Life, 6th Edition

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CHAPTER 8
Photosynthesis Energy from the Sun
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Photosynthesis

• Biochemical process in which light energy is
  converted to chemical energy
• Photos light
• synthesis to put together
• In plants, photosynthesis takes place in
  chloroplasts.
• It involves many enzyme controlled steps

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Photosynthetic Reactants and Products

• 6 CO2 12 H2O light ? C6H12O6 6 O2 6 H2O

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Figure 8.1
figure 08-01.jpg

• Figure 8.1

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Photosynthesis

• Photosynthesis can be divided into two pathways
• The light reaction - driven by light energy
  captured by chlorophyll. Consists of Photosystem
  I and Photosystem II. It produces ATP and NADPH
  H.
• The CalvinBenson cycle - does not use light
  directly. It uses ATP, NADPH H, and CO2 to
  produce sugars.

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Properties of Light and Pigments

• Light is the source of energy that drives
  photosynthesis
• Molecules that absorb light energy in the visible
  range are called pigments.

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Figure 8.5
figure 08-05.jpg

• Figure 8.5

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Properties of Light and Pigments

• When light and a pigment molecule meet, one of 3
  things happen
• Reflection the light bounces off the molecule
• Transmission the light passes through the
  molecule
• Excitation the light is absorbed by the
  molecule. If absorbed, the molecule goes from its
  ground state to and excited state of higher
  energy
• An electron is boosted to another orbital

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Pigments

• When a beam of white light shines on an object,
  and the object appears to be red in color, it is
  because it has absorbed all other colors from the
  white light except for the color red.
• In the case of chlorophyll, plants look green
  because they absorb green light less effectively
  than the other colors found in sunlight and
  reflect the green light not absorb

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Properties of Light and Pigments

• Different pigment molecules absorb different
  wavelengths of light
• The particular set of wavelengths that a pigment
  absorbs is called its absorption spectrum
• Review Figures 8.7

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Figure 8.7
figure 08-07.jpg

• Figure 8.7

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Properties of Light and Pigments

• Chlorophylls are the most important pigments in
  photosynthesis
• Chlorophyll a is the primary pigment in
  photosynthesis.
• Chlorophylls and accessory pigments trap light
  and transfer energy to a reaction center

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Chlorophyll
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• An excited pigment molecule may
• lose its energy by emitting light of longer
  wavelength or
• transfer the absorbed energy to another pigment
  molecule as a redox reaction.

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Figure 8.8 Energy Transfer and Electron Transport
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• There are two different systems for transport of
  electrons in photosynthesis.
• 1. Noncyclic electron transport produces
  NADPH H and ATP and O2.
• 2. Cyclic electron transport produces only ATP.

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Noncyclic

• In noncyclic electron transport, two photosystems
  are required.
• Photosystems consist of many chlorophyll
  molecules and accessory pigments bound to
  proteins.

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Photosystem I

• Photosystem I uses light energy to reduce NADP
  to NADPH H.
• The reaction center contains a chlorophyll a
  molecule called P700 because it best absorbs
  light at a wavelength of 700 nm.

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Photosystem II

• Photosystem II uses light energy to split water,
  producing electrons, protons, and O2.
• The reaction center contains a chlorophyll a
  molecule called P680 because it best absorbs
  light at a wavelength of 680 nm.
• To keep noncyclic electron transport going, both
  photosystems must constantly be absorbing light.

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• After absorbing light energy
• an energized electron leaves the Chl in the
  reaction center and participates in a series of
  redox reactions.
• the electron flows through a series of carriers
  in the thylakoid membrane.
• producing ATP

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Figure 8. 9 Noncyclic Electron Transport Uses Two
Photosystems (Part 1)
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Figure 8. 9 Noncyclic Electron Transport Uses Two
Photosystems (Part 2)
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Cyclic Electron Transfer

• Cyclic electron transport produces only ATP.
• The electron passes from an excited P700 molecule
  and cycles back to the same P700 molecule.
• No O2 is released.
• In cyclic electron flow, photosystem I acts on
  its own.

