all right so now we're gonna talk about
kind of one of the big wrinkle in this
whole process so photorespiration
probably sounds like something that
sounds good it's like photo light
respiration oh that sounds good photo
respiration is generally speaking bad so
photorespiration happens because stomata
if you remember those are the pores on
the underside of the leaf that allow
gases to come in and out of the leaf and
the problem is when it's hot and dry the
plant has to worry about (I'm using
anthropomorphic language) but the plant
will basically close the stomata to keep
from drying out dying and the problem is
then it can't exchange gas with the air
around it so what happens is over time
the photosynthetic apparatus will
actually use up the carbon dioxide it
will fix all of it into sugars and so
when you get a below about 50 parts per
million (PPM) and currently carbon dioxide
levels are around 400 parts per million
it's actually quite elevated from what
it used to be but we'll talk about that
later but as the carbon dioxide drops
you get photorespiration and the reason
for this as we talked about Rubisco is
Ribulose 1,5 bisphosphate carboxylase
oxygenase so it can also act as an
oxygenase and when it does that it will
fix oxygen instead of carbon dioxide and
the problem is this is bad as far as the
plant is concerned it has to expend a
bunch of enzymes and energy and time to
basically get rid of the stuff that's
left behind and some of the chemicals
are actually poisonous to parts of the
photosynthetic apparatus some of the
other enzymes are actually inhibited by
them so that's bad so this is actually
an image this is a space-filling image
of Ribulose 1,5 Bisphosphate carboxylase
oxygenase also known as Rubisco
so here's the process so the normal
process is up here carboxylase we get
3-pga we get our three carbon compound
we go whoo hoo that's awesome when we
get oxygenase instead of getting a three
carbon compound we get this funny thing
and we end up with glycolate and this is
one of these compounds that can actually
slow down photosynthesis this can go
onto the Calvin cycle but basically we
didn't fix any carbon were essentially
losing carbon and that's bad so this is
actually an outline of the whole process
of basically getting rid of glycolate so
I mentioned the peroxisome we
talked about it so glycolate it goes in
here basically we get H2O2 which is
hydrogen peroxide also very damaging so
then we have to have Catalase to get
rid of that let's go over to the
mitochondrion we're releasing a little
bit of CO2 back
we've got NADH actually being picked up
and then being expended over here then
we're expending some ATP all the way to
get back to the calvin cycle so it's
really really crazy so basically you
know is this a bad thing so you might
say well why haven't plants figured out
a way to go to get around this problem
and in some extent they have although
not all plants is actually a fairly
small number of them
and one of the ways of getting around
this is something called C4
photosynthesis so this is the 4 carbon
pathway so if you remember I just talked
about in normal photosynthesis the
initial carbon fixed is C3
it has 3 carbons these guys actually
start with 4 carbons they still use
Rubisco because for whatever reason
nothing has really figured out anything
that's better than Rubisco at doing what
it does but it adds basically an extra
step on top of (normal photosynthesis) C4 plants have
something called Krantz Anatomy
basically it's this wreath shape so this
wreath shape is got these these cells
the bundle sheath cells so this is the
vascular bundle this will actually
transmit water and nutrients through the
leaf if you haven't learned about yet
but we will well we are right now
xylem and phloem so it's going to
transport this as the plumbing of the
leaf and these are the cells that go
around it and the cells that go around
it these have actually specialized to
essentially split the two halves of
photosynthesis the light reactions and
the calvin cycle to be in different
cells so we've got large chloroplasts
with a few to no grana that's those
membranes and the bundle sheath cells
and then we've got smaller chloroplasts
with very well-developed grana in the
mesophyll cells so there's split these
two halves so you might guess these
mesophyll cells if they got well
developed grana that's those
membranes that's where the light
reactions take place so these are going
to be specialized in the light reactions
these are going to be specialized in
the Calvin cycle because they don't have
very many grana and they've got these
relatively large cells they've got lots
of stroma so lots of stroma that's where
all those enzymes are that's where
Rubisco is so this is where the carbon
fixation is going to take place you
might say first of all why does that
help so first of all you're releasing
oxygen in the light reactions so you
keep oxygen over here and then we don't
have to deal with oxygen over here
because we're not doing the light
reactions so we don't produce as much
oxygen there but there's actually an
extra step that they do which makes this
even more efficient
so c4 plants have an extra enzyme PEP
carboxylase so this is another important
enzyme I I said it only have you guys
remember Rubisco but this is one that's
specific to c4 so c4 PEP carboxylase is
basically doing to take this carbonic
acid (CO2) and it's going to fix it into
oxaloacetate, malate and aspartate so
