BIO 10 Lecture 8

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BIO 10 Lecture 8

• THE VITAL FORCE
• PHOTOSYNTHESIS

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• Photosynthesis the process by which living
  organisms harvest the energy of the sun to reduce
  CO2 and H2O to energy-rich carbohydrates
• Recall that reduction involves adding electrons
  to molecules and is endergonic (requires energy)

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• Only certain organisms can photosynthesize
• Plants
• Cyanobacteria
• Algae
• The entire ecosystem runs on the work done by
  these organisms
• Organisms like ourselves, who rely entirely on
  respiration for energy, can only do so because
  the photosynthesizing organisms make the
  energy-rich molecules for us
• We can then eat those molecules and break them
  down into useable energy "chunks" as ATP

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The Nature of Light

• Light is very mysterious
• Electromagnetic waves as well as quantum
  packets of energy called photons
• Light waves always travel at the same speed
  through a vacuum
• Doesnt travel with respect to you, the Earth, or
  anything else
• Doesnt wiggle anything
• Not at all like sound waves or water waves
• Wavelength determines the amount of energy
  carried by each photon

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• Light waves have different wavelengths in a
  spectrum from gamma rays (most energetic,
  shortest) to radio waves (least energetic,
  longest), only a portion of which are visible.
• Photosynthetic organisms are green because the
  pigment chlorophyll absorbs only certain
  wavelengths of light (mostly red and blue)
• The wavelengths it does not absorb are radiated
  back to our eyes and, collectively, these appear
  green to our eyes

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The Leaf Site of Photosynthesis in Plants
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Where Photosynthesis Takes Place
Stroma
Photosynthesis begins in the thylakoid membrane
-Contains the chlorophyll molecules on
Photosystems Photosynthesis ends in the stroma
(with starch) -The stroma surrounds the
thylakoids and is enclosed by the double
membrane of the chloroplast
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• Photosynthesis Occurs in Two Phases

Light Dependent Phase Step 1 Chlorophyll absorbs
a photon at Photosystem II and one of its
electrons is kicked off onto an electron carrier
molecule Step 2 Now lacking an electron,
chlorophyll grabs one from a water molecule in
the stroma, splitting the water into H and
O2 Step 3 The O2 diffuses out of the cell and is
expired by the plant Step 4 H begins to build
up in the stroma
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Light Dependent Phase Step 5 The electron
carrier molecule now transfers the electron it
received from chlorophyll down a transport chain,
similar to that in the mitochondrion Step 6 At
each step, the energy released by the transport
of the electrons is used to pump H ions against
their gradient into the stroma This process
produces an H gradient, with much higher
concentrations of H inside the stroma than
outside Step 7 The H ions flow out of the
stroma with their concentration gradient through
an ATP synthase enzyme pore, driving the
production of ATP from ADP P -This ATP will be
used in the Light Independent Reaction to
drive the synthesis of starch
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Light Dependent Phase Step 8 At the end of the
electron transport chain, the electrons are
dumped onto a second set of chlorophyll molecules
at Photosystem I Step 9 Light hits these
chlorophyll molecules and kicks the electrons off
them onto electron carriers Step 10 The
electrons are passed down a second electron
transport chain and are finally dumped onto NADP
to form NADPH, a molecule that will carry
electrons and hydrogens to the Light Independent
(starch-building phase) of photosynthesis

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ATP to Calvin cycle
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Movement of ATP NADPH to the Light Independent
Reaction  
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• Phase 2 Light-independent Reactions
• ATP and NADPH are not good permanent storage
  molecules, so the plants convert the energy into
  carbohydrates
• Electrons from carriers are added to CO2
  (breathed in by the plant) and H2O to reduce
  these molecules to starch
• Reactions occur in the stroma of the thylakoid
• Main cycle of reactions is called the Calvin Cycle

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Summary of Photosynthesis
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Chemical Equations

• Photosynthesis
• 6CO2 6H2O sunlight ? C6H12O6 6O2
• Respiration
• C6H12O6 6O2 ? 6CO2 6H2O ATP

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Short Review of Lecture 8

• Compare the light dependent and light independent
  reactions of photosynthesis. What goes into each
  reaction and what is produced by each?
• In what ways are the processes of photosynthesis
  and respiration similar? In what ways are they
  different?
• How can you account for the similarities between
  the two processes?