Mass transport in plants

Question 1

Define what transpiration is

Answer 1

The process where evaporation of water from the leaves pulls a stream of water through the xylem vessels of the stem

Question 2

Explain how water moves through the leaf

Answer 2

Water evaporates from the cell walls of the mesophyll cells into the air spaces in the leaf.
This lowers water potential in mesophyll cell and water moves by osmosis from neighbouring cells
The process is repeated through the cells
Replaced by water from the xylem

Question 3

Explain how water moves up the xylem

Answer 3

Cohesion-tension theory
Water evaporates from mesophyll cells
Water molecules form hydrogen bonds between each other - cohesion
Water forms continuous, unbroken column through mesophyll cells and xylem - transpiration pull
This causes a negative pressure within the xylem
No energy required other than that from the sun which drives evaporation

Question 4

Describe the mass flow mechanism for the transport of organic substances in the phloem

Answer 4

Glucose produced in photosynthesising cells converted to
sucrose
Hydrogen ions actively transported out of companion cell
Hydrogen ions return to companion cell through co-transport protein with sucrose (facilitated diffusion)
Sucrose moves into sieve tube elements lowering water potential
Water moves from the higher water potential in xylem to the sieve tube element creating a higher hydrostatic pressure
Sucrose at the sink is used in respiration or converted to starch Sucrose is actively transported from sieve tube element to sink cells lowering water potential
Water leaves sieve tubes down water potential gradient via osmosis creating a lower hydrostatic pressure
Sucrose moves by mass flow down the hydrostatic pressure gradient from source to sink

Question 5

Summarise the evidence for the mass flow mechanism

Answer 5

-Pressure in sieve tubes (sap released when cut)
-Concentration of sucrose higher in leaves than roots
-Downward flow in phloem only occurs in the light
-Increase in sucrose in leaf followed later by similar increase in sucrose in phloem
-Lack of oxygen inhibits translocation of sucrose in phloem
-Companion cells have many mitochondria

Question 6

Summarise the evidence against mass flow mechanism

Answer 6

-Function of sieve plates unclear - hinder mass flow
-Not all solutes move at same speed - they would in mass flow
-Sucrose delivered same rate to all regions - not dependent on sucrose concentration of sink as mass flow would suggest

Question 7

Tracer experiment

Answer 7

-plants are provided with radioactively labelled carbon which is absorbed over time and used in photosynthesis
-creates sugars which contain the labelled carbon dioxide
-Thin slices from the stem are cut and placed on x-ray film that turns black when exposed to radioactive material
-Shows sugars are transported in the phloem

Question 8

Evidence that translocation of organic molecules occurs in phloem

Answer 8

• when a phloem is cut, a solution of organic molecules flow out
• plants provided with radioactive CO2 can be shown to have radioactively labelled carbon in phloem after a short time
• aphids are a type of insect that feed on plants. They have mouthparts which penetrate the phloem. They can tf be used to extract the contents of sieve tubes.
  These contents show daily variations in the sucrose content of leaves that are mirrored a little layer by identical changes in the sucrose content of the phloem
• the removal of a ring of phloem around the circumference of a stem leads to the accumulation of sugars above the ring and their disappearance from below it.

Question 9

Ringing experiment

Answer 9

A section of the outer layers is removed around the complete circumference of the woody stem. After a period of time, the region of the stem immediately above the missing ring of tissue is seen to swell. Samples of the liquid accumulated in this region were found to be rich in sugars and other dissolved organic substances. Some non-photosynthetic tissues in the region below the ring are found to wither and die, while those above the ring continue to grow

phloem is removed from section of stem. find sugars above the cut but no below.

Question 10

Describe the cohesion-tension theory of water in the xylem

Answer 10

1. Water lost from leaf because of transpiration/Transpiration
2. Lowers water potential of mesophyll / leaf cells
3. Water pulled up xylem (creating tension)
4. Water molecules cohere / ‘stick’ together by hydrogen bonds;
5. (forming continuous) water column;
6. Adhesion of water (molecules) to walls of xylem

Question 11

The mass flow hypothesis is used to explain the movement of substances
through phloem.
Use your understanding of the mass flow hypothesis to explain how
pressure is generated inside this phloem tube.

Answer 11

Sucrose actively transported (into phloem);
2. Lowering/reducing water potential
OR
More negative water potential;
3. Water moves (into phloem) by osmosis (from xylem);