light independent reactions
DESCRIPTION
Light Independent Reactions. SBI4U1. CALVIN CYCLE. A.K.A. Calvin Benson Cycle, Light Independent Reaction “Dark Reaction” Carbon Fixation Discovered by “ Melvn Calvin”. Overall BIG Picture: Calvin cycle converts CO 2 from the atmosphere to carbohydrates (glucose) - PowerPoint PPT PresentationTRANSCRIPT
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Light Independent ReactionsSBI4U1
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CALVIN CYCLE
• A.K.A.Calvin Benson Cycle,Light Independent Reaction“Dark Reaction”Carbon Fixation
Discovered by “Melvn Calvin”
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Overall BIG Picture:Calvin cycle converts CO2 from the atmosphere to carbohydrates (glucose)– Requires ATP and NADPH ( from light dependent
rx.)
• These rxns occur in presence or absence of light– Light independent (previously called dark rxns)
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Occurs in the stroma
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Why is the Calvin cycle known as C3 Photosynthesis?
–The first compound formed in photosynthesis has 3 carbons! ( Hence plants that undergo this type of photosynthesis are called C3 plants!–This compound is “PGA”
Note*For the next few slides, please have your diagrams of Calvin cycle infront of you!
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STEP 1) CARBON FIXATION
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1. Carbon Fixation (C3 photosynthesis)
• Chemical bonding of 5-carbon RuBP and 1-carbon CO2
• It is very unstable, so it immediately splits into TWO 3-carbon molecules called PGA
• Plant that use this method are calle C3 plants
Summarized Equation: CO2 + RuBP unstable C6 2PGA
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STEP 2) REDUCTION
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2. Reduction
• PGA is in a low energy state• Therefore, ATP is used to create 1,3-BPG• Following this, NADPH is oxidized to form G3P • Of the G3P produced:– some will exit cycle to form sugars (e.g. Glucose)– Remaining ( most) will move to the 3rd stage ( Regeneration)
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STAGE 3) REGENERATION OF RuBP
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3. Regenerating RuBP• Most G3P molecules go on to make more
RuBP
• Energy from ATP breaks and reforms the bonds to make the 5 C RuBP
Calvin cycle must be completed 6X to create molecule of glucose.
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Net Equation:6CO2 + 18ATP + 12NADPH + water 2G3P + 16Pi + 18ADP + 12NADP+
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ACCOUNTING
• The accounting is complicated– 3 turns of Calvin cycle = 1 G3P
– 3 CO2 1 G3P (3C)
– 6 turns of Calvin cycle = 1 C6H12O6 (6C)
– 6 CO2 1 C6H12O6 (6C)
– 18 ATP + 12 NADPH 1 C6H12O6
– any ATP left over from light reactions will be used elsewhere by the cell
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Animation: http://highered.mcgraw-hill.com/sites/0070960526/student_view0/chapter5/animation_quiz_1.html
• The G3P produced can be used for the synthesis of other molecules for plants– Majority transported out of chloroplasts into
cytoplasm– Where it used to produce sucrose, starch, or cellulose
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Recall: Anatomy of a Leaf…
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Stomata: opening surface of a leaf that allows exchange of gases (CO2 in and H2O out)
Guard Cells: control the opening and closing of stomata. They surround the stomata
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• When the weather is hot/dry, guard cells close stomata. It does this as a response to conserve water loss ( transpiration)
• As a result…If stomata is closes CO2decreases in the leaves & O2 increases
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Adaptations to Photosynthesis
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What is Photorespiration?
• Rubisco…the most abundant enzyme on earth!
Well….both O2 and CO2 are subsrates to Rubsico and can bind to its active site. Therefore they compete for it’s active site.
Photorespiration= O2 reacts with Rubisco, as a result 2 new products are formed: 2-carbon compound (phosphoglyerate) and a 3-carbon phosphoglycerate
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In summary…
• In cool/moist conditions:– Stomata opens CO2 in leaves are sufficient
– CO2 binds to Rubisco– Calvin Cycle can begin
• In hot/dry conditions:– Stomata closes O2 is higher than CO2
– O2 binds to Rubisco– Calvin cycle cannot begin
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• Under normal conditions C3 plants lose 20% of their energy to fix one CO2 molecule
• Most plants are C3 plants (e.g. Wheat, rice, spinach, cotton, grasses)
• Some plants have evolved mechanisms to reduce photorespiration: C4 and CAM Plants
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C4 Plants• Inhabit hot, dry, environments• E.g. Corn, sugarcane, millet
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Structure of C4 Plants ( Pg. 169)C4 plants use energy to “pump” carbon dioxide into the bundle-sheath cells, where it becomes concentrated.
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• In the outer layer of mesophyll cells, CO2 is fixed by addition of a 3C phosphoenolpyruvate (PEP)– Produces 4C oxaloacetate– Converted to 4C malate– Transported to bundle sheath cell– Converted to pyruvate and CO2
– Pyruvate is converted to PEP and CO2 enters the Calvin cycle (taking place in the bundle sheath cell)
– Since CO2 is high = Calvin cycle is efficient
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CAM Plants
• Water storing plants• E.g. Cacti and pineapples• Thrive in hot, arid conditions• CO2 fixation is separated from Calvin cycle by
time of day, rather than cell types
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CAM PLANTS-The opening and closing of stomata in CAM plants, are opposite from most plants. - stomata are open at night and closed in the daytime. ---When CO2 is removed from the four-carbon compound malate in the daytime, it cannot leave the cell because the stomata are closed.- In cool climates, CAM plants are very inefficient b/c they use energy to drive the reactions that store carbon dioxide.
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Comparing Aerobic Respiration & Photosynthesis ( Similarities/Differences)• Rxns that capture light energy and convert it
to organic material are related to aerobic respiration rxns
• Products of aerobic respiration are starting substrates for photosynthesis
• Products of photosynthesis are starting substrates for aerobic respiration
*Know Table 4.1 ( pg 170) Hint..Hint
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Learning Expectations...
• Relationship b/t light dependent and independent rxns
• Purpose of the Calvin cycle• 3 stages in the Calvin cycle• Net equation for Calvin cycle• Adaptations to photosynthesis • Difference between C3, C4, and CAM plants