1. Chapter 10 PHOTOSYNTHESIS
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2. PHOTOSYNTHESIS – What is it?
a. PHOTOSYNTHESIS: The conversion of LIGHT ENERGY to chemical energy is stored
in sugars or other organic compounds; occurs in plants, algae, and certain prokaryotes.
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3. PHOTOSYNTHESIS – Who participates?
a. AUTOTROPHS: An organism that obtains organic food molecules WITHOUT EATING
anything derived from other living beings.
b. Autotrophs use energy from the sun to produce their organic molecules from CO2 and
other inorganic raw materials obtained from the environment.
c. Plants are specifically photoautotrophs using light as their energy source.
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4. PHOTOSYNTHESIS – Who benefits?
a. Heterotrophs: An organism that obtains organic food molecules by eating other
organisms or substances derived from them.
b. The biospheres consumers (pictured).
c. Heterotrophic decomposers (mostly fungi): consume the remains of dead organisms.
d. Almost all heterotrophs are dependent directly or indirectly on photoautotrophs for food
and oxygen, a by-product of photosynthesis.
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5. PHOTOSYNTHESIS – Where does it occur?
a. Chloroplasts: an organelle found in plants and photosynthetic protists that absorbs
sunlight and uses it to drive the synthesis of organic compounds from carbon dioxide
and water.
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6. PHOTOSYNTHESIS – Where is the chloroplast located?
a. Chlorophyll: the pigment tissue of a leaf sandwiched between the upper and lower
epidermis and specialized for photosynthesis.
b. Chloroplasts are found mainly in the thylakoid of the mesophyll. Of the chloroplasts
c. A typical mesophyll cell has half a million chloroplasts.
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7. PHOTOSYNTHESIS – What can be found in the LEAF?
a. STOMATA (pl.): a microscopic pore surrounded by guard cells in the epidermis of
leaves and stems that allows gas exchange between the environment and the interior of
the plant.
8. PHOTOSYNTHESIS– What can be found in the chloroplast?
a. _______________________________: a flattened ____________________________
______________________________ inside a chloroplast.
b. Thylakoids exist in an interconnected system in the chloroplast and contain the
_____________________________________________ used to convert light energy to
chemical energy.
9. PHOTOSYNTHESIS– What can be found in the chloroplast?
a.
______________________ (pl.); granum (s.): A stack of membrane-bound
___________________________ in the chloroplast.
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b.
_________________________: Within the chloroplast, the _____________________
_______________________________ of the chloroplast surrounding the thylakoid
membrane; involved in the _____________________________of organic molecules from
carbon dioxide and water.
10. PHOTOSYNTHESIS– What can be found in the chloroplast?
a. _________________________________: The green ___________________ that gives
leaves their color and resides in the _______________________________________ of
the chloroplasts.
b. _________________________ (a.k.a. biochrome): substances produced by living
organisms that have color resulting from selective color absorption.
c. About _______________________________________ chloroplasts per square
millimeter of leaf surface!!
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11. More on Chlorophyll
a. Chlorophyll a: A photosynthetic pigment that participates directly in the light reactions,
which convert solar energy to chemical energy.
b. Chlorophyll b: An accessory photosynthetic pigment that transfers energy to chlorophyll
a.
c. Note difference (in the functional groups).
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12. TRACKING ATOMS
a. SUMMARY OF PHOTOSYNTHESIS:
b. 6 CO2 + 12 H2O + Light Energy  C6H12O6 + 6 O2 + 6 H20
c. Using glucose to simplify relationship
d. Direct product is a 3 carbon sugar used to make glucose.
e. 6 CO2 + 6 H2O + Light Energy  C6H12O6 + 6 O2
f. (Net consumption of water shown here.)
g. Can you see it is the reverse of cellular respiration?
h. In 1930’s, C.B. van Neil of Stanford University hypothesized that the oxygen came from
the splitting of water in photosynthesis, not from carbon dioxide.
i. 20 YEARS LATER HIS WORK WAS CONFIRMED USING A HEAVY ISOTOPE OF
OXYGEN (oxygen-18)!!
