Transport in Plants Flashcards
what do xylem tissues consist of?
xylem vessels/ tracheids
xylem parenchyma
xylem fibres (sclerenchyma)
describe xylem vessels
Xylem vessels are long, hollow tubes with walls made of cellulose and lignin
Xylem vessels have a large empty lumen
Xylem vessels have pits in the walls (non-lignified areas)
There are no horizontal cross-walls
Forces of adhesion can occur between the water molecules and the walls of the vessels enabling upward movement of the columns of water.
why is there no cytoplasm in xylem vessels?
so a large volume of water can fit inside of the lumen.
function of pits in xylem vessels
they allow horizontal movement of water and minerals through the cellulose walls to neighbouring cells
why are there no horizontal cross-walls in xylem vessels?
so there is no obstruction to the free upward movement of columns of water and minerals inside of xylem vessels
what is the purpose of long tubes for xylem vessels?
The long tubes provide a large surface area over which water molecules can be attracted to the walls as they move upwards over long distances.
what are tracheids?
These are non-living cells with lignified walls but they have horizontal cross-walls present
what is lignin?
strong, hard substance that is impermeable to water
how is lignin deposited in walls of xylem vessels?
Annular (rings)
Pitted
Helical
Reticulate
Scalariform
function of lignin in xylem vessels
Lignin strengthens the walls and prevents them from collapsing when columns of water inside of them are under tension.
what is the function of pits in the tracheids?
to allow the movement of water and minerals between tracheids and into other cells
what is xylem parenchyma?
These are living cells with cytoplasm which are found filling spaces between other types of xylem cells.
They have thin walls
what is xylem sclerenchyma?
These are dead, empty cells with thin walls, smaller than xylem vessels.
They provide strength and support.
what are the three pathways of water from one cell to another?
Apoplast
Symplast
Vacuolar pathway
how does the apoplast pathway work?
water moves along by mass flow along the cell walls without entering the cytoplasm of the cell.
This is the pathway of least resistance
how is water transported from one cell to another by the vacuolar pathway?
In this case the water molecules move from one cell to another by passing through the cell wall, cell surface membrane → cytoplasm → tonoplast →into the vacuole of one cell → then moving across the tonoplast, cytoplasm, cell surface membrane and cell wall out of the plant cell → into another vacuole in the same way.
In this case, water moves by osmosis from one vacuole to another due to differences in water potential between the 2 cells.
How does the symplast pathway work?
In this pathway, water moves by mass flow from 1 cell to another by entering into the cytoplasm after crossing the cell wall and cell surface membrane → then flowing within the cytoplasm via the plasmodesmata, into an adjacent cell.
The water does not enter the vacuole of the cell but remains in the cytoplasm as it flows along from one cell to another.
what is the endodermis?
This is the innermost layer of cells in the cortex of the root. It separates the cortex from the stele or vascular tissue of the root.
what does the stele consist of?
xylem
pericycle
parenchyma
phloem
features of the endodermis
made up of a single layer of cells and surrounds one or two layers of cells which make up the pericycle of the root
have a hydrophobic material called suberin deposited in bands in their walls
what is the purpose of suberin deposited in bands in the endodermis walls?
It forms the Casparian Strip, which blocks the free movement of water and minerals along the apoplast pathway
When water and minerals travelling the apoplast pathway reach the Casparian Strip, they must face the partially permeable cell surface membrane of the endodermal cells. only allow certain minerals and other substances
what are the roles of the endodermis?
- To regulate the movement of water and minerals between the cortex and the xylem of the root.
- To ensure a one-way flow of minerals from the soil to the xylem and the rest of the plant
- To protect the plant from the entry of pathogens and toxic ions that can be present in the soil water.
- It plays a role in the development of root pressure in the roots of plants, which is needed to assist in the upward movement of minerals and water in the plant.
how does water move into plant roots
water and mineral ions are present in soil solution. WP of soil > WP of vacuole in root hair epidermal cell
water enters by osmosis across cell wall, cell surface membrane, cytoplasm, tonoplast and enters vacuole.
vacuole becomes dilute and WP of cell increases
water moves along water potential gradient in vacuolar pathway by osmosis until it reaches xylem vessels
in xylem vessels, WP is low compared to WP of soil solution
how is water potential of xylem vessels made lower?
by the pumping of mineral ions from the endodermis into the xylem
what happens to water potential as you go up the plant?
