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LESSON 2: THE PATHWAYS
OF PHOTOSYNTHESIS
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Lessons:
1 | 2 |
Overview
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Photosynthesis is responsible for
virtually all of the organic carbon in the biological world.
During photosynthesis, carbon is fixed in the form of carbon
dioxide. Carbon dioxide is reduced, using the energy of the sun,
to form carbohydrates. The carbohydrates produced during
photosynthesis quite literally feed the earth.
Photosynthesis also has other great
impacts on the biosphere. Prior to the evolution of
photosynthesis, our atmosphere had considerably less oxygen than
it does today. As a result, it is likely that life consisted
only of anaerobic, single-celled organisms. The oxygen that
results as a byproduct of photosynthesis greatly increased the
oxygen content of our atmosphere, starting around 2.8 billion
years ago. This increase in oxygen made possible the evolution
of multicellular, aerobic organisms, such as protists, plants,
fungi, and animals.
Learning Objectives
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Understand the qualities of
visible light that make it useful for photosynthesis.
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Be able to follow the flow of
energy through photosynthesis, from sunlight to
carbohydrate.
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Understand the differences between
the light reactions and the carbon fixation reactions
(Calvin-Benson cycle) of photosynthesis.
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Understand the basic structure of
photosystems.
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Know the enzyme RuBisCo, and its
functions.
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Understand different forms of
autotrophy.
Topics covered in this Lesson
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 The light that allows us to see
everyday objects and color around us is called, appropriately
enough, visible light. Visible light is part of a larger
spectrum of
electromagnetic radiation, which includes,
infrared radiation,
UV light, and
X-rays.
Photosynthetic organisms capture
different wavelengths of visible light, most notably the red and
blue wavelengths, to power the production of carbohydrates.
Because many photosynthetic organisms absorb only red and blue
wavelengths of visible light, they reflect green wavelengths,
and so appear green to us.
Learning Objectives
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Understand that light travels
in waves of very short wavelength.
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Understand the relationship of
wavelength and energy.
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Be able to define
pigment, and give several examples of pigments.
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Know which wavelengths of light
are most useful for photosynthesis, and how this would
be shown on an
absorption spectrum and an
action spectrum.
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 Photosynthesis is broken up into two
sets of reactions – the light reactions and the carbon fixation
reactions (the Calvin-Benson cycle). During the light reactions,
light is absorbed by pigments located in photosystems. The
energy thus absorbed is used to power an electron transport
chain, much like the electron transport chain of aerobic
respiration, resulting in the production of ATP by oxidative
phosphorylation.
The light reactions also reduce a
molecule called NADP. Both ATP and reduced NADP are used in the
second set of reactions, the carbon fixation cycle.
Learning Objectives
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Understand the location of
photosynthetic
pigments in prokaryotic and eukaryotic cells.
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Know the basic structure of a
photosystem, including the antenna system and the type of
molecule present in the reaction center.
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Understand why photosystems
contain different types of pigments.
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Be able to describe what
happens when light is absorbed by a photosystem.
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Know how oxygen is produced
during the light reactions.
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Know the role of redox
reactions in photosynthetic electron transport and ATP
production.
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Understand the designations
P680 and P700.
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Be able to describe the basic
differences between non-cyclic and cyclic electron flow,
and why cyclic electron transport is important.
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Understand why stacks of
thylakoid membranes in chloroplasts are analogous to
Reese’s Peanut Butter Cups®.
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 The light reactions of photosynthesis
are significant because light energy is captured and transferred
into the chemical bonds of ATP and NADP. During the carbon
fixation reactions, carbon dioxide is fixed and then reduced
into carbohydrate, using the energy stored in the ATP and NADP
produced by the light reactions. The fixation of carbon dioxide
is catalyzed by the most abundant enzyme on earth, RuBisCo.
The carbon fixation reactions represent
the entry of inorganic carbon (in the form of carbon dioxide)
into the biological world.
Learning Objectives
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Understand the link between the
light reactions and the
carbon fixation reactions of photosynthesis.
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Know the three stages of the carbon
fixation reactions, and the basic outcome of each.
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Know which enzyme catalyzes the
fixation of carbon dioxide, and its substrate.
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Know which molecule leaves the
carbon fixation reactions to become carbohydrate.
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Understand why the carbon fixation
reactions are referred to as a ‘cycle’ (the Calvin-Benson
cycle)
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Know what it means to say that
RuBisCo has both carboxylase and oxygenase activity.
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Be able to define
photorespiration and understand why it is a problem for many
plants.
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Be able to explain how
C4 and
CAM plants solve the problem of photorespiration.
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Know how the enzyme
PEP carboxylase helps C4 and CAM plants avoid photorespiration.
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Be able to give examples of C3, C4
and CAM plants.
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 Autotrophs are defined as ‘self-feeders’, meaning that they are able to
produce all of the organic compounds that they need to survive.
When we think of autotrophic organisms, plants are usually the
first to come to mind. However, there are a number of organisms
that have the ability to synthesize organic compounds from
inorganic compounds using chemical energy. Such chemoautotrophs
make up a very small percentage of autotrophic organisms and
tend to be found in extreme environments, but the processes they
use to harvest energy can be quite diverse and fascinating.
Learning Objectives
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Know what an
autotroph is (hint: they’re not all plants).
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Know the differences between
photoautotrophs and
chemoautotrophs.
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Know what types of molecules
are used as chemical energy sources by chemoautotrophs
and how their energy is released.
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Know one habitat where
chemoautotrophs are found.
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