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13.
COMPARISON
a. Cellular Respiration and Photosynthesis
b. Both mechanisms involve redox reactions.
c. Cellular respiration: energy is released from sugar when electrons associated with
hydrogen are transported by carriers to oxygen, forming H2O. Energy used to produce
ATP.
d. Photosynthesis reverses electron flow.
e. Water is split, and its electrons are transferred along with H+ from water to CO2,
reducing it to surgar.
f. Endergonic – energy provided by the sun.)
g. 6 CO2 + 6 H2O + Light Energy  C6H12O6 + 6 O2
h. CO2 becomes reduced.
i. H2O becomes oxidized.
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14. PHOTOSYNTHESIS – Stage One
a. Light reactions: The first of two major stages in photosynthesis (preceding the Calvin
cycle).
b. These reactions, which occur on the thylakoid membranes of the chloroplast, convert
solar energy to the chemical energy of ATP and NADPH , releasing oxygen in the
process.
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15. PHOTOSYNTHESIS – Stage Two
a. Calvin cycle : The second of the two major stages in photosynthesis (following the light
reactions), involving fixation of atmospheric CO2 and reduction of the fixed carbon to
carbohydrate by the addition of electrons (provided by NADH).
b. IMAGE: Mel Calvin
16. PHOTOSYNTHESIS – Electron Acceptor
a. Light absorbed by chlorophyll drives a transfer of the electrons and hydrogen ions from
water to an acceptor calle.
b. NADP+: Nicotinamide adenine dinucleotide phosphate, an _______________________
____________________________ that when ______________________ becomes
________________________ and temporarily stores energized electrons produced
during the light reactions.
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17. PHOTOSYNTHESIS – Generation of ATP
a. Light reactions generate ATP using chemiosmosis to power the addition of a phosphate
group to ADP, a process called:
b. PHOTOPHOSPOHORYLATION: The process of generating ATP from ADP and
phosphate by means of a proton-motive force generated across the thylakoid
membrane of the chloroplast.
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18. PHOTOSYNTHESIS - Reducing CO2 to Sugar
a. The Calvin Cycle begins by incorporating atmospheric CO2 into organic molecules
present in the chloroplast in a step called:
b. CARBON FIXATION” The initial incorporation of carbon from CO2 into an organic
compound by an autotrophic organism.
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19. PHOTOSYNTHESIS: What is the nature of sunlight?
a. Light is an energy form, or radiation, known as the ELECTROMAGNETIC SPECTRUM;
it travels in rhythmic waves called wavelengths (ranging in size from less than a
nanometer to 6than a kilometer).
b. WAVELENTH: The distance between two crests of waves. Light behaves in many ways
as a:
c. PHOTON: a quantum, or discrete quantity, of light energy that behaves as if it were a
particle.
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20. PHOTOSYNTHESIS: What is the nature of sunlight?
a. The segment of the electromagnetic spectrum most important to LIFE IS a narrow band
with wavelengths ranging from 380 nm to 750 nm and is known as
b. VISIBILE LIGHT : that portion of the electromagnetic spectrum that can be detected as
various colors by the human eye.
c. ROY G. BIV
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21. WHY LEAVES ARE GREEN
a. When light meets matter, it may be reflected, transmitted or absorbed. Substances that
absorb sunlight are known as pigments.
b. Different pigments absorb different wavelengths
c. Leaves appear green because they reflect or transmit green light.
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22. OTHER PIGMENTS
a. Chlorophyll a is the key light capturing pigment participating in the light reactions.
Chlorophyll b CAROTENOIDS are accessory pigments.
b. By absorbing wavelengths of light that chlorophyll a cannot, accessory pigments
broaden the spectrum of colors that can drive photosynthesis as demonstrated by this
absorption spectra. .
