An Overview of Photosynthesis

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An Overview of Photosynthesis

• Jim Zubricky
• EDCI 367 - W 930-1220
• Wednesday, 2 October 2002

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Objectives

• Summarize how energy is captured from sunlight in
  the first stage of photosynthesis
• Analyze the function of electron transport chains
  in the second stage of photosynthesis
• Relate the Calvin cycle to carbon dioxide
  fixation in the third stage of photosynthesis
• Identify three environmental factors that affect
  the rate of photosynthesis

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Overview of Photosynthesis

• Stage 1 Energy is captured from sunlight.
• Stage 2 Light energy is converted to chemical
  energy
• Stage 3 Chemical energy powers the formation of
  organic compounds.
• This can be summarized in the following equation
  
• 6CO2 6H2O -light-gt C6H12O6 6O2

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Stage 1

• The chemical reactions in Stages 1 and 2 are
  called the light reactions
• They are dependent on light - so if light is not
  present, they cant occur.
• Light energy is used to make energy-storing
  compounds
• Light is a form of radiation
• Its energy in the form of waves that travel
  through space.
• You see it as visible light - the wavelengths of
  visible light are seen as different colors

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Stage 1 (2)

• So how does a leaf absorb light?
• Leaves have pigments that absorb only certain
  wavelengths and reflect all the others.
• Chlorophyll is the primary pigment involved in
  photosynthesis
• It absorbs mostly blue and red light and reflects
  green and yellow.
• This is why most plants look green and yellow

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Stage 1 (3)

• Plants contain two types of chlorophylls
  (chlorophyll a and chlorophyll b)
• The pigments that produce the yellow and orange
  fall leaf colors are carotenoids.
• They absorb wavelengths of light different from
  those absorbed by chlorophyll
• What does this mean?
• If both work together, they can absorb MORE light
  energy during photosynthesis than just one
  separately.

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Stage 1 (4)

• The pigments involved in plant photosynthesis are
  located in the chloroplasts of leaf cells
• Clusters of these pigments are called thylakoids
• Disk-like structures
• When light strikes one of these, energy is
  transferred to electrons in chlorophyll and other
  pigments
• This causes the electrons to jump to a higher
  energy level (theyre excited)
• This is how electrons capture energy from sunlight

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Stage 1 (5)

• The excited electrons jump from one chlorophyll
  molecule to others in the thylakoids
• These electrons power the second stage
• The leaving electrons from the chlorophylls have
  to be replaced
• Theyre replaced from water (H2O) molecules
• Water molecules are split apart by an enzyme, and
  the electron from the Hydrogen atom joins the
  chlorophyll, leaving a H ion.
• Remaining oxygen atoms combine to form O2

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Stage 2 (1)

• Excited electrons that leave chlorophyll
  molecules are used to produce new molecules that
  temporarily store chemical energy
• This includes ATP
• An electron jumps from one molecule in one
  thykaloid to another molecule in a nearby
  thykaloid so that it looks like the electron is
  being passed from person to person
• This series of molecules passing electrons along
  the thykaloid membrane is called electron
  trnasport chains

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Stage 2 (2)

• How are electron transport chains used to make
  molecules that store energy?
• One type of thykaloid membrane contains a protein
  that acts as a pump
• Excited electrons lose energy once they go
  through this pump
• This lost energy is used by the pump to push in
  the hydrogen ions into the membrane
• H becomes concentrated in the membrane,
  producing a concentration gradient

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Stage 2 (3)

• H ions will leak (or diffuse) out out of the
  thylakoid down their concentration gradient
  through a specialized protein
• As this happens, it passes a channel in which a
  chemical reaction occurs that adds a phosphate
  group (PO3-) to a molecule of ADP..this forms
  ATP
• This powers the third stage

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Stage 2 (4)

• A second electron transport chain is used to add
  a H to NADP to form NADPH
• NADPH is an electron carrier that provides the
  high energy electrons needed to make
  carbon-hydrogen bonds in stage 3

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Summary Thus Far

• Pigment molecules in the thylakoids of
  chloroplasts absorb light energy
• Electrons in the pigments are excited by light
  and move through electron trnasport chains in
  thylakoid membranes
• These electrons are replaced by electrons from
  water molecules
• Oxygen gas forms
• H ions accumulate, sets up a concentration
  gradient, provides energy to make ATP and NADPH

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Third Stage (1)

• In this stage, carbon atoms from CO2 are used to
  make organic compounds
• The transfer of carbon dioxide to organic
  compounds is called carbon dioxide fixation
• Also called the light independent reaction
• How is carbon dioxide fixed?

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Calvin Cycle

• Most common way is the Calvin Cycle
• Calvin Cycle is a series of enzyme-assisted
  chemical reactions that produce a 6 carbon sugar
• There are 4 steps

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Calvin Cycle

• Step 1 Each molecule of CO2 is added to a
  five-carbon compound by an enzyme
• Step 2 The resulting 6 carbon compound splits
  into two 3-Carbon compounds
• Phosphate groups from ATP and NADPH break off and
  add themselves to the carbon compounds
• Step 3 One of the sugars is used to make
  organic compounds
• Step 4The other is used to regenerate the cycle

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Requirements for the Calvin Cycle

• 3 molecules of CO2 to make the organic compounds
• ATP and NADPH - from step 2

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Factors that affect Photosynthesis

• Light
• Rate of photosynthesis increases as light
  intensity increases UNTIL all the pigment has
  been used.
• Carbon dioxide
• Rate of photosynthesis will increase until the
  plant is bombarded with CO2
• Temperature
• Many temperature-dependent enzymes work in this
  process