3.4.2 Mass Transport in Plants

Question 1

What enviromental factors INCREASE the rate of transpiration

Answer 1

• Increasing temperature
• Decreasing humidity
• Increasing light intensity
• Increasing wind speed

Question 2

What enviromental factors DECREASE the rate of transpiration

Answer 2

• Decreasing temperature
• Increasing humidity
• Decreasing light intensity
• Decreasing wind speed

Question 3

Why does the rate of transpiration increase with increasing light intensity

Answer 3

Causes stomata to open

Question 4

Why does the diameter of the xylem decrease as the rate of transpiration increases

Answer 4

• Water lost from the leaf in transpiration
• Creates more tension and pulling force on water molecules in xylem

Question 5

What does water lost from the leaf in transpiration creating more tension and pulling force on water molecules in xylem create

Answer 5

The diameter of the xylem to decrease

Question 6

What device is used to measure the rate of transpiration

Answer 6

Potometers

Question 7

What are the vital procedures to setting up a potometer

Answer 7

• Cut stem underwater, and at a slant
• Ensure no air bubbles are present
• Seal joints with vasaline to ensure airtight

Question 8

Why do potometers not truly measure the rate of transpiration

Answer 8

• Water used for support
• Water used in photosynthesis
• Water produced in aerobic respiration
• Apparatus not fully sealed

Question 9

What is the benefit of xylem cells being dead, and therefore empty with no cytoplasm contents

Answer 9

Allows water to flow easier

Question 10

What xylem specialisation allows water to flow easier

Answer 10

Cells are dead, and therefore empty with no cytoplasm contents

Question 11

What is the benefit of xylem cells being long, forming tubes, with no end walls, meaning it’s hollow

Answer 11

Allows for continuous columns of water

Question 12

What xylem specialisation allows continuous columns of water to flow

Answer 12

Cells are long, forming tubes, with no end walls meaning thay are hollow

Question 13

What is the benefit of xylem cells having a thickened cell wall with cellulose and lignin

Answer 13

Support, and waterproofing

Question 14

What xylem specialisation allows for support, and waterproofing

Answer 14

Thickened cell walls with cellulose and lignin

Question 15

What is the benefit of xylem cells have pits in walls

Answer 15

Allows lateral movement of water

Question 16

What xylem specialisation allows for lateral movement of water

Answer 16

Pits in walls

Question 17

Describe and explain the cohesion-tension theory

Answer 17

• Water lost from the leaves by evaporation, due to transpiration
• Lowers the water potential of mesophyll cells, so water drawn out of xylem
• Tension created pulls water up xylem
• Due to cohesive forces between water molecules, caused by hydrogen bonding
• Allows a continous column of water to be pulled
• Water molecules adhere to the xylem walls

Question 18

In the cohesion-tension theory how is water lost from the leaves

Answer 18

By evaporation, due to transpiration

Question 19

In the cohesion-tension theory why is water drawn out of the xylem

Answer 19

The water potential of mesophyll cells has been lowered

Question 20

In the cohesion-tension theory what does tension created dp

Answer 20

Pulls water up xylem

Question 21

In the cohesion-tension theory what causes cohesive forces between water molecules

Answer 21

Hydrogen bonding

Question 22

In the cohesion-tension theory what allows a continous column of water to be pulled

Answer 22

Cohesive forces between water molecules, due to hydrogen bonding

Question 23

In the cohesion-tension theory what do water molecules adhere to

Answer 23

Xylem walls

Question 24

In the phloem solution, what key molecules are transported

Answer 24

Sucrose and amino-acids

Question 25

What key component are within the sieve tube element of the phloem

Answer 25

Sieve plates

Question 26

What is the specialisation of companion cells in the phloem

Answer 26

Have lots of mitochondria, to actively transport sucrose into phloem from source

Question 27

What is the most general way of describing route of mass-flow hypothesis

Answer 27

From source to sink

Question 28

Describe and explain the Mass-Flow Hypothesis

Answer 28

* **Sucrose actively transported** into sieve tube element, from source, **by companion cells** * This **lowers the water potential** in the sieve tube element, so **water moves in by osmosis** from the xylem * Produces a **high hydrostatic pressure** , as the volume in the sieve tube element increases * **Mass** flow of sucrose from source to sink * At sink, sucrose is unloaded and used or converted to starch for storage

Question 29

In the Mass-Flow Hypothesis what actively transports sucrose into sieve tube element from source

Answer 29

Companion cells

Question 30

In the Mass-Flow Hypothesis what does a low water potential in the sieve tube element allow

Answer 30

Water to move in by osmosis from the xylem

Question 31

In the Mass-Flow Hypothesis what does the movement of water into the sieve tube element by osmosis from the xylem create

Answer 31

High hydrostatic pressure

Question 32

In the Mass-Flow Hypothesis where is the **mass** flow

Answer 32

From source to sink

Question 33

In the Mass-Flow Hypothesis what happens at the sink

Answer 33

Sucrose is unloaded and used or converted to starch for storage

Question 34

Explain how the following provides evidence for the Mass-Flow Hypothesis: Translocation slows if **respiration inhibitors** used

Answer 34

No active transport of sucrose from companion cells at source

Question 35

Explain how the following provides evidence for the Mass-Flow Hypothesis: If phloem punctured, contents flows out

Answer 35

Occuring at **high pressure**

Question 36

Explain how the following provides evidence for the Mass-Flow Hypothesis: Plants can be supplied with radioactive CO2 and traced

Answer 36

Sucrose moves from source, up and down stem, to sinks

Question 37

Explain how the following provides evidence for the Mass-Flow Hypothesis: **ring** of bark removed, so that xylem remains, but phloem removed

Answer 37

* Bulging above ring due to build-up of fluid in phloem * Tissues die below, as no sucrose for respiration