Photosynthesis

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Photosynthesis

• Tuesday, July 22

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Sunlight the ultimate source of energy

• Heterotroph
• Obtain organic material by consuming other
  organisms
• Carnivores/Herbivores - animals
• Decomposers fungi and bacteria
• Autotroph
• Produce organic material from CO2 and inorganic
  raw materials
• Photoautotroph
• Light is the source of energy for organic
  synthesis
• Plants and algae
• Chemoautotroph
• Ammonia and sulfur is the energy source
• bacteria

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Photosynthesis
6 CO2 12 H2O LIGHT ? C6H12O6 6 H2O
6O2

• Photosynthesis
• Light energy from the sun is captured and
  converted into chemical energy (sugar)
• Occurs in chloroplasts in mesophyll cells found
  in the tissue of plant leaves
• Chlorophyll is the green pigment of plants that
  absorbs light energy
• CO2 enters and O2 exits through small pores
  called stomata

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Figure 10.3 Tracking atoms through photosynthesis
Photosynthesis splits water using heavy oxygen
(18) CO2 2 H2O ? CH2O H2O O2 CO2 2 H2O
? CH2O H2O O2
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An overview of photosynthesis cooperation of the
light reactions and the Calvin cycle

• Light reactions
• Converts solar energy to chemical energy
• Generates ATP (energy) and NADPH (reducing power)
• Occurs in the thylakoid membranes
• Calvin cycle
• Incorporates CO2 into organic molecules (carbon
  fixation) which are then reduced to sugar
• Occurs in the stroma

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Figure 10.4 An overview of photosynthesis
cooperation of the light reactions and the Calvin
cycle
Light is absorbed by chlorophyll
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Figure 10.4 An overview of photosynthesis
cooperation of the light reactions and the Calvin
cycle
Phosphate is added to ADP forming ATP
(photophosphorylations)
Electrons and hydrogen are transferred from water
to NADP (temporary storage of energized
electrons), releasing oxygen
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Figure 10.4 An overview of photosynthesis
cooperation of the light reactions and the Calvin
cycle
The calvin cycle uses the energy (ATP) and
reducing power (NADPH) from the light reactions
to make sugar
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Figure 10.5 The electromagnetic spectrum
Visible light drives photosynthesis
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Figure 10.6 Why leaves are green interaction of
light with chloroplasts
Pigment molecules of chloroplast absorb red and
blue light, reflecting the green light that we
see.
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Figure 10.7 A spectrophotometer measures the
absorption spectrum
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Figure 10.8 Evidence that chloroplast pigments
participate in photosynthesis absorption and
action spectra for photosynthesis

• - Chlorophyll a participates directly in the
  light reactions
• Chlorophyll b transmits energy to chlorophyll a
• Carotenoids protects chlorophyll by absorbing
  and dissipate excessive light energy

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Light Absorption

• Chlorophyll a absorbs a photon elevating an
  electron (ground state) to a higher energy
  orbital (excited state)
• Only photons with the amount of energy equal to
  the difference between the ground and excited
  state can be absorbed
• Chlorophyll absorbs only specific wavelengths
• The unstable excited electron falls back to its
  ground state emitting the excess energy as heat
  or light (fluoresence)

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Figure 10.10 Excitation of isolated chlorophyll
by light
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Figure 10.9 Location and structure of
chlorophyll molecules in plants
Photons are absorbed by clusters of pigment
molecules embedded in the thylakoid membrane
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Photosystems harvesting light energy

• Photosystem - cluster of a few hundred
  chlorophyll a, chlorophyll b, and carotenoid
  molecules
• The number and variety of pigments allows
  harvesting light over a large surface and large
  portion of the spectrum
• Photosystem I (P700) absorbs light of 700nm
  wavelength
• Photosystem II (P680) - absorbs light of 680nm
  wavelength
• Energy is passed from molecule to molecule until
  it reaches the reaction center chlorophyll
• A molecule called the primary electron acceptor
  traps the high-energy electron from the
  chlorophyll before it can return to the ground
  state

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Figure 10.11 How a photosystem harvests light
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Figure 10.12 Noncyclic electron flow generates
ATP and NADPH
Photosystem II absorbs light which excites an
electron that is eventually captured by the
primary electron acceptor. The chlorophyll,
missing an electron, is now a very strong
oxidizing agent.
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Figure 10.12 Noncyclic electron flow generates
ATP and NADPH
The electron from the chlorophyll is replaced by
electrons extracted by splitting water. In this
process, oxygen is released as a waste product.
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Figure 10.12 Noncyclic electron flow generates
ATP and NADPH
The excited electron is passed from photosystem
II to I by an electron transport chain consisting
of a mobile carrier plastoquinone (Pq), a complex
of two cytochromes, and a copper containing
protein plastocyanin (Pc).
Electrons move to lower energy levels, releasing
energy which is used by the thylakoid membrane to
produce ATP (photophosphorylation).
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Figure 10.12 Noncyclic electron flow generates
ATP and NADPH
The final electron acceptor of the ETC is the
reaction center chlorophyll of photosystem I
which has lost its excited electron to the
primary electron acceptor.
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Figure 10.12 Noncyclic electron flow generates
ATP and NADPH
The electron acceptor of photosystem I passes its
excited electron down another ETC which transmits
them to ferredoxin (Fd). NADP reductase
transfers the electrons from Fd to NADP
producing NADPH.
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Figure 10.13 A mechanical analogy for the light
reactions
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Figure 10.14 Cyclic electron flow
The Calvin cycle requires more ATP than NADPH, so
when ATP is low, excess NADPH can stimulate the
cyclic flow of electrons to make up the difference
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Figure 10.15 Comparison of chemiosmosis in
mitochondria and chloroplasts
Protons pumped in and leak out
Protons pumped out and leak in
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Figure 10.16 The light reactions and
chemiosmosis the organization of the thylakoid
membrane
At least 3 steps contribute to the proton gradient
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Figure 10.17 The Calvin cycle
3 x 1C
6 x 3C
3 x 5C
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Figure 10.17 The Calvin cycle
The carboxyl group is reduced to a carbonyl group
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Figure 10.17 The Calvin cycle
To produce one molecule of sugar, the calvin
cycle requires 9 ATP and 6 NADPH
Through a series of steps, the 5 G3P molecules
are rearranged to form 3 RuBP molecules
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Alternate Mechanisms

• Stomata allow exchange of materials
• Intake of CO2 and release of O2
• Transpiration evaporative loss of water
• Stomata close on a hot, dry day to conserve water
  thereby reducing photosynthetic yield
• Photorespiration- C3 plants use Rubisco
• Excess oxygen is added to the calvin cycle,
  producing a 2 carbon product which is then broken
  down to CO2
• Alternate strategies to fix carbon
• C4 photosynthesis (forms 4-carbon compound as its
  first product)
• CAM

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Figure 10.18 C4 leaf anatomy and the C4 pathway
In the mesophyll, CO2 is first added to PEP to
form a 4C molecule which is transported into
bundle-sheath cells where it releases CO2 and
enters the calvin cycle. This process maintains
CO2 concentration in the bundle-sheath, which
favors photosynthesis over photorespiration.
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Figure 10.19 C4 and CAM photosynthesis compared

• CAM crassulacean acid metabolism
• Succulent (water-storing) plants keep stomata
• Closed during the day to conserve water
• Open at night to take up CO2
• Incorporate CO2 into a variety of organic acids
  which are stored in mesophyll vacuoles until
  morning

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Figure 10.20 A review of photosynthesis
Thylakoid membrane
Stroma