- [Instructor] Lecture eight quiz review
is on photosynthesis.
This is one that's also fairly difficult
if you don't know what you're looking for
for the quiz, as I mentioned
with lecture seven's quiz review,
these are the harder lectures because
it just ramps things up.
So pay close attention as
you're going forward here.
There are some simple questions,
but there are some hard questions as well.
Some of the simpler questions are
what goes into photosynthesis
and what comes out?
Sometimes I use the terms,
reactants or substrates
and then I use the term products
or reactants or substrates,
that's light, carbon dioxide and water.
Products are things like
glucose and oxygen, right?
So that's an easy one.
But there's also some harder ones too,
based on what we've
learned in the lecture.
And that is things like
let's say I were to tag
this oxygen right here.
This carbon dioxide oxygen with an isotope
meaning it's a different,
heavier version of that atom.
And we can distinguish
it from the other atoms.
And scientists have done
this and this is why
I use this as a scenario.
And I say, where does this oxygen end up?
Well, that requires that
you understand or remember
what we've talked about in this lecture.
And that is, where do the atoms go?
So again, let's jump ahead real quick.
Carbon dioxide, these
atoms go into glucose.
So that's what you would
say, is that this oxygen
would end up over here in
glucose, not in this oxygen
that is released as oxygen
gas and not into the water.
That's one of the reactants, right?
It's one of the reasons why this picture
is such a good picture as it illustrates
the separation of some of these processes.
So again, same thing if I asked,
if we tagged this oxygen in
water, where does it end up?
It ends up in the air,
the oxygen in the air,
it doesn't end up in the glucose here.
Okay, and they've done
these experiments before.
So that's another question.
That's a harder question,
is where does the oxygen go, right?
There's another one that
also illustrates a concept.
And that is, I'll ask you
where most of the organic mass
of the plant comes from as it goes
through this photosynthesis process.
And I give you a little story
about how these scientists
were measuring the soil and determining
how much nutrients were
taken up from the soil,
you know, ammonia and
nitrate and things like that.
And they found that most
of the biological mass
didn't come from the soil.
It came from the air, the carbon dioxide
because this glucose,
remember is a precursor
for all carbohydrates,
but also that Calvin cycle
is used to essentially make
other things like amino acids
and lipids and other things like that.
So make sure that you understand that
most of the biological organic
mass that is incorporated
into a plant basically comes from the air,
from carbon dioxide.
Now, there will be a question
on photosynthetic pigments,
make sure you know, the
chlorophyll and carotenoids
because I'm going to test you on to some
of the general wavelengths that are used.
For example, chlorophyll
absorbs blue and red,
while carotenoids absorbs blue and green.
So make sure you know the
pigments as it relates to energy.
You're not gonna be given questions
on all the different spectrum of energy.
But you are gonna be given
questions on generally
what colors these
photosynthetic pigments absorb.
And so make sure you understand
chlorophyll and carotenoids.
Those are the only two that
are gonna show up in this.
Now there's some
hypothetical questions that
are related to energy absorption
that you're also going to get.
I usually use a forest
tree scenario with trees
that have black leaves and
white leaves and green leaves
and orange and yellow
leaves and all of that.
And it comes down to the
concept of energy absorption.
So remember that if the leaves are black,
that's the maximum amount
of energy absorption
because they're absorbing
all visible light.
That's why solar panels are black,
it's because they're trying
to absorb all visible light
to turn it into electrical energy.
Whereas plants they have
to have this trade off
too much energy absorption,
and it causes the plant to heat up.
And that's not good.
And that's generally why they don't absorb
all visible light, but they absorb
as much as they can handle.
And that's generally
the chlorophyll pigments
are the optimal for that.
That's why most photosynthetic
organisms are green,
because green is one of the least
absorbed wavelengths of light.
Now, other questions may
ask which colored leaves
are absorbing the least?
And now this is a hypothetical question.
And in those you would
look for the white leaves,
because white remember is a
reflection of all visible light.
So while you go outside on
a hot day with a white shirt
not a black shirt, otherwise,
you're gonna get hotter
than what you normally would
because your shirt pigment
would be absorbing more energy.
So you wear a white
shirt because it reflects
most of the visible
light and doesn't cause
the material to heat up.
