Exchange - Mass Transport In Plants

Question 1

How do water molecules stick together?

Answer 1

Hydrogen bonds between molecules

Question 2

State what is meant by transpiration

Answer 2

The transport of water throughout a plant

Question 3

Explain the cohesion tension theory

Answer 3

• Evaporation of water from cells in leaves
• Reduces water potential in the cells and a water potential gradient is established
• Cohesion between water molecules by hydrogen bonding
• Adhesion between water molecules and sides of xylem vessels
• Tension created in xylem
• Continuous column of water pulled up the xylem
• Water is replaced through the roots

Question 4

Explain how xylem tissue is adapted to its function

Answer 4

• Lignin tubes with no end walls so allows water to move in a continuous column
• No organelles so allows easier water flow
• Thick wall contains lignin so provided support
• Pits in walls so allows lateral movement

Question 5

Effects of light intensity on rate of transpiration

Answer 5

• Increased light intensity increases rate of transpiration
• More stomata are open so increased surface area for water to evaporate

Question 6

Effects of temperature on rate of transpiration

Answer 6

• Increased temperature increases rate of transpiration
• Increased diffusion as water molecules have more kinetic energy

Question 7

Effects of humidity on rate of transpiration

Answer 7

• Increased humidity decreases rate of transpiration
• Water potential gradient is reduced

Question 8

Effects of air movement on rate of transpiration

Answer 8

• High air movement means increased rate of transpiration
• Higher water potential gradient is established

Question 9

Describe how a simple potometer may be used to measure the rate of water uptake

Answer 9

• Used to estimate rate of transpiration
• Measures water uptake by a shoot
• Assumes all water taken up is transpired
• However some water may be used to make cells turgid, for hydrolysis or for photosynthesis

Question 10

Explain how the phloem tissue it adapted to its function

Answer 10

• Sieve tubes have no nucleus, little cytoplasm and few organelles so it allows unobstructed flow of solutes
• End walls of sieve tubes have pores so it allows continuous flow of substances through sieve tube
• Companion cells contain many mitochondria to make ATP for active transport

Question 11

What is an assimilate

Answer 11

Sollutes

Question 12

What is meant by a source and a sink

Answer 12

• A source is where sugars are made/stored and are therefore in high concentration
• A sink is whee the sugars are used in respiration or converted into starch for storage

Question 13

Describe the evidence for the mass flow hypothesis

Answer 13

• If a ring of bark is removed from a woody stem a bulge forms above the ring
• Pressure in the phloem can be investigated using aphids and stomach contents can be analysed
• Radioactive tracer can be used to track the movement of organic substances
• Translocation can be stopped using a respiratory poison

Question 14

Describe the evidence against the mass flow hypothesis

Answer 14

• Sucrose travels to many different sinks not just the one with the highest water potential
• Sot all solutes in the phloem move at the same rate
• The role of the sieve plates is unclear

Question 15

Describe mass flow in the phloem

Answer 15

1. Active transport of sucrose into phloem via sucrose transport proteins
2. Reduces water potential in the phloem
3. Water moves down a concentration gradient (xylem —> phloem) by osmosis
4. High hydrostatic pressure
5. Pressure gradient (mass flow) from source to respiring cells in the sink
6. Sucrose is removed at the sink by active transport for storage or respiration
7. Increases water potential in the phloem
8. Water moves down water potential gradient (phloem —> xylem) by osmosis