BIOL10004 Chapter Notes - Chapter 17: Phloem, Maximum Exposure, Primordium
29 Jun 2018
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Leaf Structure and Function
Plants have adapted to different environments
Photorespiration: competition between carbon dioxide and oxygen
Enzyme that catalyses carbon fixation (Rubisco) also use oxygen as substrate and oxygenate
RuBP
With subsequent production of CO2 from product of oxygenation is Photorespiration
Normal atmospheric conditions, O2 is more abundant than CO2
O2 competes effectively for binding site on Rubisco
oDespite enzyme having higher affinity to CO2
Photorespiration occurs only in light
oProduces CO2 and consumes O2
Photorespiration
oEarly photosynthetic organisms evolved in oxygen lacking atmosphere
oNot a problem
oToday's atmosphere
Photorespiration results in CO2 being released
Undoing binding of carbon that would be carbohydrate synthesis
Consumes ATP
Reduces efficiency of carbon fixation
Plants lose 25-50% of carbon that would be fixed during photosynthesis
Plants have adapted to different environments
The C4 pathway of photosynthesis
First stable compound of carbon fixation is 4-carbon compound
Plants with C4 photosynthesis have distinctive leaf anatomy
oVascular bundles surrounded by cylinder of bundle sheath cells
Outer layer of mesophyll cells
oBundle sheath and mesophyll cells contain chloroplasts
Both have different structure and function
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Plants have additional carboxylation enzyme
oPhospho-enolpyruvate (PEP) carboxylase
oOperates in cytoplasm of leaf mesophyll cells
oEnzyme catalyzes carboxylation of 3-carbon PEP
oProduct is 4-carbon organic acid, Oxaloacetate
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oImmediately converted to another 4-carbon compound (Malate)
oMesophyll chloroplasts provide ATP and NADPH to make PEP
oLack Rubisco
oOnly have enzymes of Calvin-Benson cycle needed to reduce PGA to PGAL
Once in chloroplasts of bundle sheath cells
oMalate decarboxylated to CO2 + pyruvate
oCO2 then fixed into carbohydrates by Rubisco and other Calvin-Benson cycle
enzymes
oPyruvate transported back into mesophyll cells
Converted back into PEP to complete cycle
oC4 decarboxylation reactions generates NADPH
Consumed in bundle sheath chloroplasts during PGA reduction
oBundle sheath chloroplasts have only few Granal stacks and little PS2 activity
oLittle capacity for light-dependent NADPH formation
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find more resources at oneclass.com
Document Summary
Enzyme that catalyses carbon fixation (rubisco) also use oxygen as substrate and oxygenate. With subsequent production of co2 from product of oxygenation is photorespiration. Normal atmospheric conditions, o2 is more abundant than co2. O2 competes effectively for binding site on rubisco: despite enzyme having higher affinity to co2. Early photosynthetic organisms evolved in oxygen lacking atmosphere: not a problem o. Undoing binding of carbon that would be carbohydrate synthesis. Plants lose 25-50% of carbon that would be fixed during photosynthesis. First stable compound of carbon fixation is 4-carbon compound. Plants with c4 photosynthesis have distinctive leaf anatomy: vascular bundles surrounded by cylinder of bundle sheath cells. Phospho-enolpyruvate (pep) carboxylase: operates in cytoplasm of leaf mesophyll cells o o. Immediately converted to another 4-carbon compound (malate: mesophyll chloroplasts provide atp and nadph to make pep o. Lack rubisco: only have enzymes of calvin-benson cycle needed to reduce pga to pgal.