it's actually going to bind these up
into these organic acids so it can do
this basically what it does is it's
going to concentrate carbon dioxide. Why
is this important? Because PEP
carboxylase notice it's just
carboxylase it's not an oxygenase and that
means this enzyme
doesn't get confused by high levels of
oxygen so it's going to concentrate the
co2 and what cells is it going to
concentrate this in hmmm well initially
it's actually going to fix it in the
mesophyll cells and the reason for that
this is the stoma so this
is where the gas is going to come in so
the initial place where carbon dioxide
is going to land is right here so the
PEP carboxylates is actually going to
act here but those organic acids are
going to be transported to the bundle sheath cells
because this is where the Calvin cycle
is going to take place and those organic
acids will actually be turned back into
carbon dioxide once they get over here
so this is a electron micrograph diagram
from your textbook that shows this so
here we've got CO2
we've got oxaloacetate PEP carboxylase
it's going to be turned into malate
notice we're using some NADPH to do this
so we get all the way over here we're
transporting malate use some NADPH we go
back to co2 and of course what enzyme is
going to carry out this reaction it's
still gonna be Rubisco so Rubisco will
still carry out the calvin cycle just
like normal the only thing that's
different is the light reactions are
over here so that NADPH and ATP have to
be transported over here and the organic
acids have to be transported over here
but you notice that the chloroplasts
here are very different they have very
few grana versus over here they have
these super thick very active grana
right here so light reactions are taking
place in the mesophyll cell this is the
bundle sheath cell so it's split up and
now we've got the Calvin cycle taking
place over here so it's really a neat
process
so this is kind of an overview so a lot
of this takes place in grasses although
not all grasses are c4 a lot of them are
not a lot of them are c3 there are some
economically important grasses that are
c4 for example sugarcane but also corn
and a couple of others but wheat is a c3
grass so right here we've got CO2 coming
in we have a c4 pathway that's the
capturing of those enzymes with PEP
carboxylase turning them into organic
acids like malate and oxaloacetate then
It's gonna transport
that to the bundle sheath cell where
they can be used
we've got Rubisco which is still gonna
grab carbon dioxide do the calvin
cycle like normal that's what happens in
the bundle sheath cells that's c4
photosynthesis so you might say well why
don't all plants do this again because
the c4 pathway actually costs a little
bit of energy to do this now I also said
that this is mostly found in grasses
there are c4 we haven't talked about
them yet but there are c4 dicots that's
a whole the other group of angiosperms
so here's here's the deal is there's
kind of a competition so light intensity
versus increase in photosynthesis so at
low light you notice that the c3 plant
actually has a better photosynthetic
rate than the c4 plant however if you
get high enough light intensity the c4
plant is actually be able to respond to
it more so you're gonna find a c4 plant
usually in a very bright open location
so a lot of prairie grasses and
grassland plants are c4 plants then we
can look over here same thing with
temperature so as temperature increases
the c3 plant it goes down the c4 plant
doesn't really matter as far as
temperature because it can concentrate
that carbon dioxide
they can keep it stomata open (closed) for a lot
longer so also relatively hot dry open
that's the kind of places you're going
to find more c4 plants versus C3 plants
but the thing to know is that in terms
of the number of species on earth there
are far more c3 plants that's that
normal photosynthesis without the extra
steps then there are c4 plants
so there's one other wrinkle to this
which is CAM photosynthesis CAM
photosynthesis is kind of the even more
extreme form of photosynthesis that
allows plants to survive in dry
environments so this you're going to
find in something like succulents
especially cacti and things it's fairly
common in a lot of house plants so
they're actually going to be able to
survive really really dry environments
so what they do is because they have
they've now split well they don't really
split their parts of photosynthesis they
use those same enzymes that we saw in
the C4 they can capture co2 but what
they do is they do it at night so at
night time it's cooler using a little
bit more moist so if you're a desert
plant that's the time to open your
stomata so you're not going to lose as
much water of course the problem is you
can't do photosynthesis at night so
instead they're using our good friend
PEP carboxylase again and it's going to
bind up all of the stuff into acids
that's actually what the name is the
name of this stands for Crassulacean
acid metabolism because people that were
studying these they noticed that the
tissue of these plants became more
acidic at night so basically these guys
are going to grab onto the co2 you're
going to bind it up as acids and then
during the day when the stomata is
closed they can take all of that acid
turn it back into co2 and then basically
carry on photosynthesis as usual with
Rubisco on all those types of things so
really useful for plants that are
dealing with really low levels of water
and lots of light
