Why do plants need transport systems?
- In large multicellular plants the epithelial (surface) cells, which are close to the supply, can gain all they need by simple diffusion.
- But there are many cells inside the plant that are further from the supply. These cells would not recieve enough oxygen and nutrients to survive.
- The particular problem in plants is that the roots can obtain water fairly easily, but cannot absorb sugars from the soil. The leaves can produce sugars, but cannot obtain water from the air.
What substances need to be moved?
The transport system in plants moves water in special tissues called VASCULAR TISSUE.
- Water and soluble minerals travel UPWARDS in xylem tissue.
- Sugars flow UP or DOWN in phloem tissue.
The vascular tissues
- Vascular tissue= distributed throughout the plant.
- The Xylem and Phloem are found together in vascular bundles. These bundles often contain other types of tissue that give the bundle strength and help support the plant.
Xylem and Phloem in the young root and in the stem
- The vascular bundle is found at the centre of the young root.
- Large central core of xylem, often in the shape of an X.
- Phloem found between the arms of the X- shaped Xylem.
- This arrangements provides strength in order to withstand the pulling force the root is exposed to.
- Vascular bundle found near the outer edge of stem.
- Non- woody plants= bundles are separate and discrete.
- Wood plants= separate in young stems but become continuous in older stems.
- This means there is a complete ring of vascular tissue just under the bark of a tree= this arrangement provides strength and flexibility to withstand the bending forces to which stems and branches are exposed to.
- Xylem= towards the inside of the bundle.
- Phloem= towards the outside of teh bundle.
- Layer of cambium inbetween Xylem and Phloem.
- Cambium= a layer of meristem cells that divide to produce new Xylem and Phloem.
Xylem and Phloem in the leaf
- The vascular bundles form the midrib and veins of a leaf.
- There are 2 major groups of flowering plants, dicotyledons and monocotyledons- these names are based on the number of first leaves (seed leaves or cotyledons) they have.
- A plant transport tissue that carries water from the roots to the rest of the plant.
- It consists of hollow columns of dead cells lined end-to-end and reinforced with ligin.
- It provides important support for the plant.
- A plant transport system that carries the products of photosynthesis to the rest of the plant.
- It consists of sieve tube elements and companion cells.
Structure of Xylem
- Consists of tubes to carry water and dissolved minerals.
- Consists of fibres to help support the plant and living parenchyma cells.
- These are long cells with thick walls that have been impregnated by ligin.
- As the xylem develops the ligin waterproofs the walls of the cells.
- As a result the cells die, and their end walls and contents decay.
- This leaves a long column of dead cells with no contents- a tube with no end walls- A XYLEM VESSEL.
- The ligin thickening forms patterns in the cell wall. This prevents the vessel from being too rigid and allows flexibility of the stem or branch.
- In some cases liginification= not complete= it leaves pores in the walls of the vessel which are called PITS or BORDERED PITS. These allow water to leave one vessel and pass into another adjacent vessel OR pass into living parts of the plant.
Adaptations of xylem to its function
-It is made from dead cells alligened end-to-end to form a continuous column
-The tubes are narrow so the water column does not break easily and capillary action can be effective.
-Pits in the lignified walls allow water to move sideways from one vessel to another.
THE FLOW OF WATER IS NOT IMPEEDED BECAUSE;
-There are no end walls.
-There is no nucleus or cytoplasm.
Structure of Phloem
- Consists of 2 types of cell: the sieve tube elements and the companion cells.
- Not true cells- they contain very little cytoplasm and no nucleus.
- They are lined up end-to-end to form a tube, in which the plant transports sugars (usually surcrose). The sucrose is dissolved in the water to form sap.
- Unlike xylem vessels, this tube contains cross-walls at intervals. These cross-walls are perforated by many pores to allow the sap to flow.
- Inbetween the sieve tubes
- They have a large nucleus and dense cytoplasm
- They have numerous mitochondria to produce ATP needed for active processes.
- They carry out the metabolic processes needed by the sieve tube elements.
