Why plants need a transport system
Size- A plant is so large that cell on the epithelial cells could gain all they needed by diffusion but others would get no nutrition. The main problem is that while the roots can easily collect water, they cannot get CO2 for photosynthesis. Whereas the leaves have the oppostie problem, having a large supply of CO2 but no water this means that substances must be transported around the plant in order for it to be effective.
Surface area to volume - The surface area to volume ratio show sthat the larger something volume gets the smaller the surface area gets in comparison.
What goes where?
Vascular tissues move substances around the plant.....
- Water and minerals move up from the roots to the leaves via xylem tissue.
- Sugar travels down the Phloem tissue to the roots from the leaves.
Xylem and Phloem in the root
The Xylem are the cross in the middle.
The Phloem are the dots sorrounding it.
Xylem and phloem in the stem
The Xylem are the parts coloured red.
The phloem are the orange parts.
The white line inbetween is called the cambium.
Xylem and phloem in the leaves
The xylem and phloem are both present in the part of the leaf called the central midrib.
Xylem tubes are used to transport water from the roots to the leaves.
They are long tubes constructed of lignin impregnated cells, these cells are all dead.
- The tubes are very narro allowing the tube to be relatively strong and not break easily.
- Pits in the lignin walls allows water to move from one vessel to another.
- Lignin deposited into the xylem tube in spirals this allows them to be more flexible then if it was lignified entirely.
- There are no wall and no cell contents so water can pas through unaffected.
Phloem (Sieve tubes and companion cells)
The phloem is used to transport sugars from the leaves to the roots, they consist of two parts the sieve tubes and the companion cells.
These cells are lined up end to end to form a tube. They are all hollow containing no nucleus and very little cytoplasm. Inbetween each cell is a wall with many holes in it this is called the Sieve plate.
Next to the sieve tubes are companion cells, they are attached to the sieve tubes by plasmodesmata (small holes in the cell wall which material can be moved through). They conatin a nucleus and cytoplasm along with alot of mitochondria. The mitochondria provide energy for Sucrose to be actively loaded from the companion cells into the sieve tubes where it can be transported as a sap.
The loss of water vapour from the leaves of a plant in order to gain CO2 and increase the pressure difference so that more water may be moved into the leaf.
What effects transpiration?
- Number of leaves
- Number size and position of stomata
- The presence of a way cuticle, reduce evaporation.
- Light levels
- Availabilty of water
This piece of equipment measures the amount of water that the plant takes in.
This means that it DOES NOT measure transpiration, but since 99% of the water taken into a plant is lost via transpiration it gives a reasonably accurate measurement.
- To set up a potometer you must first get a sample of a plant.
- Cut this piece from its source at an angle and preferably underwater. This prevents air bubbles entering the source and damaging the plant.
- Attach the cut end of the stem of the plant to the potometer underwater and seal it.
Transpiration! A consequence of gaseous exchange!!
Transpiration is an unavoidable consequence of gaseous exchange because water that evaporates within the leaf is lost to the air as it evaporates within the leaf.
This allows the plant to gain CO2.
Adaptations to reduce water loss
- A waxy cuticle reduces water loss due to evaporation.
- Stomata on the underside of the leaf.
- Closing the stomata at night.
- Losing their leaves in the winter when water in the ground may be frozen so needs to be conserved.
Water potential is the measurement of water molecules tendency to diffuse from one place to another.
Water will move from an area of high water potential to one of lower water potential until the two are equal.
Water can move through cells that are touching down water potential gradients, when moving from the roots to the xylem by osmosis it can take one of three pathways.
Apoplast pathways - The cell walls have many water filled spaces inbetween water can travel directly down these.
Vacuolar pathway - This moves through both the cytoplasm of the cell and also its vacoule. It does this via plasmodesmata.
Symplast pathway - Water travels through the cytoplasm of each cell and into the next cell via plasmodesmata.
Using these pathways water can travel across the root to the xylem. In order to make sure all water goes into the xylem the root has a Casparian ***** this is a waterproofed area of the cell that prevents water moving along the apoplast pathway and moves it into the centre of the cell where it can be moved into the xylem.
Role of the casparian *****
What does it do?
- The Casparian ***** (CS) blocks the apoplast pathway.
- This ensures that water and other dissolved ions move into the cell membrane
- Inside the cell membrane there are transporter proteins.
- From here they move into the xylem from which they cannot returr.
Movement from the Xylem to the leaf
Water from the xylem moves into the leaf where it evaporates and moves out of the leaf through the stomata.
It is moved across the leaf by osmosis.
This momement of water out of the xylem causes low hydrostatic pressure and thusly tension which pulls water up the xylem.
How does water move up the stem?
There are 3 processes which help to move water up the stem...
- Root pressure, the process of the endodermis moving minerals into the root by active transport means the minerals have to be pushed out the way and thsly up the stem.
- Transpiration pull, this is when tranpiration causes water vapour to move out of the xylem and into the leaf, but since the water molecules have cohesion when pulled they act like one big chain and this pulls up more molecules.
- Capillary action, The same force which attracts water molecules together also pulls on the sides of the vessel creating adhesion, since the vessels are very narrow this pulls the water up the stem.
Adaptations of leaves to reduce water loss
- Lower surface area, for example cacti have needle like leaves.
- Dense spongy mesophyll layer
- Thick waxy cuticle
- Closing stomata
- Hairs on the surface of the leaf can trap humid air.
- Pits to trap air
- Rolling the leaf so the lower epidermis is not exposed.
-Low water potential inside the leaf.
The movement of sucrose up and down the plant. From Source (area where sucrose is produced) to Sink (area where it is used).
From the source - Sucrose enters the sieve tube element which reduces its water potential. As a result water moves intot the sieve tube element. This increases the hydrostatic pressure.
At the sink - At this point sucrose is being used so its concentration in the surrounding cells has been decreased. Sucrose moves into the surrounding cells diffusion or active transport. This causes the water potential to increase causing it to move into surrounding cells, decreasing the hydrostatic pressure.
Hydrostatic pressure - Pressure created by a fluid pushing against the walls of a container.
This is when a flow of of water is created because it is continually moving down a hydrostatic pressure gradient either up or down a plant.
Evidence for translocation
How we know
The phloem is used...
- If a plant is given a radioactive source of carbon dioxide we can traceit, and see that it moves to the pholem.
- Ringing a tree (removing its bark in a ring) causes sugar to collect above the ring where the phloem stops.
- Aphids feed by pressing their mouthparts into the phloem and feeding off the sugars.
Uses Active processes...
- Companion cell have many mitochndria
- Translocation is stopped when the production of ATP is stopped.
- Sugars move to fast to be attributed to diffusion alone.
How we know it uses this mechanism
- The concentration of sucrose is higher in the sink.
- Role of sieve plates unclear
- Sugars move to all parts of plants at equal rates