3.1.3 - Transport in plants Flashcards
Why do plants need a specialised transport system ?
- To move products of photosynthesis, water, and oxygen around from their place of origin to where they are needed
- Most plants have to transport substances over large distances
- SA: V ratio is small in plants (even though for leaves SA:V high)
Define dicotyledonous
Plants that produce two seed leaves
How do you tell if a plant is mono- or di- cotyledonous from its leaves ?
- Monocots have vessels parallel to the leave
- Dicots have vessels which branch out from a central vessel
What are dicotyledonous plants (dicots) ?
They make seeds that contain two cotyledons
What are cotyledons ?
They are organs that act as food stores for the developing embryo plant
What do cotyledons form ?
They form the first leaves when the seed germinates
What are herbaceous dicots ?
- They have soft tissues and a relatively short life cycle
- They die down at the end of the growing season to the soil level
What are arborescent dicots ?
They are woody dicots, which have hard lignified tissues and a long life cycle
Define vascular system
A system of transport vessels in plants
What are the two main types of transport vessels in herbaceous dicots ?
Xylem and phloem
Define vascular bundle
- The vascular system made up of xylem and phloem tissue
- The transport tissues are arranged together packed in vascular bundles
What is the function of the xylem ?
- The transport of water and mineral ions
- Support
What does the xylem carry ?
Water and mineral ions
What is the flow of the xylem like ?
Up from the roots to the shoots and leaves
What is the xylem made up of ?
- It is made up of dead cells
- Long hollow structures made by several columns of cells fusing together end to end
What are the two tissues associated with the xylem in herbaceous dicots ?
- Thick walled parenchyma
- Tannin
What is the role of the parenchyma ?
It packs around the xylem vessels, storing food and containing tannin deposits
What is the role of tannin ?
It is a bitter astringent tasting chemical that protects plant tissues from attacks by herbivores
What is the role of lignin in the xylem ?
To provide extra mechanical strength
How can the lignin be laid in the walls of the xylem ?
- Rings
- Spirals
- Solid tubes with lots of small unlignified areas called bordered pits
What is the function of the phloem ?
To transport food in the form of organic solutes around the plant from the leaves where they are made by photosynthesis
What does translocation supply the cell with ?
It supplies the cells with the sugars and amino acids needed for cellular respiration and for the synthesis of other molecules
What is the flow of the phloem like ?
It can go both up and down the plant
What is the main transporting vessel of the phloem ?
Sieve tube elements
What is the structure of the phloem like ?
- Many cells joined end to end to form a long hollow structure
- Not lignified
- Sieve plates
- Companion cells
What is the role of the sieve plates ?
To allow the contents of the phloem to flow both ways
What is the result of the pores in the cell wall, caused by the sieve plates ?
The pores in the cell walls cause the tonoplast, the nucleus and some other organelles to break down
What does the phloem become, overall ?
A tube filled with phloem sap and the mature phloem cells have no nucleus
What are companion cells ?
They are cells that are closely linked to the sieve tube elements by plasmodesmata
What are plasmodesmata ?
Microscopic channels through the cellulose cell walls linking the cytoplasm of adjacent cells
Why are the companion cells active ?
They act as a life support system for the sieve tube cells, which have lost most of their normal cell functions
Why is water key in plants ?
- Turgor pressure as a result of osmosis in plant cells provides a hydrostatic skeleton to support the stems and leaves.
- The loss of water by evaporation helps the plant keep cool
- Mineral ions are transported in water
- It is a raw material for photosynthesis
What are root hair cells ?
An exchange surface in plants where water is taken into the plant from the soil
In which part of the root is water taken up ?
Root hairs
What is a root hair ?
It is a long thin extension from a root hair cell
How are root hairs adapted as exchange surfaces ?
- Small so they can easily penetrate through soil particles
- Large SA:V ratio
- Thin surface layer
- Solutes in its cytoplasm maintain a water potential gradient between the soil water and the cell
How does water enter the root ?
By osmosis
Explain the difference in water potential between the soil and the root hair cell
- Soil water has a very low concentration of dissolved minerals so it has a high water potential
- The cytoplasm of the root hair cell has a high concentration of different solvents so it has a low water potential
What must the water potential of the soil be relative to the root in order for water to be taken up ?
The water potential of the soil must be higher than the water potential of the root
How does the plant ensure the water potential of the root is more negative than the soil ?
Ions from the soil are actively pumped into the root
How is water transported in the plant ?
Via the xylem
What pathway is taken by the water to go from the soil to the xylem ?
- Root hair cell
- Cortex
- Endodermis
- Pericycle
- Xylem
What is the function of the pericycle ?
It has meristematic tissue and produces lateral roots
What is the function of the endodermis ?
It contains a ring of suberin, called the casparian strip, which is impermeable to water
What is the function of the cortex ?
Stores a large amount of starch
What are the two pathways that water can move through ?
- Symplast pathway
- Apoplast pathway
What is the symplast pathway ?
- Water travels through the cytoplasm and from cell to cell via plasmodesmata, by osmosis.
- From high water potential to a low water potential
What is the apoplast pathway ?
Water travels through the cellulose cell wall
What is the advantage of travelling through the cellulose cell wall?
It offers the path of least resistance
What is the Casparian strip ?
It is a band of waxy material that runs around each of the endodermal cells forming a waterproof layer
What is the Casparian strip made of ?
Suberin
What happens to water travelling via the apoplast pathway when it reaches the Casparian strip ?
It cannot continue and so is diverted to the symplast pathway
What is the advantage of water being diverted into the symplast pathway?
