Photosynthesis
How is energy being obtained by
the organisms in this picture?
Autotrophs & Heterotrophs
(Review)
 Plants are able to use light energy from the sun
to produce food. - Autotrophs
 Organisms that obtain energy from the foods
they eat . - Heterotrophs
Photosynthesis
 Definition:
 Photosynthesis is the
process by which
light energy is
converted to
chemical energy in
form of glucose.
Energy
 Energy is the ability
to do work and
there’s always work
to be done by the cell
First Law of
Thermodynamics
 Energy cannot be
created or
destroyed—it can
only be changed in
form.
Energy is changed from potential
energy to kinetic energy (the
energy of motion)
Light energy is changed to
chemical energy
Jan Van Helmont’s
Experiment-1600s
 Question: When a
seedling with a mass
of only a few grams
grows into a tree with
a mass of several
tons, where does the
tree’s increase in
mass come from?
From the soil? The
air? The water?
Jan Van Helmont’s
Experiment
 Hypothesis: Since a
plant’s roots are in
the soil, perhaps the
tree is eating the soil
and therefore, its’
mass comes from
the soil.
He imagined tiny faces on the
ends of roots “eating” the soil.
Jan Van Helmont’s
Experiment
 Procedure: Van
Helmont carefully
found the mass of a
pot of dry soil and a
small seedling.
 He planted the
seedling in the pot of
soil, watering it
regularly for 5 years.
Jan Van Helmont’s
Experiment
 Results: At the
end of 5 years,
the seedling
had gained 75
kilograms.
 The soil and
the pot had
only
decreased a
few grams.
Jan Van Helmont’s
Experiment
 Van Helmont’s
Conclusion: He
concluded that most
of the mass of the
tree must have come
from the water
because that was the
only thing that was
added to the pot.
Trees are made of water??????
Van Helmont was
incorrect!
 Van Helmont’s
experiment accounts
for the hydrate or
water portion of the
carbohydrate
produced by
photosynthesis. But
where does the
carbon of the carbo
portion come from?
Glucose—the most common and
important carbohydrate/sugar
made by plants in photosynthesis
The Rest of the Story that Van
Helmont didn’t know: CO2 !
 Although Van
Helmont did not
realize it, carbon
dioxide in the air
made a major
contribution to the
mass of his tree. It is
the carbon in CO2
that is used to make
carbohydrates.
Carbon Dioxide
Joseph Priestley’s
Experiment-1780
 Priestly took a candle,
placed a glass jar over it,
and watched as the flame
gradually died out.
 Something in the air was
necessary to keep the
candle burning.
 Today we call that
substance oxygen.
Plants Produce Oxygen
 Priestley than found out that if he
put a sprig of mint under the jar
and allowed a few days to pass,
the candle could be re-lighted and
would remain lighted longer.
 The plant produced the substance
needed for the candle to burn.
 Later it was shown that this would
only occur if the plant was placed
in the light.
Requirements for
Photosynthesis
 Photosynthesis
requires:
 Carbon dioxide
 Water
 Light
 Pigments such as
chlorophyll to absorb
the light energy
Formula For
Photosynthesis:
 6 CO2 + 6 H2O + Sunlight → C6H12O6 + 6 O2
 Carbon Dioxide + Water + Light→ Glucose + Oxygen
Sunlight
 Nearly all organisms
on Earth depend on
the sun for energy.
 Sunlight contains
wavelengths of
energy that can be
absorbed by plants.
Electromagnetic Spectrum
 Energy
exists in
different
wavelengths.
 Shorter
wavelengths
have more
energy
Why are leaves green?
 Leaves contain the
pigment chlorophyll.
 This pigment is green
because it absorbs all
other colors of light
except green.
 It reflects green light.
Pigments Absorb Light
Energy
Chlorophyll
 The process of
photosynthesis
begins when light is
absorbed by
pigments in the plant
cell.
 The most important
pigment is
chlorophyll-a green
pigment.
Energy-Storing
Compounds
 Light energy that is absorbed by chlorophyll is
transferred to ATP molecules.
Photosynthesis: The Light
and Dark Reactions
 Photosynthesis occurs in 2 sets of reactions:
 Light Reactions-trap sunlight energy and use it to make
ATP (The “photo” part of photosynthesis)
 Dark Reactions-use energy from ATP plus CO2 and
H2O to build glucose. (the “synthesis” part of
photosynthesis).
Glucose
Chloroplast
 Photosynthesis
takes place in the
chloroplast.
 Light energy is
trapped by
chlorophyll
molecules
embedded in the
membranes of
the chloroplast.
Draw and label all parts- color the
thylakoid membranes green!
Summary of Dark
Reactions
 Also called the LightIndependent
Reactions.
 In these reactions
(called the Calvin
Cycle), CO2 is used
to build glucose.
Overview of
Photosynthesis
 In summary, light
energy is changed to
chemical energy that
is stored in a
molecule of glucose.
 Plants and almost all
other organisms use
glucose to provide
them with energy for
cellular work.