CONCEPT 10.3: The light reactions convert solar energy to the chemical energy of ATP
and NADPH
• Chloroplasts are solar-powered chemical factories
• Their thylakoids transform light energy into the chemical energy of ATP and NADPH
The Nature of Sunlight
• Light is electromagnetic energy, also called electromagnetic radiation
• Electromagnetic energy travels in rhythmic waves
• Wavelength is a measure of the distance between crests of electromagnetic waves
• It can range from less than a nanometer (gamma rays) to more than a kilometer (radio
waves)
• The electromagnetic spectrum is the entire range of electromagnetic energy, or radiation
• Visible light (wavelengths 380 nm to 740 nm) drives photosynthesis and produces the
colors seen by the human eye
• Light also behaves as though it consists of discrete particles, called photons
• Each photon has a fixed quantity of energy which is inversely related to the wavelength
of light; shorter wavelengths have more energy per photon of light
Photosynthetic Pigments: The Light Receptors
• Pigments are substances that absorb visible light
• Different pigments absorb different wavelengths, and the wavelengths that are absorbed
disappear
• Wavelengths that are not absorbed are reflected or transmitted
– For example, most leaves appear green because chlorophyll absorbs violet-blue
and red light while reflecting and transmitting green light
–
• A spectrophotometer measures a pigment’s ability to absorb various wavelengths
, • It sends light through pigments and measures the fraction of light transmitted at each
wavelength
• An absorption spectrum is a graph plotting a pigment’s light absorption versus
wavelength
• Three types of pigments in chloroplasts include:
– Chlorophyll a, the key light-capturing pigment that participates directly in light
reactions
– Chlorophyll b, an accessory pigment
– Carotenoids, a separate group of accessory pigments
• The absorption spectrum of chlorophyll a indicates that violet-blue and red light will
work best for photosynthesis, while green is the least effective
• The action spectrum for photosynthesis, a profile of the relative effectiveness of
different wavelengths, confirms the effectiveness of violet-blue and red light
• The action spectrum of photosynthesis was first demonstrated in 1883 by Theodor W.
Engelmann
• He exposed different segments of a filamentous alga to different wavelengths of light and
used the growth of aerobic bacteria as a measure of O2 production
• The action spectrum for photosynthesis is broader than the absorption spectrum of
chlorophyll
• Accessory pigments, such as chlorophyll b, broaden the spectrum used for photosynthesis
• The difference in the absorption spectrum between chlorophyll a and b is due to a slight
structural difference between the pigment molecules
• In the last decade, two other forms of chlorophyll were discovered—chlorophyll d and
chlorophyll f—that absorb higher wavelengths of light
• The cyanobacterium, Chroococcidiopsis thermalis, uses chlorophyll f in place of
chlorophyll a in shaded conditions
• Other accessory pigments called carotenoids, are yellow or orange because they absorb
violet and blue-green light
and NADPH
• Chloroplasts are solar-powered chemical factories
• Their thylakoids transform light energy into the chemical energy of ATP and NADPH
The Nature of Sunlight
• Light is electromagnetic energy, also called electromagnetic radiation
• Electromagnetic energy travels in rhythmic waves
• Wavelength is a measure of the distance between crests of electromagnetic waves
• It can range from less than a nanometer (gamma rays) to more than a kilometer (radio
waves)
• The electromagnetic spectrum is the entire range of electromagnetic energy, or radiation
• Visible light (wavelengths 380 nm to 740 nm) drives photosynthesis and produces the
colors seen by the human eye
• Light also behaves as though it consists of discrete particles, called photons
• Each photon has a fixed quantity of energy which is inversely related to the wavelength
of light; shorter wavelengths have more energy per photon of light
Photosynthetic Pigments: The Light Receptors
• Pigments are substances that absorb visible light
• Different pigments absorb different wavelengths, and the wavelengths that are absorbed
disappear
• Wavelengths that are not absorbed are reflected or transmitted
– For example, most leaves appear green because chlorophyll absorbs violet-blue
and red light while reflecting and transmitting green light
–
• A spectrophotometer measures a pigment’s ability to absorb various wavelengths
, • It sends light through pigments and measures the fraction of light transmitted at each
wavelength
• An absorption spectrum is a graph plotting a pigment’s light absorption versus
wavelength
• Three types of pigments in chloroplasts include:
– Chlorophyll a, the key light-capturing pigment that participates directly in light
reactions
– Chlorophyll b, an accessory pigment
– Carotenoids, a separate group of accessory pigments
• The absorption spectrum of chlorophyll a indicates that violet-blue and red light will
work best for photosynthesis, while green is the least effective
• The action spectrum for photosynthesis, a profile of the relative effectiveness of
different wavelengths, confirms the effectiveness of violet-blue and red light
• The action spectrum of photosynthesis was first demonstrated in 1883 by Theodor W.
Engelmann
• He exposed different segments of a filamentous alga to different wavelengths of light and
used the growth of aerobic bacteria as a measure of O2 production
• The action spectrum for photosynthesis is broader than the absorption spectrum of
chlorophyll
• Accessory pigments, such as chlorophyll b, broaden the spectrum used for photosynthesis
• The difference in the absorption spectrum between chlorophyll a and b is due to a slight
structural difference between the pigment molecules
• In the last decade, two other forms of chlorophyll were discovered—chlorophyll d and
chlorophyll f—that absorb higher wavelengths of light
• The cyanobacterium, Chroococcidiopsis thermalis, uses chlorophyll f in place of
chlorophyll a in shaded conditions
• Other accessory pigments called carotenoids, are yellow or orange because they absorb
violet and blue-green light