WP is highest in soil and root and decreases going up due to increasing amounts of dissolved substances
then it is lowest in the air
how so minerals enter the root?
either passively or actively
Minerals that enter passively move by mass flow along the apoplast pathway and travel along the cell walls until they reach the endodermis (Casparian Strip that blocks the apoplast pathway)
The ions would have to be able to cross the partially permeable membranes of the endodermal cells in order to diffuse further.
Some ions can also cross the partially permeable membranes of the epidermal cells via facilitated diffusion.
what affects passive movements?
no ATP required
occurs along diffusion gradient
higher temperatures speeds up kinetic energy of ions and increases the rate of diffusion
how are cations taken up into root hair cells?
1 The root hair cells secrete H+ out of the root cells into the soil solution using H+ pumps in the cell surface membrane. This creates a slight negative charge in the root epidermal cells .
- Cations are then moved actively from the soil solution into the root cells via carrier proteins (located in the cell surface membranes) with the expenditure of ATP.
This movement is assisted by the negative charge in the root epidermal cells since the cations will be attracted into the cells.
what mechanism is used to transport anions across membranes of root hair cells?
cotransport mechanism
what occurs in the cotranport mechanism?
H+ pumps in the cell surface membranes of root epidermal cells actively move H+ from the root epidermal cells into the soil solution using ATP.
A high H+ concentration is established outside of the root in the soil solution.
Both the H+ and the Cl- ions then move passively from the soil solution via a cotransport protein that functions only if both types of ions are present.
This mechanism is called indirect active transport. The active part of the process is affected by oxygen concentration etc.
what do ions do once they enter endodermal cells?
they can move from those cells via the symplast pathway or be pumped Into the pericycle and then move into the xylem.
a high concentration of ions in the xylem lowers the water potential and attracts water from the cells around the stele, the cortex, and the soil solution.
what is root pressure?
a high pressure that develops in the xylem as water enters it.
It pushes water and minerals a short way upwards into the xylem of the stem
what factors is root pressure affected by?
temperature
availability of ions in soil
absence of oxygen
how does temperature affect root pressure?
higher temperatures increase root pressure
how does the availability of ions in the soil affect the root pressure?
there is less pressure when fewer ions are available
how does a lack of oxygen affect root pressure?
it lowers the root pressure
what is the cohesion-tension theory about?
explaining how water can move upwards in a plant against gravity from soil into the air
what is water potential
the tendency of water to move out of a system by osmosis via a partially permeable membrane
define mass flow
bulk movement of molecules together from one point to another due to differences in hydrostatic pressures
what is hydrostatic pressure?
pressure exerted by a column of liquid against its containing vessel
explain cohesion
force of attraction due to hydrogen bonding between water molecules for energy
define adhesion
force of attraction between water molecules and the walls of the xylem vessels to aid in the upward movement of water
what is capillarity?
the upward movement of water molecules in the xylem vessels due to the force of cohesion and adhesion
explain transpiration pull
a suction effect in the leaves when water diffuses out and also evaporates from cells into the atmosphere.
followed by a pulling effect on water molecules in the xylem due to cohesion
in order for it to occur, there must be a loss of water from the leaves
what is the tensile strength of a water column?
refers to the ability of a column of water to remain unbroken despite the transpiration pull and force of gravity
where is the stomata found?
in the epidermis of leaves, stems and flowers
how many guard cells are in a stoma?
2 with a pore between them
characteristics of guard cells
firmly joined at both ends but can separate in the mid region of their length
have unevenly thickened walls (wall near he pore is thick and wall far away is thin)
have bundles of cellulose microfibrils which are arranged as hoops around the width of the cell
why are there bundles of microfibrils in guard cells
So that when the guard cells become turgid, they cannot expand in width, but can expand in length.
However, because the ends are joined together, the guard cells will curve and therefore, the pore will open
what are the factors that affect transpiration?
temperature
relative humidity in the air inside or outside of the leaf
air movements
watter supply or availability of water in the soil for the plant
light
number of stomata present
surface area of the leaf
thickness of the cuticle on surface of the leaf
whether the stomata are at the surface of the leaf or sunken
how does light affect transpiration?
