Acquisition of water & minerals Flashcards
Describe plants’ need for transport
- As land plants evolved, and increased in number, competition for light, water and nutrients also increased.
- So, the simple ways of transporting water and minerals became inadequate, leading to the evolution of vascular tissues, consisting of xylem and phloem to carry out long distance support in plants
Ex: - Xylem transports water and minerals from roots to shoots
- Phloem transports the products of photosynthesis from where they are made/stored to where they are needed
How did the competition for light, water and nutrients increase?
Land plants evolving and increasing in number
How does the transport from xylem and phloem occur?
- Xylem transports water and minerals from roots to shoots
- Phloem transports the products of photosynthesis from where they are made/stored to where they are needed
Define passive transport
Passive transport occurs spontaneously and it doesn’t require metabolic energy (ATP)
Define active transport
Movement of some materials across membranes takes place using ATP and that process is called active transport
Describe the methods of passive transport
- Diffusion
- Osmosis
- Imbibition
- Facilitated Diffusion
- Bulk flow —> Long distance
Describe diffusion
- Molecules have an energy called thermal energy, due to their constant motion. One result of this motion is diffusion
- In the absence of other forces, the movement of molecules of a substance from a place where it’s more concentrated to a place where it is less concentrated due to the random motion of molecules is diffusion
- The motion of a molecule is random, but movement of a population of molecules by diffusion is directional
- So, diffusion takes place according to a concentration gradient spontaneously, not using metabolic energy (ATP)
- Diffusion takes place across the membrane too, if the membrane is permeable to those molecules
Ex: 1. Water and soluble materials can diffuse through the cellulose cell wall
- O2 and CO2 can diffuse through the plasma membrane
Give 2 examples for diffusion
- Water and soluble materials can diffuse through the cellulose cell wall
- O2 and CO2 can diffuse through the plasma membrane
Define diffusion
In the absence of other forces, the movement of molecules of a substance from a place where it’s more concentrated to a place where it is less concentrated due to the random motion of molecules is diffusion
What is diffusion a result of?
Thermal energy of molecules in constant motion
Define osmosis
The diffusion of free water molecules across a selectively permeable membrane is osmosis
Describe osmosis
- Osmosis is a special type of diffusion
- The diffusion of free water molecules across a selectively permeable membrane is osmosis
- Free water is water molecules that are not bound to solutes or surfaces
What is free water?
Free water is water molecules that are not bound to solutes or surfaces
Define imbibition
The physical adsorption of water molecules by hydrophilic materials is imbibition
Ex: Adsorption of water molecules by the cellulose cell walls
Give an example for imbibition
Adsorption of water molecules by cellulose cell walls
Define facilitated diffusion
Movement of water and hydrophilic solutes across the membranes passively with the help of transport proteins that span the membrane is facilitated diffusion
Describe facilitated diffusion
- Movement of water and hydrophilic solutes across the membranes passively with the help of transport proteins that span the membrane is facilitated diffusion
- Transport proteins are very specific
- They transport some substances but not the others.
- This movement also takes place along a concentration gradient and it’s a passive movement
Define bulk flow
- Bulk flow is the movement of liquid and the materials (entire solution) in response to the pressure gradient.
Describe bulk flow
- Bulk flow is the movement of liquid and the materials (entire solution) in response to the pressure gradient.
- The bulk flow always transports materials from higher pressure to lower pressure region
- It is a long-distance transport method
- This flow doesn’t occur through the membranes and occurs at much greater speed than diffusion
- This method of transport is independent of solute concentration gradient
What is bulk flow independent of?
This method of transport is independent of solute concentration gradient
What is water potential?
The physical property that predicts the direction in which water will flow governed by solute concentration and applied pressure is called water potential
What is water potential related to?
Potential energy of water molecules
What conditions can affect water potential?
Solute concentration and physical pressure
Describe the effect of solute potential on water potential
- Solute potential is directly proportional to the molarity of the solution (solute potential is also called osmotic potential since solutes affect the direction of osmosis)
- The solutes in plants are typically mineral ions and sugars
- Solute potential of pure water is 0 MPa
- When solutes are added, they bind to water molecules and reduce free water molecules, reducing the capacity of water to move and do work
- In this way, an increase in solutes has a negative effect on water potential
- Therefore, solute potential of a solution is always expressed as a negative value,
- As solute concentration increases, water potential becomes more negative.
