Photosynthesis
ATP
• Adenosine
Triphosphate is the
energy currency of
the cell.
• It does not get
destroyed (just like
money), but cycles
between 2 states.
– ATP: 3 phosphates  high
energy!
– ADP: 2 phosphates  low
energy!
Photosynthesis: Overview
• Photosynthesis is a process used by plant
cells to convert light energy from the sun
into chemical energy.
•
6CO2 + 6H2O + light  C6H12O6 + 6O2
• Photosynthesis combines carbon dioxide
and water with light energy and creates
sugar (glucose) and oxygen gas.
6CO2 + 6H2O + light C6H12O6 + 6O2
• How do we know that this
is the equation?
– Van Helmont’s Experiment
in the 1600s saw that water
contributed to the creation of
sugars.
– Priestley (1700s) showed
that plants produced
something that could keep
flames burning.
– Ingenhousz showed that
Priestley’s experiment only
worked when plants are
exposed to light
Showed that H2O is required
Showed that O2 is a product
Showed that light is a necessary!
Light
• Carbon Dioxide and water are converted
into sugars and Oxygen gas in the
presence of light. HOW???
• Plants capture light energy through the
use of a pigment called chlorophyll.
• 2 Types: chlorophyll a and chlorophyll b
Chlorophyll
• Only certain colors of
light are absorbed by
chlorophyll.
• Infact, chlorophyll
absorbs red and blue
light wavelengths
best!
• Leaves of plants
reflect green light,
and chlorophyll
absorbs poorly in
green light.
Function of Chlorophyll
• In the chlorophyll molecule the
light absorbed excites
electrons in the molecule.
• It is these high-energy
electrons that drive
photosynthesis
Reactions of Photosynthesis
• Photosynthesis takes
place in the
chloroplasts of plant
cells.
– Thylakoid membrane
forms structures called
Grana
– The area outside the
thylakoid is known as
the Stroma
Photosynthesis
• There are 2 parts of photosynthesis.
– Light Reactions
– Light-independent Reactions (Dark
Reactions) (Calvin Cycle)
• Each take place in a different part of the
chloroplast.
Light-Dependent Reactions
• In the thylakoid chlorophyll is arranged in the
membrane by proteins into structures called
Photosystems.
• As sunlight is absorbed by the photosystems, the
excited electrons jump from chlorophyll to a carrier
molecule called NADP+
• The purpose of the light reaction is to produce ATP
and NADPH
Light-Dependent Reactions
• The purpose of the light reaction is to
produce ATP and NADPH
Light-Dependent Reactions
• A. Light is absorbed in Photosystem II
– The electrons become excited and passed down an
electron transport chain to create energy.
– These missing electrons must be replaced!
– On the inside of the thylakoid water molecules are
split into H+ and O2 and 2 electrons.
– The electrons from the split water replace the
excited electrons.
– Hydrogen ions are released on the inside of the
Thylakoid. Oxygen gas is released as a product!
Light-Dependent Reactions
• B. The high energy electrons are passed down
Photosystem I.
– Some of the energy in the excited electrons is used to
PUMP H+ ions into the inside of the Thylakoid from
the Stroma.
• C. In Photosystem I the high energy electrons
are combined with H+ ions from the thylakoid
and transferred to NADP+ to create NADPH
Light-Dependent Reactions
• D. As electrons are passed, and NADPH is
created, lots of H+ ions are being pumped into
the inside of the thylakoid  makes it positively
charged and the outside (Stroma) negatively
charged.
• This difference in charge is used to create ATP!
• E. Another protein pump called ATP Synthase
transports the H+ ions from the thylakoid to the
Stroma like a water turbine, to create ATP.
– ADP + Phosphate group  ATP
Light-Dependent Reactions
• Summary:
– The Light-Dependent Reactions absorb light
energy to excite electrons. The excited
electrons are used:
• A. To pump H+ ions into the Thylakoid of a
Chlorplast
• B. To create NADPH
• To power ATP Synthase in creation of ATP
Light Independent Reactions
• The Calvin Cycle takes the energy
products from the Light Reactions and
uses them to synthesize sugar (glucose).
• AKA Calvin Cycle, Dark Reactions.
• These reactions do not require light
energy to occur.
Light Independent Reactions
• Light reactions occur in the Thylakoid
membrane.
– Creates NADPH on the stroma side of
Thylakoid
– ATP Synthase creates ATP on the stromal
side of the Thylakoid.
• Dark reactions occur in the Stroma of the
chloroplast.
Light Independent Reactions
• Named after Melvin Calvin (hence the
name Calvin Cycle).
• Converts Carbon Dioxide into sugar.
• Requires the energy of ATP and NADPH
to power the cycle.
Rubisco
• Rubisco is the most
abundant enzyme on the
planet!
• It catalyzes the first
reaction in the Calvin
Cycle.
• It combines the incoming
CO2 with the five Carbon
compound called
Ribulose-bisphosphate.
Calvin Cycle
Light Independent Reactions
• A. 6CO2 enters the cycle and through a chemical
reaction is combined with 6 5-carbon
compounds.
– Keep track of your Carbon.
• 6* (CO2) = 6 Carbons
• 6* (C5) = 30 Carbons
• 36 Carbon atoms total
• The result of the reaction is 12 molecules that
contain 3 Carbon atoms each.
• 12*(C3) = 36 Carbons
• 36 Carbon atoms total
Light Independent Reactions
• B. From the 12 3-Carbon compounds, 2 leave
and are used to make glucose.
• 36 Carbon atoms total
• 2 of the 3-Carbon molecules are used to create glucose
(C6H12O6) = 6 Carbons
• That leaves 30 Carbons. 10*(C3)= 30 Carbons.
• C. The remaining 10 3-Carbon compounds are
rearranged back to the original 6 5-Carbon
compounds the Calvin Cycle started with.
• 10*(C3) = 30 Carbons
• 6*(C5) = 30 Carbons
Light Independent Reactions
• The two 3-Carbon molecules are used to
create glucose!
• The remaining ten 3-Carbon molecules
are recycled, to keep the Calvin Cycle
going.
• For each turn of the Calvin Cycle ONE
glucose is made.
Summary
Summary
• In the Light Reactions:
– Water is split to generate electrons for the
Photosystem II.
• The split water produces O2 (waste product).
• H+ ions are pumped into the lumen of the Thylakoid to
generate a charge across the membrane.
– The excited electrons (via light) are passed along
ultimately being picked up by NADP+ to create
NADPH. This occurs in Photosystem I.
– The charge across the Thylakoid membrane is used
by ATP Synthase to create ATP.
Summary
• In the Dark Reactions:
– The ATP and NADPH created in the Light
Reactions are used to fuel the Dark
Reactions.
– CO2 is combined through a number of
reactions with other carbon compounds to
generate ONE glucose molecule.
– The remaining carbon is recycled to keep the
Dark Reactions going.