AP Biology
Lecture #17
Photosynthesis Part 2
7.3 The Light Reactions
1. Two paths operate within the thylakoid membrane
noncyclic and
*straight line
cyclic
*in a circle
2. Both paths use ATP, but the noncyclic also produces NADPH
3. PHOTOPHOSPHORYLATION = ATP production
1. Light hits photosystem II and exites an electron, H20
2. The primary electron acceptor passes the electron down the ETC and generates ATP
3. Light is required for PSI, but not water, it generates NADPH
Something trivial....
Photosystem I and Photosystem II are named
based on when they were discovered, PSI was
established first.
Figure 7.5
Potential to transfer energy (volts)
Energy Changes (Light RXN)
second
transfer
chain
e–
first
transfer
chain
e–
e–
NADPH
e–
(Photosystem I)
(Photosystem II)
H2O
1/2O2 + 2H+
Indicate which system
(PS1 or PS2 or BOTH)
____1. Splits water
____2. Produces NADPH
____3. Has an electron transport chain
____4. Requires light
____5. Utilizes a primary electron acceptor
____6. Occurs in the thylakoid
____7. Requires the input of H20
____8. The cyclic path
____9. Uses chlorophyll
____10. Releases oxygen
Are you still confused? This is pretty
hard to visualize, but through the magic
of technology, we can watch these
processes as animations
McGraw Hill Animation
Forest Biology - The Light Reactions
7.3
A.
B.
C.
Light Reactions
Two Pathways
Noncyclic
Cyclic
D. ATP Production --> CHEMIOSMOSIS
When H20 is split, two H+ remain
These H+ are pumped from the stroma into the thylakoid
This creates a gradient used to produce ATP from ADP
ATP is the whole point of Photosystem II and will be used to
power the Light Independent Reactions (Calvin Cycle)
Figure 7.7
Chemiosmosis is difficult to visualize.
So... you get to color it!
Yay! colorin
g!
Chemiosmosis Video/Animations
Animation of Photosynthesis, Chemiosmosis (ATP Synthesis)
The Calvin Cycle
Also called
*The Light Independent Reactions
*The Dark Reactions
*Named after Melvin Calvin, who
used a radioactive isotope of carbon
to trace the reactions.
The Calvin Cycle
is a series of reactions producing carbohydrates.
carbon dioxide fixation, carbon dioxide reduction,
and regeneration of RuBP.
FIXATION
REDUCTION
REGENERATION
B. Fixation of Carbon Dioxide
1. CO2 fixation is the attachment of CO2 to an organic
compound called RuBP.
2. RuBP (ribulose bisphosphate) is a five-carbon molecule that
combines with carbon dioxide.
3. The enzyme RuBP carboxylase (rubisco) speeds this
reaction; this enzyme comprises 20–50% of the protein content
of chloroplasts, probably since it is a slow enzyme.
Calvin Cycle Animation
C. Reduction of Carbon Dioxide
1. With reduction of
carbon dioxide, a
PGA
(3-phosphoglycerate [C3])
molecule forms.
2. Each of two PGA
molecules undergoes
reduction to PGAL in
two steps.
3. Light-dependent
reactions provide
NADPH (electrons) and
ATP (energy) to reduce
PGA to PGAL.
D. Regeneration of RuBP
1. Every three turns of
Calvin cycle, five
molecules of PGAL are
used to re-form three
molecules of RuBP.
2. Every three turns of
Calvin cycle, there is
net gain of one PGAL
molecule; five PGAL
regenerate three
molecules of RuBP.
Figure 7.8
E. The Importance of the Calvin Cycle
1. PGAL, the product of
the Calvin Cycle can be
converted into all sorts of
other molecules.
2. Glucose phosphate is
one result of PGAL
metabolism; it is a
common energy
molecule.
sunlight
water uptake
carbon dioxide uptake
ATP
LIGHT
DEPENDENT
REACTIONS
ADP + Pi
NADPH
LIGHT
INDEPENDENT
REACTIONS
NAD+
P
oxygen release
glucose
new water
Figure 7.9
In order for photosynthesis to occur, plants must open tiny
pores on their leaves called STOMATA.
Opening these pores can lead to loss of water.
Figure 7.10
C4 plants and CAM
plants use an
alternate pathway to
FIX carbon dioxide
from the air.
Figure 7.11
Plant Pathways: Making more
sugars even under heat stress
• Photorespiration: Plants in hot dry environments
close their stomata to conserve water but reduce
the CO2 entry and increase the O2 buildup from
photosynthesis
• Photorespiration is nonproductive
• Plants have evolved C4 and CAM pathways to
counter hot environments and still produce
sugars
Plant Pathways: Making more sugars even
under heat stress (C4, CAM)
• C3 = Calvin-Benson Cycle = Dark RXN
– Basswood, bluegrass, beans
– PGA (phosphoglycerate) is a 3-C intermediate molecule
• C4:
– Corn, sugarcane, grass
– Fix carbon twice in different cells (mesophyll and bundle
sheath)
– 4-C intermediate molecule: oxaloacetate
– Open stomates at night
– Store CO2 as intermediate molecule, then forms pyruvate
which goes through Calvin-Benson cycle during day without
opening stomates
Plant Pathways: Making more sugars even
under heat stress (C4, CAM)
• CAM: (Crassulacean Acid Metabolism)
– Succulents, cactus, orchids, pineapples
– Fix carbon at night (stomates open), store
intermediate product (malate) for use in
photosynthesis the next day in CalvinBenson cycle
– Fix carbon in the same cell at different times (unlike
C4)
Quick Practice
Quick Practice
grana
thylakoid
stroma
O2
Pg 129b
Light & H2O
CO2
ADP
NADP
ATP
NADPH
O2
glucose
A = photosystem II
B = photosystem I
C = H20
D = Electron Transport Chain
E = ATP Synthase
AB = ATP
AC = phospholipids
AD = light (energy)