• Chapter 7
Photosynthesis
AP Bio
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Ch 7 Goals Photosynthesis
Distinguish b/t autotrophic and heterotrophic nutrition.
Distinguish b/t photosynthetic autotrophs and chemosynthetic autotrophs.
Describe the location and structure of the chloroplast.
Explain how chloroplast structure relates to its function.
Write a summary equation for photosynthesis.
Explain the role of REDOX reactions in photosynthesis.
Describe the wavelike behaviors of light.
Describe the relationship b/t an action spectrum & an absorption spectrum.
Explain why the absorption spectrum for chlorophyll differs from the action spectrum for
photosynthesis.
List the wavelengths of light that are most effective for photosynthesis.
Explain what happens when chlorophyll or accessory pigments absorb photons.
List the components of a photosystem and explain their function.
Trace electron flow through photosystems II & I.
Compare cyclic and noncyclic electron flow and explain the relationship b/t these components of the
light reactions.
Summarize the light reactions with an equation and describe where they occur.
Describe important differences in chemiosmosis b/t oxidative phosphorylation in mitochondria &
photophosphorylation in chloroplasts.
Summarize the carbon-fixing reactions of the Calvin cycle & describe changes that occur in the
carbon skeleton of the intermediates.
Describe the role of ATP & NADPH in the Calvin cycle.
Describe what happens to rubisco when the oxygen concentration is much higher than carbon dioxide.
Describe the major consequences of photorespiration.
Describe two important photosynthetic adaptations that minimize photorespiration.
Describe the fate of photosynthetic products.
Photosynthesis in nature
• Autotrophs - biotic producers;
photoautotrophs – E from
sunlight
chemoautotroph: E from
inorganic chemicals
• Heterotrophs
- biotic
consumers; obtains organic food
by eating other organisms or
their by-products (includes
decomposers)
How do chemoautotrophs do this?
• Sulfur bacteria. Certain colorless bacteria share the ability of
chlorophyll-containing organisms to manufacture carbohydrates
from inorganic raw materials, but they do not use light energy for
this. They secure the necessary energy by oxidizing some reduced
substance present in their environment. The free energy released by
the oxidation is harnessed to the manufacture of food.
• For example, some chemoautotrophic sulfur bacteria oxidize H2S in
their surroundings (e.g., the water of sulfur springs) to produce
energy:
2H2S + O2 → 2S + 2H2O; ΔG = -100 kcal
• They then use this energy to reduce carbon dioxide to carbohydrate
(like the photosynthetic purple sulfur bacteria).
2H2S + CO2 → (CH2O) + H2O + 2S
The chloroplast
Sites of photosynthesis
Pigment: chlorophyll
Plant cell: mesophyll
Gas exchange: stomata
Double membrane
Thylakoids, grana,
stroma
• Pigments located in
thylakoid membrane
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Overall Reaction
Sun’s E +
6H2O + 6CO2 ------ C6H12O6 + 6O2
Reduced?
Oxidized?
CO2 gets reduced when energized e- from water get added to
it, along with H+ making sugar!
Can you feel the goose bumps? Does it not give you chills?
Look at Jimmy Pratte; he’s got ‘em!
Nature of Light
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order of colors is determined by wavelength.
The longer the wl of visible light, the more red
the color.
The shorter the wl the more violet
energy of is inversely
porportional to the
wavelength: longer
wavelengths have less
energy than do
shorter ones.
Pigments
Light absorbing molecule
color comes from the wl of light reflected ( those not absorbed).
chlorophyll absorbs all wl of visible light except green
Black pigs absorb all wl that strike them.
White pigments/lighter colors reflect all or almost all
All photosynthetic orgs have chloro a.
Accessory pigs absorb E – broaden range
– Chloro b, xanthophylls, carotenoids
• Photosynthetic pigs absorb mostly from red & blue wl
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xanthophylls
Action & absorption spectrum
• The action spectrum of photosynthesis is the relative effectiveness of
different wavelengths of light at generating electrons.