- The cytoplasm of the companion cells and the sieve tube elements are linked through many PLASMODESMATA.
PLASMODESMATA= Gaps in the cell walls allowing communication and flow of substances between the cells.
Is the measure of the tendency of water molecules to diffuse from one place to another.
- Water always moves from a region of higher water potential to a region of lower water potential.
- The water potential of pure water= 0
- In a plant cell, the cytoplasm contains salts and sugars (solutes) that will reduce the water potential. This is because there are always fewer 'free' water molecules available than in pure water.
- As a result, the water potential in plant cells is always negative.
Osmosis (plant cells in pure water)
- If a plant cell is placed in pure water, it will take up water molecules by osmosis. This is because the water potential in the cell is lower (more negative) than the water potential in the water.
- However, the cell will not continue to absorb water until it bursts- this is because the cell has a strong cellulose cell wall.
- Once the cell is full of water it is described as being TURGID
- The water inside the cell begins to exert pressure on the cell wall called the PRESSURE POTENTIAL.
- As this builds up it reduces the influx of water.
Osmosis- (plant cell placed in solution)
- If a plant cell is placed in a solution with a very low water potential, it will lose water by osmosis.
- This is because the water potential of the cell is higher than the water potential of the solution. So water diffuses down its potential gradient out of the cell.
- The cell loses its turgidity... if water loss continues, the cytoplasm and vacuole shrink.
- Eventually the cytoplasm no longer pushes against the cell wall (INCIPIENT PLASMOLYSIS).
- If water loss continues, the PLASMA MEMBRANE will lose contact with the wall= a condition known as PLASMOYLSIS.
How does water move between cells?
- When plant cells are touching each other, water molecules can pass from one cell to another.
- The water molecules will move from the cell with the higher water potential (less negative) to the cell with the lower water potential (more negative).
The route water can take between cells
The Apoplast pathway
- The cellulose cell walls have many water-filled spaces between the cellulose molecules. Water can move through these spaces and between the cells.
- In this pathway the water does not pass through any plasma membranes. This means that dissolved mineral ions and salts can be carried with the water.
The Symplast pathway
- Water enters the cell cytopalsm through the plasma membrane.
- It can then pass through the plasmodesmata (singular: plasmodesma) from one cell to the next.
PLASMODESMATA= Gaps in the cell wall that contain a thin strand of cytoplasm.
The Vacuolar pathway
- Similar to the symplast pathway.
- Water is not confined to the cytoplasm it can enter and pass through the vacuole too.
Water uptake from the soil
- The outermost layer of cells (the epidermis) contains root hair cells that increase the surface area of the root.
- These cells absorb minerals from the soil by active transport using ATP for energy.
- The minerals reduce the water potential of the cell cytoplasm.
- This makes the water potential in the cell lower than that in the soil.
- Water is taken up across the plasma membrane by osmosis as the molecules move down the water potential gradient.
Movement across the root
- The movement of water across the root is driven by an active process that occurs at the endodermis. The endodermis is a layer of cells surrounding the xylem.
- The endodermis consists of the CASPARIAN STRIP.
- THE CASPARIAN STRIP blocks the apoplast pathway, forcing water into the symplast pathway.
- The endodermis cells move minerals by active transport from the CORTEX into the xylem. This decreases the water potential in the xylem. As a result, water moves from the cortex through the endodermal cells to the xylem by osmosis.
- This reduces the water potential in the cells just outside the endodermis.
- This combined with the water entering the root hair cells, creates a water potential gradient across the whole cortex. Therefore the water is moved along the symplast pathway from the root hair cells, across the cortex and into the xylem.
- At the same time it can pass through the apoplast pathway and join with the symplast pathway before passing the endodermis.
The role of the casparian strip
- Blocks the apoplast pathway between the cortex and the Xylem. This ensures that water and nitrate ions have to pass into the cell cytoplasm through cell membranes.
- There are transporter proteins in the cell membranes.
- Nitrate ions are actively trasnported from the cytoplasm of the cortex cells into the Xylem. This lowers the water potential in the Xylem so water from the cortex cells follows into the Xylem by osmosis.
