Photosynthesis

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Photosynthesis
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Sunlight
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The Wave-Particle Duality of Light
Electromagnetic radiation, such as light from the
sun, exhibits properties of both waves and
particles
The waves are composed of particles, or packets
of energy called photons
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The light given off by a light bulb, for
instance, is carried by photons
The brighter the bulb, the greater is the number
of photons emitted each second
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Enormous amounts of light energy is delivered to
earth from the sun via photons
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These photons are captured by pigment molecules
in photosynthetic organisms, and their energy is
used to drive the process of photosynthesis
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The wave part of light, or electromagnetic
radiation, has crests
The distance between these crests is called the
wavelength
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Wavelengths in the electromagnetic spectrum range
in length from less than a nanometer (gamma rays)
to more than a kilometer (radio waves)
The segment of the spectrum most important to
life is a narrow band that ranges from about 380
to 750 nm in wavelength
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This radiation is known as
Visible Light
Because it is detected as various colors by the
human eye
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• As light meets matter, it may be
• Reflected
• Transmitted
• Absorbed

Substances that absorb visible light are called
pigments
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Different pigments absorb light of different
wavelengths The wavelengths that are absorbed
disappear
If a pigment is illuminated with white light, the
color most reflected by the pigment is the color
we see
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Chlorophyll is the light absorbing pigment in
plants We see green when we look at a leaf
because chlorophyll absorbs red and blue light
while transmitting and reflecting green light
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Why does chlorophyll absorb red and blue light
while reflecting green? Light can perform work
only if it is absorbed, and the absorption
spectra of chlorophyll is in the red and blue
range
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The ability of a pigment to absorb various
wavelengths of light can be measured with the
spectrophotometer
We can then plot a pigments light absorption
versus the wavelength of the light to get an
absorption spectrum
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Determining an absorption spectrum
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The absorption spectra of chloroplast pigments
provide clues to the relative effectiveness of
different wavelengths for driving photosynthesis,
since light can perform work in chloroplasts only
if it is absorbed
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How does photosynthesis work?
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Overview of Photosynthesis
The light reaction and the Calvin cycle cooperate
in converting light energy to the chemical energy
of food
In the chloroplast, the light reactions occur in
the thylakoid membrane, and the Calvin cycle
occurs in the stroma
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Details
What happens when chlorophyll and other pigments
absorb photons? When a pigment molecule absorbs a
photon, one of the molecules electrons is
elevated to an orbital where it has more
potential energy Potential energy is energy of
position
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A pigment molecule whose electron is in its
normal orbital is said to be in its ground state
After absorption of a photon boosts an electron
to an orbital of higher energy, the pigment
molecule is said to be in an excited state
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The energy of an absorbed photon is converted to
the potential energy of an electron raised from
the ground state to an excited state But the
electron cannot remain there long the excited
state, like all high-energy states, is unstable
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How can plants harvest the potential energy of
electrons in their excited states?
In the thylakoid membrane, chlorophyll is
organized along with proteins and other kinds of
organic molecules into photosystems
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A photosystem has a light-gathering antenna
complex consisting of a cluster of chlorophyll
a, chlorophyll b, and carotenoid molecules
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When an antenna molecule absorbs a photon, the
energy is transmitted from pigment to pigment
molecule until it reaches a chlorophyll a
molecule in the reaction center
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Sharing the reaction center with the chlorophyll
a molecule is a specialized molecule called the
primary electron acceptor The excited-state
electron in this chlorophyll a molecule is
grabbed by the primary electron acceptor before
it has a chance to drop back down to its ground
state.
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If our chlorophyll a moleculethe one in the
reaction centerloses an electron to the primary
acceptor, it must get another to replace it from
somewhere else
The question is Where?
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From Water!
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Water is split during photosynthesis, and
electrons from the water are used to replace any
lost to the primary electron acceptor
Water is split into two hydrogen ions (H) plus
two electrons, and one oxygen atom the oxygen
atom combines with another to form O2, and is
released as a waste product
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Lets take a look at the whole process of
photosynthesis
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These are the end products of photosynthesis
Glucose, water, and oxygen
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