Photosynthesis

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Photosynthesis

• Synthesizing carbohydrates through a series of
  Biochemical Reactions

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Figure 7.18 The chloroplast, site of
photosynthesis
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Key structures INSIDE chloroplasts

• Thylakoid saclike structure in chloroplasts
  made of photosynthetic membranes these sacs
  are made up of lipid bilayers
• Granum a stack of thylakoids
• Stroma region outside of the thylakoid
  membranes
• Chlorophyll molecules are embedded in the
  thylakoid membranes.

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Figure 10.9 Location and structure of
chlorophyll molecules in plants
The pigment molecules have a large head section
that is exposed to light in the surface of the
membrane the hydrocarbon tail anchors the
pigment molecules into the lipid bilayer.
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• There are 2 stages in Photosynthesis
• 1. Light dependent reactions
• 2. Light independent reactions
• (Calvin Cycle)

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Figure 8-7 Photosynthesis An Overview
Section 8-3
Chloroplast
Chloroplast
NADP
ADP P
Light- Dependent Reactions
Calvin Cycle
ATP
NADPH
Go to Section
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2 stages of Photosynthesis

• Stage 1 Light dependent reactions
• require presence of light
• occur in thylakoids of chloroplasts
• use energy from light to produce ATP and
  NADPH (a temporary, mobile energy source that
  helps store even more energy)
• oxygen gas is produced as a by-product

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Figure 10.4 An overview of photosynthesis
cooperation of the light reactions and the Calvin
cycle (Layer 1)
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Figure 10.4 An overview of photosynthesis
cooperation of the light reactions and the Calvin
cycle (Layer 2)
So what is happening in the thylakoids?
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First, a refresher on PIGMENTS

• Chlorophyll a - directly involved in
  transformation of photons to chemical energy
  (reds, blues, violets)
• Chlorophyll b - helps trap other wavelengths and
  transfers it to chlorophyll a (reds, blues,
  violets)
• Carotenoids cannot transfer sunlight energy
  directly to the photosynthetic pathway, but must
  pass their absorbed energy to chlorophyll. (reds,
  oranges, yellows)
• Chlorophyll c d found only in certain types of
  algae

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Figure 10.11 How a photosystem harvests light
Chlorophyll a
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Photosystems II I

• Pigments form aggregates on the thylakoid
  membrane called photosystems.
• The purpose of these photo systems is to collect
  energy over a "broad" range of wavelengths and
  concentrate it to one molecule called a reaction
  center which uses the energy to pass one of its
  electrons on to a series of enzymes.
• This aggregate of different proteins is called an
  antennae complex.

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Figure 10.13 A mechanical analogy for the light
reactions
There are two kinds of Photosystems in most
photosynthetic eukaryotes. When working together,
they absorb enough energy from the sun to split
an molecule of water.2H2O ? O2 4e- 4H

• Photosystem II
• When a photon of light strikes the reaction
  center of Photosystem II, it excites an electron.
  
• Two water molecules bind to an enzyme that splits
  water into hydrogen ions (aka protons) and
  releases an oxygen atom.
• Two electrons are released in this process, and
  these electrons can be traced through photosystem
  
• II and photosystem I.

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Figure 10.13 A mechanical analogy for the light
reactions

• Photosystem I
• The electron that was used in Photosystem II is
  just sitting around, all de-energized but its
  story is not finished.
• A small protein carries the electron to
  Photosystem I.
• Light absorbed by photosystem I energizes this
  electron and passes it to another primary
  electron acceptor.
• An enzyme transfers these electrons to NADP to
  form NADPH.
• The electron is now on its way
• to the Calvin Cycle as part of
• an NADPH molecule.

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• Photosystem II -- Light is absorbed by pigment.
  Energy is transferred to e-, which go into ETC
  (electron transport chain.) Hydrolysis breaks
  water up into e-, H, and O2
• ETC moves H ions from stroma into inner
  thylakoid.
• Photosystem I -- light is absorbed by pigments,
  energy goes to e-, NADPH is formed
• Hydrogen movement makes inside positively
  charged.
• As H diffuses through ATP synthase, ADP is made
  into ATP.

Figure 8-10 Light-Dependent Reactions
Section 8-3
Hydrogen Ion Movement
Photosystem II
ATP synthase
Inner Thylakoid Space
Thylakoid Membrane
Stroma
Electron Transport Chain
Photosystem I
ATP Formation
Go to Section
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• Stage 2 Light Independent Reactions
• do not require light also known as the
  Dark Reactions or the Calvin Cycle
• take place in the stroma of chloroplasts
• ATP and NADPH produced during light
  dependent reactions are used to make glucose

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Figure 10.x1 Melvin Calvin
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Figure 10.4 An overview of photosynthesis
cooperation of the light reactions and the Calvin
cycle (Layer 3)
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Figure 8-11 Calvin Cycle

• 6 CO2s combine with 6 5-C molecules make 12
  3-C molecules
• C. 2 of the 12 3-C molecules are made into
  glucose
• D. Other 10 3-C molecules are broken down into
  six 5-C molecules to start cycle over

Section 8-3
CO2 Enters the Cycle
Energy Input
ChloropIast
5-Carbon Molecules Regenerated
6-Carbon Sugar Produced
Sugars and other compounds
Go to Section
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Factors affecting Photosynthesis

• Amount of water available too little, stop
  photosynthesis
• Temperature best between Oo Celsius and 35o
  Celsius (too high, damage enzymes too low, stop
  photosynthesis)
• Intensity of light up to a point, increasing
  light intensity increases rate of photosynthesis