Chapter 6 Section 6 2 The Calvin Cycle

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Chapter 6 Section 6. 2

Chapter 6 Section 6. 2

The Calvin Cycle n n n Second set of reactions in photosynthesis Produces organic

The Calvin Cycle n n n Second set of reactions in photosynthesis Produces organic compounds using the energy stored in ATP and NADPH during the light reactions Named after Melvin Calvin (American scientist who identified the details of the pathway)

Carbon Fixation n n The incorporation of CO 2 into organic compounds The carbon

Carbon Fixation n n The incorporation of CO 2 into organic compounds The carbon atoms from CO 2 become bonded or “fixed” into organic compounds

Steps of the Calvin Cycle n 1. CO 2 combines with Ru. BP to

Steps of the Calvin Cycle n 1. CO 2 combines with Ru. BP to form two molecules of PGA Ru. BP = 5 -carbon carbohydrate (Ribulose biphosphate) PGA = 3 -carbon molecule (Phosphoglycerate)

n n n 2. Each molecule of PGA is converted into a molecule of

n n n 2. Each molecule of PGA is converted into a molecule of PGAL (3 -phosphoglyceraldehyde) Two part process: First, each PGA receives a phosphate group from a molecule of ATP. Second, the resulting compound receives a proton from NADPH and releases a phosphate group producing PGAL

n n 3. Most of the PGAL is converted back into Ru. BP Some

n n 3. Most of the PGAL is converted back into Ru. BP Some PGAL is used to make a variety of organic compounds

n n Each turn of the Calvin cycle fixes one CO 2 molecule Since

n n Each turn of the Calvin cycle fixes one CO 2 molecule Since PGAL is a 3 -carbon molecule, it takes three turns of the cycle to produce each molecule of PGAL

n n n For each turn of the Calvin cycle, 2 ATP and 2

n n n For each turn of the Calvin cycle, 2 ATP and 2 NADPH molecules are used in step 2 One more ATP molecule is used in step 3 Three turns of the Calvin cycle uses 9 molecules of ATP and 6 molecules of NADPH

Alternative Pathways n n Calvin Cycle: the most common pathway for carbon fixation C

Alternative Pathways n n Calvin Cycle: the most common pathway for carbon fixation C 3 plants: fix carbon only through the Calvin cycle

n n n Alternative pathways are found in plants from hot, dry climates Water

n n n Alternative pathways are found in plants from hot, dry climates Water loss occurs through stomata (small pores) located on the underside of leaves Stomata also allow CO 2 to enter and O 2 to leave

C 4 Pathway n n n Enables plants to fix CO 2 into 4

C 4 Pathway n n n Enables plants to fix CO 2 into 4 -carbon compounds Stomata are partially closed during hottest part of the day Examples: corn, sugar cane, crabgrass

The CAM Pathway n n Found in plants that open stomata at night and

The CAM Pathway n n Found in plants that open stomata at night and close them during the day Examples: Cacti and pineapples

Rate of Photosynthesis n Rate is affected by the plant’s environment n Most important

Rate of Photosynthesis n Rate is affected by the plant’s environment n Most important factor is light intensity n Higher light intensity causes more electrons to get excited

n n Rate is also affected by CO 2 Increased levels of CO 2

n n Rate is also affected by CO 2 Increased levels of CO 2 stimulates photosynthesis

n n n Temperature is another environmental factor that affects photosynthesis When temperature rises,

n n n Temperature is another environmental factor that affects photosynthesis When temperature rises, the rate of photosynthesis increases until it reaches a certain point If temperature is too high, enzymes cannot function

The End

The End

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