Ex: solute potential of the 0.1M sugar solution is -0.23 MPa
What is solute potential also called?
Osmotic potential: because solutes affect the direction of osmosis
What are the solutes affecting the solute potential?
Mineral ions and sugars
What is the solute potential of pure water?
0 MPa
Why does solute potential of a solution always affect water potential negatively?
When solutes are added, they bind to water molecules and reduce free water molecules, reducing the capacity of water to move and do work
Describe the effect of pressure potential on water potential
- Pressure potential is the physical pressure on a solution
- Pressure potential can be positive or negative relative to atmospheric pressure
Ex: Pressure potential of a xylem vessel is usually less than -2 MPa as xylem vessels are under tension (negative pressure)
- Pressure potential of a living cell is a positive value because the living cell is usually under positive pressure by osmotic uptake of water
- The cell contents press the plasma membrane against the cell wall
Then press against the protoplast, producing a pressure called turgor pressure - When turgor pressure increases, the water potential of the cell also increases
Define pressure potential
It is the physical pressure on a solution
Give an example for where pressure potential is negative
Ex: Pressure potential of a xylem vessel is usually less than -2 MPa as xylem vessels are under tension (negative pressure)
Why is pressure potential usually a positive value?
Pressure potential of a living cell is a positive value because the living cell is usually under positive pressure by osmotic uptake of water
What is turgor pressure?
The cell contents press the plasma membrane against the cell wall
Then press against the protoplast, producing a pressure called turgor pressure
Describe the entry of water into a vacuolated cell across the cell membrane
- If a cell is placed in a solution, direction of water movement depends on the water potential of external solution and protoplast.
- Take a fully flaccid cell as a result of losing water
- The water potential of that cell is 0. So pressure potential is 0, water potential = solute potential
- Solute potential of pure water is 0, and addition of solutes will increase the negative value of solute potential will become more negative
- Suppose this flaccid cell is placed in a solution of higher solute concentration (more negative solute potential) than the cell itself, since the external solution has a lower (more negative) water potential, water diffuses out the cell,
- The protoplast of the cell shrinks and pulls away from the cell wall. This is plasmolysis
Describe the process of placing a flaccid cell in pure water.
- Suppose this flaccid cell is placed is pure water. ( Water potential = 0 MPa)
- The cell has a lower water potential than pure water as it contains solutes.
- So, water enters the cell by osmosis.
- Then the protoplast of the cell begins to swell and press the plasma membrane against the cell wall.
- The partially elastic cell wall, exerting a turgor pressure, pushes back against pressurized protoplast.
- Pressure potential is thus increased gradually.
- The maximum value of pressure potential = solute potential of the cell. Water potential becomes 0.
- This matches the water potential of the extracellular environment (0 MPa)
- Then a dynamic equilibrium is formed and there’s no further net movement of water
- If the cell has maximum value for pressure potential (equal to cell’s solute potential), the cell is fully turgid.
- (Fully turgid/ flaccid cells aren’t found in nature)
Write the functions of turgor pressure
- If a non-woody tissue is placed in a solution with higher water potential, it’s stiffened and is very rigid. So, turgor pressure helps support non-woody plants
- Turgor pressure is also important in cell elongation
- Loss of turgor results in wilting, a condition where leaves and stem droop
What is wilting?
Wilting is a condition where leaves and stem droop due to the loss of turgor
How does turgor pressure help support non-woody plants?
If a non-woody tissue is placed in a solution with higher water potential, it’s stiffened and is very rigid. So, turgor pressure helps support non-woody plants
Describe the movement of water and minerals from the soil solution to the plant root
- The cells near the root tips are important because most of the absorption of water and minerals occur there.
- In this region, the epidermal cells are permeable to water and many are differentiated to root hairs.
- Root hairs account for much of the absorption of water by roots, due to increase of surface area
- Root hairs absorb the soil solution which consists of water molecules and dissolved mineral ions that are not tightly bound to soil particles.
- This absorption takes place across the plasma membrane.
- Water can enter root hair by osmosis, a passive movement across the concentration gradient.
