www.soulcare.orgSid Galloway

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Photosynthesis 6.1 Light Cycle6.2 Dark Cycle
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Autotrophs make their own food, and most use
photosynthesis to do it. Plants are the most
common, but algae and some bacteria use it.
All life depends on autotrophs (directly or
indirectly) for food. Nearly all living things
obtain energy either directly (like plants) or
indirectly (like heterotrophs that eat plants)
from the suns energy captured during
photosynthesis.
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Photosynthesis (Simply Summarized) Leaves of
plants have chloroplasts that are filled with
chlorophyll, which capture the energy from the
sunlight). Water enters through the roots.
CO2 enters through stomata openings on the
underside of the leaves. Water and CO2 move to
the chloroplasts. Chemical reactions there,
produce (O2) and sugars like glucose (C6H12O6).
Cells then use the energy in the sugars
(carbohydrates) to function.
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Photosynthesis

• Plant chloroplast capture light energy and
  converts it to chemical energy, which is stored
  in the bonds of sugar and other organic molecules
  synthesized from carbon dioxide and water
• This captured light energy is converted and
  stored as chemical energy know as photosynthesis

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Water
Sun Light
Carbon Dioxide
Photosynthesis
Sugar
Oxygen
Cellular Respiration
Energy - ATP
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(Lab) Chloroplasts in Elodea
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Chloroplast

• Chloroplasts are the site of photosynthesis
• Are found in all green plant parts
• The leaves are the major site of photosynthesis
  in most plants
• Chlorophyll is green pigment in chloroplast that
  gives leaf its color
• Chloroplast are found primarily in cells of the
  mesophyll ( of the leaf)
• Each mesophyll contains 30-40 chloroplast

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Parts of Chloroplast 3

• Stroma fluid filled space outside thylakoid,
  Calvin cycle occurs here
• Thylakoid flattened membranous sac inside
  chloroplast, light reaction occurs here.
  
  Grana whole stack, while thylakoid is
  each individual
• Intermembrane space a double membrane that
  partitions its contents from cytosol

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Formula for Photosynthesis
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ATP Molecule
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• The overall photosynthetic equation has been
  known since the early 1880s
• Glucose is the major product of photosynthesis
• Water appears on both sides of the equation
  because 12 molecules are consumed and 6 newly
  formed during photosynthesis

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• The discovery in 1930s that oxygen given off by
  plants is derived from water and not from carbon
  dioxide was one of the earliest clues to the
  mechanisms of photosynthesis and indicates that
  chloroplasts split water into hydrogen and oxygen
• Products of photosynthesis are glucose, oxygen,
  water and ATP

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2 Stages of Photosynthesis

• 1. Light Reaction occurs in the thylakoid
• 2. Calvin Cycle occurs in stoma, also called
  dark reaction

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Light Reaction

• Occurs in Thylakoid
• Involves the conversion of light energy into
  chemical energy (photolysis)
• Water is split during this process and oxygen is
  released as a by productreason plants make oxygen

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• The coenzyme NADP picks up hydrogen and
  electrons from the split water molecules and
  stores them for use in the Calvin cycle (dark
  reaction)
• ATP is also generated and stoed for energy for
  the dark reaction

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The Light Reaction in the Thylakoid Membrane
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The Light Reaction in the Thylakoid Membrane
Primary electron acceptor
Primary electron acceptor
Photosystem II
Electron Transport Chain
Photosystem I
Electron Transport Chain
Animation here http//www.stolaf.edu/people/giann
ini/flashanimat/metabolism/photosynthesis.swf
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Electron flow

• 2 possible routes for electrons to flow during
  light reaction
• 1. Cyclic flow simplest pathway, generates ATP
  only
• 2. Noncyclic flow light reaction usually
  happens this way NADP is formed

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Calvin Cycle ( dark reaction)

• During this cycle, carbon dioxide from the air is
  fixed to a 5 carbon sugar (ribulose bisphosphate
  ) by the addition of electrons from NADP from
  the light reaction ( this is called carbon
  fixation) resulting in a new unstable 6 carbon
  sugar

