Light Independent ReactionsSBI4U1

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 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)

Occurs in the stroma

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!

STEP 1) CARBON FIXATION

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

STEP 2) REDUCTION

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)

STAGE 3) REGENERATION OF RuBP

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.

Net Equation:6CO2 + 18ATP + 12NADPH + water 2G3P + 16Pi + 18ADP + 12NADP+

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

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

Recall: Anatomy of a Leaf…

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

• 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

Adaptations to Photosynthesis

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

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

• 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

C4 Plants• Inhabit hot, dry, environments• E.g. Corn, sugarcane, millet

Structure of C4 Plants ( Pg. 169)C4 plants use energy to “pump” carbon dioxide into the bundle-sheath cells, where it becomes concentrated.

• 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

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

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.

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

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