Photosynthesis Light Reaction

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Photosynthesis(Light Reaction)
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Introduction to Photosynthesis

• Life is solar powered.
• photosynthesis captures light energy from the sun
  and converts into chemical energy that is stored
  in sugars and other organic molecules, ATP..
• Photosynthesis nourishes almost all of the living
  world directly or indirectly. Some Bacteria do
  not need it.
• Photosynthesis has 3 stages. 1. Energy is
  captured from the sun light. 2. Light energy is
  converted into chemical energy which is stored as
  ATP and NADPH. 3. The energy stored in ATP and
  NADPH powers the formation of organic compounds
  using CO2.

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Cont.

• Autotrophs are the producers of the biosphere
  they get their energy from in organic sources
  like light.
• There are photoautotrophs and chemoautotrophs.
• Heterotrophs live on organic compounds produced
  by other organisms, autotrophs. Consumers.
• Almost all heterotrophs are entirely relianton
  photoautotrophs for food and for oxygen
• So the circle of life starts with photosynthesis.

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Where Does Photosynthesis Occur

• Occurs in the chloroplasts of plant or
  photosynthetic organism cells.
• Any green part of a plant contains chloroplasts
• But, leaves are the major site of photosynthesis
  for most plants.
• There are about 500,000 chloroplasts per square
  millimeter of leaf surface.
• color of a leaf comes from chlorophyll, the green
  pigment in the chloroplasts. It reflects green
  wavelengths of light.
• Chloroplasts are found mainly in mesophyll, the
  middle tissue of a leaf.

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Cont.

• O2 exits and CO2 enters the leaf via tiny pores,
  stomata, in the leaf.
• Veins carry water from the roots and carry off
  sugar from mesophyll cells to other plant areas

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Chloroplast Structure
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How Light is Absorbed
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How it Works
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The Light Reactions (Step 1)

• The thylakoids convert light energy into the
  chemical energy of ATP and NADPH.
• How When a molecule absorbs a photon of light,
  one of that molecules electrons is elevated to
  an orbital with more potential energy so it is
  now Excited.
• Photons get absorbed by clusters of pigment
  molecules in the thylakoid membranes called
  photosystems.
• chlorophyll is organized along with proteins and
  smaller organic molecules into photosystems.
• photosystems act as a light-gathering antenna
  complex made of chlorophyll a, chlorophyll b,
  and carotenoid pigment molecules.

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Light Reaction Cont
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Light Reaction Cont

• When any antenna molecule absorbs a photon, the
  photon gets passed from molecule to molecule
  until it arrives at a specific chlorophyll a
  molecule, the reaction center.
• Here it finds a primary electron acceptor that
  removes the excited electron from the reaction
  center or chlorophyll a molecule. This starts the
  light reactions
• Each photosystem - - reaction-center chlorophyll
  and primary electron acceptor surrounded by an
  antenna complex - works in the chloroplast as a
  light-gathering unit.

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Light Reaction Cont..

• 2 Kinds of Photosystems Photosystem I has a
  reaction center chlorophyll, the P700 center,
  that has an absorption peak at 700nm. Photosystem
  II has a reaction center with a peak at 680nm.
• They work together to use light energy to make
  ATP and NADPH.
• There are 2 routes the excited electron can take
  the Non-Cyclic path and the Cyclic path the
  non-cyclic is more common so we will focus on it.
  

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Light Reaction Cont. Non-Cyclic Path

• Main route, produces both ATP and NADPH.
• Step 1. photosystem II absorbs light, the excited
  electron is captured by the primary electron
  acceptor, and departs from the reaction center
• Step 2. An enzyme splits water, converting into 2
  H ions and O2. The Hydrogen replaces the
  electrons that just left and the oxygen is
  released as a byproduct, lucky for us.
• Step 3. The excited electrons pass along an
  electron transport chain, ETC, before ending up
  at the photosystem I reaction center

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Light Reaction Cont. Non-Cyclic Path

• Step 4. As these electrons move along the ETC
  their energy is used to make ATP.
• Step 5. The electrons gather in the P700 center
  of photosystem I and are then captured by a 2nd
  primary electron acceptor which sends them down
  another ETC.
• Step 6. these electrons are passed from the
  transport chain to NADP, creating NADPH.
• NADPH will carry these high-energy electrons to
  the Calvin cycle. Also know as Dark Reactions
  since they do not need light to run.

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Overview of Light Reactions