So now we've learned about the
background to some extent, the machinery
that the plant is going to use. Then we
have to think about the plant itself -
what is it? Well, it's basically a support
structure for chloroplasts, that's what
it's there for.
It's rather rather like The Selfish Gene
view you can argue that plants exist
only to support the chloroplasts that are
within it.
So we have typically a support structure -
well, first we start the very bottom,
we've got the roots which you've already
remarked are essential for pulling the
nutrients out of the soil that the plant
uses to make the machinery which in
photosynthesises with. Then there's a
trunk that typically holds the plant up.
It's, to be honest, it's not always quite
clear what the function of a trunk is. In
many cases it helps to keep the leaves
above predation, or it may move them up
into sunlight where so the
photosynthesis becomes more effective.
But if you take it to the extreme and
you look at for example why some trees
are taller and others are short, it's not
at all apparent why that's the case. So
there are some subtleties involved in
there. So that trunk is mostly made out of
carbohydrates which at the very
beginning we learned came from the seed
but all the others since then have been
produced by the photosynthesis to plant
itself. So that you've got the trunk and
you got the branches and they serve to
distribute the leaves, and the leaves
are simply organs to photosynthesise.
They are little structures to house
chloroplasts. So the trunk and the roots
bring the water up that the plant needs,
and the leaves themselves are broad and
flat, and optimised to capture sunlight
and to give a high surface area to which
CO2 can be brought into the
mechanisms that will capture it and
convert it into sugars. The leaves
themselves typically they're sort of a
bit of a compromise between structural
strength and making the best out of the
resources that are available.
Then of course the majority of the leaf
is made out of out of carbohydrates, but
there are the materials in there such as
lignans as well which start their lives
also as compounds but synthesised from
carbon dioxide with water. So the leaf
has little holes in the bottom that are
called stomata and these stomata served
to allow carbon dioxide into the chloroplasts where the sunshine will illuminate
the chlorophyll within them and can be
converted to energy. The holes in the
bottom of the leaf, the little stomata,
they lead into usually a rather spongy
tissue called mesophyll, where much of
the chloroplasts are. Though they're also in the
upper part of the leaf just below the
surface. The light penetrates fairly
deeply into the leaf and often the
maximum photosynthesis is well into the
thickness of the leaf. Now what the little
holes do? Well, one of the things they do
is lose water. And the water, actually
losing water is not really an essential
part of the plant's function. The plant
spends a lot of effort and energy to
throw water out of the soil up through
the branches and out through the leaves.
Why does it do it? Well water cools the leaves a bit, which on some
circumstances can be useful. But it's
mostly it's because they want to capture
CO2. To capture CO2, the little stomata in
the bottom of the leaf have to be open
and when they're open
typically a plant loses something like
400 times as much water as the CO2 that
it gains. That's partly because water
being a lighter molecule diffuses faster
than CO2 does, and partly of course because
there's a lot more water than CO2, for
which we can be grateful. Something like
25,000 ppm of water versus, at the moment, about 400 ppm of CO2. But the plant
doesn't want to lose the water it's just
stuck with that situation. And some
plants such as cacti, will go out of
their way to avoid that situation. So a
cactus is a bit cunning about the way
that it separates its photosynthetic
processes. The stomata open at night and the cactus captures CO2 through the open
stomata while
the temperature is lower and the
relative humidity is higher so loses
less water. It then stores the compounds
that's created with the stored carbon
dioxide, closes its stomata, and waits for
sunshine. And then when the sunshine
comes it moves into the light processes
of photosynthesis and it starts creating
ATP and NADPH and all those other
substances
there'll be necessary to drive the dark
cycles that can continue in the night
and actually synthesise the sugars.
