mass transport in plants

Question 1

Describe how xylem vessels structure is related to their function.

Answer 1

long hollow tubes with no end walls SO continuous water columns
Tubes contain no cytoplasm or organelles SO no obstruction to continuous water column
Lignin SO provides support
Pits in walls SO allows lateral movement of water when vessel blocked
Lignin SO waterproof preventing water loss

Question 2

What is meant by negative pressure in the xylem vessels

Answer 2

pressure inside the xylem is lower than the atmosphere as water is under tension

Question 3

Name and explain the factors that can increase the rate of transpiration

Answer 3

1. Higher Light intensity -> more stomata open as higher rate of photosynthesis ->……….
2. Higher Temperature -> more kinetic energy of H2O molecules so more evaporation at leaf ->……
3. Lower Humidity (dryer conditions) -> increased water potential gradient from water in air spaces to outside ->…..
4. More Windy conditions -> carries H20 away from stomata creating a larger
   water potential gradient of water vapour from inside to outside leaf…..

A02 extensions: ……-> increased rate of evaporation/transpiration ->
increased rate of flow of continuous column of water held by cohesion:
->adhesion to walls so reducing diameter of xylem more
->more negative pressure

Question 4

1. Describe and explain precautions a student should take when setting up the potometer

Answer 4

1. Seal joints (and anything exposed to the air with Vaseline/oil) so no water evaporates here
2. Cut shoot under water
3. Cut shoot at a slant-prevents damage and ensures continuous water column
4. Insert into apparatus under water-no air bubbles
   5.Dry leaves so water potential gradient between air spaces and atmosphere isn’t affected

Question 5

2. In an experiment to see how a named factor effects transpiration, what two measurements would you need to make if the units are:

cm3 per minute
Grams per hour per group of xylem vessels

Answer 5

Length that air bubble moves (to turn into volume using pieR2) and time taken in minutes. = cm3 per min
Initial and final mass of everything in grams, time taken for movement in hours and number of groups of xylem vessels. =Grams per hour per group of xylem vessels

Question 6

Givetworeasons why the potometer/rate of transpiration doesnottruly measure the rate of transpiration

Answer 6

1.Water used in photosynthesis
2.Water produced in respiration
3. Water used for support / turgidity
4.Apparatus not sealed / ‘leaks’
5. water used in hydrolysis or produced
in condensation reactions

Question 7

Describe translocation (mass flow/ carbohydrate/ sucrose/ sugars/ amino acid transport)

Answer 7

1.Companion cells actively transport sucrose into the phloem

2.Lowers water potential in phloemandwater enters by osmosis

3.Produces higher hydrostatic pressure

4.causing mass flow transporting sucrosefromsite of photosynthesis (leaf)torespiring cells

5. Movement through the leaf via plasmodesmata

Question 8

Describe the cohesion-tension theory (transpiration/movement of water and ions)

Answer 8

Water transpired from leaf through stomata
Lowers water potential of leaf cells
Water drawn out of xylem
Creates tension, pulling water up in xylem
Cohesion, hydrogen bonds between water molecules, maintains continuous water column

 / Adhesion of water molecules to walls of xylem  Pulling walls inwards and results in tension /