Topic 3.4 - Mass transport in 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

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

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

Describe how it affects rate of transpiration and explain why:
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

Describe how it affects rate of transpiration and explain why:
Humidity

Answer 5

· High humidity in the atmosphere à 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 6

Describe how it affects rate of transpiration and explain why:
Temperature

Answer 6

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 7

What is the phloem and what is its function?

Answer 7

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 8

Describe how the process of translocation works in plants

Answer 8

· 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 9

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

Answer 9

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