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CO 7
Chapter 8
Photosynthesis
Photosynthetic Organisms
Photosynthesis transforms solar
energy
Organic molecules built by
photosynthesis provide both the
building blocks and energy for
cells.
Plants use the raw materials:
carbon dioxide and water
Chloroplasts carry out
photosynthesis
Chlorophylls and other
pigments involved in absorption
of solar energy reside within
thylakoid membranes of
chloroplasts
Plants as Solar Energy Converters
Solar Radiation - Only 42% of solar radiation that hits the
earth’s atmosphere reaches surface; most is visible light.
Photosynthetic Pigments - Pigments found in chlorophyll absorb
various portions of visible light; absorption spectrum.
1. Two major photosynthetic pigments are chlorophyll a and chlorophyll
b.
2. Both chlorophylls absorb violet, blue, and red wavelengths best.
3. Very little green light is absorbed; most is reflected back; this is why
leaves appear green.
4. Carotenoids are
yellow-orange
pigments which
absorb light in
violet, blue, and
green regions.
5. When
chlorophyll breaks
down in fall, the
yellow-orange
pigments in
leaves show
through.
Absorption and action spectrum - A spectrophotometer
measures the amount of light that passes through a
sample of pigments.
1) As different wavelengths are passed through, some
are absorbed.
2) Graph of percent of light absorbed at each
wavelength is absorption spectrum
.
3) Photosynthesis produces
oxygen; production of oxygen is
used to measure the rate of
photosynthesis.
4) Oxygen production and,
therefore, photosynthetic activity
is measured for plants under
each specific wavelength; plotted
on a graph, this produces an
action spectrum.
5) Since the action spectrum
resembles absorption spectrum,
this indicates that chlorophylls
contribute to photosynthesis.
Photosynthetic Reaction
1. In 1930 C. B. van Niel showed that O2 given off by
photosynthesis comes from water and not from CO2.
2. The net equation reads:
Two Sets of Reactions in Photosynthesis
1. Light reactions cannot take place unless light is present. They
are the energy-capturing reactions.
b. Chlorophyl within thylakoid membranes absorbs solar energy
and energizes electrons.
c. Energized electrons move down the electron transport
system; energy is captures and used for ATP production.
d. Energized electrons are also taken up by NADP+, becoming
NADPH.
2. Calvin Cycle Reactions
a. These reactions take
place in the stroma; can
occur in either the light
or the dark.
b. These are synthesis
reactions that use
NADPH and ATP to
reduce CO2.
What you should know by now..
1. The equation for photosynthesis
2. The structure of a chloroplast
3. Which spectrum(s) of light is used by plants and the
pigments associated with light spectrum.
4. The two stages of photosynthesis and their products
**Things are about to get much more difficult**
The Light Reactions
1. Two paths operate within the thylakoid membrane
noncyclic
*straight line
and
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.
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?
These reactions are 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
One More Time…
Light Reactions
A. Two Pathways
B. Noncyclic
C. 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)
Chemiosmosis is difficult to visualize.
So... you get to color it!
Yeah!
coloring!
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
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
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.
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.
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.
C4 plants and CAM
plants use an
alternate pathway to
FIX carbon dioxide
from the air.
Quick Practice
Quick Practice
grana
thylakoid
stroma
O2
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)
Compare Photosynthesis to Respiration
1. Where?
2. What are the products?
3. What compounds are needed to start the processes?
4. Which has:
Electron Transport Chain
Phospholipid membrane
ATP Synthase
Proton Pumps
Chlorophyll
The production of glucose / The use of glucose