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Photosynthesis
Chapter 8
Autotrophs vs. Heterotrophs
Autotrophs
• make their own food
•
include plants, some
protists, and some bacteria
Heterotrophs
• CANNOT make their own food;
must obtain energy from
outside sources
• Includes animals, fungi, and
some bacteria
Chemosynthesis
• Chemosynthesis is one way in
which organisms can produce
their own food by converting
chemicals into useable
nutrients.
– Example: Chemosynthetic
bacteria living in the depths of
the ocean use inorganic
materials and heat from ocean
vents to produce food.
Photosynthesis
• Photosynthesis is the process in
which plants use sunlight, carbon
dioxide, and water to produce
oxygen gas and stored energy in
the form of sugar.
• Photosynthesis occurs in
chloroplasts, primarily those found
in the leaves of a plant.
Chemosynthesis vs. Photosynthesis
Chloroplast
• Chloroplasts have
two membranes
– The outer membrane is
like a normal membrane
surrounding other
eukaryotic organelles
– The inner membrane is
folded into specialized
sacs called thylakoids.
Chloroplast
• Thylakoids are stacked
on top of each other to
form columns called
grana (singular:
granum)
• The space in the
chloroplast is called the
stroma.
Leaf Cross-Section
• Cuticle = waxy
covering on top of
the leaf
• Upper Epidermis =
top layer of cells on
the leaf
Leaf Cross-Section
• Palisade layer = is just
under the upper
epidermis; primary
location of
photosynthesis
• Spongy Mesophyll =
below the palisade
layer; has rounded
cells and air space for
gas exchange
Leaf Cross-Section
• Veins = tube-like
tissues that deliver
water from the roots
to the top of the plant
and carry sugars from
the leaves to the
lower parts of the
plant.
Leaf Cross-Section
• Lower Epidermis =
bottom layer of the
leaf
• Stomata / Stomata
= specialized cells
on the underside of
the leaf; used for
gas exchanged
Stomata
• Plants “breath in” carbon
dioxide and release oxygen
through openings on the
underside of leaves called
stomata / stomates
• Guard cells form the “lips”
of the mouth while the
stomate is the opening.
Stomata (cont.)
• Stomata open when they
need carbon dioxide.
• Plants lose water through
evapo-transpiration when
stomates are open, so they
close if they lose too much
water.
Parts of Photosynthesis
1. Light Dependent Reactions
2. Light Independent Reactions
(Calvin Cycle)
Light Reactions / Light Dependent
• Occurs in thylakoid membrane
• Sunlight absorbed by a green pigment called
chlorophyll within thylakoids
Light Reactions (cont.)
• Absorbed light energy causes proteins within
the thylakoids to split water molecules into
oxygen gas and hydrogen ions; oxygen is
released.
– Ions are charged atoms because they have
unequal numbers of protons (+) and electrons (-)
– Hydrogen normally only has one proton and one
electron
Light Reactions (cont.)
• Electrons move through PSII, electron
transport chain, and PSI.
– PSII and PSI = photosystems I & II; contain
pigments and give energy to ions
– The electron transport chain move the charged
electrons from PS II to PS I
Light Reactions (cont.)
• As the electrons move through the thylakoid
membrane, the protons build up a positive
charge outside of the cell.
• Protons are forced back into the membrane
because of diffusion, more specifically,
chemiosmosis
• They move through a special protein channel
called ATP synthetase, which causes ADP+P to
become ATP
Light Reactions (cont.)
• At the end of the light reactions, the electrons
and protons are captured by an electron
carrier called NADP+ to become NADPH
Light Reactions
Light Reactions Summary
• STARTS WITH:
–
–
–
–
Water
Sunlight
NADP+
ADP+P
• ENDS WITH:
– Oxygen gas (O2)
– NADPH
– ATP
Calvin Cycle / Light Independent
• Takes place in the stroma of chloroplast.
• An enzyme called rubisco attaches a carbon
dioxide molecule to a 5-carbon sugar called
RuBP.
Calvin Cycle / Light Independent (cont)
• This molecule is unstable and immediately
breaks into two 3-carbon sugars called PGA.
• This part of the Calvin Cycle is called “carbon
fixation.”
Calvin Cycle / Light Independent (cont)
• Using ATP and NADPH from the light
reactions, each molecule of PGA is converted
to PGAL.
• ATP becomes ADP+P while NADPH becomes
NADP+. Both of these molecules are recycled
and sent back to the light reactions.
Calvin Cycle / Light Independent (cont)
• PGAL is converted to 4-, 5-, and 6-carbon sugars
throughout the cycle.
• After three turns of the cycle, five molecules of RuBP
are produced that can later be recycled in the Calvin
Cycle. One molecule of sugar is made.
• Usually plants produce and store sucrose, but books
generally state glucose is stored.
Calvin Cycle
Calvin Cycle Summary
• STARTS WITH:
–
–
–
–
–
Carbon dioxide
Rubisco
RuBP
ATP
NADPH
• ENDS WITH:
– Sugar (sucrose or
glucose usually)
– RuBP (more made for
future Calvin Cycles)
– ADP + P (sent to light
reactions)
– NADP+ (sent to light
reactions)
Photosynthesis as a Chemical Equation
Factors Influencing Rates of
Photosynthesis
• Carbon Dioxide Levels
• As CO2 increases, rate
increases
• Light Intensity
• As light intensity
increases, rate increases
BUT then plateaus
Factors Influencing Rates of
Photosynthesis
• Temperature
• Oxygen Levels
• As temperature increases,
rate increases BUT then
plateaus
• As O2 increases, rate
decreases