It allows the plant to control the movement of water into the root via osmosis, it takes the route of the lowest resistance
How does water enter the xylem from the endodermal cells?
- Endodermal cells actively pumps salts into the xylem
- This makes the water potential of the xylem more negative than the endodermal cells
- This means water enters the xylem via osmosis
- Ensures a large water potential gradient is maintained
What is root pressure ?
It is a process involved in the movement of water up the stem and through the leaf
What causes root pressure ?
- Endodermal cells actively pump ions into the xylem vessels
- A water potential gradient is generated so water enters the xylem vessel by osmosis
- This is because the water potential in the xylem is more negative than the water potential in endodermal cells
- Pressure in the xylem increases, forcing water upwards
What is the evidence for root pressure ?
- Root pressure relies on the active pumping of ions which requires ATP
- If there is no ATP no ions are pumped actively and no water potential gradient is created
- Therefore water cannot enter the xylem by osmosis
- So there is no root pressure
What makes up the vascular bundle, from inwards out ?
- Xylem
- Cambium
- Phloem
- Sclerenchyma
What is the function of the cambium ?
It is meristematic and differentiates to form more xylem and phloem as the plant grows
What is the advantage of xylem vessels not being completely lignified ?
It allows for the stem to flex and move slightly e.g. in the wind
What is the function of lignin in xylem ?
It strengthens the walls to resist the forces generated by water moving up the stem
What force is created by the bonds between water molecules ?
Cohesion
What force is created by the interaction between water molecules and the vessel holding the water ?
Adhesion
How is the continuous stream of water created ?
Cohesion and adhesion
What 3 processes are involved in the movement of water up the stem and through the leaf ?
- Root pressure
- Capillary action
- Cohesion-tension theory
What causes capillary action?
- The adhesive forces between xylem vessels and the water molecules
- This pulls a water molecule up
- Due to the cohesive forces between water molecules, other water molecules are also pulled up with it
Why is the xylem a bundle of very narrow vessels rather than one wide vessel?
- Greater heights of liquid are achieved in thinner tubes due to capillarity
- With a smaller tube there is a greater contact with the vessel wall compared with the volume of water in the centre
- So greater cohesive forces
- Easier to go up the xylem
What is transpiration ?
Transpiration is the loss of water vapour from leaves and stems as a result of evaporation from cell surfaces inside the leaf and diffusion down a concentration gradient out through the stomata
Why is the transpiration pull reffered to as the cohesion-tension theory ?
- The cohesive force between water molecules pulls other molecules upwards as it is a continuous stream
- This puts pressure on the column of water
- This inward pressure is called tension
What is the evidence for the cohesion-tension theory ?
- Changes in the diameter of trees. When transpiration is at its height during the day, the tension in the xylem vessels is at its highest too. As a result the tree shrinks in diameter. This can be tested by measuring the circumference of a suitably sized tree at different times of the day
- When a xylem vessel is broken air is drawn in to the xylem rather than water leaking out. The plant can then no longer move water up the stem as the continuous stream of water molecules held together by cohesive forces has been broken.
What happens if a xylem vessel is broken ?
The continuous stream of water is broken so no water can be taken up
If a xylem vessel becomes blocked can water still reach the leaves ?
Yes
Why can water still reach the leaves if a xylem vessel becomes blocked ?
The pits in the xylem allows for the movement of water
How is the rate of transpiration controlled ?
The opening and closing of the stomatal pores. This is a turgor driven process.
How do guard cells differ from the lower epidermal cells ?
- They contain chloroplasts
- They have extra cellulose thickening on the inner side of the cell, allowing them to open up (turgor pressure driven process)
Outline how guard cells open
- Cells surrounding guard cells actively pump K+ ions into the guard cells, making their water potential more negative
- Water enters by osmosis as the water potential of guard cells is more negative than that of the surrounding cells
- Guard cells swell but because inner wall is thicker than outer wall as cell swells a pore opens up
What is crucial about the cell walls of guard cells ?
The inner wall is thicker than the outer wall
Outline how guard cells close
- K+ ions diffuse out of the guard cells and back into epidermal cells
- No longer has a lower water potential in the guard cells so water leaves by osmosis
- Guard cells become flaccid and closes
Give 4 ways plants generally conserve water
- Waxy cuticle
- Stomata on underside of leaf
- Closable stomata
- Roots that grow down to the water in the soil
What factors affect the rate of transpiration ?
- Light
- Relative humidity
- Temperature
- Air movement
How does light affect the rate of transpiration ?
Increasing the light intensity will increases the number of open stomata which will increase the rate of transpiration as more water will be evaporated from the surface of the leaf
What is relative humidity ?
The amount of water vapour in the air compared to the total amount of water the air can hold
How does high relative humidity the rate of transpiration ?
- High relative humidity will lower the rate of transpiration because of the reduced water vapour potential gradient between the inside of the leaf and the outside air
- Very dry air has the opposite effect and increases the rate of transpiration
How does an increase in temperature affect the rate of transpiration ?
An increase in temperature increases the KE of the water molecules and therefore increases the rate of evaporation from the spongy mesophyll cells into the air spaces of the leaf
How does air movement affect the rate of transpiration ?
- Anything that increases the diffusion gradient will in turn increase the rate of transpiration
- Air movement of wind will increase the rate of transpiration
- A long period of still air will reduce transpiration
How does soil water availability affect the rate of transpiration ?
- The amount of water available in the soil can affect the rate of transpiration.
- If it is very dry the plant will be under water stress and the rate of transpiration will be reduced
