Photosynthesis

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Photosynthesis

• Chapter 7 (Starr Taggart 2001)

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• Photosynthesis process by which the chloroplasts
  of plants capture light energy and convert it to
  chemical energy stored in the bonds of sugar

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• RELEVANCE Life on planet Earth is
  solar-powered.
• 1. Try to name something you eat that isnt from
  a plant or from an animal that ate a plant.
• Food is used for
• a) Creating macromolecules like glucose and amino
  acids.
• b) More importantly, food is used to generate
  cellular energy, ATP.
• 2. Oxygen is produced as a by-product of
  photosynthesis.

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energy input from sun
PHOTOAUTOTROPHS (plants, other producers)
nutrient cycling
HETEROTROPHS (consumers, decomposers)
energy output (mainly heat)
Fig. 7.4a, p. 116
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Discovery of Important Elements

• Experiments by 17th century scientists, Van
  Helmont and Joseph Priestly.
• Van Helmont demonstrated that soil and H2O,
  although important, were not sole sources of food
  for plants.
• Experiment Helmont planted willow seedling in a
  pot with known amount of soil and added known
  volumes of water. At end of 5 years tree gained
  74 kg of biomass, while soil only lost 57 kg of
  biomass. Water volumes alone also could not
  account for difference in weight.

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Priestly demonstrated plants were O2-evolving.

• Observation 1 When a candle and a small plant
  placed in a sealed chamber, the candle would burn
  10 days longer than it would have without the
  plant.

• Observation 2 A mouse living in a
  closed chamber with a plant will live far longer
  than a mouse in a chamber with no plant.

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Light Energy
12H2O 6CO2
6O2 C2H12O6 6H2O
OXYGEN
WATER
CARBON DIOXIDE
GLUCOSE
WATER
in-text, p. 113
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What type of light do plant require for
photosynthesis?

• Sunlight is a form of energy known as
  electromagnetic energy also called radiation.
• Electromagnetic energy travels in rhythmic waves
  and the distance between these waves is termed
  the wavelength.
• The shorter the wavelength the more energy.
• The entire range of radiation is called the
  electromagnetic spectrum.

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How do plants absorb light? With pigment
molecules.

• Chlorophyll molecules (green pigments) are the
  primary and required pigments for photosynthesis.
• Absorb radiation in blue and red ranges.

Absorption spectrum a diagram that shows how
effectively a pigment absorbs light of different
wavelengths
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How do plants absorb light? With pigment
molecules.

• Carotenoid and phycobilin (orange and yellow
  pigments) are accessory pigments for
  photosynthesis.
• Expand the range of radiation that can be used.

phycoerythrin (a phycobilin)
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(combined absorption efficiency across entire
visible spectrum)
chlorophyll b
chlorophyll a
carotenoids
chlorophyll b
chlorophyll a
phycoerythrin (a phycobilin)
phycoerythrin (a phycobilin)
Absorption spectrum a diagram that shows how
effectively a pigment absorbs light of different
wavelengths
Fig. 7.6, p. 117
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• Photosynthesis driven by only part of the visible
  spectrum (blue and red).
• Why are plants green? Plant pigments in the
  green plant reflects green and absorbs blue and
  red.

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Where are photosynthetic pigments found?
Figure 8.3 (Krogh 2000)
In the thylakoid membranes of chloroplasts.
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How are the pigments in the thylakoid membrane
organized?
Figure 8.4 (Krogh 2000)
They are organized in discrete units called
photosystems.
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Photosystems

• 250-400 pigment molecules
• reaction center containing one pair of
  chlorophyll molecules
• antennae or accessory pigments gathering
  radiation to transfer to reaction center
• two interconnected photosystems (PS II and PS I)
  work together to provide optimum absorption

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What happens to absorbed energy?
1. Captured by atoms of a photosystem reaction
center
2. Escape as fluorescent light (fluoresce longer
wavelengths blue wavelengths fall to lower
energy red)
3. Lost as heat
Figure 8.4 (Krogh 2000)
Fig. 7.9, p. 119
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water-splitting complex
thylakoid compartment
H2O
2H  1/2O2
P680
P700
pool of electron transporters
acceptor
acceptor
PHOTOSYSTEM II (light green)
PHOTOSYSTEM I (light green)
stroma
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• Photosynthesis occurs in two phases.
• PHASE 1
• Light-dependent reactions
• PHASE 2
• Light-independent reactions (dark)
• Other names for these reaction are the Calvin
  Cycle or the C3 Cycle.

