Mass Transport in Plants

Question 1

What are some features of the xylem?

Answer 1

• Dead
• Hollow tube
• No end walls
• Continuous tube
• Thick-walled
• Lignified

Question 2

Why do xylem vessels have no end walls?

Answer 2

To allow for continuous columns of water.

Question 3

Why do xylem vessels have no cytoplasm / organelles?

Answer 3

To allow for easier, unimpeded water flow.

Question 4

Why are xylem vessels lignified?

Answer 4

• To provide support to withstand tension in the water column.
• It is waterproof to keep water in cells.

Question 5

Why does the xylem have pots in the walls?

Answer 5

To allow for lateral movement to get around blocked vessels.

Question 6

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Question 7

What are some abiotic factors that affect transpiration?

Answer 7

• Temperature
• Air movement
• Percentage humidity
• Water diffusion gradients

Question 8

What are some biotic factors that affect transpiration?

Answer 8

• Surface area to volume ratio
• Waxy cuticle thickness
• Stomata opening / closing
• Stomata density
• Rolled leaves
• Sunken stomata
• Hairy leaves

Question 9

What is transpiration?

Answer 9

A passive process and is the main driving force for drawing water up the stem against gravity.

Question 10

Describe the cohesion tension theory of water transport in the xylem.

Answer 10

• Water evaporates from the leaves (transpiration) mostly through the stomata (some directly through the cuticle).
• This decreases the water potential in the mesophyll cells.
• This creates a water potential gradient that draws water out of the xylem by osmosis.
• This creates tension (negative pressures) in the column of water in the xylem.
• Cohesion between water molecules due to hydrogen bonding allows water to be pulled up as a continuous column in the xylem.

Question 11

How does cohesion occur in the xylem?

Answer 11

Due to hydrogen bonding between water molecules.

Question 12

How does adhesion occur in the xylem?

Answer 12

Due to hydrogen binding between water molecules and the walls of the xylem vessel.

Question 13

Capillary action CHECK

Question 14

How does the diameter of a tree trunk provide evidence for cohesion tension theory?

Answer 14

• The diameter of a tree changes during the day.
• Negative pressures in the xylem draw the vessels inwards, due to cohesion-tension in the column and adhesion between water and the xylem walls.
• This decreases the overall diameter when transpiration rate and water loss is highest.

Question 15

How can a porous pot provide evident for cohesion tension theory?

Answer 15

• Evaporation creates tension and negative pressures within the xylem.
• This is shown by the movement of mercury or water uk the tube.

Question 16

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Question 17

What is a potometer?

Answer 17

Equipment used for measuring the rate of transpiration.

Question 18

How can a potometer measure the rate of transpiration?

Answer 18

If cells of the plant are fully turgid, then the rate of water uptake / absorption is similar to the rate of transpiration.

Question 19

Why, when using a potometer, should you cut the shoot and out it in the potometer underwater?

Answer 19

To prevent air entering and breaking the continuous water column.

Question 20

What measurements do you need to make to measure rate of water uptake in cm^3 min ^-1 using a potometer?

Answer 20

• Measure the distance the bubble has travelled.
• Measure the time it has taken from the bubble to travel.
• Measure the radius / diameter of the capillary tube.

Question 21

Why may water uptake not be equivalent to the rate of transpiration?

Answer 21

• Water may be used to provide turgidity / support for cells.
• Water may be used in photosynthesis or produced in respiration.
• Apparatus may not be properly sealed (‘leaks’j.

Question 22

What does the reservoir in the potometer do?

Answer 22

It can be used to return the bubble to the start so you can gain repeated measures.

Question 23

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Question 24

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Question 25

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Question 26

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Answer 27

- Phloem tissue transports organic molecules in flowering plants. - It transports mostly sucrose and amino acids, as well as some mineral ions. - Transoort in the phloem can occur in both directions. The movement is called translocation and is active (uses ATP).

Question 28

Describe and explain the features of sieve cells.

Answer 28

- Sieve elects are connected by sieve players at their traverse ends, which are porous to enable flow between cells. - Sieve elements have no nuclei and reduced numbers or organelles to ,aximise space for the translocation of materials. - The sieve elects also have thick walls and rigged cell walls to withstand the hydrostatic pressure which facilitate flow (positive pressure gradients).

Question 29

Why are sieve elements connected by sieve plates at their traverse ends (which are porous)?

Answer 29

To enable flow between cells.

Question 30

Why do sieve elements have no nuclei and a reduced number or organelles?

Answer 30

To maximise space for the translocation of materials.

Question 31

Why do sieve elements have thick and rigid cell walls?

Answer 31

To withstand the hydrostatic pressure which facilitate flow.

Question 32

What is the function of companion cells?

Answer 32

To provide metabolic support for sieve element cells and facilitate the loading and unloading of materials at the source and the sink.

Question 33

Describe and explain the function of companion cells.

Answer 33

- Infolding of the cell-surface membrane increases surface area to volume ration to allow for more material exchange. - Many mitochondria to fuel the active transport of materials between the sieve tube and the source or sink. - Active transport and cotransport proteins within the plasma membrane move materials into or out of the sieve tubes.

Question 34

Why do companion cells have an infolding of the cell-surface membrane?

Answer 34

To increase surface area to volume ration to allow for more material exchange.

Question 35

Why do companion cells have many mitochondria?

Answer 35

To release ATP to fuel the active transport of materials between the sieve tube and the source or sink.

Question 36

Why do companion cells have active transport and cotransport proteins within the plasma membrane?

Answer 36

To move materials into or out of the sieve tube.

Question 37

Why do companion cells have lots of ribosomes?

Answer 37

To make carrier proteins needed for transport across membranes.

Question 38

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Question 39

Why are sieve elements unable to sustain independent metabolic activity without the support of a companion cell.

Answer 39

Sieve element cells have no nuclei and fewer organelles.

Question 40

What do the plasmodesmata do?

Answer 40

They exist between sieve elements and companion cells in relatively large numbers, connecting the cytoplasm of the two cells, mediating the exchange of metabolites.

Question 41

What is a source in a plant?

Answer 41

Where sucrose is made e.g. photosynthesis in the leaves.

Question 42

What is a sink in a plant?

Answer 42

Where sucrose is used (for respiration and storage) e.g. roots.

Question 43

Describe the mass flow hypothesis.

Answer 43

- Active transport of sucrose into the sieve tubes from the companion cells **lowers the water potential** inside them. - Water follows by **osmosis** down its water potential gradient out of the xylem and into the sieve tubes. - This **increases the hydrostatic pressure** near the source. … - Active transport of sucrose into the sink cells **increases the water potential** in the phloem. - Water follows, leaving the sieve tube by **osmosis** down its water potential gradient into the xylem. - This **decreases the hydrostatic pressure** near the sink.

Question 44

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Question 45

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Question 46

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Question 47

Name two types of experiments used to investigate transport in plants.

Answer 47

Ringing and tracer

Question 48

Describe a ringing experiment.

Answer 48

- Outer layers (bark and phloem removed), xylem left intact. - Tissues below the ring die, tissues above the ring grow. - These swollen regions are rich in sugars. - Shows phloem is responsible for translocation. - Swelling above the ring shows evidence of pressure gradients.

Question 49

Describe a tracer experiment.

Answer 49

- A leaf is supplied with radioactive carbon dioxide. - The radioactively labelled carbon is incorporated into glucose and then sucrose and is found in the phloem over time. - Radioactivity is shown by exposure to X ray film.