Water Transport in Plants
Water enters a plant through its root hair cells. Water has to get from the soil to the root and into the xylem. The root is covered in root hairs which increases the surface area speeding up water uptake. Water moves from a high water potential (soil) to a low water potential (root). This maintains a gradient.
Water travels through the apoplast pathway (non living parts of the cells) through the cell walls. However when the water gets to the endodermis its path is blocked by a waxy strip called the casparian strip so the water has to take the symplast pathway. The symplast pathway (living parts of the cells) goes through the cytoplasm which is good as it has to go through the cell membrane which controls what enters and leaves the cell. The cytoplasm of neighbouring cells are connected via plasmodesmata. Once past the barrier water moves into the xylem.
Cohesion and tension help water move up to the leaves. Water transpires from the leaves which creates a tension and pulls more water into the leaves. Water molecules are cohesive so when some are pulled up the rest follow so all the water in the xylem moves up. Adhesion is when the xylem also sticks to the water molecules and gets pulled in slightly. Root pressure also helps water move upwards. When water is transported into the xylem from the roots it creates a pressure that pushes more water up.
Transpiration is the evaporation of water from a plants surface. Water evaporates from the cell walls and accumulates in the spaces between the cells in the leaf. When the stomata open it moves out of the leaf down the concentration gradient.
Four main factors affect transpiration rate
- Light - the lighter it is the faster the transpiration rate, stomata open in the light
- Temperature - the higher the temperature the faster the transpiration rate. The water molecules will have more kinetic energy and increases the concentration outside the leaves.
- Humidity - the lower the humidity the faster the transpiration rate as there will be a higher concentration gradient.
- Wind - the windier it is the faster the transpiration rate lots of air movement blows the water molecules away which increases the concentration gradient.
Routes through the root
Water can travel through roots into xylem by:
Symplast Pathway - goes through living parts of cells (cytoplasm). Cytoplasm of neighbouring cells connect through plasmodesmata.
Apoplast Pathway - goes through non-living parts of root (cell walls). Walls are very absorbent and water can diffuse through them, as well as passing through spaces between them.
When water in apoplast reaches endodermis, its path is blocked by waxy strip in cell walls (Casparian strip). Water now has to take symplast pathway. This is useful because it means water has to go through a cell membrane. Cell membranes control whether or not substances in the water get through. Once past the barrier, water moves into xylem.
Apoplast provides least resistance
How water enters a plant
- Water has to get from the soil, through the root and into the xylem
- the system vessels that transports water throughout the plant.
- the bit of the root that absorbs water is covered in root hairs. these increase the roots surface area, speeding up water intake.
- once its absorbed, the water has to get through the cortex, including the endodermis, before it can reach the xylem.
How water travels into the xylem
water can travel through the roots into the xylem by two different paths:
- the symplast pathway - goes through the living parts of cells - the cytoplasm. the cytoplasm of neighbouring cells connect through plasmodesmata (small gaps in the cell walls).
- the apoplast pathway - goes through the non-living parts of the root - the cell walls. the walls are very absorbent and water can simply diffuse through them, as well as passing through the space between them.
- when water in the apoplast pathway gets to the endodermis cells through, its path is blocked by a waxy strip in the cell walls, called the caspaarian strip. now the water has to take the symplast pathway.
- this is useful, because it means the water has to go through a cell membrane. cell membranes are able to control whether or not substances in the water get through.
- once past this barrier, the water moves into the xylem.
both pathwayss are used, but the main one is the apoplast pathway because it provides the least resistance.
How water moves up a plant
water can move up a plant in two ways:
- cohesion and tension help water move up plants, from roots to leaves, against the force of gravity.
- water evaporates from the leaves at the top of the xylem
- this creates tension (suction) which pulls more water into the leaf
- water molecules are cohesive (they stick together) so when some are pulled into the elaf others follow. this means the whole column of water in the xylem from the leaves down to the roots moves upwards.
- water enters the stem through roots
- root pressure also helps move the water upwards. when water is transported into the xylem in the roots, it creates a pressure and shoves water already in the xylem further upwards. this pressure is weal and couldn't move water to the top of bigger plant by itself. but it helps, especially in young, small plats where the leaves are still developing
transpiration is the evaporation of water from a plants surface, especially the leaves.
- water evaporates from the moist cell walls and accumulates in the space between cells in the leaf.
- when the is stomata open, it moves out of the leaf down the concentration gradient (theres more water inside the leaf than in the air outside)
transpiration is really a side effect of photosynthesis - the plant needs to open its stomata to let in CO2 so that it can produce glucose, but this also lets water out.
Factors affecting the transpiration rate
- light - the lighter it is the faster the transpiration rate. this is because the stomata open when it gets light. when its dark the stomata are usually closed, so there's little transpiration.
- temperature - the higher the temperature the faster the transpiration rate. warmer water molecules have more energy so they evaporate from the cells inside the leaf faster. this increases the concentration gradient between the inside and outside of the leaf, making water diffuse out of the leaf faster.
- humidity - the lower the humidity, the faster the transpiration rate. if the air around the plant is dry, the concentration gradient between the leaf and the air is increased, which increases transpiration.
- wind - the windier it is, the faster the transpiration rate. lots of air movment blows away water molecules from around the stomata. this increases the concentration gradient, which increases the rate of transpiration.
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 root
The Xylem are the cross in the middle.
The Phloem are the dots sorrounding it.
Xylem and Phloem in 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 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.
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 affects 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 strip 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 return.
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.
- Evidence against...
- - Role of sieve plates unclear
- - Sugars move to all parts of plants at equal rates.