3. Water in Plants - Xylem Flashcards
How does a Potometer work?
A potometer is a piece of equipment that can be used to measure the rate of uptake of water by a plant shoot.
The potometer is filled with water with no air bubbles. Under water, a leafy shoot is attached. The potometer is then removed and all joints made watertight. An air bubble is introduced into the capillary tube. As water evaporates from the leaves, water is pulled into the stem and the distance moved by the air bubble in a given time is measured. About 99% of the water taken up by a plant is lost in transpiration, which means that the rate of uptake is almost the same as the rate at which transpiration is occurring.
Xylem Vessels
The majority of water is transported through the xylem.
They have thick cellulose cell walls
As they mature, waterproof lignin is produced in the wall and the cells die
The end walls break down which allows the cells to form a continuous tube
Xylem tissue and Xylem Vessels
Xylem tissue transports water and mineral ions in solution. These substances move up the plant from roots to leaves.
Xylem vessels are part of the xylem tissue. Xylem vessels are very long tube-like structures formed from hollow cells joined end to end. There are no end walls, making an interrupted tube that allows water to pass easily.
Can you explain how each feature helps the xylem to transport water?
Total lack of cell contents
No end walls
a diameter of between 0.001 and 0.2mm
Lignified walls
Pits
Total lack of cell contents
Reduces resistance to flow
No end walls
Cohesion/tension maintained as one continuous column
a diameter of between 0.001 and 0.2mm Aids adhesion (attraction to lignin)
Lignified walls
Waterproof which reduces water loss
Pits
Lateral movement of water between xylem and other parts of plant
How are root hair cells adapted to absorb water from the soil?
Soil solution is mostly water so has a very high water potential
The root hairs and other cells of the root have sugars, amino acids etc dissolved in them so have a much lower water potential
Water moves by osmosis from soil into root hairs down this water potential gradient
apoplast pathway
The apoplast pathway goes through the non living parts – the spaces between cells and the cell walls. Spaces between the cellulose fibres in cell walls allow water to pass easily by diffusion
The walls are very absorbent and water can simply diffuse through them as well as passing through the spaces between them
Both pathways are used but the main one is the apoplast pathway because it provides the least resistance
symplast pathway
The symplast pathway goes through the living parts of cells – the cytoplasm- as a result of osmosis. Cells connect with each other by tiny openings called plasmodesmata. Each plasmadesmata is filled with a thin strand of cytoplasm.
As water enters one cell, the water potential of that cell will now be higher than its neighbour so water will then move into this neighbouring cell. The water potential of the first cell will now be lower and so a water potential gradient is set up across the cortex from root hair cell to endodermis.
Casparian strip
When water in the apoplast pathway gets to the endodermis cells, its path is blocked by a waxy strip called the casparian strip
Now the water has to take the symplast pathway
This is useful because the water has to go through a cell membrane
Cell membranes are able to control whether or not substances in the water get through
How does water enter the xylem?
Endodermal cells actively transport salts into the xylem
This process requires energy and can only occur within living cells
This creates a lower water potential in the xylem
Water now moves into the xylem by osmosis down a water potential gradient
This creates a pressure called root pressure that helps to move water up the xylem
Root Pressure
This helps to move the water upwards. When water is forced into the xylem in the root it “shoves” the water upwards. This force is quite small (helps new shoots without leaves have water move to tip against gravity).
The force may be created when the endodermal cells actively transport salts into the xylem vessels of the root. This is done to create a lower water potential in the xylem so that water enters it
How does water move up the stem?
At the other side of the endodermis are the xylem and phloem tissue – the vascular tissue. Endodermal cells actively transport salts into the xylem. This requires Energy. This creates a lower water potential in the xylem. Water now moves into the xylem by osmosis along a water potential gradient. This creates a force that helps water move up the xylem. The force is called root pressure. It is a weak pressure and cannot move water far – helpful in young small plants where the leaves are still developing.
Cohesion-tension theory
Water evaporates from the leaves (transpiration)
This transpiration puts the xylem under tension, which pulls more water into the leaf
Water molecules form Hydrogen bonds between each other and tend to stick together (cohesion). When some water molecules are pulled into the leaf, others follow
Water forms a continuous column from mesophyll cells and down the xylem
Water then enters the stem through the roots
What is transpiration and what factors affect it
Transpiration is the evaporation of water from a plant through open stomata. Water evaporates from the moist cell walls and leaves the plant through open stomata. This loss of water vapour is due to water potential gradient between cells inside the leaf and the air outside, Transpiration can be affected by: Temperature Wind speed Humidity Light intensity
