alternative pathways oi palm
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Photosynthesis Quest Number 3
Monkemeier
CP Biology 2011
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The incorporation of carbondioxide into organic compoundsis known as carbon fixation.
Carbon fixation occurs during the
light independent reactions akaCalvin Cycle or Dark Reactions.
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Carbon dioxide diffuses into the stroma of thechloroplast.
The enzyme, RibuloseBiphosphatase (akaRubisco) enables Carbon Dioxide to combinewith a 5-carbon sugar known as Ribulose
biphosphate. This six carbon compound is UNSTABLE and
splits into two 3-Carbon sugars. 3-PGA The two 3-Carbon sugars then get rearranged
and form G3P One of the G3P will go on to make glucose The other G3P will go to regenerate the 5-Carbon
sugar known as RibuloseBiphosphatase.
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The Calvin Cycle just described isknown as the C3 pathway becausethe first stable organic moleculeformed is a three carbon sugar!
Examples of C3 plants are: grass,oak trees, maple trees, rosebushes, etc.
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C3 plants keep their stomata OPEN during theday when the sun is out and CLOSED at nightwhen sun is not shining.
The carbon dioxide can get IN and Oxygencan get OUT during the day when the stomataare open.
ON hot, dry, days, water loss (transpiration) is
a PROBLEM. C3 Plants have decreased levelsof photosynthesis due to water loss!
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When the stomata are closed, carbon dioxidecan not enter the leaf and oxygen can notleave the leaf.
Oxygen competes with carbon dioxide in TheCalvin Cycle.
When too much oxygen is inside the leaf, itwill bind with RUBISCO (aka ribulose
biphosphatase) and shut down the CalvinCycle. This is known as photorespiration.
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When oxygen combines with the enzyme ofthe Calvin Cycle (Rubisco, aka ribulosebiphosphatase), it then combines with the5-Carbon sugar and the combination breaks
down.
The Calvin Cycle shuts down because: the5-Carbon sugar is not available to make asugar and it breaks down into water andcarbon dioxide.
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Plants have had to evolve indifferent climates.
In HOT and DRY climates, plants
have evolved alternativepathways. Plants fight transpiration,photorespiration and the needfor carbon dioxide to enter theleaf.
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Corn, sugar cane and crab grass. Help fight transpiration by keeping their stomata
partially closed during the day and partially closed atnight.
Fix carbon dioxide into a temporary storage moleculethat has four carbons!
It is called a C4 pathway because the first stablecompound that carbon dioxide is fixed into is a fourcarbon compound.
The four carbon compound is transported tospecialized cells where Calvin Cycle is taking place!
The four carbon compound releases carbon dioxideto run Calvin Cycle.
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Jade plants, succulent plants, pineapple, Keep stomata CLOSED during the day and
OPEN at night.
Store carbon dioxide as an organic acid.
The organic acid then releases carbon dioxidedirectly to the Calvin Cycle.
CAM plants grow very slowly, but they require
less water than C3 or C4 plants.
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C3 Plants C4 Plants CAM Plants
Stomata Open at NightClosed duringDay
Partially closedduring day andpartially closedat night
Closed duringday and Open atnight
Carbon fixation Carbon dioxideis fixed into athree carboncompound thatis stable
Carbon dioxideis temporarilystored as a 4carbon stablecompound
Carbon dioxideis temporarilystored as anorganic acid.
Water Loss Has trouble withwater loss dueto transpiration.Trouble withphotorespiration
Less water lossthan C3 plants.Photorespirationis not a problem
Grow very slowlyand noproblems withphotorespiration