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Photosynthesis
Watch this: http://youtu.be/g78utcLQrJ4
D. Photosynthetic Reaction
1. In 1930 C. B. van Niel showed that O2 given off by
photosynthesis comes from water and not from CO2.
Redox reactions ultimately transfer electrons from water to carbon dioxide
2. The net equation reads:
What is a plant anyway?
7.1 Photosynthetic Organisms
A. Photosynthesis transforms
solar energy
B. Organic molecules built by
photosynthesis provide both the
building blocks and energy for
cells.
Figure 7.1b
C. Plants use the raw materials: carbon dioxide (from
stoma) and water (from roots)
D. Chloroplasts carry out photosynthesis
Photosynthesis takes place in the chloroplasts!
Figure 7.1c
E. Chlorophylls and other
pigments involved in absorption
of solar energy reside within
thylakoid membranes of
chloroplasts
Figure 7.2
Stoma (stomata pl.)
allow for gas
exchange
Quick Check 1. Plant
2. Thylakoid
3. Photosynthesis
4. Organic Molecules
5. Grana
6. Chloroplast
7.2 Plants as Solar Energy Converters
A. Solar Radiation - Only 42% of solar radiation that hits the
earth’s atmosphere reaches surface; most is visible light. Light
is composed of packets of energy called photons.
B. 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. Most green 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.
Color
• The color that we see when looking at a
pigmented object is the wavelengths that are
reflected or transmitted by the pigmented
object. In other words, we see the reflected
wavelengths. A green leaf reflects green. Green
is not absorbed by the plant. Chlorophyll
absorbs in the red, blue and violet wavelengths.
Fall Foliage Slideshow
Fall Leaves
• Why do leaves turn orange/red/brown in the fall?
• What are the major pigments which absorb light?
• Why do leaves appear green?
• The reason why there are accessory pigments in a leaf
is to absorb energy in parts of the spectrum that
chlorophyll can’t
E. Two Sets of Reactions in Photosynthesis
1. Light reactions cannot take place unless light is present.
They are the energy-capturing reactions. They convert light
energy to chemical energy. When chlorophyll absorbs a photon,
one of its electrons is raised to the excited state.
Excited Electrons
• When a photon hits a molecule, it releases
energy as light, loss of an electron, fluorescence
and heat
• Light reactions, and photosystems are located in
the thylakoid membrane as well as the H+ ion
gradient
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.
The products of
the light reactions
are ATP and
NADPH
What you should know by now..
1. The equation for photosynthesis. Write it!
2. The structure of a chloroplast. Sketch it!
3. Compare the two stages of photosynthesis and
their products. Chart it!
**Things are about to get much more difficult**
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
(this is where we pick up electrons!)
3. PHOTOPHOSPHORYLATION = ATP production
(phosphorylation means adding a P to ADP  ATP)
4. Photosystems are located in the thylakoid membrane which is
where the light reactions occur.
1. Light hits photosystem II (yes, II comes before I)and exites an electron
in 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, PSII
was established first.
Figure 7.5
We use these electrons to
go to the Calvin Cycle
We’ve used our electrons here to
form ATP
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 Light Reactions
A. Two Pathways
B. Noncyclic
C. Cyclic
D. ATP Production --> CHEMIOSMOSIS
When H20 is split, two H+ remain, oxygen is released
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!
coloring!
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.
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.
Figure 7.9
Factors that Affect Photosynthesis
1. Light Quality (color)
2. Light intensity
3. Light Period
4. Carbon Dioxide Availability
5. Water Availability
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.
Alternative Pathways
The Calvin Cycle is the MOST Common Pathway for Carbon
Fixation. Plant Species that fix Carbon EXCLUSIVELY through
the Calvin Cycle are known as C3 PLANTS.
Plants in hot dry environments (C4 plants) have a problem with
water loss, so they keep their stomata partly closed... this
results in CO2 deficit (Used in Calvin Cycle), and the level of O2
RISES (as Light reactions Split Water Molecules).
CAM plants conserve water by opening their stomata only at night
Figure 7.10
C4 plants and CAM
(Crassulacean acid metabolism)
plants use an
alternate pathway to
FIX carbon dioxide
from the air.
Figure 7.11
THE CAM PATHWAY - Plants that use the CAM
Pathway open their stomata at night and close
during the day.
At night, CAM Plants take in CO2 and fix into
organic compounds. During the day, CO2 is
released from these Compounds and enters the
Calvin Cycle. Because they have their stomata
open only at night, they grow slow.
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)