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Photosynthesis Light Reaction

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Any green part of a plant contains chloroplasts ... Chloroplast Structure. How Light is Absorbed. How it Works. The Light Reactions (Step 1) ... – PowerPoint PPT presentation

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Title: Photosynthesis Light Reaction


1
Photosynthesis(Light Reaction)
2
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.

3
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.

4
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.

5
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

6
Chloroplast Structure
7
How Light is Absorbed
8
How it Works
9
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10
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11
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.

12
Light Reaction Cont
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14
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.

15
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.

16
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

17
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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19
Overview of Light Reactions
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