• Absorption spectrum shows the wl of light absorbed by various pigments
• Red & blue…
• Redox process
• H2O is split, e- (along
w/ H+) are
transferred to CO2,
reducing it to sugar
• 2 major steps:
1. light reactions
(“photo”)
√NADP+ (electron
acceptor) to
NADPH
√Photophosphorylation
:
ADP ---> ATP
2. Calvin cycle
(“synthesis”)
Carbon fixation:
carbon into
organics
Photosynthesis: an
overview
• Light harvesting units
of thylakoid mem
• antenna pigs, rx
center chloro a &
primary e- acceptor
• Antenna pigs (broaden
range of absorption)
struck by photons
• e- is passed to rx
center
• Excited e- from
chloro is trapped by
primary e- acceptor
Photosystems
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Photosystem II (P680):
photons excite chlorophyll eto an acceptor
e- are replaced by splitting of
H2O (release of O2)
e-’s travel to Photosystem I
down an electron transport
chain
as e- fall, ADP ---> ATP
(photophosphorylation by
chemiosmosis)
Photosystem I (P700):
fallen’ e- replace excited e- to
primary e- acceptor
2nd ETC (NADP+ reductase)
transfers e- to NADP+ --->
NADPH (...to Calvin cycle…)
These photosystems produce
equal amounts of ATP and
NADPH
http://www.youtube.com/watch
?v=BK_cjd6Evcw
Light Dep Rx
Noncyclic electron flow
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3 molecules of CO2 are ‘fixed’
into glyceraldehyde 3phosphate (G3P)
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Phases:
1- Carbon fixation~
each CO2 is attached to RuBP
(rubisco enzyme)
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2- Reduction~ electrons
from NADPH reduces to G3P;
ATP used up
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3- Regeneration~ 5 G3P
rearranged to RuBP; ATP used;
cycle continues
http://www.youtube.com/watch?v
=mHU27qYJNU0&feature=related
The Calvin cycle
Calvin Cycle, net synthesis
• For each G3P (and for 3 CO2)…….
Consumption of 9 ATP’s & 6 NADPH
reactions regenerate these molecules)
(light
• G3P can then be used by the plant to make glucose
and other organic compounds
• Alternative cycle when
ATP is deficient
• Photosystem I used but
not II; produces ATP but
no NADPH
• Is Oxygen produced?
Why?
• Why? The Calvin cycle
consumes more ATP than
NADPH…….
• Cyclic
photophosphorylation
Cyclic electron flow
Alternative carbon fixation methods, I
• Photorespiration:
hot/dry days;
stomata close; CO2
decrease, O2
increase in leaves;
O2 added to
rubisco; no ATP or
food generated
• Two Solutions…..
• 1- C4 plants: 2
photosynthetic cells,
bundle-sheath &
mesophyll; PEP
carboxylase (instead
of rubisco) fixes
CO2 in mesophyll;
new 4C molecule
releases CO2
(grasses)
Alternative carbon fixation methods, II
• 2- CAM plants:
open stomata
during night,
close during
day… why?
• (crassulacean
acid
metabolism);
cacti,
pineapples,
etc.
A review of photosynthesis
Separation of pigments by paper
chromatography – Part A of lab
retention factor (Rf value)
Rf value is the ratio between how far
the component travels and the
distance the solvent travels from
a common starting point (the
origin).
If one of the sample components
moves 2.5 cm up the paper and
the solvent moves 5.0 cm, then
the Rf value is 0.5.
You can use Rf values to identify
different components as long as
the solvent, temperature, pH, and
type of paper remain the same. In
the image below, the light blue
shading represents the solvent
and the dark blue spot is the
chemical sample.
Tape, Measure, ID &
place name on it.
Staple to lab
Chlorophyll a – bright gree
Chlorophyll b – olive green
Carotene – orange yellow
Xanthophyll – yellow (may get 2
different types)
Photo lab part B
• Step # 6 only add 1 ml of phosphate buffer to cuvette 5 – it says to add 1 ml
and then a few sentences later has you adding another ml. DON’T. Only
add 1 ml.
• positive control: a positive control should give the desired outcome of the
experiment, provided that all the reagents and equipment are functioning
properly. For example, if your experiment results in the ability of bacteria to
grow on a petri plate containing antibiotic, your positive control will be
bacteria that are known to carry the appropriate drug resistance marker.
Even if none of your experimental bacteria grow, as long as there is growth
of the positive control you know that growth was possible.
• negative control: a negative control should be designed to not give the
desired outcome of the experiment. In the example above, bacteria which
do not carry a drug resistance marker should not be able to grow on a petri
plate containing antibiotic. If growth is observed, it is a red flag that
something is wrong with the experiment. (What could be one reason for
growth?)