Download Solutions - ISpatula

Root Architecture and Acquisition of
Water and Minerals
There are short and tall trees. We have annual trees(surviving just for one growing
season) , biennial plants(that takes two years to complete its biological lifecycle)
and Perennial trees (that lives for more than two years).
The size of plant is correlated to the size of roots. For instance, perennial trees
have huge have huge roots that grows deeply in the soil to anchor the plant and to
get the maximum amount of resources that are existed in the soil.
We mean by resources the water and minerals (like nitrates and phosphates).
The angiosperms (the flowering plants) are either monocots or dicots, try to get a
root of a corn or a wheat plant , you will find it fibrous ( the root would be near the
surface), a root like that is not enough to give a basis to a tree of many meters
length.
Most monocots except palms live a short life (annual trees; they live one year)
The huge trees (the dicots), usually have taproots which grow deeply in the soil
and has branching.
If the plant needs nitrate for example, and it is far from this mineral, the root will
grow longitudinally to reach this mineral, and if the nitrate is existed in a big
amount you would find the root making more branches to get a maximum amount
of nitrates.
The nitrate (anion) needs to across the membrane, so it needs specific proteins,
these proteins are already existed but when there is a huge amount of nitrates the
cells of the root will produce more transport proteins to get more nitrates
efficiently.
Huge trees are vascular meaning that they contain phloem and xylem, while short
plants (like mosses) are not expected to have vascular tissue.
The vascular tissue provides the process of transport. The roots of these trees live
symbiotically with a kind of fungi. The hyphae of the fungi may cover the root or
get between the cells of the root (in the cell wall, it doesn`t enter the cytoplasm).
For example for each 1cm² of the root there are many meter of the hyphae, they
contribute longitudinally, and this provides a big surface area for the fungi.
The fungi gets nutrients by secretion of digestive enzymes into the external
environment, External digestion of complex molecules then absorption of simple
soluble products of digestion by diffusion and active transport. So it must have a
high surface area relative to the volume.
So the fungi help the root by absorption of lots of minerals (by its high surface
area) specially the phosphates, secrete substances that protect the plant from
many diseases and have lots of benefits. (They get nutrients for their needs and
give the plant amounts of minerals absorbed).
Mycorrhizae means mutualistic association between roots (of nearly 80% vascular
plants) and some kinds of fungi.(this relationship provides a good growth to the
plant).
Transport of photosynthesis products through the different parts of the plant.
The transport in vascular plants is processed by three steps:
1- At the cellular level (short distance transport): to across a membrane of a
cell (crossing of solutes and water the permeable membrane).
2- At tissue or organ level (short distance transport): these substances need to
across the root (the whole cortex of the root) or the mesophyll of the leaf
(crossing tissue).
Three transport routes for water and solutes are
a. The apoplastic route, through cell walls and extracellular spaces
b. The symplastic route, through the cytosol
c. The transmembrane route, across cell walls
3- At the level of whole plant transport (long distance transport).
Via xylem, water and dissolved minerals (xylem sap) are transported from the root
to the leaves, even if the distance between the root and the leaves is 100 meters.
Products of photosynthesis, leaf
By xylem vessels transport of water and minerals from the roots to leaves occurs.
Phloem sap consists primarily of water, with sugars, hormones, and mineral
elements dissolved in it and it moves from the source to the sink.
At the cellular level:
The cell wall is permeable meaning that not any substance can cross the
membrane.
At this level we need to transport important minerals like nitrates and potassium
ions from the root, transport sucrose to the sieve tube (sucrose is neutral but
potassium is cation and nitrate is anion) and transport water.
Solutes are transported either passively or actively. These solutes require specific
transport proteins that may be carrier proteins or selective channels or
cotransporters (transporting two substances together) and are found in the
membrane of the leaf.
Carrier proteins: linkage of the solute to a part of this protein carrier that makes it
changing its conformation so that it can translocates this solute from one site to
another site of the membrane.
Selective channels: they are specific to the solutes they transport, some of them
are gated (they open when they are exposed to a stimulus that changes the
voltage or we may have a chemical stimulus. Stretching of the membrane may be
the stimulus.
when we talk about passive transport we mean that solutes are transported
without using energy down their concentration gradient.
In active transport: it requires energy, usually needs carrier proteins , it is an uphill
process and it is against concentration gradient. For instance, if there is a big
amount of nitrates in the root and even that we want to transport soil solution
inside the cells against concentration gradient.
This process need at first proton pumps that use ATP to keep pumping protons.
Due to the continuous pumping of protons, electrochemical gradient is formed
across the membrane.
chemical: protons concentration differs across the membrane.
electrical: inside the membrane will be negative relating to the outside.
Electrochemical gradient can also be called proton gradient and PH gradient “more
acidic in the outside”.
A voltage is formed across the membrane so we have membrane potential.
The electrochemical gradient causes membrane potential "due to preventing the
ions from moving under their chemical gradient and electrical forces".
More pumping of protons lead to a higher membrane potential, till a particular
concentration of protons they will start getting back, and this is used to perform a
work; the work here is to transport some substances actively through
cotransporters.
