BIOL 204 Lecture Notes - Lecture 37: Leaf, Carboxylation, Pyruvic Acid
26 May 2018
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BIOL204 – C4 Cycle Pathway
Photorespiration and Temperature
- Major organ of plants is the leaf, has a waxy cuticle that prevents water loss
- The cuticle also prevents the rapid diffusion of gases (CO2) so to have high
rates of gas exchange the leaf has pores called stomata and the plant controls
the size of these
- When plants face a hot dry climate they face a dilemma – open stromata to let
CO2 in but close stomata to conserve water
- Photorespiration becomes a problem in hotter climates
o This is because of the effect of temperature on solubility of gases
o O2 and CO2 decrease solubility as temperature increases but CO2
decreases more rapidly than O2 so there is a imbalance in the CO2/O2
ratio
o As ratio decreases = phosphorespiration is greater
o Up to 50% of plants energy can be wasted through photorespiration
The C4 Pathway Circumvents Photorespiration
- Some plants have a 2nd carbon fixation pathway called the C4 cycle
- CO2 combines with PEP to produce oxaloacetate which is then reduced to
malate by electrons from NADPH
- Then malate is transported to Calvin cycle, oxidized to pyruvate, release CO2
- Then pyruvate converted back into PEP and consumes ATP
- Differences between C3 and C4 cycles
o 1st products of cycles:
▪ C4 cycle – 4 carbon oxaloacetate
▪ Calvin cycle – 3 carbon phosphoglycerate
o Carboxylation reactions
▪ C4 – reaction is catalyzed by enzyme PEP carboxylase which
has more affinity for CO2 and has no oxygenase activity
C4 Plants
- Examples: tropical plants, corn, sugar cane
- C4 cycle occurs in mesophyll cells which lie close to surface of leaves where
O2 is abundant
- Malate diffuses from mesophyll cells to bundle sheath cells where O2 is not
abundant
- Malate enters chloroplasts, converted to pyruvate and CO2 (where O2 is low
and CO2 is high) so this inhibits rubisco’s oxygenase activity and
carboxylation reaction can run
- Pyruvate returns to mesophyll cells to enter another C4 cycle
- C4 has an additional energy requirement (the hydrolysis of ATP to AMP is
required to regenerate PEP from pyruvate, meaning you need 6 more ATP
molecules for each G3P produced)
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Document Summary
Major organ of plants is the leaf, has a waxy cuticle that prevents water loss. The cuticle also prevents the rapid diffusion of gases (co2) so to have high rates of gas exchange the leaf has pores called stomata and the plant controls the size of these. When plants face a hot dry climate they face a dilemma open stromata to let. Photorespiration becomes a problem in hotter climates. Some plants have a 2nd carbon fixation pathway called the c4 cycle. Co2 combines with pep to produce oxaloacetate which is then reduced to malate by electrons from nadph. Then malate is transported to calvin cycle, oxidized to pyruvate, release co2. Then pyruvate converted back into pep and consumes atp. C4 cycle occurs in mesophyll cells which lie close to surface of leaves where. Malate diffuses from mesophyll cells to bundle sheath cells where o2 is not.