In the presence of light, photosynthesis occurs in the guard cells. Water enters them from the subsidiary cells, causing them to become turgid, swell, curve and open the stomata. Rate of transpiration is high.
the opposite occurs when there is no light
how does water availability affect transpiration?
if there is no water in the soil, stomata closes due to no water in the soils, stomata closes due to no water available to replace water that is lost from the leaves. Guard cells become flaccid and the stomata closes. Transpiration stops.
what is the role of air movements on transpiration rate?
in windy conditions, the leaders of the moist air are removed quickly from the surfaces of the leaves. This facilitates the diffusion of water vapour from the leaf spaces through the stomata.
Excessively high wind speeds cause the stomata to close because the plant is under stress. This prevents the loss of too much water from the plant.
how does relative humidity affect transpiration rate?
if the humidity of the surrounding air is high and the humidity in the air spaces in the leaf is also high, there will not exist a steep diffusion gradient, so the rate of transpiration will be very low.
On the other hand, if the humidity of the air around is low, a steep diffusion gradient will exist between the air spaces in the leaf and the air around it is low, a steep diffusion gradient will exist so rate is high
how does temperature affect transpiration?
An increase in temperature increases the water holding capacity of the air. As a result, warmer water can hold more vapour and there will be an increase in the rate of transpiration.
When temperatures are lowered, transpiration rates decrease because cooler air can hold less vapour.
what are the roles of transpiration?
- Cooling of the plant due to evaporation of water from surface of the leaf
- Supplying nutrients from the soil ie. minerals to all parts of the plant
- Ensuring that all cells reduce the water
- Allowing the exchange of gases between the plant and surrounding air.
what does the phloem tissue consist of?
phloem sieve tubes
companion cells
phloem parenchyma
phloem sclerenchyma
what are phloem sieve tubes?
A sieve tube consists of a number of elongated sieve elements or sieve tube elements that are joined end to end vertically to form a long, continuous tube whose cross walls are perforated by pores.
Describe the elements of the sieve tube
Each sieve element is living with a plasma membrane and a small amount of cytoplasm, which contains a few small mitochondria, as well as a small amount of endoplasmic reticulum.
The cytoplasm forms a thin lining inside the cell just under the cell surface membrane.
There is no nucleus, ribosomes, vacuole, or golgi.
There are proteins present in the cytoplasm of he sieve elements
There are pits between the companion cells and sieve elements
how is a sieve plate formed?
Where the end walls of the 2 sieve elements meet, a sieve plate forms. This is perforated by large sieve pores. Normally, the pores are not blocked out
strands of callose (polysaccharide) can be found blocking the pores after the elements are damaged.
what is the function of the pit between the companion cell and sieve elements?
they are allow communication between companion cells via the plasmodesmata
what organelles are found in phloem companion cells?
Each one contains a nucleus, ribosomes, vacuole, cytoplasm , many large mitochondria, golgi body, and rough endoplasmic reticulum
what is the function of companion cells?
they are involved in movement of solutes into and out of sieve tubes
features of companion cells
they are located adjacent to each sieve tube element
many plasmodesmata pass through cell walls of companion cell and adjacent sieve tube element
explain phloem parenchyma
These are living cells which fill spaces in the phloem tissue. They also play a role in collecting and transferring solute into the companion cells.
what is the phloem sclerenchyma?
This consists of non-living cells which are elongated and have thickened walls. They have deposits of lignin in the walls for strengthening.
define translocation
Translocation refers to the movement of dissolved solute by mass flow due to differences in hydrostatic pressure between the source and sink.
what is a source?
A source is that part of the plant from which dissolved food materials are moved by translocation via the phloem.
explain the sink
A sink is that part of the plant to which dissolved food materials are being translocated via the phloem.
what are the similarities between sieve tubes and companion cells
both are part of the phloem
both are living cells at maturity
both play a role in translocation
pits between them allow for communication
differences between sieve tubes and companion cells
companion cells are smaller, narrower and densely packed with cytoplasm and organelles
Sieve tubes are elongated cells that form a continuous column
Sieve tube is dependent on companion cell for metabolic activities while the companion cells are responsible for movement of solutes into and out of sieve tubes
change that was made to the mass flow hypothesis
Original: both solutes and water move at the same rate and in the same direction
Revised: movement was due to hydrostatic pressure gradients in the phloem, and that these were brought about by active processes which utilized ATP
what does translocation require for it to occur?
the establishment of a hydrostatic pressure gradient in the phloem sieve tubes and energy
where does active loading of sucrose occur?
from the photosynthetic tissues of the leaf into the phloem at the source
what happens at the sink?
