electron
flow in l
ight reac
tion
events
the electron transport chain
photosynthesis' r
eactions
light-depe
ndent
light indep
endent
-requires sunlight
-does not require sunlight
-occur in thylakoid membrane
-occur in stroma
-light reaction
-dark reaction
chloroplast - small organelles inside the cells of plants
and algae. They absorb light to make sugar in a process called
photosynthesis.
• The thylakoid membranes are phospholipid bilayers
photon-the smallest
unit of light
photon
-When Chlorophyll gets hit by
that photon, an electron
absorbs that energy and gets
excited
(goes to a higher energy state)
-this is called photoexcitation
what's a photosystem?
Photosystems are light-absorbing complexes in the thylakoid
membranes that are present in photosynthetic organisms. There are two
types of photosystems: Photosystem II (PS II) and Photosystem I (PS I)
That excited electron is now going to go on a journey designed to
extract all of its new energy and convert that energy to be used by the
plant. this is called the electron transport chain. A
series of carriers through which electrons are transported from a
higher to a lower energy state.
the chlorophyll then transfers its high-energy electron to
an acceptor molecule.
this is a special protein designed specifically for carrying
electrons electrons, also called a mobile electron carrier.
electron
since the chlorophyll lost one electron, in cooperation with the rest of
PSII ,It splits that ultra-stable molecule, H2O, stealing one of its
electrons, to replenish the one it lost.
- the bypts of this water splitting: hydrogen (single protons) and oxygen
electron
from H2O
all energized electrons from PSII have been picked up by electron carriers and
are now being transported onto our second protein complex: the
Cytochrome Complex
it does 2 things:
- it serves as an
intermediary between
PSII and PS I
-uses a bit of the energy
from the electron to
pump another proton
into the thylakoid.
So the thylakoid's starting to fill up with protons. but why?
• By pumping the thylakoid full of protons, we're creating a
concentration gradient.
• the protons then move from areas of high concentration to areas of
low concentration through an enzyme straddling the thylakoid
membrane called ATP Synthase, and that enzyme uses that
energy to convert ADP to ATP.
proton
ATP
created
All this moving along the electron transport chain requires energy, and as you
might expect electrons are entering lower and lower energy states as we move
along.
but our electron is now about to be re-energized upon delivery to Photosystem
I. PS I is a similar mix of proteins and chlorophyll molecules that we saw in PSII, but
with some different products.
electron
After a couple of photons
re-excite a couple of
electrons, the electrons pop
off, and hitch a ride onto
another electron carrier.
This time, all of that energy will be used to help make NADPH, which, like
ATP, exists solely to carry energy around. Here, yet another enzyme, called
NADP+ reductase helps combine two electrons and one hydrogen ion
with a little something called NADP+ to create NADPH
So, what we're left with now, after the light dependent reactions is
chemical energy in the form of ATPs and NADPHs. And also of course, we
should not forget the most useful useless byproduct in the history of
useless byproducts...oxygen.
yey,
we're
done!
prepared by maimai
references that
saved
me:
• https://www.ncbi.nlm.nih.gov/books/
• https://www.youtube.com/watch?v=sQK3Yr4Sc_k -especially this one
• https://chem.libretexts.org
• http://mandevillehigh.stpsb.org