- But in root hairs, concentration of mineral ions is greater than in soil solution.
- K+ concentration in root hair is hundreds of times greater than in soil solution.
- So, mineral ion transport occurs against the concentration gradient, by active transport.
- The soil solution is also absorbed into the hydrophilic walls of epidermal cells and passes freely along cell walls and extracellular spaces into the root cortex
Why does root hairs account for much of the absorption of water by roots?
due to increase of surface area
How does water enter the root hair?
By osmosis
What does the soil solution consist of?
water molecules and dissolved mineral ions that are not tightly bound to soil particles.
How does mineral ion transport into the root take place?
Against the concentration gradient by active transport
What happens after the soil solution is absorbed into the root?
The soil solution is also absorbed into the hydrophilic walls of epidermal cells and passes freely along cell walls and extracellular spaces into the root cortex
What is radial transport?
Transport of water and minerals entered from soil to root cortex into the xylem of the root is known as radial transport
What is the last checkpoint for the selective passage of minerals from cortex into vascular cylinder?
Endodermis
What should all materials which enter the root through cell walls and extracellular spaces cross?
Membranes of endodermis
What is the function of endodermis in radial transport?
Unwanted materials can be selectively excluded from the plant from endodermis
What are the 3 routes used in radial transport?
- Apoplastc route
- Symplastic route
- Transmembrane route
Describe radial transport
Transport of water and minerals entered from soil to root cortex into the xylem of the root is known as radial transport
The endodermis, the innermost layer of cells in the cortex, functions as the last checkpoint for selective passage of minerals from cortex into the vascular cylinder.
All materials which enters root through cell walls and extracellular spaces should cross the membranes of endodermis.
So, unwanted materials can be excluded.
There are 3 routes used in radial transport: apoplast route, symplastic route and transmembrane route
Describe the apoplast route
The apoplastic route consists of everything external to the plasma membrane of living cells and includes cell walls, extracellular spaces and interior of dead cells such as vessel elements and tracheids.
Water and solutes move along the continuum of the cell walls and extracellular spaces and it’s known as the apoplast route
Uptake of soil solution by the hydrophilic walls of root hairs provides access to the apoplast. Water and minerals then can diffuse into cortex along this matrix of walls and extracellular spaces
Endodermis blocks apoplast route by a barrier located in the transverse and radial walls of endodermal cells, called casparian strips.
It’s a belt made of suberin which is impervious to water and mineral salts.
Thus water and minerals can’t cross the endodermis and enter the vascular cylinder via apoplast.
So, water and minerals cross the selectively permeable plasma membrane before entering the vascular tissue and keep unneeded and toxic materials out.
The endodermis also prevents solutes that have accumulated in the xylem from leaking back into the soil solution
What does the apoplastic route consist of?
The apoplastic route consists of everything external to the plasma membrane of living cells and includes cell walls, extracellular spaces and interior of dead cells such as vessel elements and tracheids.
Define the apoplastic route
Water and solutes move along the continuum of the cell walls and extracellular spaces and it’s known as the apoplast route
What provides access to the apoplast?
Uptake of soil solution by hydrophilic walls of root hairs
What s the casparian strip?
It is a barrier located in the transverse and radial walls of endodermal cells which blocks the apoplast
It is a belt made of suberin which is impervious to water and mineral salts.
Why can’t water and minerals cross the endodermis and enter the vascular cylinder through the apoplast?
Because the casparian strip s made of suberin which is impervious to water and mineral salts
What are the 2 functions of the endodermis in the apoplast route?
- Because of the casparian strip in Endodermis, Water and minerals cross the selectively permeable plasma membrane before entering the vascular tissue and keep unneeded toxic materials out
- The endodermis also prevents solutes that have accumulated in the xylem from leaking back into the soil solution
Describe the symplastic route
The symplast consists of the entire mass of cytosol of all living cells in a plant, as well as plasmodesmata, the cytoplasmic channels that interconnect them.
In the symplastic route, water and solutes move along the continuum of the cytosol.
This route requires substance to cross a plasma membrane once, when they first enter the plant.
After entering one cell, substances can move from cell to cell via plasmodesmata
What does the symplastic route consist of?
