Unit 3 Mass transport in Plants

Question 1

Describe the role of xylem in plants

Answer 1

• Tissue that transports water in the stem and leaves of the plants
• Xylem vessels are long-tube like structures formed from dead cells

Question 2

Describe what is meant by transpiration

Answer 2

• The loss of water vapour from the stomata by evaporation

Question 3

Describe 4 factors affecting the rate of transpiration.

Answer 3

LIGHT - The lighter the faster the transpiration rate, positive correlation
TEMPERATURE - More kinetic energy in molecules so faster the transpiration rate, positive correlation
HUMIDITY - The lower the humidity, the faster the transpiration rate, negative correlation
WIND - The more wind, the faster the transpiration rate, positive correlation

Question 4

Explain the cohesion-tension theory

Answer 4

• Water lost from leaf through stomata because of transpiration/ evaporation
• Which lowers water potential of mesophyll/ leaf cells
• So water is pulled up the xylem to replace creating tension
• Water molecules stick together due to hydrogen bonds forming a continuous water column
• Also adhesion of water molecules to walls of xylem(stick) & water pulled up creating tension

Question 5

Describe the role of phloem in plants and its structures.

Answer 5

• Tissue that transports organic substances in plants
  STRUCTURES
• Sieve tube elements = Living cells containing no nucleus& few organelles
• Companion cells = Provide ATP required for transport of organic substances

Question 6

Describe the first stage of translocation.

Answer 6

1. Sucrose actively transported into phloem by companion cell using co transporter protein
2. As sucrose moves into sieve tube elements, causes lower water potential in phloem
3. Causes water to move into phloem from xylem by osmosis and increased volume of water causes higher hydrostatic pressure

Question 7

Describe the second stage of translocation

Answer 7

4. Water moves down sieve tube by mass flow towards areas of lower hydrostatic pressures /respiring cells
5. At sink sucrose diffuses into surrounding cells for respiration for storage

Question 8

Describe the use of tracers to investigate transport in plants.

Answer 8

• Tracing involves radioactively labeling carbon
• Plants are grown in presence of radioactive carbon dioxide which are converted into sugars in photosynthesis
• Using audiography we can see areas exposed to radiation to correspond to where

Question 9

Describe the use of ringing to investigate transport in plants.

Answer 9

• Ring of bark and phloem removed leaving xylem
• Causes tissue above ring to swell due to excess sucrose as tissue below begins to die
• Shows sucrose/sugars must be transported in the phloem