Sucrose diffuses out of the phloem by facilitated diffusion or sometimes it leaves the phloem sieve tube by active transport and moves into the sink.
Water follows and this lowers the hydrostatic pressure in the phloem sieve tube.
where does the hydrostatic pressure gradient start?
in the sieve tubes and not in the photosynthetic tissue or storage tissues where unloading happens
how are transfer cells adapted for active loading of solutes in phloem to occur?
internal projections on the cell walls
many mitochondria
cell surface membrane
water attracted
unloading at sink
how does the mechanism of Indirect Active Transport of Solutes from Phloem Parenchyma into Companion Cells happen?
a protein pump actively transports protons (H+) from the companion Cells into the phloem Parenchyma against the concentration gradient (H+ concentration in phloem parenchyma established)
Another transport protein in the companion cells functions as a cotransport protein and allows the H+ and sucrose to move passively across the membrane into the companion cell from a high to low concentration
water will enter the cell by osmosis and the hydrostatic pressure of the companion cells will increase
evidence to support pressure flow hypothesis
Rate of transport of solutes in phloem= 10 000 times faster than it would be if substances moved by diffusion.
Phloem sap always has a relatively high pH (about 8) as expected if H+ were actively pumped out of companion cells.
The concentration of sugars in phloem varies from 15-30% compared with 0.5% in leaf mesophyll (parenchyma) cells.
Large amounts of ATP in phloem companion cells.
Higher H+ concentration of sucrose in leaves and lower H+ concentration in roots.
criticisms of mass flow hypothesis
Mass flow is passive in the sieve tubes but sucrose transfer into the phloem is not; hypothesis does not take into consideration The need for living sieve tubes in order for mass flow to occur.
Pressure flow/ mass flow does not explain the bi-directonal movement of solutes in the same sieve tube at the same time.
The use of KCN stops respiration in the phloem and translocation stops, suggesting that pressure flow alone cannot account for the process, but an active process is also involved.
The pressure gradients that actually exist are insufficient to account for the movement along sieve pores which actually impose resistance to the mass flow of solutes in the phloem.
what drives the movement of water along a vascular plant?
The movement of water occurs along a water potential gradient, starting from the soil, through the plant, and into the surrounding air.
what is the role of transpiration in water movement?
Transpiration is a passive process where water vapour diffuses out of the leaf through open stomata, lowering water vapour concentration in the air spaces of the leaf.
How is water replaced in the mesophyll cells after evaporation?
Water evaporating from the mesophyll cells is replaced by water pulled from the xylem through cohesion.
What happens to the hydrostatic pressure in the xylem when water is lost at the top of the plant?
The loss of water reduces the hydrostatic pressure at the top of the xylem, creating a suction effect that pulls water upwards
how do cohesion and adhesion help in water movement?
Cohesion: Water molecules stick together, forming an unbroken column.
Adhesion: Water molecules are attracted to the xylem walls, aiding the upward movement.
why doesn’t the xylem vessel collapse under tension?
Xylem vessels are strengthened by bands of lignin, which keep them open despite the tension and narrowness.
what creates root pressure in the xylem?
Root pressure develops when ions are actively pumped into the xylem from the endodermis, followed by water movement, increasing hydrostatic pressure.
how does water move from the soil solution into the root?
Water moves into the root cortex along a water potential gradient, replacing water that enters the xylem.
does transpiration require ATP?
No, transpiration is a passive process and does not require ATP.
what establishes the suction effect in the leaves?
The suction effect is established by water evaporation and diffusion from the mesophyll cells, reducing water potential in the leaf.
how does water move up the xylem?
Water moves up the xylem due to a hydrostatic pressure gradient and is pulled upwards by cohesion and adhesion forces.
