Transcript SBI 4U photosynthesis 1

Photosynthesis 1:
Light-Dependent
Reactions
This may get
confusing… try to follow
along with the diagram
on p160 of your text!
Overview
 Photosynthesis transforms light energy into
chemical energy.
 6CO2 + 6H2O + energy -> C6H12O6 + 6O2
 Two sets of reactions make up
photosynthesis: light dependent and light
independent reactions
 Water enters the plant through the roots, CO2
enters through the stomata (openings in the
leaves)
 They then diffuse into the chloroplasts of the
cells
Chloroplasts
 The chloroplasts’ inner membranes form
interconnected disks called thylakoids that
look like flattened sacs and will usually be
stacked on top of one another to form grana
 Surrounding the grana is a fluid-filled interior
called the stroma
Absorption of Light Energy
 Energy is absorbed from light in packets of
energy called photons
 Each wavelength (colour) of visible light is
associated with photons of one distinct amount of
energy
 A compound that absorbs that certain amount of
light energy is called a pigment
 Pigments that absorb energy and then pass it on
to another compound are called photosynthetic
pigments
 Photosynthetic organisms have a variety of
photosynthetic pigments, but the main ones are
chlorophyll a and chlorophyll b.
 Chlorophyll a and b absorb most
wavelengths other than green ones. They
reflect the green.
 Carotenoids absorb blue and green
wavelengths
Photosystems Capture Energy
 Chlorophyll molecules act in clusters embedded in the
thylakoid membrane with proteins in the centre of them
and make up a photosystem.
 Chlorophyll molecules in these photosystems are able to
absorb light energy at various wavelengths.
 A pigment absorbs a photon, the molecule passes the
energy to a unique pair of chlorophyll a molecules called
the reaction centre.
 An antenna complex includes all the surrounding pigment
molecules that gather the light energy, they then transfer
that energy to the reaction centre
 When the reaction centre gets the energy, it “excites” an
electron (raises it to a higher energy level) and is then
passed to an electron-accepting molecule
 Chloroplasts of plants have 2
photosystems called photosystem I (P700)
and photosystem II (P680) based on their
wavelengths of light they absorb.
Light Dependent Reactions
Step 1:
 P680 molecule in the reaction centre of
photosystem II absorbs a photon, exciting an
electron pair.
 Excited electron pair leaves the photosystem II,
leaving the P680 positively charged
 P680+ attracts electrons from water and splits it
to H+ and O2 gas.
 The O2 is released by the plant
 2 H2O molecules are needed to get 4 electrons
and one O2 molecule.
 The electrons are absorbed by the P680+ and
can now excite more of them with light photon
energy.
Light Dependent Reactions
Step 2:
 From the electron acceptor, the electrons
are transferred one by one along a series
of eletron-carrying molecules (electron
transport system)
 Each transfer of electrons releases a small
amount of energy
 Energy is used by a b6-f complex to pump
hydrogen ions form the stroma across the
thylakoid membrane
 This creates an electrochemical gradient
Light Dependent Reactions
Step 3:
 While steps 1 and 2 are taking place, light
energy is also being absorbed by
photosystem I.
 Energy is transferred to the reaction centre
P700 where electrons are being excited.
 Excited electrons are passed to an electron
acceptor but this time the electrons are
replaced by electrons coming from the end of
the electron transport system of photosystem
II.
Light Dependent Reactions
Step 4:
 Electrons that were received by the
electron acceptor from photosystem I are
used by NADP+ to form NADPH.
 The reducing power of NADPH will be
used in the light independent reactions.
Chemiosmosis
 Movement of hydrogen ions through ATP
synthase creates ATP
 Called photophosphorylation
Noncyclic and Cyclic
Photophosphorylation
 Non-cyclic is what we just looked at.
NADPH accepts the electron and then
moves to the Calvin Cycle.
 The passage of one electron pair
generates 1 NADPH and slightly more
than 1 ATP.
 This doesn’t produce enough to go
through with the light-independent
reactions, so chloroplasts will do cyclic
photophosphorylation.
 Excited electron pairs leave photosystem I
and are passed to an electron acceptor
 The pass back to the b6-f complex the
proton gradient is generated in the same
manner as in noncyclic
photophosphorylation and ATP synthesis
from the electrochemical gradient can
occur.
 Neither NADPH nor oxygen is produced