The symplast consists of the entire mass of cytosol of all living cells in a plant, as well as plasmodesmata, the cytoplasmic channels that interconnect them.
How do solutes move in the symplastic route?
In the symplastic route, water and solutes move along the continuum of the cytosol.
This route requires substance to cross a plasma membrane once, when they first enter the plant.
After entering one cell, substances can move from cell to cell via plasmodesmata
Describe the transmembrane route
- The transmembrane route requires repeated crossing of plasma membranes as water and solutes exit one cell and enter the next.
Which route shows least resistance for transport?
Apoplast route
Why does more water use apoplast route for transport?
Because it shows least resistance for transport
After going through the 3 routes, where do the water and minerals enter?
Tracheids and vessel elements of xylem
Why are tracheids and vessel elements considered a part of the apoplast?
Because these water conducting cells lack protoplasts when mature
How do endodermal cells take in minerals transported by the apoplast route?
Endodermal cells and living cells of the vascular tissues discharge minerals from their protoplast to thier own cells
What 2 routes involve in transporting solutes from the symplast to apoplast?
Diffusion and active transport
How is water in vessel elements and tracheids transported to shoot system?
By bulk flow only through the apoplast
What is the ascent of xylem sap?
Water and minerals which enter the vascular cylinder are transported to the upper parts of the plant and this is ascent of xylem sap
What is the mode of transportation of xylem sap, water and dissolved minerals in the xylem?
Bulk flow, which is much faster than diffusion
What hypothesis can be put forward to explain the process involved in ascent of xylem sap?
Cohesion-tension hypothesis
Describe radial transport
Transport of water and minerals entered from the soil to the root cortex into xylem of the root is known as radial transport
The endodermis, innermost layer of cells in the cortex, functions as the last checkpoint for the selective passage of minerals from cortex into vascular cylinder.
All materials which enter the root through cell walls and extracellular spaces should cross membranes of endodermis.
So, unwanted materials can be selectively excluded.
3 routes are used in radial transport: appoplast, symplast, transmembrane
Define radial transport
Transport of water and minerals entered from the soil to the root cortex into xylem of the root is known as radial transport
Define apoplastic route
The apoplastic route consists of everything external to plasma membrane of living cells, extracellular spaces and interior of dead cells such as vessel elements and tracheids
Water and solutes move along the continuum of cell walls and extracellular spaces and its known as apoplastic route
Describe apoplast route
The apoplastic route consists of everything external to plasma membrane of living cells, extracellular spaces and interior of dead cells such as vessel elements and tracheids
Water and solutes move along the continuum of cell walls and extracellular spaces and its known as apoplastic route
Uptake of soil solution by hydrophilic walls of root hairs provide access to apoplast.
Water and minerals can then diffuse into the cortex along the matrix of walls and extracellular spaces.
Endodermis blocks apoplastic route by a barrier located in transverse and radial walls of endodermal cells, called casparian strips
It’s a belt made of suberin which is impervious to water and mineral salts
Thus, water and minerals cannot cross endodermis and enter vascular cylinder via apoplast
So, water and minerals cross selectively permeable plasma membrane before entering the vascular tissue and keeps unneeded toxic materials out
Endodermis also prevents solutes that have accumulated in xylem from leaking back into soil solution
Functions of endodermis in apoplast
water and minerals cannot cross endodermis and enter vascular cylinder via apoplast
So, water and minerals cross selectively permeable plasma membrane before entering the vascular tissue and keeps unneeded toxic materials out
Endodermis also prevents solutes that have accumulated in xylem from leaking back into soil solution
Describe endodermis in apoplast
Endodermis blocks apoplastic route by a barrier located in transverse and radial walls of endodermal cells, called casparian strips
It’s a belt made of suberin which is impervious to water and mineral salts
Thus, water and minerals cannot cross endodermis and enter vascular cylinder via apoplast
So, water and minerals cross selectively permeable plasma membrane before entering the vascular tissue and keeps unneeded toxic materials out
Endodermis also prevents solutes that have accumulated in xylem from leaking back into soil solution
How does the endodermis block apoplastic route?
Endodermis blocks apoplastic route by a barrier located in transverse and radial walls of endodermal cells, called casparian strips
It’s a belt made of suberin which is impervious to water and mineral salts