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23. EXCITATION OF CHLOROPHYLL BY LIGHT
a. When a molecule absorbs a photon of light, one of the molecule’s electrons is
elevated to an orbital where it has more potential energy. This excited state is unstable
and will return to ground state. If chlorophyll is in isolation (not in the photosynthetic
machinery of plants) it will emit heat and fluorescence (light).
- Returns from excited state back to ground state in a billionth of a
second !
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24. THE LIGHT REACTIONS USE PHOTOSYSTEMS
a. PHOTOSYSTEM: a light-capturing unit located in the thylakoid membrane of the
chloroplast consisting of a reaction-center complex surrounded by numerous light
harvesting complexes. There are two types of photosystems, I and II; they absorb light
best at different wavelengths.
b. Remember: we are in a thylakoid membrane.
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25. Photosystems Have Reaction Center Complexes
a. The reaction center complex consists of proteins associated with a special pair of
chlorophyll a molecules and a primary electron acceptor.
b. Located centrally in a photosystem, this complex triggers the light reactions of
photosynthesis. Excited by light energy, the pair of chlorophylls donates an electron to
the primary electron acceptor , which passes an electron to an electron transport
chain.
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26. Photosystems have light-harvesting complexes
a. LIGHT-HARVESTING COMPLEX: a complex of proteins associated with pigment
molecules (including chlorophyll a, chlorophyll b, and carotenoids) that captures light
energy and transfers it to reaction center pigments in a photosystem.
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27. Photosystems have Primary Electron Acceptors
a. PRIMARY ELECTRON ACCEPTOR: in the __________________ membrane of a
______________________________ a specialized _______________________that
shares the reaction-center complex with a pair of chlorophyll a molecules and that
accepts an _______________________________ from them.
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28. Two Types of Photosystems Participate in the Light Reactions
a. PHOTOSYSTEM II (PS II) functions first: One of two capturing units in a chloroplast’s
thylakoid membrane; it has two molecules of P680 chlorophyll a at its reaction center.
PHOTOSYSTEM I (PSI) functions second: One of two light-capturing units in a
chloroplast’s thylakoid membrane; it has two molecules of p700 chlorophyll a at its
reaction center.
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29. LIGHT DRIVES THE SYNTHESIS OF ATP AND NADPH in the LIGHT REACTIONS
a. LINEAR ELECTRON FLOW: a route of electron flow during the light reactions of
photosynthesis that involves both photosystems (I and II) and produces ATP, NADPH,
and O2. The net electron flow is from H20 to NADP+.
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30. Alternative Electron Paths
a. CYCLIC ELECTRON FLOW: a route of electron flow during the Light reactions of
photosynthesis that involves only photosystem I and that produces ATP but not
NADPH or O2.
b. Seen in photosynthetic bacteria (purple and green sulfur bacteria) and cyanobacteria.
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31. CHEMIOSMOSIS (comparison of cellular respiration and photosynthesis)
a. Chloroplasts and mitochondria generate ATP by the same basic mechanism:
chemiosmosis.
b. An electron transport chain assembled in a membrane pumps protons across the
membrane as electrons are passed through a series of carriers that are progressively
more electronegative.
c. Note locations.
d. What is the source of electron for each mechanism?
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32. CHEMIOSMOSIS AND THE THYLAKOID MEMBRANE
a. Thylakoid membrane of the chloroplast pumps protons from the stroma into the
thylakoid space which functions as the H+ reservoir. Two sources of H+ for the
thylakoid space: splitting of water, translocation of H+ across the membrane.
b. ATP is synthesized as the H+ diffuses from the thylakoid space back to the stroma
through the ATP SYNTHASE complexes. ATP that formed in the stroma then travels to
the CALVIN CYCLE to energize the making of sugar.
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33. Energy produced from the light reactions ( ATP and NADPH) is sent to the CALVIN
CYCLE to reduce CO2 to SUGAR.
a. The Calvin Cycle is ANABOLIC, using energy (ATP and NADPH) to build
carbohydrates from smaller molecules. STEPS:
b. CARBON FIXATION of CO2 to 5–carbon RUBP (ribulose bisphosphate) is catalyzed by
rubisco (RuBP carboxylase-oxygenase) forming a short-lived 6-carbon intermediate
which immediately splits into 2 molecules of 3-phosphoglycerate for each fixed CO2 .