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Figure 8.10 Cyclic Electron Transport Traps
Light Energy as ATP
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Z Scheme

• Photosystem I II (P680 P700) work together to
  generate ATP and NADPH.
• This pathway is called the Z scheme.
• Noncyclic

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Noncyclic Electron Flow or Z Scheme

• In Photosystem II chlorophyll a absorbs light
  energy to become energized chloropyll a
• 2 electrons are released and caught by the
  primary electron acceptor.
• H20 ? ½ O2 2 e- 2H

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• The electrons pass through a redox chain for
  chemiosmotic ATP production.
• The electron transport chain pumps protons across
  the membrane into the thylakoid space.
• The protons accumulate establishing a proton
  concentration gradient
• ATP synthases open and the protons diffuse to
  generate ATP from ADP.

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Z Scheme Contd

• The electrons are passed to P700 chlorophyll
• P700 loses electrons to Ferredoxin (Fd)
• NADP combines with H to form NADPH.
• NADPH is the source of H used to make C6H12O6

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Figure 8.11 Chloroplasts Form ATP
Chemiosmotically
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The CalvinBenson Cycle

• The CalvinBenson cycle makes sugar from CO2
• ATP and NADPH provide the needed energy
• This pathway was elucidated through use of
  radioactive tracers

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The CalvinBenson Cycle

• Three phases
• 1. Carbon Fixation RuBP CO2 ? 6 carbon sugar
  ? 3 PG (first stable product)
• The reaction is catalyzed by rubisco (ribulose
  bisphosphate carboxylase).
• 2. Series of reactions to produce G3P
• 3. Regeneration of RuBP (7 enzymatic steps)
• RuBP (ribulose biphosphate) is the initial CO2
  acceptor

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The CalvinBenson Cycle

• The end product of the cycle is glyceraldehyde
  3-phosphate, G3P.
• There are two fates for the G3P
• One-third ends up as starch, which is stored in
  the chloroplast and serves as a source of
  glucose.
• Two-thirds is converted to the disaccharide
  sucrose, which is transported to other organs.

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Figure 8.13 The Calvin-Benson Cycle
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Rubisco

• Rubisco is a carboxylase, adding CO2 to RuBP. It
  can also be an oxygenase, adding O2 to RuBP.
• These two reactions compete with each other.
• When RuBP reacts with O2, it cannot react with
  CO2, which reduces the rate of CO2 fixation.

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Photorespiration

• A specialized metabolic pathway in which rubisco
  reacts with O2 instead of CO2
• Occurs under stress conditions of hot, dry,
  bright days when the internal leaf concentration
  of O2 is greater than CO2 concentration.
• Glucose production is reduced thereby limiting
  plant growth

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C3 Plants

• Most common type of plants on earth.
• Grow best in temperate zones
• Includes rice, wheat, soybeans, bluegrass
• On hot days the stomata close, O2 builds up and
  photorespiration occurs.
• The first product is the 3-C molecule of 3PG
• CO2 RuBP ? 3 phosophoglycerate (3 C compound)

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Figure 8.16 Leaf Anatomy of C3 and C4 Plants
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C4 Plants

• C4 plants have 2 enzymes (PEP carboxylase
  rubisco) for CO2 fixation in 2 different parts of
  the leaf.
• PEP carboxylase does not have an affinity for O2
  and fixes CO2 even at very low CO2 levels.
• What is the significance of this fact?
• C4 plants include sugarcane, corn and other
  plants that grow in hot, dry climates.

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C4 Plants Contd

• CO2 PEP carboxylase ? Oxaloacetate (4 C
  compound).
• Occurs in cells near top of leaf
• Oxaloacetate diffuses into bundle sheath cells in
  the interior of the cells.
• Here oxaloacetate loses a C forming CO2
• CO2 enters the Calvin-Benson Cycle

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Crassulacean Acid Metabolism (CAM)

• CAM plants are succulents or water storing
  plants.
• Include cacti and pineapples
• CAM plants open their stomata only at night
• CO2 enters and forms malic acid which is stored
  as an acid in the vacuoles until morning
• In daylight, the CO2 is released from the acid
  and enters the Calvin Benson Cycle.

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Stomates

• Stomates close when weather is hot dry.
• O2 concentration increases, CO2 concentration
  decreases.
• Why?
• Ribulose requires high concentrations of CO2
• If sufficient CO2 is unavailable,
  photorespiration occurs.

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Metabolic Pathways in Plants

• Both photosynthesis and respiration occurs in
  plants.
• Compare photosynthesis and respiration.