3.4.2 Mass Plants

Question 1

What is the xylem and what is its function

Answer 1

The xylem is made of dead, hollow cells which form a continuous tube through the plant.
It transports water from the roots up the plant

Question 2

1. Describe what causes water to move up the xylem (according to the cohesion-tension theory)

Answer 2

• water evaporates from the leaves;
• this reduces water potential in the mesophyll cells in the leaf
• so water moves out of the xylem down a water potential gradient by osmosis
• this creates tension (negative pressure) in the xylem
• so water is pulled up the xylem as a continuous column
• there is cohesion between water molecules due to H-bonding
• the column doesn’t break because of adhesion between the water molecules and the xylem walls

Question 3

1. Why is the process that transports water up the xylem called the cohesion-tension theory?

Answer 3

• Tension is negative pressure which is created by the evaporation of water from the leaves
• Cohesion is the attraction between the water molecules as a result of the H-bonding between them

Question 4

Light intensity

Answer 4

• High light intensity causes stomata to open
• So more water evaporates from the leaves
• So more tension in the xylem
• So bigger transpirational pull
• Higher rate of transpiration

Question 5

temperature

Answer 5

• High temperature causes more water to evaporate from the leaves
• So more tension in the xylem
• So bigger transpirational pull
• Higher rate of transpiration

Question 6

humidity

Answer 6

• High humidity in the atmosphere  reduces the diffusion gradient between the inside of the leaf and the atmosphere
• So less water vapour diffuses out of the leaves
• So less tension in the xylem
• So smaller transpirational pull
• Lower rate of transpiration

Question 7

air movement

Answer 7

• More air movement in the atmosphere  increases the diffusion gradient between the inside of the leaf and the atmosphere
• So more water vapour diffuses out of the leaves
• So more tension in the xylem
• So bigger transpirational pull
• Higher rate of transpiration

Question 8

1. What is the phloem and what is its function?

Answer 8

The phloem is made up of sieve tube elements and companion cells
All the cells are alive
The sieve tube elements have no nucleus, organelles and very little cytoplasm so that substances are able to flow easily through
The companion cells control the sieve tube elements and have lots of mitochondria to produce ATP used for active transport

Question 9

1. Describe how the process of translocation works in plants

Answer 9

• In source cells (e.g. leaves) sugars are actively transported into phloem;
• By companion cells;
• Lowers water potential of sieve tube element
• water enters sieve tube element from the xylem by osmosis;
• Increase in hydrostatic pressure causes mass movement towards sink (e.g. root)
• Sugars used for respiration or converted to starch for storage in roots

Question 10

1. Describe how a high pressure is created in the phloem near the source

Answer 10

• Sucrose enters the phloem and decreases the water potential in the phloem
• So water moves into the phloem by osmosis
• The volume of water in the phloem increases so the pressure increases

Question 11

1. Phloem tissue is found in close association with xylem tissue. Explain the importance of this close association. [2]

Answer 11

• When organic substances enter the phloem, the water potential decreases and water enters the phloem from the xylem
• This is important as it increases the hydrostatic pressure so organic substances can be move by mass transport from the source to the sink

Question 12

1. What does it mean to ‘ring’ a tree? Draw a picture to show what the tree looks like once it has been ‘ringed’ and after some time….

Answer 12

‘Ringing’ a tree is when a section of bark is removed. The bark contains the phloem
The region above where the bark is removed becomes swollen because the organic substances are no longer able to move down the tree

Question 13

1. What is a tracer? What do experiments with tracers show?

Answer 13

Tracers can be fluorescent or radioactive
You introduce the tracer to a known point on the plant
Then take samples from the phloem at specific times at known distances from the site the tracer was introduced
Or expose the plant to autoradiography paper to see where the substance has been moved t

Question 14

1. What is autoradiography?

Answer 14

• The radioactive substances can be tracked using autoradiography
• This is when the plant is placed next to photographic film
• The film will change colour when exposed to radiation