Photosynthesis Chapter 10

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PhotosynthesisChapter 10

• P. Biology
• Liberty Senior High School
• Mr. Knowles

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Question

• Does eating carrots really improve your vision?

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• Photosynthesis
• Occurs in plants, algae, certain other protists,
  and some prokaryotes (photoautotrophs).

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• Plants are photoautotrophs
• They use the energy of sunlight to make organic
  molecules from water and carbon dioxide

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Chloroplasts The Sites of Photosynthesis in
Plants

• Leaves are the major sites of photosynthesis

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• Chloroplasts
• Are the organelles in which photosynthesis
  occurs contain thylakoids and grana

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Chloroplasts
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Photosynthesis

• lt 1.0 of suns energy that strikes Earth is
  converted into 150 billion metric tons of sugar.
• In cellular respiration, electrons are
  transferred from sugar to oxygen they lose
  potential energy to make ATP.
• In photosynthesis, electrons from water gain
  potential energy as they are transferred to
  carbon dioxide to make sugar the energy is
  provided by light.

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C6H12O6 O2
Free Energy ? G
CO2 H2O
Rxn Time
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Historical Perspective

• 17th Century van Helmont measured willow tree
  growth compared mass of tree to mass of soil.
• Late 18th Century J. Priestly restores air in
  a vacuum with mint.
• 18th Century Ingenhousz found air only restored
  in the presence of sunlight hypothesized CO2 ? C
  O2

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Role of Water

• 1930s Van Niel studied purple sulfur bacteria
  that performed photosynthesis w/o water
• CO2 2 H2S Light Energy? (CH2O) H2O 2 S
• The H2S serves an electron donor.
• H2O serves as donor in green plants.

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Role of Water

• In the 1950s used 18O ( an isotope of oxygen )
  in water to trace oxygen in the reaction.
• 6 CO2 12 H218O Light Energy ? C6H12O6 6
  18O2 6 H2O

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The Role of Light

• In 1900s, F. F. Blackman determined that
  photosynthesis occurred in two steps.
• Initial set of reactions were dependent on light-
  Light Reactions or Light-Requiring Reactions.
• A second set of reactions were independent of
  light-Dark Reactions or Non-Light Requiring
  Reactions- affected by temperature.
• Dark Reactions use enzymes.

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The Splitting of Water

• Chloroplasts split water into
• Hydrogen and oxygen, incorporating the electrons
  of hydrogen into sugar molecules

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The Biophysics of Light

• Light consists of units of energy- Photons.
• Not all photons have the same amount of energy.
• Photons travel in waves higher the energy the
  shorter the wavelength (measured in nm, lambda).

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• The electromagnetic spectrum
• Is the entire range of electromagnetic energy, or
  radiation

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• Reflect light, which include the colors we see

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Absorption Spectrum

• The chemical nature of the molecule light hits
  determines if the energy is absorbed.
• Each molecule has a characteristic range of
  photons absorption spectrum it can absorb.

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• Spectrophotometer
• Is a machine that sends light through pigments
  and measures the fraction of light transmitted at
  each wavelength.

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• An absorption spectrum (with a spectrophotometer)
• Is a graph plotting light absorption versus
  wavelength

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Energy Absorption

• Pigments are molecules that absorb visible light
  well.
• Two kinds of photosynthetic pigments

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Carotenoids

• Carotenoids- yellow and orange pigments that are
  highly efficient at absorbing a broad range of
  energies. Ex. Beta-carotene in carrots.
• Ex. Xanthophylls are an oxygenated form of
  carotene. These are accessory pigments.
• Other carotenoids are found in petals, birds and
  shrimp

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• Accessory pigments
• Absorb different wavelengths of light and pass
  the energy to chlorophyll a

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Beta-Carotene
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Chlorophylls

• Cholorophylls- chlorophyll a and b- absorb narrow
  ranges of spectrum but highly efficient.
  Chlorophyll a is the major photosynthetic
  pigment.

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• Chlorophyll a
• Is the main photosynthetic pigment
• Chlorophyll b
• Is an accessory pigment

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• The absorption spectra of three types of pigments
  in chloroplasts

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• The action spectrum of a pigment
• Profiles the relative effectiveness of different
  wavelengths of radiation in driving photosynthesis

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• The action spectrum for photosynthesis
• Was first demonstrated by Theodor W. Engelmann

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Excitation of Chlorophyll by Light

• When a pigment absorbs light
• It goes from a ground state to an excited state,
  which is unstable

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• If an isolated solution of chlorophyll is
  illuminated
• It will fluoresce, giving off light and heat