Download Photosynthesis - Northwest ISD Moodle

Photosynthesis
Main Idea
• Light energy is trapped and converted into
chemical energy during photosynthesis.
Overview
• Most autotrophs (only 10% of the Earth’s
species) make organic compounds, like
sugars, by photosynthesis.
• Photosynthesis occurs in two phases.
• Overall equation:
– 6CO2 + 6H2O  C6H12O6 + 6O2
Phase One: Light Reactions
• The absorption of light is the first step in
photosynthesis.
• Two energy storage molecules – NADPH
and ATP – are produced to be used in the
light independent reaction.
Chloroplasts
• Large organelles that capture light energy
in photosynthetic organisms.
• Found mainly in the cells of leaves
• Disc-shaped organelles that contain two
main compartments essential to
photosynthesis
Chloroplasts
• Thylakoids are flattened saclike
membranes that are arranged in stacks.
• The stacks are called grana.
• Light-dependent reactions occur in the
thylakoids.
• The stroma is a fluid filled space outside
the grana where the light-independent
reactions take place.
Pigments
• Light-absorbing colored molecules called
pigments are found in the thylakoid
membranes of chloroplasts.
• Different pigments absorb specific
wavelengths of light.
• Violet light has the shortest wavelength,
while red light has the longest.
• Chlorophylls are the major light absorbing
pigments in plants.
Pigments
• Chlorophyll a and chlorophyll b are the 2
most common types of chlorophyll in
autotrophs.
• These types of chlorophyll absorb most
wavelengths except green.
• This means the green light is reflected;
therefore, plants look green.
Pigments
• Other pigments are also found in leaves.
• In the fall when the chlorophyll starts to
break down, the other pigments are able
to be seen.
• Hence, the vibrant fall colors on the trees.
Electron Transport
• The thylakoid membranes have a large surface
area, which provides the space needed to hold
large numbers of electron-transporting
molecules.
• Photosystem I and photosystem II contain lightabsorbing pigments and proteins.
• Photosystems are composed of photosynthetic
pigments clustered together.
• Each pigment inside the photosystem is capable
of capturing photons, which are packets of
energy.
Electron Transport
• 1. Light energy excites electrons in
photosystem II.
• 2. Light energy causes a water molecule
to split, releasing an electron into the
electron transport system, a H+ ion into
the thylakoid space, and oxygen as a
waste product. THIS SPLITTING IS
ESSENTIAL TO PHOTOSYNTHESIS.
Electron Transport
• 3. The excited electrons move from
photosystem II to an electron-acceptor
molecule in the thylakoid membrane.
• 4. The electron-acceptor molecule tranfers
the electron along a series of electroncarriers to photosystem I.
Electron Transport
• 5. In the presence of light, photosystem I
transfers the electron to a protein called
ferrodoxin. The electrons lost by
photosystem I are replaced by electrons
shuffled from photosystem II.
• 6. Ferrodoxin transfers the electrons to the
electron carrier NADP+, forming the
energy storage molecule NADPH.
Chemiosmosis
• ATP is produced in conjunction with
electron transport by the process of
chemiosmosis.
• The breakdown of water is not only
essential for providing the original
electron, but for also providing the protons
(H+) necessary to drive ATP synthesis
during chemiosmosis.
Chemiosmosis
• Eventually H+ will accumulate in the
thylakoid interior and diffuse down the
concentration gradient to the stroma
through ion channels spanning the
membrane.
• These channels are enzymes called ATP
synthases.
• As H+ moves through ATP synthase, ATP
is formed in the stroma.
What Happens If…
• Some of the light-energized electrons may
return to chlorophyll after they’ve moved around
the electron transport chain, but most continue
on with NADPH.
• If these electrons are not replaced, the
chlorophyll will be unable to absorb light and the
light-dependent will stop and no ATP will be
made.
• To replace lost electrons, water molecules are
split by a process call photolysis.
Phase Two: The Calvin Cycle
• Because NADPH and ATP are not stable
enough to store chemical energy, there is
a second phase of photosynthesis.
• The Calvin cycle is the phase in which
energy is stored in organic molecules such
as glucose.
• AKA the light-independent reactions.
Phase Two: The Calvin Cycle
• 1. Carbon fixation – 6 CO2 molecules combine
with six 5-carbon compounds to form 12 3carbon molecules called 3-phosphoglycerate (3PGA).
• 2. Energy in ATP and NADPH is transferred to
the 3-PGA molecules to form high energy
molecules called glyceraldehyde 3-phosphates
(G3P). ATP supplies the phosphate group,
while NADPH supplies the H+ ions and
electrons.
Phase Two: The Calvin Cycle
• 3. 2 G3P molecules leave the cycle to be
used for the production of glucose and
other organic compounds.
• 4. An enzyme called rubisco converts the
remaining 10 G3P molecules into 5-carbon
molecules called ribulose 1, 5biphosphates (RuBP). These molecules
combine with new carbon dioxide
molecules to continue the cycle.
Phase Two: The Calvin Cycle
• It takes a total of 6 cycles for one glucose
to be formed.