- After this the apoplast pathway of the endodermal cell= blocked (water cannot pass back into the cortex).
How does water move up the stem?
THREE PROCESSES THAT HELP WATER MOVE UP THE STEM
- Root pressure
- Transpiration pull
- Capillary action
- The action of the endodermis moving minerals into the xylem by active transport drives water into the xylem by osmosis.
- Root pressure can push water a few meters up a stem, but cannot account for water getting to the top of tall trees.
- Water molecules are attracted to each other by forces of COHESION.
- Cohesion= The attraction of water molecules for one another.
- These Cohension forces are strong enough to hold the molecules together in a long chain or column.
- As molecules are lost at the top of the column, the whole column is pulled up as one chain....this creates the transpiration stream.
- The pull from above can create tention in the column of water...this is why the xylem vesself must be strengthened by ligin....the ligin prevents the vessel from collapsing under tension.
- This process is called the COHENSION-TENSION THEORY because it involves cohension between the water molecules and tension in the column of water.
Adhension= The attraction of water molecules to the walls of the xylem.
Because the xylem vessels are very narrow, these forces of attraction can pull water up the sides of the vessel.
How water leaves the leaf
- Most of the water that leaves the leaf exits through the stomata, tiny pores in the epidermis.
- Only a tiny amount leaves through the waxy cuticle.
- Water evaporates from the cells lining the cavity immediately below the guard cells. This lowers the water potential in these cells, causing water to enter them by osmosis from neighbouring cells.
Definition= The loss of water vapour from aerial parts of the plant due to evaporation.
The process of Transpiration
- Osmosis from the xylem to mesopyll cells.
- Evaporation from the surface of the mesophyll cells into the intercellular spaces.
- Diffusion of water vapour from the intercellular spaces out through the stomata.
The Transpiration stream
As water leaves the xylem in the leaf, it must be replaced from below.
Water moves up the xylem from the roots to replace the water lost.
How the movement of water up the stem is useful to
- Water is required in the leaves for photosynthesis.
- Water is required to enable cells to grow and elongate.
- Water keeps the cell turgid.
How to measure the rate of Transpiration
-Potometer can be used to estimate the rate of water loss.
Precautions that should be taken;
- The Potometer must be set up under water to ensure no extra air bubbles get into the system.
- All joints must be air-tight and water-tight.
- The leaf area must be dried and measured.
- The Potometer and shoot must be left to acclimatise before any readings start.
Factors that alter the rate of transpiration.
Number of leaves
-More leaves= more surface area= more water vapour can be lost.
Number, size and position of stomata
-Many large stomata= increase in the water vapour lost.
-If stomata is on lower surface= water vapour loss= slower.
Presence of a cuticle
Reduces evaporation from the leaf surface.
Light causes the stomata to open to allow gaseous exchange for photosynthesis.
What happens when the plant loses too much water
If water loss by transpiration is greater than water uptake from the soil, the plant cells will lose turgidity. The leaves of both non- woody and woody plants will wilt and eventually die.
Structural/behavioural adaptations to reduce water
- A waxy cuticle on the leaf will reuce water loss due to evaporation through the epidermis.
- The stomata are often found on the undersurface of leaves, not on the top surface. This reduces the evaporation due to direct heating from the sun.
- Most stomata are closed at night, when there is no light for photosynthesis.
Plants in arid conditions
- Smaller leaves, particularly leaves shaped like needles. This reduces the total surface area of the leaf therefore reducing the amount of water vapour lost via transpiration.
- Densely packed spongy mesophyll. This reduces the cell surface area that is exposed to the air inside the leaves. This means less water vapour will be lost due to evaporation.
- Closing the stomata when water availability is low will resudce water loss and the need to take up water.
- Rolling the leaves so that the lower epidermis is not exposed to the atmosphere can trap air that becomes saturated. This is another wat to reduce or even eliminate the water potential gradient.
Adaptations of Marram grass
-Leaf rolled to trap air inside.