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34. Calvin Cycle: REDUCTION Phase Results in 6 G3P’s
a. 3-Phosphoglycerate receives an additional phosphate group from ATP and then
NADPH donates a pair of electrons to produce G3P:
GLYCERALDEHYDE-3-PHOSPHATE: a three carbon carbohydrate that is the direct
product of the Calvin cycle; it is also an intermediate in glycolysis.
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35. Regeneration of the ________________________________ RuBP
a. After reduction phase is completed, one molecule of G3P is used as a _____________
________________________________ to produce other sugars.
b. 5 molecules of G3P are ________________________ into three molecules of
_____________ using energy from ____________________.
c. RuBP, the CO2 _____________________________, has been regenerated.
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36. Since plants arrived on land 475 million years ago – face the problems of terrestrial
life…especially dehydration.
What metabolic adaptations have been made?
Plants need to balance the need for photosynthesis while preventing excess water
loss.
Guard cells opening and closing of the stomata.
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37. Avoiding dehydration on land…
a. Photorespiration: A metabolic pathway that consumes oxygen and ATP, releases
carbon dioxide, and decreases photosynthetic output. Photorespiration generally
occurs on hot, dry, bright days, when stomata close, and the oxygen concentration in
the leaf exceeds that of carbon dioxide. Rubisco incorporates O2 producing a compound
that splits, leaves the chloroplast, releases CO2! Uses ATP!!
Produces NO sugar! Protects against excess light???
b. C3 PLANTS: a plant that uses the Calvin cycle for the initial steps that incorporate CO2
into organic material, forming a three carbon compound (3-Phosphoglycerate) as the
first stable intermediate.
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38. ALTERNATIVE METHODS OF CARBON FIXATION
a. C3 plants (wheat, soy bean) use the Calvin cycle to initiate carbon fixation.
b. During hot, dry conditions, plants will close their stomata to reduce transpiration and
conserve water. This situation reduces the amount of CO2 available for carbon fixation
.
c. Light reactions are continuing to produce O2 that is unable to exit through closed
stomata and increasing in concentration.
d. This situation results in PHOTORESPIRATION.
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39. C4 PLANTS: Alternate modes of carbon fixation have evolved in C4 plants and CAM
plants to minimize photorespiration.
a. C4 PLANTS: A plant in which the Calvin cycle is preceded by reactions that incorporate
CO2 into a four-carbon compound, the end product of which supplies CO2 for the Calvin
cycle.
b. PEP – HIGH AFFINITY FOR CO2, NO AFFINITY FOR OXYGEN! Pep carboxylase
fixes CO2 more efficiently than rubisco.
c. Keeps the concentration of CO2 high.
d. MESOPHYLL CELLS ARE A BUFFER TO PREVENT RUBISCO FROM BINDING
OXYGEN.
e. 4-C COMPOUND MOVES INTO A BUNDLE SHEATH VIA PLASMODESMA
f. SEE in Zea mays slide IN LAB!
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40. C4 PLANTS
a. BUNDLE-SHEATH CELL: In C4 plants, a type of photosynthetic cell arranged into
tightly packed sheaths around the veins of a leaf.
Plasmodesmata allows passage of 4-carbon compound
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41. CAM PLANT ADAPTATION
a. CAM PLANT: A plant that uses crassulacean acid metabolism, an adaption for
photosynthesis in arid conditions.
b. In this process, carbon dioxide entering open stomata during the night is converted to
organic acids, which release CO2 for the Calvin cycle during the day , when stomata
are closed.
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42. C4 VS. CAM
a. C4: Carbon fixation and Calvin cycle occur in different cells!
b. CAM: Carbon fixation and Calvin cycle occur at times.
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