So proton pumps generate membrane potential and proton gradient.
Potassium has a positive charge "that is existed in the soil solution" ,there is a
specific channels for transporting potassium , the higher negativity inside the
membrane leads to higher entering of potassium inside even if against
concentration gradient.
So by proton gradient the difference of charge is made, and that is beneficial for
transport of potassium ions.
In the plant cells there are lots if cotransporters and protons are always one of the
solutes , in the cotransporters of animal cells sodium must be included .
The protons will get back downhill "passively" , but this process is coupled with
active transport of sucrose "neutral solute" .
So by proton pump we could move the sucrose from a region of low concentration
to region o higher concentration "against concentration gradient to load the
sucrose to cells".
The proton-sucrose cotransporter will not change its shape unless the couple of
the proton and sucrose bind together to its binding site, at the time of binding both
of the proton and the sucrose the protein will change its configuration so that it
can translocates both protons and sucrose from one site on the membrane into the
other site.
Another benefit of proton gradient is reflected on the nitrate " the nitrate is in the
soil and as a result it is located in the root" , we have a cotransporter that couples
the downhill of protons "the passive diffusion of protons" to the uphill " the active
transport" of nitrates. Both of nitrates and protons must be binding to the
cotransporter proteins to be moved.
Water transport:
Osmosis is the net movement of water across a selectively permeable membrane .
In spite of the smallness of water molecules they can`t across the membrane due
to the phospholipid bilayer " only a very little amount can across" . they move by
protein water channels called aquaporins.
Each aquaporin allows the moving of water at the rate of about 3 billion molecules
per second, all of them are in one row.
In animal cells, the concentration of the solutes control the direction of water.
Assume that we have two chambers , each chamber contains the same volume of
water, and the 1st chamber has 5% solutes while the 2nd chamber has 20% solutes.
Dissolving solutes in water is achieved by bounding of water molecules around the
solutes making hydration shells.
Water molecules that make the hydration shell and are attracted to the solute are
called bound water, while other water molecules are called free water " not
involved in hydration shells".
Free water in the 1st chamber is more than in the 2nd chamber , so it is called
hypotonic .
The direction of osmosis has a direction from the hypotonic to the hypertonic
chamber "the water will move from its high concentration to the lower
concentration".
So in animals osmolarity just depends on solutes concentration.
If the two solutions are isotonic, the net direction of osmosis would be zero.
In animals we just have a membrane, we don`t have cell walls like in plants.
In a hypotonic medium, direction of osmosis is to the inside of the plant cell, water
fills the vacuole, so the cell (including the cell membrane) extends, and starts
pushing against the anterior of the cell wall "there will be a pressure on the cell
walls" , the elasticity of the cell wall will allow extending to a particular limit till it
starts exerting pressure to the inside of the cell "to the protoplast " preventing the
entering of water to the inside of the cell, this pressure is called turgor pressure .
Water pressure that is resulted due to the entering inside the cell is called osmotic
pressure.
When the turgor pressure equals the osmotic pressure, the net direction of
osmosis is zero " water amount that enters the cell equals the water amount that
leaves out".
Another factor that affected the movement of water " the 1st factor is the solute
concentration" is the pressure exerted by the cell wall that opposes the osmotic
pressure.
So we call the 2nd factor: the physical pressure exerted by the cell wall opposing
osmotic pressure.
Water potential (Ψ) depends on two factors:
1 –solute potential or osmotic potential.
2 – pressure potential.
Water moves from a region of higher water potential to region of lower water
potential.
Water potential : free water ability to perform work when moving since it has a
potential " energy " allowing it to do that.
megapascal = 10 atm.
If we took pure water ( distilled water that has no solutes) under standard
conditions, we would find the water potential of it = zero "by definition".
If we dissolved solutes in the water, water potential will decrease " because more
solutes leads to less free water".
So any solution must have a negative water potential.
" the osmotic or solute potential is always negative to solutions and equals zero for
pure water".
Pressure potential could be negative or positive.
You may push the water making positive pressure or pull it making it negative.
Assume that we took the two directions of the tube and separated them by a
permeable membrane to water only.
In the 1st case:
pure water is put in the left arm of tube and in the 2nd right arm a solution of .1
molar is added.
The osmotic potential depends on the molarity ( concentration of solution ).
When we want to calculate the water potential :
water potential equals zero in pure water in the left arm.
Water potential = osmotic potential + pressure potential = 0+0 =0
In the right arm, the concentration of solutes is .1 molar meaning that it equals an
osmotic potential(solute potential) which is -.23. we pushed the water here by a
pressure that is nearly equals .23 megapascal (positive pressure), so the water
potential = -.23 + .23 = 0.
So we have an equal water potential, so the direction of osmosis is zero.
In the 2nd case:
When applying more pressure to the right arm(positive pressure), the water
potential will increase, so it will move from the right arm to the left arm.
In the 3rd case:
When applying positive pressure to the right arm that contains solutes, no net
movement of water would happen; because the water potential is negative and
the pressure potential is negative and equals the water potential.
In the 4th case:
If we pull the water from the right arm (applying negative pressure), water will
move from the left arm to the right arm.