Photosynthesis & Cellular
Respiration
How plants get Energy
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Photosynthesis
• Photosynthesis - the process in which
green plants use the energy of sunlight to
convert water and carbon dioxide into
high-energy carbohydrates and oxygen
2
Photosynthesis
• There are two main steps in
Photosynthesis.
• Light reaction
• Calvin Cycle
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The Photosynthesis Equation
• The Photosynthesis Equation
• The equation for photosynthesis is:
• 6CO2 + 6H2O
Light
• carbon dioxide + water
C6H12O6 + 6O2
Light
sugars + oxygen
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Light Energy
• Light is a form of energy
– chlorophyll absorbs light.
– The energy is transferred directly to electrons
in the chlorophyll molecule raising the energy
levels of these electrons.
– high-energy electrons are what make
photosynthesis work
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Inside a Chloroplast
photosynthesis takes place inside chloroplasts.
Plant
Chloroplast
Plant cells
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Inside a Chloroplast
• Chloroplasts contain thylakoids—saclike
photosynthetic membranes.
• Thylakoids are arranged in stacks known as grana.
A singular stack is called a granum.
• Stroma – space outside of the thylakoids
Granum
Stroma
Single
thylakoid
Chloroplast
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Light and Pigments
• photosynthesis requires chlorophyll
– pigments - light-absorbing molecules that
gather the sun's energy
• The main pigment in plants is chlorophyll.
• There are two main types of chlorophyll:
– chlorophyll a
– chlorophyll b
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Light and Pigments
Estimated Absorption (%)
• Chlorophyll does not absorb light in the green
region of the spectrum. Green light is reflected by
leaves, which is why plants look green.
100
80
60
Chlorophyll b
Chlorophyll a
40
20
0
400 450 500 550 600 650 700 750
Wavelength (nm)
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Inside a Chloroplast
• Proteins in the thylakoid membrane organize
chlorophyll and other pigments into clusters called
photosystems, which are the light-collecting units
of the chloroplast in the thylakoid membrane.
Photosystems
Chloroplast
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Light Reaction Sep 1
• Light energy forces electrons in the
chlorophyll a molecules to become
excited.
• This means that have more energy than
usual.
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Light Reaction Step 2
• The electrons leave the chlorophyll a
molecule and are accepted by the primary
electron acceptor.
• This begins the start of the electron
transport chain and activates the
Hydrogen pumps.
• Hydrogen is pumped into the Thylakoid.
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Light Reaction
• However, certain electrons are donated to
NADP (cousin to NAD) instead of heading
down the chain.
• NADP takes and electron and a Hydrogen.
13
Light Reaction
• SO…. Where did the electrons that were
lost come from?
• Water!
• An enzyme in the Thylakoid breaks two
water molecules down into four hydrogen
ions (H+), their 4 electrons and O2.
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Light Reaction
• An enzyme in the Thylakoid breaks two
water molecules down into four hydrogen
ions (H+), their 4 electrons and O2.
• This is why plants don’t need to take in O2.
They make it by splitting water!
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Calvin Cycle
• Now that the plant has made energy it is
going to use that energy to make a simple
sugar.
• But WE NEED CARBON to make sugar.
• Plants take in CO2 and turn it into sugar!
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Calvin Cycle Step 1
• We need three CO2 to get started.
• We start by hooking the carbon from each
CO2 up to a 5 carbon chain with
phosphates at each end.
• This now make a 6 carbon chain with
phosphates at each end.
• We create three of these at a time.
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Calvin Cycle Step 2
• Each carbon chain breaks in half.
• You now have 6 three carbon chains with
a phosphate at each end.
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Calvin Cycle Step 3
• The ATP and NADPH created in
photosynthesis gives a phosphate and a
hydrogen to each of the three carbon
chains.
• There is now a phosphate attached at
each end.
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Calvin Cycle Step 3 Cont.
• The three carbon chains now give off a
phosphate.
Yes we gained 6 phosphates to lose 6
phosphates. CRAZY!
But this give the three carbon chains the
energy needed to rearrange slightly.
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Calvin Cycle Step 4
• One of the new three carbon chains
leaves to become a carbohydrate.
• We have made a molecule of sugar!!!
• The rest of the 3 carbon chains use 3
ATPs to recombine into the 5 carbon
chains.
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Inside a Chloroplast
H2O
CO2
Light
NADP+
ADP + P
Lightdependent
reactions
Calvin
Calvin
cycle
Cycle
Chloroplast
O2
Sugars
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Photosynthesis Reactions
– light-independent reactions (Calvin cycle)
• takes place in the stroma
• ATP and NADPH not stable enough to store the
energy they carry for more than a few minutes
• uses ATP and NADPH energy to build high-energy
sugars for long term storage
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Factors Affecting
Photosynthesis
• Many factors affect the rate of
photosynthesis, including:
• Water
• Temperature
• Intensity of light
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Comparing Photosynthesis and Cellular
Respiration
• The energy flows in photosynthesis and
cellular respiration take place in opposite
directions.
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Comparing Photosynthesis and Cellular
Respiration
• On a global level, photosynthesis and
cellular respiration are also opposites.
– Photosynthesis removes carbon dioxide from
the atmosphere and cellular respiration puts it
back.
– Photosynthesis releases oxygen into the
atmosphere and cellular respiration uses that
oxygen to release energy from food.
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