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• Photosynthesis occurs in two phases.
• PHASE 1
• Light-dependent reactions
• Occurring on the thylakoid membrane of
  chloroplasts
• Power of sunlight excites electrons in pigment
  molecules.
• Excited electrons used to make
• ATP (adenosine triphosphate) from ADP (adenosine
  diphosphate)
• NADPH (nicotinamide dinucleate phosphate) from
  NADP

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ATP is the chief usable form of energy for cells
Figure 6.4 (Krogh 2000)
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Summary of Light Reactions
1. Two electrons acquired from the breaking of a
water molecule are excited when PS II captures
light energy. 2. The electron acceptor of PS
II captures these electrons. 3. Electrons pass
down an electron transport chain, get shunted to
membrane and give off energy to make ATP.
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Summary of Light Reactions
sunlight
THYLAKOID COMPARTMENT
H2O
second electron transport system
photolysis
e
ATP SYNTHASE
e
NADPH
NADP
first electron transport system
ATP
ADP Pi
PHOTOSYSTEM I
PHOTOSYSTEM II
STROMA
Fig. 7.12a, p. 121
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• Photosynthesis occurs in two phases.
• PHASE 2
• Light-independent reactions (dark)
• ATP and NADPH are not good permanent storage
  molecules, so the plants covert energy into
  several bonds in a glucose molecule.
• Occurs in the stroma (liquid portion of
  chloroplast)
• CO2 becomes part of a carbohydrate by process
  called carbon fixation

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Figure 8.6 (Krogh 2000)
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• Light-Independent Reactions
• CARBON FIXATION
• Enzyme called rubisco brings together three
  molecules of CO2 from the atmosphere and three 5C
  sugar molecules called RuBP (ribulose
  bis-phosphate).
• Six-carbon product splits into 3 3-carbon
  products called PGA (3-phosphoglyceric acid).

Figure 8.7 (Krogh 2000)
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Light-Independent Reactions Energizing the
Sugar In two reactions, ATPs give phosphates to
3-PGAs. These derivative molecules gain
electrons from NADPH and are transformed into an
energy rich sugar called glyceraldehyde
3-phosphate (G3P).
Figure 8.7 (Krogh 2000)
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Light-Independent Reactions Exit of Product One
G3P exits the cycle and can be used for energy or
transformed into materials for other plant uses.

Figure 8.7 (Krogh 2000)
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Light-Independent Reactions Regeneration of
RuBP Remaining G3Ps are transformed back to RuBP
with the help of some more ATP.
Figure 8.7 (Krogh 2000)
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Summary of Photosynthesis
sunlight
water uptake
carbon dioxide uptake
ATP
ADP Pi
LIGHT INDEPENDENT-REACTIONS
LIGHT DEPENDENT-REACTIONS
NADPH
NAD
glucose
P
oxygen release
new water
in-text, p. 115
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• Glitch in the systemPhotorespiration
• Process whereby Rubisco combines with O2 instead
  of CO2 thus binding O2 with RuBP unproductively.
• Occurs one O2 for every three CO2.
• Especially problematic in hot weather because of
  evaporation of water. Plant closes stomata in
  leaves to prevent evaporation, but as water is
  kept in, CO2 is kept out. As the light-dependent
  reactions continue, O2 builds up, combining with
  RuBP unproductively.

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CO2 or O2
CO2 H2O
Rubisco affixes O2 to RuBP.
one glycolate only one PGA (not two) decreased
CO2 uptake, fewer sugars can form
CALVIN- BENSON CYCLE
C3 PLANTS. With low CO2 / high O2,
Photorespiration predominates.
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• Photorespiration and Plant Adaptation
• C4 plants (warm climate adaptation)
• Use a different enzyme that does not bind to O2
  intermediate product is a 4C sugar
• 4C derivative shuttles CO2 to bundle-sheath cells
  where it can be used for the Calvin Cycle even if
  stomates are closed
• Costs ATP to do this in sunny climates this is
  not an issue, because with abundant sunlight, ATP
  is plentiful.
• Grasses, corn, sugarcane, and sorghum

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CO2
carbon fixation in mesophyll cells
oxaloacetate
that carbon fixed again in bundle-sheath
cells CO2 level in leaf enhanced no
photorespiration
CALVIN- BENSON CYCLE
C4 PLANTS. With low CO2 / high O2,
Calvin-Benson cycle predominates.
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• Photorespiration and Plant Adaptation
• CAM Photosynthesis
• Close stomata during the day, open at night
• Start C4 metabolism at night by fixing CO2 but
  wait for day to use abundant ATP to finish.
• Cactus, pineapple, mint, and orchid

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CAM PLANTS. With low CO2 / high O2,
Calvin-Benson cycle predominates.
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Comparison of three strategies
Figure 8.12 (Krogh 2000)
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Plants capture energy from the sun, which drives
photosynthesis.
1. Sunlight energy is converted to chemical bond
energy in ATP. Typically, NADPH also forms.
Carbon dioxide, water are required
Oxygen is released
2. ATP and NADPH are used in reactions that form
glucose, other enery-rich organic compounds.
1. Usable energy is released when cells break
down glucose and other organic compounds.
Carbon dioxide, water are released
Oxygen is required
2. Released energy is coupled to electron
transfers that bring about the formation of many
ATP molecules.
Fig. 7.2, p. 113
ATP is available to drive cellular tasks