Photosynthesis: Using Light to Make Food

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Photosynthesis Using Light to Make Food

• Chapter 7

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Photosynthesis

• The process by which plants use light energy to
  make sugars and other organic food molecules from
  carbon dioxide and water
• Most important chemical process on earth because
  it provides food for virtually all organisms
• For photosynthesizers and those that eat them

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Photosynthesis

• Autotrophic (self-feeder) organisms use light
  energy to make sugar and oxygen gas from carbon
  dioxide and water

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Autotrophs

• Sustain themselves without eating other organisms
  or organic molecules
• Primary producers of food in the biosphere
• On land, plants such as oak trees and cacti are
  the predominant producers
• In aquatic environments, algae and photosynthetic
  bacteria are the main food producers

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Photosynthesis and Chloroplasts

• In most plants, photosynthesis occurs primarily
  in the leaves, in the chloroplasts
• A chloroplast contains
• stroma, a fluid
• grana, stacks of thylakoids
• The thylakoids contain chlorophyll
• Chlorophyll is the green pigment that captures
  light for photosynthesis

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Chloroplasts
stomata
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Photosynthesis

• The O2 liberated by photosynthesis is made from
  the oxygen in water

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Photosynthesis is a redox reaction

• When the water molecules are split apart they are
  actually oxidized
• They loose electrons along with hydrogen atoms
• These electrons and H ions are transferred to
  CO2, producing sugar
• The CO2is reduced because it gains electrons and
  H ions

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Photosynthesis and Cellular Respiration are redox
reactions
Photosynthesis
Respiration
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Photosynthesis

• 2 stages, each with multiple steps
• Light reactions- convert light energy to chemical
  energy and produce 02 waste
• Calvin Cycle- assemble sugar molecules using CO2
  and the energy products from the light cycles

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The Light Reactions of Photosynthesis

• Occur in the thylakoid membranes of the
  chloroplasts grana
• Light energy is used to make ATP
• The ATP is used to drive the transfer of an
  electron from water to NADP, to make NADPH
• Temporarily stores energy as NADPH and water is
  split giving off O2
• Absorb solar energy and convert it to chemical
  energy stored in ATP and NADPH

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The Calvin Cycle

• Occurs in the stroma of the chloroplast
• Carbon fixation- incorporation of CO2 into
  organic compounds
• After incorporation of CO2 enzymes in the cycle
  make sugars by reducing the carbon with the H
  provided by NADH and using the energy provided by
  ATP
• Does not require light to run, but needs products
  provided by the light reactions

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Photosynthesis
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Visible Radiation

• Sunlight gives off energy in radiation at
  different wavelengths
• As sunlight shines on a leaf, certain
  wavelengths of light are absorbed and put to use
  in photosynthesis
• Pigments in the leaves absorb the wavelengths
• Mainly blue-violet and red-orange are absorbed
• Green wavelengths are transmitted and reflected
  by the pigments

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Chlorophyll and Chloroplasts

• Chlorophyll a- blue and red light, directs energy
  to the light reactions
• Chlorophyll b- blue and orange light, directs
  energy to chlor. a
• Broadens range of useable wavelengths

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Key steps of the light reactions

• Absorption of light energy
• By the photosystems and chlorophyll
• Excitation of electrons
• Electrons of chlorophyll a
• Formation of ATP and NADPH
• By using an electron transport chain

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How photosystems capture solar energy

• Light energy is in packets called photons
• Pigments absorb the photons and the pigments
  electrons gain energy
• Very unstable state, so it transfers the energy
  to an electron acceptor
• This acceptor carries the energy to other
  compounds in the next phase

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Photosystems

• Reaction center- chlor. a, and primary electron
  acceptor
• Antenna molecules- chlor. b, carotenoid pigments
• Photosystem I and PS II- gather light at
  different wavelengths

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Formation of ATP and NADPH

• Similar to the electron transport chains in
  cellular respiration
• Series of oxidation/reduction reactions shuttle
  the electron from one carrier to the next
• PS II captures light and receives an electron
  from H2O
• Electron is excited by light energy and passed
  down ETC to PS I
• It is excited again, passed along an ETC and
  made into NADPH

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Formation of ATP and NADPH

• The electron transport chains are arranged with
  the photosystems in the thylakoid membranes and
  pump H through that membrane
• The flow of H back through the membrane is
  harnessed by ATP synthase to make ATP
• In the stroma, the H ions combine with NADP to
  form NADPH

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Formation of ATP and NADPH
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Calvin Cycle

• Takes CO2 from the air and ATP and NADPH from the
  light reactions and converts it to the
  energy-rich sugar molecule G3P (glyceraldehyde
  3-phosphate)
• The Calvin cycle occurs in the chloroplasts
  stroma
• This is where carbon fixation takes place and
  sugar is manufactured

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Calvin Cycle

• 3 molecules of CO2 fixed using the enzyme rubisco
  to form RuBP (ribulose biphosphate)
• 6 molecules of 3-PGA (3-phosphoglyceric acid) are
  formed
• ATP and NADPH are used to reduce the 6 3-PGA to 6
  G3P
• One G3P is released and the go back into the
  cycle and are converted to the rubisco
• Cycle goes around twice to produce 2 G3P
• 2 G3P make one glucose

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Calvin Cycle
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Overview
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Photorespiration

• Most plants are C3 plants, which take CO2
  directly from the air and use it in the Calvin
  cycle
• In these types of plants, stomata on the leaf
  surface close when the weather is hot
• This causes a drop in CO2 and an increase in O2
  in the leaf (reduces loss of water)
• Photorespiration may then occur and produce CO2
  for the Calvin cycle

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Photorespiration

• Rubisco incorporates O2 instead of CO2
• Makes a 2 carbon compound
• Breaks the 2 carbon compound into CO2 and water
• Runs the Calvin cycle with the CO2

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C4 Plants

• Some plants have special adaptations that enable
  them to save water
• Special cells in C4 plantscorn and
  sugarcaneincorporate CO2 into a four-carbon
  molecule
• This molecule can then donate CO2 to the Calvin
  cycle

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Global Warming and the Greenhouse Effect

• Due to the increased burning of fossil fuels,
  atmospheric CO2 is increasing
• CO2 warms Earths surface by trapping heat in the
  atmosphere
• This is called the greenhouse effect
• Because photosynthesis removes CO2 and moderates
  the greenhouse effect
• Deforestation may cause a decline in global
  photosynthesis

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CAM Plants

• The CAM plantspineapples, most cacti, and
  succulentsemploy a different mechanism
• They open their stomata at night and make a
  four-carbon compound
• It is used as a CO2 source by the same cell
  during the day