbohydrates. Let’s quickly review the dark reaction….the Calvin-Bensen cycle.
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3-phosphoglyceric acid (3-carbon molecule) Rubisco is the most abundant enzyme on the planet. Glyceraldehyde-3- phosphate 3 The net product of Ps C 3 Pathway Fig. 6.1 Carboxylation
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3-phosphoglyceric acid Rubisco causes photorespiration , which competes with the carboxylation step and reduces the efficiency of C 3 photosynthesis Glyceraldehyde-3- phosphate 3 Rubisco O 2 CO 2 Photorespiration Carboxylation C 3 Pathway
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Photorespiration rubisco RuBP + O 2 CO 2 As temperature increases, rubisco promotes oxygenation of RuBP over the carboxylation reaction, so net Ps declines. CO 2 declines in leaves on hot, dry days (stomata close), while O 2 remains relatively high. Photorespiration generates no ATP and no photosynthate. Net Ps can be reduced by 30-50%. Photorespiration occurs only in the presence of light and is often referred to as light respiration.
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Dark Respiration ( Oxidation of carbohydrates) (CH 2 O) 6 + 6O 2 6CO 2 + 6H 2 O + ATP (energy) Takes place only in the mitochondria. Once thought to be carried out only in the dark, but it can occur at a reduced rate in the presence of light. Often partitioned into two components Growth and synthesis Maintenance respiration Proportional to dry weight of the living tissue Temperature sensitive Normally maintenance requirements are met first
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Alternative photosynthesis pathway C 4 Photosynthesis CO 2 reacts with phosphoenolpyruvate PEP to form the 4-carbon compound oxaloacetate (OAA), which is rapidly transformed into malic and aspartic acids (also 4-carbon molecules). Reaction is catalyzed by PEP carboxylase. CO 2 is released (decarboxylation of malic and aspartic acids) in the bundle sheath cells, then fixed again via C 3 pathway. Pyruvic acid is used to regenerate PEP (phosphoenolpyruvate). Fig. 6.4
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• CO 2 reacts with PEP in the mesophyll to form 4-carbon compounds (oxaloacetate, then malic and aspartic acid). • CO
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This note was uploaded on 04/09/2008 for the course BIO 311C taught by Professor Satasivian during the Fall '08 term at University of Texas at Austin.

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Plant Adaptations to the Environment '06 - Plant...

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