Download The Light Reactions

1. Pick up a copy of the notes
and the diagram.
The Light Reactions
Light Reactions
• The light reactions consist of those reactions
that require the presence of light in
photosynthesis.
• Light energy from the sun and water are used
to produce ATP and another energy carrier
known as NADPH.
• These two products are important because
they are necessary for the actual production
of organic molecules (such as glucose) that
occurs during the dark reactions.
The Photosynthesis Equation
6CO2 + 6H2O
light
C6H1206 + 6O2
What’s necessary for the light
reactions?
• NADP, ADP, water, and sunlight.
What’s produced in the light
reactions?
• Products of the light reactions (light
dependent reactions) go on to the Calvin
Cycle
• NADPH, ATP, and oxygen as a byproduct.
Light Reactions Step by Step
• Recall that photosynthesis occurs within
the chloroplast of a cell. The light
reactions occur in the thylakoids of the
chloroplasts. The thylakoid membrane is
composed of a phospholipid bilayer that is
very similar to the membrane found in a
cell.
• Within the membrane of the thylakoid there
are two clusters of pigments that we refer
to as photosystem I and photosystem II.
Remember that pigments absorb light.
The pigments present in these
photosystems are chlorophyll a,
chlorophyll b, and caretonoids. The most
important event associated with both
photosystems is the capturing of light.
• When a photon of light strikes
photosystem II, it excites an electron. At
the same time an enzyme binds to two
water molecules and splits the water into
hydrogen ions (H+ or protons) and
releases an oxygen atom (O2). Note:
This is why water is necessary for
photosynthesis to occur and this is where
oxygen is released as a byproduct of
photosynthesis.
• Two electrons are released in the process
of photolysis described above. These
electrons can be traced all the way
through photosystems I and II. The
proteins and enzymes that pass these
electrons through out the thylakoid are
known as the Electron Transport Chain.
• A primary electron acceptor accepts the
light-energized electrons that have been
energized in photosystem II and passes
them to a proton pump (“pp” in the
diagram).
• This proton pump moves the protons (H+)
across the thylakoid membrane against their
concentration gradient where they build up in the
thylakoid interior. This will have importance
later. This is a form of active transport. The
protons then exit the thylakoid interior through a
special protein channel that is ATP synthase.
Recall that ATP synthase is the enzyme that is
associated with the synthesis of ATP molecules.
• As protons pass through the ATP synthase
down the concentration gradient, they fuel
ATP synthesis. In this process a
phosphate is added to a molecule of ADP
(adenosine diphosphate) and ATP is made
and released into the stroma of the
chloroplast. This process is known as
photophosphorylation.
• Now back to the electrons that left
photosystem II and energized the proton
pump. After fueling the proton pump (pp)
the electrons are now de-energized but
they are still not finished. Another electron
carrier carries the electrons to
photosystem I (EC).
• These electrons are again reenergized in
Photosystem I as light is absorbed. These
reenergized electrons are passed to
another primary electron acceptor (“PEA2”
in the diagram) and carried to an enzyme
called NADP Reductase (“R” in the
diagram). Electrons that leave
Photosystem I are replaced by the
electrons from Photosystem II.
• The NADP Reductase combines NADP
with protons (H+) found in the stroma to
make NADPH. The NADPH combined
with the ATP produced in the light
reactions now move on to the Calvin
Cycle.
The Dark Reactions
• The dark reactions are also known as the
light independent reactions or the Calvin
Cycle. Named after Melvin Calvin.
• The light independent reactions do not
require the presence of light to occur.
• The dark reactions are the “synthesis” part
of “photosynthesis.”
• Carbon fixation occurs.
What’s Necessary for the Dark
Reactions?
• The dark reactions cannot take place
without the ATP and NADPH that was
produced during the light reactions. Recall
that ATP and NADPH are energy carriers.
• Carbon dioxide from the environment is
also necessary for the dark reactions.
What’s produced by the dark
reactions?
• Glucose (a 6 carbon sugar) and other
energy storing organic compounds are
made.
• NADP+ and ADP are produced to be
reused by the light reactions.
Glucose
• Glucose is a monosaccharide
• C6H12O6
• One Molecule of glucose
Stores 90 Times More
Chemical Energy Than One
Molecule of ATP
Energy is added. The six-carbon
molecules split to form threecarbon molecules. More energy
is added and the molecules are
rearranged into higher-energy
molecules.
Carbon dioxide
molecules enter
the cycle and are
added to fivecarbon
molecules. 6Carbon
molecules are
formed.
A high-energy three-carbon
molecule exits for every 3
CO2 molecules that enter.
After 2 three-carbon
molecules have exited, they
bond to form 1 six-carbons
sugar.
Refer to 111
in the book.
Three-carbon molecules are
changed back to five-carbon
molecules by energy from
ATP.
Alternative Pathways
• Many plants in extreme conditions
(particularly hot and dry environments)
have alternative photosynthesis pathways
to make organic compounds.
• C4 Plants
• CAM Plants
C4 Plants
• These plants fix carbon dioxide into fourcarbon molecules instead of three-carbon
molecules during the Calvin Cycle.
• These plants partially close stomata
(pores found in leaves) during the day to
avoid water loss.
• Examples: Corn and sugar Cane
CAM Plants
• Occurs in plants that live in extreme heat.
• Allow carbon dioxide to enter only at night.
• Examples: cacti, orchids, and pineapples
Factors Affecting the Rate of
Photosynthesis
• Amount of available water
• Temperature
• Amount of available light
energy