Ch.10 Photosynthesis

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Ch.10 Photosynthesis

• Sarah Burton and Lauren Thompson

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Plants and other autotrophs are the producers of
the biosphere

• Autotrophs- organisms that obtain organic matter
  without eating other organisms or substances
  derived from organisms.
• Photoautotrophs- organism that harnesses light
  energy to drive the synthesis of organic
  compounds from carbon dioxide.
• Chemoautotrophs- organism that needs only carbon
  dioxide as a carbon source but that obtains
  energy from oxidizing inorganic substances.
• Hertotrophs- organism that obtains organic food
  molecules by eating other organisms or their
  by-products.

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Chloroplasts are the sites of photosynthesis in
plants

• Chlorophyll- green pigment located within
  chloroplasts of plants.
• Parts of the Leaf
• Mesophyll- ground tissue of a leaf, sandwiched
  between upper lower epidermis specialized for
  photosynthesis.
• Vascular bundle- part of the transport system
  which exists and either the xylem and phloem.
• Stomata- microscopic pore surrounded by guard
  cells in the epidermis of leaves and stems that
  allows gas exchange.

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• Chloroplasts- organelle found only in plants
  photosynthetic protists that absorbs sunlight
  uses it to drive the synthesis of organic
  compounds from carbon dioxide water.
• Stroma- fluid in the chloroplast surrounding the
  thylakoid membrane involved in synthesis of
  organic molecules from carbon dioxide and water.
• Thylakoids- flattened membrane sac inside the
  chloroplasts, used to convert light energy to
  chemical energy.
• Grana- stacked proton of the thylakoid membrane
  in the chloroplast

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Photosynthesis Equation
6CO26H2OLight Energy?C6H12O66O2
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• Photophosphorylation- process of generating ATP
  from ADP and phosphate by means of proton motive
  force motivated by the thylakoid membrane of
  chloroplast during light reactions of
  photosythesis.
• NADPH2- acceptor that temporarily stores
  energized electrons produced during the light
  reactions.
• Carbon fixation- incorporation of carbon from CO2
  into an organic compound by an autotrophic
  organism.

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Calvin Cycle

• The Calvin cycle uses ATP and NADPH from the
  light-dependent reactions to convert CO2 into
  sugar that the plant can use. CO2 is obtained
  from the outside environment though gas-
  exchanging organs on the plants surface known as
  stomata. The process of carbon fixation
  incorporates the CO2 into organic molecules. The
  incorporation of CO2 is possible because of the
  energy-rich enzyme rubisco (ribulosebiphosphatecar
  boxylase, or RuBP), a protein made during the
  light-dependent reactions of photosynthesis and
  abundant in plant leaves. A CO2 molecule binds to
  RuBP. The molecule then splits into two 3-carbon
  molecules of PGA (3-phosphoglycerate). A series
  of reactions occur to convert the PGA into the
  3-carbon sugar molecule glyceraldehyde
  3-phosphate. This 3-carbon sugar molecule can
  then be used to make other sugars, including
  glucose and sucrose. The production of a single
  3-carbon sugar molecule requires 3 CO2, 9 ATP,
  and 6 NADPH.

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Wavelike properties of light

• Electromagnetic spectrum- entire spectrum of
  radiation ranging in wavelength for less than a
  nanometer to more than a kilometer.
• Visible light- portion of electromagnetic
  spectrum detected as various colors by the human
  eye, ranging in wavelength from about 380 nm to
  about 750 nm.

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Photosynthetic Pigments Light Receptors

• Pigments- material that changes the color of
  light it reflects as the result of selective
  color absorption
• Absorption spectrum- shows the fraction of
  incident electromagnetic radiation absorbed by
  the material over a range of frequencies.
• Action spectrum- rate of a physiological activity
  plotted against wavelength of light.

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• Accessory Pigments

• Chlorophyll a- type of blue green photosynthetic
  pigment that participates directly in light
  reactions

• Chlorophyll b- type of yellow green accessory
  photosynthetic pigment that transfers energy to
  chlorophyll a.
• Carotenoids- either yellow or orange in the
  chloroplast of plants broaden the spectrum of
  colors that can drive photosynthesis.

