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Chapter 6
Photosynthesis:
Using Light to Make Food
1. The Basics of Photosynthesis
2. The Light Reactions
3. Then comes the Calvin Cycle
4. Water Saving Adaptations
5. The Environmental Impact of Photosynthesis
1. The Basics of Photosynthesis
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1. The Basics of Photosynthesis
Photosynthesis is the process by which
plants, use the energy from sunlight to
produce sugar.
Almost all plants are photosynthetic
autotrophs, as are some bacteria and
protists, which means that they generate
their own organic matter through
photosynthesis.
A. Chloroplasts:
Sites of Photosynthesis
– Are found in the interior cells of
leaves.
– Contain stroma, a thick fluid.
– Contain thylakoids, membranous
sacs.
B. The Overall Equation for
Photosynthesis
The reactants and products of the reaction:
Energized electrons are added to carbon
dioxide to make sugar. Sunlight provides the
energy.
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C. A Photosynthesis Road Map
Photosynthesis is
composed of 2
processes:
1. The light
reactions convert 1 2
solar energy to
chemical energy.
2. The Calvin cycle
makes sugar from
carbon dioxide.
2. The Light Reactions:
Converting Solar Energy to
Chemical Energy
2. The Light Reactions:
Converting Solar Energy to
Chemical Energy
Chloroplasts are chemical factories
powered by the sun;
• That convert solar energy into
chemical energy.
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A. The Nature of Sunlight
So what’s INCREASING ENERGY
sunlight made
of?
• It’s a type of
energy called
radiation or VISIBLE LIGHT
electromagnetic
energy.
• The full range of
radiation is
called the
electro-magnetic
spectrum.
B. The Process of Science:
What Colors of Light Drive
Photosynthesis?
In 1883, German biologist Theodor Engelmann
demonstrated that photosynthesis takes place
inside the chloroplasts of plants and that the
chloroplasts receive more energy from red and blue
light than from other colours
1. He illuminated an alga
with light passed through
a prism, exposing the alga
to different wavelengths.
2. Aerobic bacteria, which
grow in the presence of
oxygen, were used to
determine where the alga
was releasing the most
oxygen and thus
photosynthesizing most.
3. They accumulated under
red and blue light,
showing that only some
parts of the
electromagnetic spectrum
are used
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Chloroplasts absorb select wavelengths of light
that drive photosynthesis.
Reflected
light is
green, so
you see
the
leaves
Absorbed green!
light is red
and blue
C. Chloroplast Pigments
Chloroplasts contain several pigments:
– Chlorophyll a
– Chlorophyll b Green
– Carotenoids orange-yellow-brown
Trees respond to the
decreasing amount of
sunlight by producing less
and less chlorophyll.
Eventually, a tree stops
producing chlorophyll.
When that happens, the
carotenoid already in the
leaves can finally show
through.
D. How Photosystems Harvest
Light Energy
A photosystem is an organized group of
chlorophyll and other molecules. It is a light-
gathering antenna.
Light behaves as photons, discrete packets
of energy.
Chlorophyll molecules absorb photons.
– Electrons in the pigment gain energy.
– The energy is released and used.
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How the Light Reactions Generate ATP and
NADPH
The light reactions of photosynthesis:
2 types of photosystems cooperate in the
light reactions.
An electron
transport chain
• Connects the 2
photosystems.
• Releases energy
that the
chloroplast uses
to make ATP.
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And it all takes place in the thylakoid
membrane of the chloroplast.
3. Then comes the Calvin Cycle:
Making Sugar from Carbon
Dioxide
3. Then comes the Calvin Cycle:
Making Sugar from Carbon
Dioxide
It functions like a sugar factory within a
chloroplast and regenerates the starting
material with each turn.
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4. Water-Saving Adaptations
4. Water-Saving Adaptations
All plants are classified as to their ability to convert
sunlight into plant matter.
Water enters leaf
CO2 enters through stomata
C3 plants
– Use CO2 directly from the air.
– Are very common and widely
distributed.
– Adaptive Value: efficient under cool
and moist conditions and under normal
light because requires less machinery
(fewer enzymes and no specialized
anatomy).
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C4 plants
– Close their stomata to save water
during hot and dry weather.
– Can still carry out photosynthesis.
– Example: corn
CAM plants
– Open their stomata only at night to
conserve water, and therefore carry
photosynthesis during the night.
– Example: cactus, orchids
5. The Environmental Impact of
Photosynthesis
5. The Environmental Impact of
Photosynthesis
Photosynthesis has an enormous impact
on the atmosphere. It swaps O2 for CO2.
And the atmospheric oxygen we breathe is
a by-product of photosynthesis….
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How Photosynthesis Moderates
Global Warming
Greenhouses are used to grow plant indoors.
They trap sunlight that warms the air inside.
A similar process,
the greenhouse
effect:
• Warms the
atmosphere.
• Is caused by
atmospheric CO2.
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