Capturing Light Energy and the Electromagnetic Spectrum

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Capturing Light Energy and the Electromagnetic
Spectrum
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The Big Picture The Conversion
of Light Energy

• Light Reactions of Photosynthesis convert light
  energy (from the sun) into chemical energy (ATP
  and NADPH)
• Chloroplasts in plant cells act as chemical
  factories powered by the sun

• The thylakoids in the chloroplast convert the
  light energy into chemical energy
• To understand the conversion we are going to
  take a look at some important properties of
  light!

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Sunlight (Contains Visible Light)

• Sunlight is a form of electromagnetic energy!
• Photosynthesis transforms (converts)
  electromagnetic energy into chemical energy of
  organic molecules (sugars)
• Sunlight is part of the electromagnetic spectrum.

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Sunlight Contains Visible Light

• Sunlight (contains Visible Light) is a form of
  electromagnetic energy.
• Electromagnetic energy travels as a wave, but is
  composed of particle-like bundles of energy.
• Sunlight travels as a wave and contains bundles
  of energy known as photons.

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Sunlight Wave Behavior

• Wave Theory
• Light travels through space as a wave
• Waves have the following characteristics
• Wavelength- the distance between the crests of
  electromagnetic waves
• Energy- shorter the wavelength the greater the
  energy of each photon
• Frequency- a measure of the number of wavelengths
  in a given amount of time

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Sunlight Light Particle behavior

• Light behaves as though it consists of particles
  called photons
• Each photon has a fixed quantity of energy
• Amount of energy is inversely related to the
  wavelength of light
• The shorter the wavelength the greater the
  energy of each photon of that light
• Ex photon of violet light packs almost twice as
  much energy as a photon of red light

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Sunlight and The Electromagnetic Spectrum

• Electromagnetic Spectrum the entire range of
  
  radiation.
• Visible Light the segment most important to
  life
• the narrow band from about 380nm to 750nm in
  wavelength
• it is detected as various colors by the human eye
• The sun radiates the full spectrum of
  electromagnetic energy, but atmosphere is
  selective and allows only visible light to pass
  through.

Visible light part of the spectrum we can see
drives photosynthesis
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The Electromagnetic Spectrum
White light is a mixture of all wavelengths of
visible light. A prism can sort
white light into its component colors.
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Pigments

• Light may be either
• reflected, transmitted, or absorbed.
• Pigments- substances that absorb visible light.
• The color we see an object to be is the color
  most reflected or transmitted by the pigment.
• EX Leaves look green because the main pigment in
  them (chlorophyll) absorbs violet-blue and red
  light while it transmits/reflects green light.

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Absorption Spectrum

• Absorption Spectrum is a graph that plots a
  pigments light absorption versus wavelength of
  light.

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The three pigments shown below differ in the
colors of light they absorb.
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Absorption Spectrum

• The three pigments shown in the prior graph
  differ in the colors of light they absorb.
• Where a curve has a peak is where much of the
  light at that wavelength is absorbed
• Where there is a trough, much of the light at
  that wavelength is reflected or transmitted.
  (what we see)

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Reexamine the Absorption Spectrum
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Light and Pigments

• Pigments- compounds that absorb light/ most
  absorb certain colors more strongly than others.
• Several pigments are located in the membrane of
  the thylakoid
• Chlorophyll a
• Chlorophyll b
• Carotenoids

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Chlorophyll a

• Absorbs violet-blue and red light
• Allows green light to be reflected/transmitted
• Is the main pigment within the photosystems.
• Directly involved in the light reactions of
  photosynthesis

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Accessory Pigments

• Pigments with different absorption spectra than
  chlorophyll a.
• Are clustered with chlorophyll a within
  photosystems.
• Help the leaves to capture the maximum amount of
  light energy
• Chlorophyll b
• Carotenoids

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Chlorophyll b

• An accessory pigment
• Assists chlorophyll a in capturing light energy
• Almost identical to chlorophyll a but there is a
  slight structural difference
• Absorbs blue light

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Carotenoids

• Yellow, orange, and brown
• Accessory pigments
• Absorbs colors that chlorophyll a cannot
  enables plant to capture more energy
• In fall when plants lose chlorophylls their
  leaves take on the rich hues of the carotenoids

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When Pigments Absorb Light

• Colors corresponding to the absorbed wavelengths
  disappear from the spectrum
• When a molecule absorbs a photon of light, one of
  the molecules electrons is elevated (excited) to
  where it has more potential energy
• Each pigment absorbs only photons corresponding
  to specific wavelengths, which is why each
  pigment has a unique absorption spectrum
• Excited state is unstable so the excited
  electrons drop back down releasing their excess
  energy.
• As excited electrons drop back down, energy is
  given off.

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Photosystems

• Cluster of pigments and proteins embedded within
  the thylakoid membrane.
• Contain accessory pigments that surround
  chlorophyll a, the main pigment of
  photosynthesis.
• Are able to absorb the maximum amount of light
  possible due to the accessory pigments and
  chlorophyll a.

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Photosystems

• Photosystems are found embedded within the
  thylakoid membrane of the chloroplast.

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Photosystems Are embedded in the Thylaoid
Membrane of the Chloroplast.
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Next time Photosystems and Chloroplasts!!!