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• This unstable 6 carbon sugar begins going through
  various chemical reactions
• The 2 necessary components for carbon fixation to
  occur here are an electron source (NADP) and an
  energy source (ATP)both supplied by the light
  reaction
• Water is released as a by-product

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• Glucose is made as the plant energy/food supply
• Ribulose bisphosphate is the 5 carbon sugar made
  at the end of the cycle and is again used to
  combine with carbon dioxide for carbon fixation
• 9 molecules of ATP and 6 molecules of NADPH are
  needed to synthesis one glucose molecule

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• Also called dark reactiondoes not require light
  directly, but need the products of light reaction
  to occur.

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Three Steps of the Calvin Cycle a. CO2 combines
with RuBP to form two molecules of PGA.b. Each
molecule of PGA is converted into a molecule of
PGAL.c. Most of the PGAL is converted back
into RuBP, but some PGAL can be used later to
make different organic compounds. RuBP
five-carbon carbohydrate. PGA and PGAL are
both three-carbon molecule C3 Plants (produce
the 3-Carbon PGA), and use only the Calvin
Cycle for carbon fixation.
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The Two Processes Visualized
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Calvin Cycle
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Calvin Cycle
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Calvin Cycle
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Photosynthetic pigments

• Pigments substances that absorb visible light
• Different pigments absorb light of different
  wavelengths

Types of pigments include Chlorophyll A ( bright
green)
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Types of pigments include

• Chlorophyll A ( bright green)
• Chlorophyll B ( yellow green)
• Chlorophyll contains the ion magnesium
• Carotene faint yellow
• Xanthrophyll yellow
• Anthrocyanin red

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- Accessory Pigments indirectly assist other
pigments. ( Chlorophyll b is an accessory
pigment assisting a.) Carotenoids are other
accessory pigments (yellow, orange, brown,
etc.) (They absorb blue and green
light.) (In the fall, plant leaves turn color
because they lose chlorophylls, which reflect
green light.)
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Photosystems

• Found in the thylakoid membrane of chloroplast
• Only 1 chlorophyll a ( photosynthetic pigment) is
  needed to start the light reaction
• All of the other chlorophyll a molecules,
  chlorophyll b molecules are carotenes function as
  light gathering antenaes

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• The entire complex of light gathering molecules
  is called a photosystem.
• There are 2 types of photosystems
• 1. photosystem I called P700..absorbs light up
  to 700nm wavelength
• 2. Photosystem II called P600absorbs light up
  to 600 nm wavelength

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Electron Transport System of the light
reactions. Photosystem cluster of pigment
molecules grouped in the thylakoid membrane. Two
types Photosystem II Actually begins the
process Photosystem I Called 1 is believed
to have evolved first. Accessory pigment
molecules start the light reactions by absorbing
light energy. (It is passed to other pigment
molecules until it reaches chlorophyll a
molecules.)
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Alternative Pathways First, remember that C3
Plants are those that only use the Calvin Cycle
to fix carbon. - They are called C3 plants, since
they fix CO2 into a compound with 3 carbons
(PGAL). Others in hot, dry climates
supplement the Calvin cycle with alternatives (C4
or CAM), because their stomata openings in the
leaves must close to preserve moisture, so the
amount of CO2 they absorb is reduced.
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C4 Pathway use an enzyme which fixes CO2 into
compounds with 4 carbons, which are then
transported to other cells where CO2 is available
to then use the Calvin Cycle.(corn, sugar cane,
are examples) CAM Pathway These plants open
the stomata only at night to reduce water
loss.They take in and fix it into compounds,
which then release it during the day for use in
the Calvin Cycle.(cactuses, pineapples, etc.)
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Rate of Photosynthesis Increases as either
light intensity or CO2 increase, but eventually
plateaus at a maximum. Increases as the
temperature increases, up to a certain
temperature.(Beyond a certain high temperature,
the rate of photosynthesis decreases.)