Transcript Photosynthesis.pptx

Photosynthesis
Chloroplasts in Elodea, 1250x.
Lessons From Thin Air
• In 1995, filmmakers from the
Harvard Smithsonian Center for
Astrophysics approached some
new graduates from Harvard
and the Massachusetts Institute
of Technology.
• One of the questions they posed:
“Here’s a seed. Imagine I planted
that seed in the ground, and a
tree grew.
…Where did all that weight come
from?”
“The President,” a giant
3,200 year-old sequoia
in California with a
height of 247 feet.
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Autotrophs
• Plants, algae, and some bacteria have the ability to
use sunlight to generate their own storage
molecules of energy.
• They are photoautotrophs.
– From the Greek photo- meaning “light,” autos- meaning
“self” and -troph meaning “nutrition.”
• The basic knowledge that
plants need water,
sunlight, and soil has been
around since ancient
times.
–
The working hypothesis
was that plants grew by
“eating” soil through their
roots.
• Jean Baptiste von Helmont
wanted to verify this – to
isolate the exact source of
the increased mass of trees
as they grew.
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The Willow Tree Experiment
• Von Helmont planted a 5pound willow tree in 200
pounds of dry, potted soil.
• He massed the tree, grew it
for 5 years, then massed it
and the soil again.
• The tree gained over 160
pounds of mass.
–
The soil only decreased in
mass by about 2 ounces.
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Matter
• According to the law of conservation of mass, mass
cannot be created nor destroyed. It only changes
form.
– Where then, does the dry mass of wood come from?
• Part of the mystery of how plants worked was due
to a lack of understanding of air.
• For many centuries, scientists believed that air was
a pure, elementary substance.
– This stemmed from
the Greek philosopher
Aristotle’s idea of all
matter being composed
of four elements: earth,
fire, wind, and water.
Priestley’s Experiment
• Joseph Priestley, a British chemist, believed that
air was not a single “elementary substance”, but
a “composition” of gases.
• During one experiment, he discovered that a
candle placed in a sealed jar would extinguish
very quickly.
– He called the air “injured,” because it was unable to
support fire.
• Placing a mouse in the jar
would have a similar
effect, and it would die.
– Priestley had discovered
oxygen!
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• Priestley’s hypothesis
about the composition
of air was correct.
• Air is primarily made
of:
–
–
–
–
–
78% nitrogen
21% oxygen
0.9% argon
0.03% carbon dioxide
78%
21%
0.02%
0.02% water
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Ingenhousz’s Experiment
• A Dutch physician, Jan
Ingenhousz, decided to repeat
Priestley’s experiment, with a
few changes.
• Two sealed containers were
studied – one exposed to
sunlight, the other left in the
shade.
– The sunlit container showed an
increase in oxygen compared to
the shaded one.
– Plants must be releasing oxygen!
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Photosynthesis
• The final piece of the puzzle was solved by a Swiss
botanist named Nicolas de Saussure.
– He enclosed the plants in a sealed container of carbon
dioxide and compared the air and plant masses before
and after growth.
• Based on these measurements, he concluded that
plants were primarily composed of:
– Water from the soil.
– Carbon from the air.
• The process was called photosynthesis, because
light (“photo”) was required to make (“synthesize”)
the plant tissues.
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• The concept of photosynthesis difficult to
understand, because the idea of producing a heavy
plant from air and water is not intuitive.
– Converting the matter in air and water to plants
requires a great deal of energy.
Energy
• There are many different forms of energy in the
universe.
• Potential energy is stored energy. It exists as a
result of position or chemical structure.
Position.
Position.
Chemical Structure.
• Kinetic energy is in motion.
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Electromagnetic Spectrum
• Kinetic energy travels in the form of waves. Each
type of energy has its own wavelength.
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Energy and Life
• Most of the energy that supports life on Earth
originates from the sun.
• Solar radiation primarily contains three ranges of
wavelengths of energy:
– Visible light, which we are
able to detect with our eyes.
– Ultraviolet, which has a
shorter wavelength than
visible light and is able to
penetrate living tissue.
– Infrared, or heat, which has a
longer wavelength than
visible light.
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Photosynthesis
• Plants have the ability to harness some wavelengths
of visible light to convert matter in the air to that of
their own tissues.
• The primary location of photosynthesis is in leaves.
– Cells within leaves
have a high
concentration
of chloroplasts,
each of which
contains a green
pigment called
chlorophyll.
• The complete chemical reaction of photosynthesis is
summarized like this:
• Carbon dioxide and water are used to synthesize
glucose, which can then be used to produce starch
or cellulose.
– Oxygen is produced as a waste product.
Color
• Plants have multiple pigments in their leaves
besides chlorophyll to help absorb the energy from
visible light.
• Collectively, these pigments are able to absorb most
wavelengths of visible light.
• The only
wavelengths
that plants do
not utilize are
green.
– Plants are
green because
that light is
reflected.
Products of Photosynthesis
• The end result of the glucose produced by
photosynthesis depends on the needs of the plant.
• If a plant needs to grow, the glucose will be used to
generate the polysaccharide cellulose.
• Excess glucose can be stored as starch to be used at
a later time.
– Potatoes and other plants
have specialized storage
organelles called amyloplasts
to produce and store starch.
• In addition to chloroplasts, plant cells also contain
mitochondria and do perform cell respiration.
– This allows them to process glucose into ATP when the
plant cells are in need of chemical energy.