12 Plant Nutrient Uptake S2019

12 Plant Nutrient Uptake S2019


This is a video lecture to finish up the
last portion of the 12 Plant Nutrient Uptake lecture. We’ll be focusing on
primary and secondary active transport. Recall that we have been talking about
how plants spend energy to take up nutrients from the soil. We’ve been
focusing on potassium as an example of a cation, and now we’re going to switch
over to think about nitrate as an example of an anion. Again we’re staying
at the molecular level to concentrate on how these ions are able to cross the
cell membrane from the soil into the root hair cell. This falls in the context
of thinking about how molecules pass from one side of the membrane to the
other. Remember that transporting ions requires
transport proteins because they’re so bulky with their sphere of hydration
that they’re unable to pass across the membrane by themselves. Additionally,
because these ions are moving against their concentration gradient, the cell
has to expend energy to bring them across the membrane, and it’s for these
two reasons that what we’re talking about is considered active transport. Last time, we talked about potassium
transport and the use of the proton pump as a form of primary active transport.
Remember, the proton pump is ATP synthase running
in reverse, and it hydrolyzes ATP to provide energy to move protons against
their concentration gradient, in the process creating a very strong
electrochemical gradient where there’s a net positive charge outside of the cell
and a net negative charge inside of the cell.
Again, this is classified as primary active transport because it’s driven by
energy from ATP hydrolysis itself, and it’s moving protons against their
concentration gradient. To get potassium ions into the root hair cell, then, uses a
form called secondary active transport which relies on the primary active
transport to set up that electrochemical gradient. Because potassium is moving
from its area of low concentration outside the cell to its area of high
concentration inside the cell, it requires an input of energy to do so. And
again this energy is coming from ATP hydrolysis and the proton pump
establishing this strong electric gradient. Potassium will then move from
its area of low concentration outside the cell to its area of high
concentration inside the cell because it’s drawn in by that strong negative
charge inside the cell and repelled by that positive charge outside of the cell.
But if we think about anions like nitrate, this strong electric gradient no
longer is sufficient to bring them from their area of low concentration outside
the cell to the area of high concentration inside the cell. Instead,
we’re going to be looking at a different form of secondary active transport
called a co-transporter. And as shown by the
image on the right, this co-transporter brings protons into the cell down their
electric and chemical gradient, and uses the energy released by that to also
bring in a nitrate against both its chemical and its electric gradient. So
let’s take another look at this. Recall that again we’re spending energy
using primary active transport, hydrolyzing ATP to get protons to move
from their area of low concentration to their area of high concentration and
because of this we’ve established a positive charge outside the cell a
negative charge inside the cell. And it’s this electric gradient which is then
sufficient for potassium ions to move against their chemical gradient but
along their electric gradient to move from outside the cell to inside the cell.
Finally, with nitrate this co-transporter is needed and it operates by bringing
protons in (we’ll go back to the color red). Protons now have such a high
concentration outside the cell and such a low concentration inside the cell that
there’s a very strong drive, right, it’s very energetically favorable for them to
want to come inside the cell, so they’re moving down both their concentration
gradient – very strong drive into the cell (so you see for chemical) – and a very
strong electric drive inside the cell. Again, that’s being repelled by that
positive charge outside the cell, attracted by that negative charge inside
the cell, and so both the electric and the chemical gradients are very aligned
for hydrogen. So again it’s very energetically favorable for that
hydrogen to move from outside the cell to inside the cell. And what a co-
transporter does is use this release of energy
as protons move down their gradients, and couples this to moving nitrate ions
into the cell, as well, even though this goes against both the electric and the
chemical gradient for nitrate. Remember nitrate is moving from an area of low
concentration to an area of high concentration, so it’s going against its
chemical gradient. Because nitrate is negatively charged it’s also moving
against its electric gradient, as it’s moving from a positive area to a
negative area. But again, this is possible because of this co-transporter that
couples the exergonic release of energy as a proton moves down its
electrochemical gradient – more than enough energy is released to drive this
endergonic movement of nitrate ions against their electrochemical gradient
into the cell.

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