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Hey everybody and welcome to Transport in Angiospermophytes, this is topic 9.2 from
the additional high level content in the DP biology syllabus.
In flowering plants, the structure responsible for the intake of nutrients and water are
the roots. Because of their important role in the water
and nutrient uptake, roots have structures designed to maximize absorption. On the image
on the right, you can see the first one: the image is dominated by a root going down the
middle with little hairs coming off of it. Those are called “root hairs”, because
in Biology, we like to keep names simple and easy to remember. They are cylindrical cells
responsible for the increase in surface area for absorption.
You probably have already seen a picture like the one on the left, with a tap root shooting
towards the ground and then smaller roots branching off of it. These branchings are
also there to maximize surface area and increase the water and nutrient uptake.
The main nutrients taken in by flowering plants are nitrogen in the form of nitrate and ammonium,
potassium as potassium ion, phosphorous as phosphate and calcium as calcium ion.
There are essentially three methods of nutrient intake.
Diffusion, which basically means a high concentration of nutrients going down a gradient, moving
towards the inside of the root where the concentration is low.
Fungal hyphae is an association of fungus and the plant root. The plant provides the
fungus with carbohydrates (food) while the fungus absorbs nutrients and relays it to
the plant. This is particularly crucial to plants because the fungal hyphae can reach
further distances than just the roots by themselves. Thirdly, a mass flow of water through the
soil which will then take dissolved nutrients into the plant as it moves through the soil.
You remember from topic 2.4 that diffusion does not require energy; whether it is unassisted
by protein channels (or simple diffusion) or assisted by protein channels (facilitated
diffusion). Flowering plants can, however, also intake
nutrients by active transport, which requires energy.
Evidence for this is the fact that when roots are deprived of oxygen, plants die. Ever wondered
why water in excess can kill plants? Terrestrial plants are adapted to absorb oxygen from pockets
of air in the soil, but not the oxygen dissolved in water.
There are basically three steps in active transport for the intake of mineral ions in
plant roots. First, the active pumps will pump out hydrogen ions, which turns the interior
of the cell very negative. This fact attracts positively-charged ions towards the inside
of the root. Lastly, the hydrogen that is pumped out, eventually returns through diffusion,
and negatively-charged ions tend to follow it when it does. This way, roots can absorb
both positive and negative ions through active transport.
You also remember from topic 2.3 that plant cells and animal cells are distinguished by
many aspects. One of them is the present of a cell wall made of cellulose in plants. This
adds support to plants because their cells are rigid, and can literally stack up on top
of each other. The interaction of the cell wall and the water
contained in a plant cell create an additional source of support called turgor. The cell
you see on the right is turgid, which means that it contains a lot of water, and this
water is causing pressure against the inside of the wall making it even more rigid. In
cases where there is less water, the cell membrane becomes flaccid and doesn’t offer
any additional support to the cellulose wall. Once the secondary meristem is activated,
lignin begins to deposit on the cells of the xylem. This creates a sturdy, continuous structure
inside of the stem and cortex of trees and shrubs. We’ll take a look at these cells
in a minute, in their transport role, but know that they’re also involved in supporting
the structure of plants with lateral growth. We’ve seen how water enters the plants;
now let’s take a quick look at how it leaves it. Flowering plants lose water through transpiration,
defined as the loss of water vapour from the leaves and stems of plants.
The transpiration stream moves from the roots towards the leaves. This direction of flow
is one of the details to keep in mind about transpiration. The other is that the xylem,
through which the water flows in flowering plants, is made up of cells that are lignified
– contain deposits of lignin – and therefore are no longer alive.
The lignin is not deposited randomly. In fact, it almost decorates the xylem with different
patterns. As you recall in Classification, traits like these can and are used to help
taxonomists identify species using a dichotomous key, as particular species will have their
own characteristic patterns of lignifications. The patterns allow water to flow not only
vertically, but also laterally, as cells along the stem also need water. On average, a medium-sized
tree will move 1000L of water per day. You also remember from topic 3, when you studied
the properties of water, that water molecules attract each other through hydrogen bonds,
the resulting property called cohesion. This is evident in transpiration as one water molecule
evaporates from a leaf at the top of the tree, it will actually pull a ‘chain’ of water
molecules up. This is known as the transpiration pull. Water is also attracted to the wall
of the xylem because of another property, called adhesion (very similar to the water
running up the inside of a micropipette, for example).
Some plants adapted for dry environments have developed thick waxy cuticles to prevent water
loss. However, this also prevents gas exchange. The development of stomata (one stoma, many
stomata) solves this problem, as they are located in the bottom of leaves and can open
and close in response to environmental conditions. You can see them in this picture, and the
stoma are actually the holes you see in this picture.
When plants lose water, guard cells, lose turgor and the stomata close to prevent further
water loss. The guard cells are actually the cells you can see around the holes in the
picture. The opposite occurs when plants have excess
water. This mechanism can also be controlled by a plant hormone produced in the root called
abscisic acid. Just like you observed in Topic 3, where several
abiotic factors affect the rate of photosynthesis, these can also affect the rate of transpiration
– or how quickly the plant loses water. Light will cause transpiration to increase
because the stomata close in when the plant is in dark.
Temperature rise will increase transpiration simply because there will be more energy available
to evaporate the water in the leaves. Wind also plays a role in transpiration. If
there are no wind currents, droplets of water will form around the stomata to prevent further
water loss. The wind will blow those away and cause new ones to form. The stronger the
wind, the more this cycle will repeat itself, causing the plant to lose more water.
Water will diffuse out of the leaf is there’s a concentration gradient. The lower the humidity
outside of the leaf, the more transpiration will occur.
It makes sense that all of these details are necessary to make water go up the plant, against
gravity. Sugars, on the other hand, have a much simpler task. They must just have to
go down! Sugars resulting from photosynthesis are produced
in the leaves and have to go pretty much everywhere. Because leaves are typically at the top, this
usually means sugars have to travel downwards. Unlike water, sugars will move through a living
tissue called ‘phloem’, and have to be moved using active transport. However, the
fact that these molecules are being moved in the same direction as gravity makes matter
much simpler. Xerophytes, from the Greek xyro – dry – and
phuton – plant, are organisms that are highly adapted for very exteme, water-deficient environments.
Some of these adaptations include reduced or rolled up leaves, spines, thickened waxy
cuticles, low growth form, a reduced number of stomata and stomata located within pits,
surround by hairs (as opposed to sitting on a flat surface on the bottom of the leaf).
As an excellent opportunity to study, you should research specific examples for each
of these traits, including species name, and create a collage with the images you find.
Remember to cite your sources and I’ll see you guys in class!