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Absorption Spectrum
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Photosystems Light-Harvesting Complexes of the
Thylakoid Membrane

• The light-harvesting (or antenna) complex of
  plants is an array of protein and chlorophyll mole
  cules embedded in the thylakoid membrane which
  transfer light energy to one chlorophyll a molecul
  e at the reaction center of a photosystem.
• The function of the reaction center chlorophyll
  is to use the energy absorbed by and transferred
  to it from the other chlorophyll pigments in the
  photosystemto undergo a charge separation, a
  specific redox reaction in which the chlorophyll
  donates an electron into a series of molecular
  intermediates called an electron transport chain.
• Primary electron acceptor a specialized
  molecule sharing the reaction center with
  chlorophyll a molecule it accepts an electron
  from chlorophyll a molecule.

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• Photosystem I- one of the two light harvesting
  units of a chloroplasts thylakoid membrane used
  the P700 reaction center chlorophyll.
• Photosystem II- one of the two light harvesting
  units of a chloroplasts thylakoid membrane used
  P680 reaction center chlorophyll.

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How a photosystem harvests light

• When a photon strikes a pigment molecule, the
  energy is passed from molecule to molecule until
  it reaches the reaction center.
• At the reaction center, an excited electron from
  the reaction-center chlorophyll is captured by a
  specialized molecule called the primary electron
  acceptor.

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Steps in creating NADPH

• Photosystem II absorbs solar energy in the form
  of light.
• The solar energy excites electrons in the
  reaction center of photosystem II, which then
  enter an electron transport chain. These
  electrons originate from the splitting of water,
  which produces free electrons and O2.
• As electrons pass down the electron transport
  chain, protons are pumped into the thylakoid
  membrane space of the chloroplast. Protons
  diffuse out of the thylakoid membrane space
  through an ATP synthase protein, creating ATP.
• PhotosystemI accepts electrons from the electron
  transport chain and uses light energy to excite
  the electrons further.

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Noncyclic Electron Flow

• Noncyclic electron flow a route of electron
  flow during the light reactions of photosynthesis
  that involves both photosystems and produces ATP,
  NADPH, and oxygen. The net electron flow is from
  water to NADP
• Noncyclicphotophosphorolation the production of
  ATP by noncyclic electron flow

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Cyclic Electron Flow

• Cyclic electron flow - a route of electron flow
  during the light reactions of photosynthesis that
  involves onlyphotosystem I and that produces ATP
  but not NADPH or oxygen
• Cyclic photophosphorolation - the generation of
  ATP by cyclic electron flow

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Alternative Mechanisms of Carbon Fixation Have
Evolved in Hot, Arid Climates

• Plants are divided into three different
  categories depending on their method of carrying
  out photosynthesis the C3 pathway, the CAM
  pathway, and the C4 pathway.

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C3 Plants

• Photorespiration presents a major problem for C3
  plants because they have no special adaptations
  to reduce the process. The problem is exacerbated
  in hot, arid climates, where the rate of
  photorespiration increases as the temperature
  goes up. Consequently, C3 plants are rarely found
  in these climates. Most plants, including wheat,
  barley, and sugar beet, are C3 plants.

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CAM Plants

• CAM (crassulacean acid metabolism) plants reduce
  photorespiration and conserve water by opening
  their stomata only at night. CO2 enters through
  the stomata and is fixed into organic acids,
  which are then stored in the cells vacuole.
  During the day, the acids break down to yield
  high levels of CO2 for use in the Calvin cycle.
  Through the periodic opening and closing of the
  stomata, CAM plants maintain a high CO2 to O2
  ratio, minimizing the rate of photorespiration.
  CAM plants, such as cacti and pineapple, are most
  common in dry environments.

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C4 Plants

• C4 plants use the enzyme PEP carboxylase to fix
  CO2 in the mesophyll cells of their chloroplasts.
  The fixed CO2 is then shuttled to specialized
  structures known as the bundle-sheath cells,
  where it is released and incorporated into the
  Calvin cycle. This process is energetically
  expensive, but it limits photorespiration by
  allowing high concentrations of CO2 to build up
  in the bundle-sheath cells. C4 plants, such as
  corn and sugar cane, are common in warm
  environments.

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Differences between C4 and CAM plants
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3 Pathways of Carbon Dioxide Fixation
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Overview of Photosynthesis

• Light Reactions
• Are carried out by molecules in the thylakoid
  membranes.
• Convert light energy to the chemical energy of
  ATP and NADPH.
• Split H2O and release O2 to the atmosphere

• Calvin Cycle Reactions
• Take place in the stroma
• Use ATP and NADPH to convert CO2 to the sugar G3P
• Return ADP, inorganic phosphate, and NADP to the
  light reactions