-Thick waxy cuticle to reduce water evaporation from surface.
-Hairs on lower surface to reduce movement of air.
-Stomata in pits to trap air with moisture close to the stomata.
The movement of sucrose and other substances up and down a plant.
Source & Sink
A Source= Releases sucrose into the phloem.
A Sink= Removes sucrose from the phloem.
Active loading= Using ATP (energy ) to transport sugars into the phloem.
How does sucrose enter the Phloem?
- Hydrogen ions are used to help in active loading.
- They are pumped out of the companion cells and allowed to diffused back in.
- As they diffused back in, they come through special cotransporter proteins that allowt hem through only if they are attached to a sugar molecule.
Movement of sucrose along the phloem at SOURCE
- Sucrose entering the sieve tube elements reduces the water potential inside the sieve tube.
- As a result water molevules move into the sieve tube element by osmosis from the surrounding tissues.
- This increases the hydrostatic pressure in the sieve tube at the source.
Movement of sucrose along the phloem at SINK
- Surcrose is used in the cells surrounding the phloem. The sucrose may either be converted to startch for storage or used in metabolic processes such as respiration.
- This reduces the concentration of sucrose in these cells.
- Sucrose molecules move by diffusion or active transport from the sieve tube elements into the surrounding cells.
- This increases the water potential in the sieve tube elements, so water molecules move into the surrounding cells by osmosis.
- This reduces the hydrostatic pressue in the phloem at the sink.
Movement of sucrose along the phloem
- Water entering the phloem at the source, moving down the hydrostatic pressure gradient and leaving the phloem at the sink, produces a flow of water along the phloem (MASS FLOW).
- Mass flow can occur in either direction (up or down) depending on where sugars are needed.
- Mass flow may occur up or down in the same phloem but at different times.
- It may be moving assimilates up the plant in some tubes and down the plant in other tubes are the same time.
Where you find sources and sinks
- A source is a leaf as sugars made during photosynthesis are converted into sucrose and then loaded into the phloem.
- A sink would occur in early spring, when the leaves are growing and need energy. This energy is supplied from stores in other parts of the plant, and the leaves act as sinks.
- Another source would be the roots where stored carbohydrates are released into the phloem.
- A sink would occur during the summer and autumn when the roots store sugar and starch. Thus the roots can act as a source during parts of the year and a sink during other parts of the year.
Evidence for the mechanism of translocation
How we know the phloem is used
- If a plant is supplied with radioactively labelled carbon dioxide (which will be used in photosynthesis) the labelled carbon soon appears in the phloem.
- An aphid feeding on a plant stem can be used to show that mouthparts are taking food from the phloem.
How we know it needs metabolic energy (ATP)
- The companion cells have many mitochondria.
- Translocation can be stopped by using metabolic poison that inhibits the formation of ATP.
How we know it uses this mechanism
- The pH of the companion cells is higher than that of surrounding cells. As the hydrogen ions are pumped out of the companion cells, this leaves fewer hydrogen ions in the cells. This means a higher pH.
Evidence against the mechanism
- Not all the solutes in the phloem sap move at the same rate.
- The role of sieve plates is unclear.
How an aphid can be used to show that sugars travel in the phloem
- An aphid uses its proboscis to pierce the phloem and drink the sap.
- Once an aphid has places its proboscis in the phloem, the aphid can be removed to show that sugary sap drips from the cut end of the proboscis.
- Further investigation can reveal that the other end of the proboscis is in the phloem.
Factors that alter the rate of transpiration (part
Will increase the rate of water loss by;
-Increasing the rate of evaporation from the cell surfaces so that the water potential in the leaf rises.
-Increase the rate of diffusion through the stomata because the water molecules have more kinetic energy.
-Decreases the rate of water loss. This is because there will be a smaller water potential gradient beween the air spaces in the leaf and the air outside.
Air movement or wind
Air moving outside the leaf will carry away water vapour that has just diffused out of the leaf.
If thre is little water int he soul, then the plant cannot replace the water that is lost.