Chapter 6
Section 6.2
The Calvin Cycle

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

The incorporation of CO2 into organic
compounds

The carbon atoms from CO2 become bonded
or “fixed” into organic compounds
Steps of the Calvin Cycle

1. CO2 combines with RuBP to form two
molecules of PGA
RuBP = 5-carbon carbohydrate
(Ribulose biphosphate)
PGA = 3-carbon molecule
(Phosphoglycerate)

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

3. Most of the PGAL is converted back into
RuBP

Some PGAL is used to make a variety of
organic compounds

Each turn of the Calvin cycle fixes one CO2
molecule

Since PGAL is a 3-carbon molecule, it takes
three turns of the cycle to produce each
molecule of PGAL

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

Calvin Cycle: the most common pathway for
carbon fixation

C3 plants: fix carbon only through the Calvin
cycle

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 CO2 to enter and O2 to
leave
C4 Pathway

Enables plants to fix CO2 into 4-carbon
compounds

Stomata are partially closed during hottest part
of the day

Examples: corn, sugar cane, crabgrass
The CAM Pathway

Found in plants that open stomata at night and
close them during the day

Examples: Cacti and pineapples
Rate of Photosynthesis

Rate is affected by the plant’s environment

Most important factor is light intensity

Higher light intensity causes more electrons to
get excited

Rate is also affected by CO2

Increased levels of CO2 stimulates
photosynthesis

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