Photosynthesis
Chapter 5
Outline
I. Photosynthesis
A. Introduction
B. Reactions
Photosynthesis
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Method of converting sun energy into chemical
energy usable by cells
Autotrophs: self feeders, organisms capable of
making their own food
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Photoautotrophs: use sun energy e.g. plants
photosynthesis-makes organic compounds (glucose)
from light
Chemoautotrophs: use chemical energy e.g.
bacteria that use sulfide or methane
chemosynthesis-makes organic compounds from
chemical energy contained in sulfide or methane
Overall Reaction of Photosynthesis
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6CO2 + 12 H2O + light
energy → C6H12O6 + 6O2+ 6H2O
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Carbohydrate made is glucose
Water appears on both sides because 12 H2O molecules
are required and 6 new H2O molecules are made
Water is split as a source of electrons from hydrogen
atoms releasing O2 as a byproduct
Electrons increase potential energy when moved from
water to sugar therefore energy is required
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Photosynthesis
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Photosynthesis takes place in specialized
structures inside plant cells called chloroplasts
(Light absorbing pigment molecules)
– Chlorophyll a
– Chlorophyll b
– Accessory pigments (auxiliary pigments)
 Carotenoids (are usually red, orange, or yellow
pigments)
Photosynthesis - Chlorophyll’s
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When you shine white light on chlorophyll, its
molecules will absorb certain colors of light. The
light that isn’t absorbed is reflected, which is
what our eyes see. Green is what we see and all
the rest red, orange, yellow & blue are absorbed.
Light-dependent Reactions
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Overview: On the thylakoid membrane the light
energy is absorbed by chlorophyll molecules-this
light energy excites electrons and boosts them to
higher energy levels. They are trapped by
electron acceptor molecules that are poised at
the start of a neighboring transport system. The
electrons “fall” to a lower energy state, releasing
energy that is harnessed to make ATP &
NADPH.
Light-dependent Reactions
What happens to the water?
Photosystem II’s stolen electron is
replenished by photolysis, or the
splitting of H2O to form H+and O2
(note: the H+ is kept inside the
thylakoid membrane). The O2
resulting is the source of all oxygen
in our atmosphere.
Light-dependent Reactions
What happens to the water?
Light-dependent Reactions
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Photosystem: light capturing unit, contains chlorophyll,
the light capturing pigment
Electron transport system: sequence of electron
carrier molecules that shuttle electrons, energy released
to make ATP
Electrons in chlorophyll must be replaced so that cycle
may continue-these electrons come from water
molecules, Oxygen is liberated from the light reactions
Light reactions yield ATP and NADPH used to fuel the
reactions of the Calvin cycle (light independent or dark
reactions)
Stroma
PHOTOSYNTHESIS
(All happens inside chloroplast)
Calvin Cycle (light independent or
“dark” reactions)
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ATP and NADPH generated in light reactions
used to fuel the reactions which take CO2 and
break it apart, then reassemble the carbons into
glucose.
Called carbon fixation: taking carbon from an
inorganic molecule (atmospheric CO2) and
making an organic molecule out of it (glucose)
Simplified version of how carbon and energy
enter the food chain