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Biology 9 Unit 7 - Photosynthesis

Autotrophs use inorganic
material: carbon dioxide from
the air, and minerals and water
from the soil to make their own
organic materials, including
carbohydrates, lipids, proteins,
and nucleic acids.
On the other hand, heterotrophs,
meaning “other feeders”, cannot
make organic materials from
inorganic materials. That is
why we must eat fruits,
vegetables, meats, and grains.
Heterotrophs depend on
autotrophs for nutrients.
Autotrophs depend on a
process called
photosynthesis for
We will examine the
process of photosynthesis
in the following Learning
Guide slides.
Photosynthesis is the
process in which light
energy is used to create
organic material.
Photosynthesis cannot
occur without the help of
Chloroplasts are disc
shaped organelles found in
certain plant cells that
carry out the process of
Chloroplasts are located
within the mesophyll of the
leaf. The mesophyll, is the
green tissue found inside the
interior of the leaf.
Also an important part to the
photosynthetic processes are
tiny pores located on the leaf
surface called stomata.
The stomata allow carbon
dioxide to enter the leaf and
allows oxygen to exit the
Found throughout the leaves and
body of the plant are vital
components called xylem and
Xylem consists of dead cells
designed for transporting water
and minerals from roots to
Phloem cells consist of living
cells that distribute sugars from
the leaves to roots and other
parts of the plant that are not
Another important
component of the
photosynthesis process
is the ability of plants to
receive water through
As seen in the diagram
to the right, the water
cycle plays a vital role
in delivering a vital
resource to all plants.
The chemical equation below provides a look at the
reactants and products of photosynthesis. You will notice
that the reactants of photosynthesis, carbon dioxide and
water, are also the waste products of cellular respiration.
Photosynthesis is a very
complex process.
The process of
photosynthesis has two
stages. The two stages
include the Light
Reactions and the Dark
Reactions/Calvin Cycle.
Sunlight, a type of energy
produces an assortment of
wavelengths. The full
range of the electromagnetic spectrum is
shown to the right. Note:
plants only use the band of
energy referred to as visible
Visible light, is only a small
portion of this spectrum. It
consists of those
wavelengths that our eyes
see as different colors.
Electromagnetic Spectrum
In the first step called the Light
Reaction, sunlight shines on
pigmented materials called
chlorophyll a and chlorophyll b
found within the chloroplasts.
Chlorophyll a - absorbs the blueviolet and red wavelengths
produced by sunlight.
Whereas, Chlorophyll b primarily
absorbs the blue and orange wave
lengths and reflects the yellowgreen wavelengths. This is the
reason why most plants are green
in color.
When a pigment
molecule, like
that of
gains energy, it
will not only
absorb this
energy it will
also emit light
and heat.
The light reaction
generates ATP,
oxygen and
hydrogen by
splitting water.
This step creates
the necessary
energy and
hydrogen that is
required for the
second step of
Energized electrons
from the watersplitting photosystem
pass down an
electron transport
chain to the NADPHproducing
photosystem (2) .
The chloroplast uses
the energy released
by this electron “fall”
to make ATP.
* Light reaction process taken from
Campbell and Reece’s Essential Biology
textbook. 2001
The NADPHproducing
photosystem transfers
its photo-excited
electrons to NADP ,
reducing it to NADPH
(3) .
The electron transport
chain replaces the
electrons lost from the
Calvin Cycle
Plants then make
sugar molecules
during the second
stage of
using carbon
dioxide during a
process known as
the Calvin cycle
or the Dark
In the first step of the
Calvin cycle, Carbon
atoms (three gray
CO2 balls enter the
cycles (1) .
An enzyme adds the
CO2 to RuBP (a
ribulose bisposphate, a
five-carbon sugar
already present in
chloroplast. The
product created is now
called 3PGA or 3phosphoglyceric acid)
In step two of the
Calvin cycle, ATP
and NADPH from the
light reactions
provide energy and
electrons (2) .
Enzymes use the ATP
energy and highenergy electrons from
NADPH to convert
the 3-PGA to threecarbon sugar, G3P
In step three of the
Calvin cycle, Carbon
exits the cycle as sugar
(Glucose) (3) .
Therefore, three CO2
molecules have been
converted to one
molecule of sugar G3P.
This is the direct product
of photosynthesis but
plant cells can use the
G3P to make glucose and
other organic compounds
for growth and fuel.
In the fourth and final step of
the Calvin cycle, the cycle
regenerates its starting material.
Note that of the six G3P
molecules produced in step 3
only one of them represents net
sugar output (4) . That’s
because we started with a total
of 15 sugar carbons in the three
RuBP molecules that accepted
CO2 back in step one.
Enzymes now regenerate the
RuBP by rear-ranging the five
G3P molecules that are left after
one of those sugars exits the
Throughout the evolution of plants natural
selection has forced plants to adapt their
photosynthesis capabilities.
Plants that use CO2 directly from the air are
called C3 plants because the first organic
compound produced, by these plants, is a
three-carbon compound called 3-PGA.
Another type of plant, a
C4 plant has special
adaptations that save
water without shutting
down the process of
These C4 plants keep
their stomata/pores
closed during the
daytime to conserve
water, yet at the same
time continue to make
Examples of these plants
include -
Cacti, ice plants, and jade
plants are classified as
CAM plants.
CAM plants conserve
water by opening its
stomata and taking in
CO2 at night.
The O2 is released during
the day through the
Calvin cycle.
Pineapple plant
Works Cited
After completing the test, move onto Unit #8.