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3.8.1 State that photosynthesis involves the conversion of light
energy into chemical energy
3.8.2 State that light from the sun is composed of a range of
wavelengths (colors)
3.8.3 State that chlorophyll is the main photosynthetic pigment
3.8.4 Outline the differences in absorption of red, blue, and
green light by chlorophyll
3.8.5 State that light energy is used to produce ATP, and to split
water molecules (photolysis) to form oxygen and hydrogen
3.8.6 State that ATP and hydrogen (derived from photolysis of
water) are used to fix carbon dioxide to make organic molecules
3.8.7 Explain that the rate of photosynthesis can be measured
directly by the production of oxygen or the uptake of carbon
dioxide, or indirectly by an increase in biomass
3.8.8 Outline the effects of temperature, light intensity, and
carbon dioxide concentration on the rate of photosynthesis
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Plants produce foods that
begin food chains
◦ The sunlight that strikes our
planet must be converted into
a form of chemical energy to
be useful to all non
photosynthetic organisms
 Most common chemical energy
produced from photosynthesis
is the molecule glucose
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The vast majority of plant leaves appear
green to our eyes
◦ Plants contain a variety of pigments in chloroplasts
◦ The photosynthetic pigment that dominates in most
plant species in the molecule chlorophyll
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Plants make use of the same part of the
electromagnetic spectrum that our eyes are
able to see
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Sunlight is actually a mixture of different
colors of light
The visible light spectrum includes many
colors, but for the purpose of considering
how chlorophyll absorbs light energy
Three regions of spectrum
◦ Red end of spectrum
◦ Green middle of spectrum
◦ Blue end of spectrum
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Substances can do one of only two things
when they are struck by a particular
wavelength of light, they can:
◦ Absorb that wavelength (if so, energy is being
absorbed and may be used)
◦ Reflect that wavelength (if so, the energy is not
being absorbed and you will see the color)
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Chlorophyll is a green pigment
◦ This means that chlorophyll reflects green light and
therefore must absorb the other wavelengths of the
visible light spectrum
 When a plant leaf is hit by sunlight, the red and blue
wavelengths of light are absorbed by chlorophyll and
used for photosynthesis
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Photosynthesis produces sugar molecules as
a food source for the plant.
Glucose molecules are held together by
covalent bonds
◦ It requires energy to create those covalent bonds
and the source of that energy can ultimately be
traced back to the sun
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The first stage of photosynthesis is a set of
reactions typically referred to as the lightdependent reactions
◦ Chlorophyll absorb light energy and convert that
energy to a form of chemical energy (ATP)
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In additions, light energy is also used to
accomplish a reaction that is called photolysis
of water
 Photolysis-water molecule is split into its component
elements: hydrogen and oxygen
 The oxygen that is split away due to photolysis of water is
typically released from the plant leaf as a waste product
 The useful products formed during this stage of
photosynthesis are ATP and hydrogen
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The second stage of photosynthesis is a
series of reactions collectively referred to as
the light-independent reactions
◦ ATP and hydrogen are used as forms of chemical
energy to convert carbon dioxide and water into
useful organic molecules for the plant
 CO2 is inorganic
 Glucose is organic (requires 6 CO2 molecules to form
one glucose molecule)
 6CO2 + 6H2O  C6H12O6 + 6O2
 This conversion of an inorganic form of an element to an
organic form is known as fixation
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Photosynthesis can be described as a series
of reactions in which carbon dioxide and
water are fixed into glucose, and oxygen is
produced as a by-product
Fixation reaction requires energy
◦ The energy to create the glucose comes directly
from the ATP and hydrogen created in the first
stage of photosynthesis
 This energy can be traced back to sunlight
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Knowing that plants do both photosynthesis
can cell respiration might lead you to the
conclusion that the two processes ‘cancel
each other out’
At any given time of year, any one plant has a
fairly consistent throughout the day and
night.
◦ Plants do not have muscle and other ATPdemanding tissues as do animals. They need ATP
for various biochemical process, but the level is
typically far below that required by an animal
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Photosynthetic rate is highly dependent on
many environmental factors including
intensity of light and air temperature
During the daytime, especially on a warm
sunny day. The rate of photosynthesis may
be very high for a particular plant
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If so, the rate of carbon dioxide taken in by
the plant and the rate of oxygen released will
both be very high
At night, the rate of photosynthesis may drop
to zero.
 At this time, a given plant may be giving off carbon
dioxide and taking in oxygen to maintain its relatively
low and consistent rate of cell respiration
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Measuring the rate of oxygen production or
carbon dioxide intake is considered to be
direct measurement of photosynthetic rate as
long as a correction is made for cell
respiration
◦ Another common approach is to measure
photosynthesis by keeping track of the change in
biomass of experimental plants.
 Massing of plants is considered to be an indirect
reflection of photosynthetic rate as the increase or
decrease in biomass may be traced to a whole variety
of factors besides photosynthetic rate
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The effect of increasing light intensity on the
rate of photosynthesis
◦ Figure 3.27
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Effects of Changing temperature
◦ Figure 3.28
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Effect of changing carbon dioxide
concentration
◦ Figure 3.29