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Photosynthesis
The story so far…
Photosynthesis requires H2O,
CO2, and light energy
o H2O is taken in by the roots
o CO2 from the air enters
through stomata
o Light energy is used by
chloroplasts
Light dependent reactions take place in the thylakoid
membrane within the chloroplast
o The big picture: water is used, oxygen is produced
o Light energy (photons) are absorbed by chlorophyll and
antenna pigments in Photosystem II (P680) and
Photosystem I (P700)
o Energy excites electrons, starting the electron transport
chain
o Water is split (photolysis), releasing O2, electrons, and two
H+ ions
o End products:
ATP and NADPH (energy carrier), which are used in the
light-independent reactions
Oxygen, which is released
through the stomata
Light-independent reactions (Calvin Cycle)
The big picture: CO2 is used, sugar is produced
Takes place in the stroma
Three phases:
o Carbon fixation – CO2 is changed into a three-carbon
compound
CO2 + RuBP → unstable 6C molecule → 2 PGA (3C)
Catalyzed by the enzyme rubisco (ribulose bisphosphate
carboxylase)
o Reduction – C3 molecules are activated by ATP to bring
them to a higher energy state, then reduced by NADPH
Produces two G3P molecules
o Regenerating RuBP – allows the cycle to continue
Energy from ATP breaks and reforms chemical bonds to
make more RuBP from G3P molecules
Calvin Cycle must be repeated six times to synthesize one
molecule of glucose
o 12 G3P are produced in six cycles
10 are used to regenerate RuBP
2 are used to make one glucose molecule
o Net equation: 6CO2 + 18 ATP + 12 NADPH + water → 2
G3P + 16 Pi + 18 ADP + 12 NADP+
G3P move out of chloroplasts and into cytoplasm, where it
can be used to make:
• Glucose and other sugars
• Starch (in times of intensive photosynthesis)
• Cellulose
• Plant oils (corn oil, safflower oil, olive oil, etc.)
• Amino acids (also requires a source of nitrogen)
Adaptations to Photosynthesis
Challenge
Photorespiration: reaction of
In hot, dry conditions, plants
oxygen with RuBP
tend to lose water through the
Reduces efficiency of
stomata
Stomata close to prevent
photosynthesis
water loss
CO2 cannot enter through
closed stomata!
Adaptation
C4 plants (ex. corn, sorghum,
CAM plants (ex. cacti,
pineapples)
sugarcane, millet, grasses)
In palisade cells, CO2 is fixed Uses the same biochemical
into a 4C compound which is
pathways as C4 plants,
pumped into bundle sheath
except that reactions take
cells and turned back into
place in the same cell
CO2
To prevent water loss,
stomata remain closed during
High concentration of CO2 in
the day and open at night
bundle sheath cells makes
the Calvin Cycle much more
efficient