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SL Topic 2.8
Photosynthesis
The Chloroplast
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Chloroplast surrounded by
two plasma membranes
Thylakoids- disk shaped
structures with membrane
surrounding chlorophyll
Grana (granum) – stacks of
thylakoids
Stroma – fluid that fills in
the chloroplast space
Lamellae – connect
neighboring stacks of
granum
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Light energy is turned into
chemical energy
Producers (plants, algae and
some bacteria) can contain
chlorophyll, mostly in
organelles called chloroplasts
Chlorophyll can trap light
energy (photons-packets of
light)
Chlorophyll converts light
energy into ATP, chemical
energy
These energy molecules are
then converted into
carbohydrates
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White light from the sun is
composed of a range of
wavelengths (colors)
Red light has a longer
wavelength with less
energy
Blue light has shorter
wavelengths with more
energy
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Chlorophyll is the main
photosynthetic pigment
There are other pigments that
can be seen by
chromatography or more
naturally by leaves turning
colors in the fall
Structure of the pigment
molecule allows it to absorb
light
For chlorophyll, the tail region
is embedded in the thylakoid
membrane and protein on top
traps the photons
Absorption of light
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Shows the rate of
photosynthesis at different
wavelengths of light
Maximum rates of
photosynthesis are at the blue
ends and red ends of the visible
light spectrum
The lowest rates are in the green
and yellow wavelengths
Thus, red and blue light is
absorbed by pigments to do
photosynthesis, while green and
yellow are reflected (that’s why
plants look green)
Draw this but don’t
copy wavelengths
Photolysis (aka Light Reactions)
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Light is absorbed the
chlorophyll molecules
embedded in the thylakoid
membranes of the
chloroplast
Light energy is converted to
chemical energy that splits
incoming water molecules
into H+ and O2
These split molecules then
give rise to the production
of ATP, cell energy and
NADPH
ATP-ADP cycle
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ADP (adenosine diphosphate) takes in energy
from food or sunlight and
uses energy to form a high
energy phosphate bond
Now becomes ATP
(adenosine tri-phosphate)
When cellular work needs to
be done, high energy bond is
broken to release the energy
Process repeats
Making Organic Molecules (Dark
Reactions)
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H+, in the form of NADP-H,
and ATP molecules made
during photolysis in the
thylakoids now move out into
the chloroplast space, stroma
CO2 is brought into plant
through stomata on leaves
and makes its way into the
chloroplasts
In the stroma, a cyclic
metabolic pathway called the
Calvin cycle takes place
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Calvin cycle takes energy
molecules made in
photolysis and Carbon
from CO2 and fixes them
into organic molecules
(carbohydrates, proteins
and lipids)
Chemical energy from
photolysis is transferred
to chemical bonds in
these organic molecules
which can be used in the
future for energy
Measuring the Rate of
Photosynthesis
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CO2 + H2O ---- C6H12O6 + O2
Rate can be directly measured by the use of
CO2 or the production of O2
Rate can be measured indirectly by increase in
Biomass (organic molecules will build up
during plant growth)
Factors affecting rate of photosynthesis
Light Intensity: (temperate)
 a) As the intensity of light
is increased the rate of
reaction of photosynthesis
increases.
 (b) Light intensity has
saturated the plants. That is
the rate now remains
constant for any further
increase in light intensity.
 (c) Note that light intensity
to achieve maximum rate of
photosynthesis is less than
the intensity of light in
summer.
Carbon Dioxide Levels
 (a). As the concentration of
carbon dioxide is increased
the rate of photosynthesis
increases.
 (b).The concentration of
carbon dioxide has saturated
the process. The maximum
rate of reaction has been
achieved. Further increases in
carbon dioxide do not increase
the rate. The rate is now
constant.
 (c) Note this is the normal
concentration of carbon
dioxide in the atmosphere.
Temperature
 Photosynthesis has many
enzyme molecules.
 The optimum temperature in
a temperate climate is about
25° C.
 However. Temperature has
many effects on a plant and
the graph should be treated
with caution.
 Temperature has just as many
effects on respiration
(changing food to energy),
transpiration (intake of gases)
and translocation (movement
of electrons) all of which in
turn affect photosynthesis.