Topic 3: Xylem

Question 1

What does the xylem transport?

Answer 1

The tissue transports water and mineral ions in solution

Question 2

In which direction does the xylem transport water and mineral ions?

Answer 2

They move up the plant
From the roots to the leaves

Question 3

Name 4 features of the xylem tissue

Answer 3

They are long tubular cells with no living contents
They are dead tissue
They have strengthened cell walls
They are hollow tubes

Question 4

Why are xylem tissues made up of long tubular cells in comparison to normal cells?

Answer 4

They have no living organelles or cytoplasm, more room to transport

Question 5

Why are xylem tissues dead?

Answer 5

They do start of as live
They die from the lack of water to form a hollow tube

Question 6

Why is it important that the cell walls in xylem tissues are strengthened/ what are they strengthened by?

Answer 6

So they can withstand pressure
Are also waterproof- no leaking=better transport
Strengthened by a woody substance called lignin

Question 7

What do the hollow tubes of xylem tissues allow?

Answer 7

Unrestricted water flow as a continuous column
Maintains the transpiration scheme

Question 8

What are tracheids?

Answer 8

They are found in conifers (and some other plants)
They are like support systems that run along the xylem

Question 9

What are the differences between xylem and phloem?

Answer 9

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

Question 10

Define transpiration

Answer 10

The movement of water and mineral ions through a plant, down a water potential gradient

Question 11

Which of the properties of water is relevant to transpiration ?

Answer 11

Cohesion
Weak hydrogen bonds between molecules means they are stuck together
Means water can move on mass as a continuos column

Question 12

What does evaporation produce for plant?

Answer 12

A pulling action known as transpirational pull

Question 13

How does water move up a stem?

Answer 13

Cohesion
Tension
Adhesion
Strengthened xylem walls

Question 14

How does cohesion allow water to move up the stem?

Answer 14

Water molecules stick to other water molecules because of hydrogen bonds. This forms a continuous column through the xylem

Question 15

Why would air bubbles slow the rate of respiration?

Answer 15

They would break up the continuos column of water

Question 16

Why does tension allow water to move up the stem?

Answer 16

Tension is negative pressure created by evaporation
Water evaporating pulls more water up the xylem- transpiration pull

Question 17

How does adhesion allow water to move up the stem?

Answer 17

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

Question 18

What kind of process is transpiration?

Answer 18

Passive because it doesnt require any metabolic energy

Question 19

How do the strengthened xylem walls allow water to move up the stem?

Answer 19

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

Question 20

What is the cohesion tension theory?

Answer 20

The different forces caused by transpiration (tension) and cohesion are mainly responsible for water moving up the stem

Question 21

What evidence is there to support cohesion tension theory?

Answer 21

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

Question 22

What are the factors that affect the rate of transpiration?

Answer 22

Light
Temperature
Humidity
Air movement