Chapter 6Chapter 6

Section 6.2Section 6.2

The Calvin CycleThe Calvin Cycle

Second set of reactions in photosynthesisSecond set of reactions in photosynthesis

Produces organic compounds using the energy Produces organic compounds using the energy stored in ATP and NADPH during the light stored in ATP and NADPH during the light reactionsreactions

Named after Melvin Calvin (American Named after Melvin Calvin (American scientist who identified the details of the scientist who identified the details of the pathway)pathway)

Carbon FixationCarbon Fixation

The incorporation of COThe incorporation of CO22 into organic into organic

compoundscompounds

The carbon atoms from COThe carbon atoms from CO22 become bonded become bonded

or “fixed” into organic compoundsor “fixed” into organic compounds

Steps of the Calvin CycleSteps of the Calvin Cycle

1. CO1. CO22 combines with RuBP to form two combines with RuBP to form two molecules of PGAmolecules of PGA

RuBP = 5-carbon carbohydrateRuBP = 5-carbon carbohydrate(Ribulose biphosphate)(Ribulose biphosphate)

PGA = 3-carbon moleculePGA = 3-carbon molecule(Phosphoglycerate)(Phosphoglycerate)

2. Each molecule of PGA is converted into a 2. Each molecule of PGA is converted into a molecule of PGAL (3-phosphoglyceraldehyde)molecule of PGAL (3-phosphoglyceraldehyde)

Two part process: First, each PGA receives a Two part process: First, each PGA receives a phosphate group from a molecule of ATP.phosphate group from a molecule of ATP.

Second, the resulting compound receives a Second, the resulting compound receives a proton from NADPH and releases a phosphate proton from NADPH and releases a phosphate group producing PGALgroup producing PGAL

3. Most of the PGAL is converted back into 3. Most of the PGAL is converted back into RuBPRuBP

Some PGAL is used to make a variety of Some PGAL is used to make a variety of organic compoundsorganic compounds

Each turn of the Calvin cycle fixes one COEach turn of the Calvin cycle fixes one CO22

moleculemolecule

Since PGAL is a 3-carbon molecule, it takes Since PGAL is a 3-carbon molecule, it takes three turns of the cycle to produce each three turns of the cycle to produce each molecule of PGALmolecule of PGAL

For each turn of the Calvin cycle, 2 ATP and 2 For each turn of the Calvin cycle, 2 ATP and 2 NADPH molecules are used in step 2NADPH molecules are used in step 2

One more ATP molecule is used in step 3One more ATP molecule is used in step 3

Three turns of the Calvin cycle uses 9 Three turns of the Calvin cycle uses 9 molecules of ATP and 6 molecules of NADPHmolecules of ATP and 6 molecules of NADPH

Alternative PathwaysAlternative Pathways

Calvin Cycle: the most common pathway for Calvin Cycle: the most common pathway for carbon fixationcarbon fixation

CC33 plants plants: fix carbon only through the Calvin : fix carbon only through the Calvin

cyclecycle

Alternative pathways are found in plants from Alternative pathways are found in plants from hot, dry climateshot, dry climates

Water loss occurs through Water loss occurs through stomata stomata (small (small pores) located on the underside of leavespores) located on the underside of leaves

Stomata also allow COStomata also allow CO22 to enter and O to enter and O22 to to

leave leave

CC44 Pathway Pathway

Enables plants to fix COEnables plants to fix CO22 into 4-carbon into 4-carbon

compoundscompounds

Stomata are partially closed during hottest part Stomata are partially closed during hottest part of the dayof the day

Examples: corn, sugar cane, crabgrassExamples: corn, sugar cane, crabgrass

The CAM PathwayThe CAM Pathway

Found in plants that open stomata at night and Found in plants that open stomata at night and close them during the dayclose them during the day

Examples: Cacti and pineapplesExamples: Cacti and pineapples

Rate of PhotosynthesisRate of Photosynthesis

Rate is affected by the plant’s environmentRate is affected by the plant’s environment

Most important factor is light intensityMost important factor is light intensity

Higher light intensity causes more electrons to Higher light intensity causes more electrons to get excitedget excited

Rate is also affected by CORate is also affected by CO22

Increased levels of COIncreased levels of CO22 stimulates stimulates

photosynthesisphotosynthesis

Temperature is another environmental factor Temperature is another environmental factor that affects photosynthesisthat affects photosynthesis

When temperature rises, the rate of When temperature rises, the rate of photosynthesis increases until it reaches a photosynthesis increases until it reaches a certain pointcertain point

If temperature is too high, enzymes cannot If temperature is too high, enzymes cannot functionfunction

The EndThe End