There are other questions,
though, that may say,
oh, the chlorophyll doesn't break down.
What color are the leaves of the forest?
Well, in that case, it'd be green
because if there's still
chlorophyll in the leaves,
then they're gonna reflect
green while absorbing
the other wavelengths of light.
So that is a question that
everyone's gonna get to
hypothetical, you know, forest question
that basically teaches you the principles
of energy absorption as we've been talking
about the different wavelengths of energy,
but how it applies, especially to plants.
Now the other questions you're gonna get
are on some of the
particulars of photosynthesis.
Photosystem two, photosystem one,
chemiosmotic phosphorylation,
where water is involved,
remember, when water is not available,
everything shuts down.
Not just ATP and NADPH production,
but also the Calvin cycle
because where is the Calvin Cycle
gonna get energy if not
from the light reactions?
So everything shuts down
if there's no water.
Sometimes I refer to the light reactions
as the thylakoid reactions,
and sometimes I refer
to the Calvin cycle reactions
as the stroma reactions, why?
Well, like in the
beginning, talked about some
of the parts of the chloroplast.
The phylakoids are where the
light reactions take place.
And the stroma is the fluid
the Calvin cycle takes place,
and okay, so I use those
interchangeably at times.
So just keep an eye out for that.
Make sure that you know
that the antenna complex
is the chlorophyll and the carotenoids.
Sometimes people just miss that process
that I talked about the
antenna complex is the pigment.
Think about it.
It's like a radio antenna, it
picks up the radio signals,
and then you're able to
interpret the information.
Well, chlorophyll and the carotenoids
are the antenna complex they
absorb the light energy,
and then they are able to
transfer to an electron,
which travels out the reaction center
and then down the
electron transport chain.
Make sure you know which one does what.
Photosystem two, makes ATP
through chemiosmotic phosphorylation.
Photosystem one makes NADPH,
a special name for that one.
The Calvin cycle make
sure you know the name
of the enzyme that starts it.
It's called rubisco.
A lot of times people confuse rubisco
with the sugar called RuBP, why?
Well, they get their
names after each other.
Rubisco is the shorthand
for ribulose biphosphate
carboxylase oxygenase.
Yeah, nobody wants to say that, right?
So we call it rubisco.
Well, ribulose biphosphate
RuBP, that's the sugar.
So people always confuse those two.
Rubisco enzyme, RuBP sugar.
So rubisco combines
carbon dioxide with RuBP.
Make sure you know that.
And the only reason why
I want you to know that
is because of the C4 pathway
and the discussion we
had on photo respiration.
That's the only reason,
you don't even need to know
all these other enzymes that
are involved in this process.
Over here make sure you
understand that in PGAL synthesis,
the energy comes from the light reactions.
This is where ATP and NADPH
from the light reactions
gives us energy to
energize these molecules
and then a couple of them
can be made into amino acids
or glucose or lipids or whatever,
whatever the plant needs, right?
It's not just sugar that the plant
is making through photosynthesis.
One question on photo respiration,
make sure you understand
that the C4 pathway is in
addition to the Calvin cycle.
It does not replace the Calvin cycle.
I will not distinguish
between C4 and CAM plants
and all that just make
sure you understand that,
95% of the plants on the
planet don't use the C4 pathway
they just use the Calvin cycle.
100% of the plants on the
planet use the Calvin cycle,
but only 5% use the C4.
These are the C4 pathway,
these are the ones that
live in the desert.
That have to avoid photo respiration.
And this is what photo respiration is.
They conserve water, oxygen builds up
through the photosynthesis process,
it interrupts rubisco's ability
to combine carbon dioxide
with the sugar, and instead
of capturing carbon dioxide,
they release it and
you get this stalemate.
And that's why the C4 pathway
is a traffic workaround
to capture carbon dioxide
separate from rubisco
so that the Calvin cycle
can keep making that
even when oxygen starts building
up in these cells, okay?
So that's the only one
that's a little difficult.
All the others are pretty straightforward.
But make sure you understand
the different parts of these processes.
And generally, about you
know, half the questions
are pretty straightforward.
The other half require you
to do some critical thinking
which is kind of the point of all of this
is to think about how all of this matters
and why it matters and how
it applies and everything.
