Year 12 - Mass transport in plants

Question 1

Describe how xylem vessel’s 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

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

Answer 2

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

Question 3

What is meant by negative pressure in the xylem vessels

Answer 3

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

Question 4

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

Answer 4

Higher Light intensity so more stomata open as higher rate of photosynthesis so … ​

Higher Temperature so more kinetic energy of H2O molecules so more ​evaporation at leaf ​so …

Lower Humidity so increased water potential gradient from water in air spaces to outside so …

More Windy conditions so carries H20 away from stomata creating a larger ​water potential gradient of water vapour from inside to outside leaf so …

(… = AO2, increased rate of transpiration, increased rate of flow of continuous column of water held by cohesion)

Question 5

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

Answer 5

Seal joints so no water lost via evaporation

Cut shoot under water so no air bubbles form and block xylem vessels

Cut shoot at a slant so no air bubbles form and block xylem vessels

Insert apparatus under water so no air bubbles form and block xylem vessels

Dry leaves so water potential gradient between air spaces and atmosphere isn’t affected

Question 6

Give two reasons why the potometer/rate of transpiration does not truly measure the rate of transpiration

Answer 6

Water used in photosynthesis​
Water produced in respiration​
Water used for support / turgidity​
Apparatus not sealed / ‘leaks’

Question 7

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

Answer 7

Companion cells actively transport sucrose (/amino acids) into the phloem ​
Lowers water potential in phloem and water enters by osmosis​

Produces higher hydrostatic pressure​

Causing mass flow transporting sucrose from site of photosynthesis to respiring cells (leaves to roots = source to sink)

Movement through the leaf via plasmodesmata