Topic 3: Xylem Flashcards
What does the xylem transport?
The tissue transports water and mineral ions in solution
In which direction does the xylem transport water and mineral ions?
They move up the plant
From the roots to the leaves
Name 4 features of the xylem tissue
They are long tubular cells with no living contents
They are dead tissue
They have strengthened cell walls
They are hollow tubes
Why are xylem tissues made up of long tubular cells in comparison to normal cells?
They have no living organelles or cytoplasm, more room to transport
Why are xylem tissues dead?
They do start of as live
They die from the lack of water to form a hollow tube
Why is it important that the cell walls in xylem tissues are strengthened/ what are they strengthened by?
So they can withstand pressure
Are also waterproof- no leaking=better transport
Strengthened by a woody substance called lignin
What do the hollow tubes of xylem tissues allow?
Unrestricted water flow as a continuous column
Maintains the transpiration scheme
What are tracheids?
They are found in conifers (and some other plants)
They are like support systems that run along the xylem
What are the differences between xylem and phloem?
Xylem is made of dead cells and they only transport mineral and ins upwards
Phloem are made up of living cells and they can transport organic substances up or down
Xylem are made up of of one part, they have no companion cells or cytoplasm but does have lignin
Phloem are made up of 2 elements, including companion cells and they have some cytoplasm but no lignin
Define transpiration
The movement of water and mineral ions through a plant, down a water potential gradient
Which of the properties of water is relevant to transpiration ?
Cohesion
Weak hydrogen bonds between molecules means they are stuck together
Means water can move on mass as a continuos column
What does evaporation produce for plant?
A pulling action known as transpirational pull
How does water move up a stem?
Cohesion
Tension
Adhesion
Strengthened xylem walls
How does cohesion allow water to move up the stem?
Water molecules stick to other water molecules because of hydrogen bonds. This forms a continuous column through the xylem
Why would air bubbles slow the rate of respiration?
They would break up the continuos column of water
Why does tension allow water to move up the stem?
Tension is negative pressure created by evaporation
Water evaporating pulls more water up the xylem- transpiration pull
How does adhesion allow water to move up the stem?
Water mo;eches stitch to the inside walls of the xylem because of hydrogen bonds
The thinner the lumen of the xylem vessel, the greater the amount of adhesion (capillary action)
Means water remans in the tube if transpiration stops
What kind of process is transpiration?
Passive because it doesnt require any metabolic energy
How do the strengthened xylem walls allow water to move up the stem?
They are strengthened with lignin to stop them collapsing inwards
Lignin helps to keep the shape so it doesnt break apart and continuos flow can happen
What is the cohesion tension theory?
The different forces caused by transpiration (tension) and cohesion are mainly responsible for water moving up the stem
What evidence is there to support cohesion tension theory?
The diameter of the tree trunk changes according to the rate of transpiration- e.g. the trunk shrinks in diameter when transpiration is high in the day and there is more tension
If the xylem vessel is broken and air enters, the tree cant draw up water
A continuos column of water is needed and this would be broken by the air bubble
What are the factors that affect the rate of transpiration?
Light
Temperature
Humidity
Air movement
