Biology
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How do plants get food?
Photosynthesis overview
Leaf structure
Chloroplast structure
Pigments
Two Stages
◦ 6a. Light Reactions
◦ 6b. Dark Reactions
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7. Relationship Between the 2 Stages
8. Factors Affecting the Photosynthetic Rate
9. Alternate Pathways
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Plants are autotrophs (meaning “self-feeders” in
Greek)
Often referred to as the producers of the biosphere
because they produce its food supply
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Photo, from the Greek word for light, refers
to the first stage.
Synthesis, meaning “putting together” refers
to the sugar construction in the second stage
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The main purpose of photosynthesis is to make
organic molecules (carbohydrates).
Overall equation:
6 CO2 + 6 H20  C6H12O6 +
6 O2
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Occurs in the leaves of plants in the chloroplasts.
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Oxygen is also produced in this process.
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Most photosynthesis occurs in the leaf.
Gas exchange of CO2 and O2 occurs at openings
called stomata surrounded by guard cells on the
lower leaf surface.
Stomata are able to open and close because water
is also evaporated through them into the
atmosphere from the plant.
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Similar to mitochondria, chloroplast has an outer
membrane and an inner membrane, with an
intermembrane space between them.
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Inner membrane is filled with a thick fluid called
stroma
*Stroma is where sugars are made from carbon
dioxide and water
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Within stroma is a system of interconnected
membranous sacs called thylakoids
◦ contains thylakoid space
◦ Built into thylakoid membranes are the chlorophyll
molecules that capture light energy
◦ concentrated in stacks called grana.
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Pigment is any molecule that is able to
absorb light .
 Only light that is absorbed by pigments is useful for
photosynthesis.
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Chlorophyll a is the most important
photosynthetic pigment.
Other pigments called antenna or accessory
pigments are also present in the leaf.
 Chlorophyll b
 Carotenoids (orange / red)
 Xanthophylls (yellow / brown)
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1. Light Reactions
2. Dark Reactins (Calvin Cycle)
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Light reactions
◦ Occurs in thylakoid membranes
◦ light energy splits water into H2 and O2
◦ H2 is used to store energy in ATP and NADPH
◦ O2 gas is a waste product.
Dark reactions, or Calvin Cycle
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Occurs in the stroma
Does not require light directly
CO2 is turned into glucose through carbon
fixation
Process is powered by ATP and NADPH from light
reactions
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Photosynthetic rate is depended on
environmental factors:
◦ Amount of light available
◦ Level of carbon dioxide
◦ temperature
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Light intensity
◦ Up to a certain intensity, photosynthesis increases
as more light is available to the chlorophyll.
◦ When all the chlorophyll molecules are activated
(saturated) by the light, more light has no further
effect.
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Temperature:
◦ Increased temperature increases photosynthetic
rate until an optimal temperature is reached.
◦ Above the optimal temperature, enzymes cannot
function properly and photosynthesis will decrease.
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Carbon Dioxide Levels:
◦ Increased carbon dioxide levels increases
photosynthesis, unless limited by another factor,
then levels off.
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These pathways adapt to perform
photosynthesis in dry and hot environment
They are more efficient than the traditional
C3 pathway which use CO2 directly from the
air
Plants with alternative pathways have a
slightly different Calvin cycle.
◦ In C4 plants the location of the Calvin cycle is
different
◦ In CAM plants the timing is different
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C4 Pathway:
◦ CO2 fixation and the Calvin cycle take place in two
separate location.
◦ Examples of C4 plants: corn, sugar cane
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CAM
◦ Occurs in succulent plants (cacti)
◦ Carbon fixation (trapping CO2) takes place at night
when the stomata are open
◦ Calvin cycle takes place during the day, when
stomata are closed
◦ This way plants do not lose much water during hot
and dry days.