3.3 Topic 3 - 3.3.4 Mass Transport - 3.3.4.2 Mass transport in plants

Question 1

What does xylem tissue transport?

Answer 1

Water and mineral ions in solution.

Question 2

Where does xylem tissue transport water and mineral ions in solution?

Answer 2

The substances move up the plant from the roots to the leaves.

Question 3

What does phloem tissue transport?

Answer 3

Phloem tissue transports solutes [mainly sugars like sucrose] round plants.

Question 4

Where does phloem tissue transport organic substances like sugars [also in solution]?

Answer 4

The substances move up and down the plant.

Question 5

What are both the xylem and phloem?

Answer 5

Mass transport systems.

Question 6

What do mass transport systems do?

Answer 6

Move substances over large distances.

Question 7

What are xylem vessels part of?

Answer 7

Part of the xylem tissue that actually transports the water and ions.

Question 8

Describe the structure of xylem vessels …

Answer 8

… very long, tube- like structures joined end to end.

Question 9

Are xylem vessels formed from dead or live cells?

Answer 9

Dead cells [vessel elements].

Question 10

Are there any end walls of xylem cells?

Answer 10

No, making an uninterrupted tube that allows water to pass up through the middle easily.

Question 11

What are two things which help water move up plants, from roots to leaves, against the force of gravity?

Answer 11

Cohesion and tension.

Question 12

What is the cohesion-tension theory of water transport [transpiration]?

Answer 12

1] Water evaporates from the leaves at the ‘top’ of the xylem [transpiration].
2] This creates tension [suction], which pulls more water into the leaf.
3] Water molecules are cohesive [they stick together] so when some are pulled into the leaf others follow. This means the whole column of water in the xylem, from the leaves down to the roots, moves upwards.
4] Water enters the stem through the leaves.

Question 13

Transpiration in simple terms:

Answer 13

Loss of water by evaporation from a plants surface [especially the leaves].

Question 14

Where does water evaporate from?

Answer 14

It evaporates from the moist cell walls and accumulates in the spaces between cells in the leaf.

Question 15

What happens when the stomata open?

Answer 15

Water moves out of the leaf down the concentration gradient [there’s more water inside the leaf than in the air outside].

Question 16

What is transpiration really a side effect of? + how?

Answer 16

Photosynthesis - the plant needs to open its stomata to let in CO2 so that it can produce glucose, but this also lets water out.

Question 17

What are the 4 main factors that affect transpiration rate?

Answer 17

Light, temperature, humidity and wind.

Question 18

How does light affect transpiration?

Answer 18

The lighter it is the faster the transpiration rate [there’s a positive correlation between light intensity and transpiration rate]. This is because the stomata open when it gets light to let in CO2 for photosynthesis. When it’s dark the stomata are usually closed, so there’s little transpiration.

Question 19

How does temperature affect transpiration?

Answer 19

The higher the temperature the faster the transpiration rate. Warmer water molecules have more energy so they evaporate from the cells inside the leaf faster.
This increases the concentration gradient between the inside and outside of the leaf, making water diffuse out of the leaf faster.

Question 20

How does humidity affect transpiration?

Answer 20

The lower the humidity, the faster the transpiration rate [there is a negative correlation between humidity and transpiration rate].
If the air around the plant is dry, the concentration gradient between the leaf and the air is increased, which increases transpiration.

Question 21

How does wind affect transpiration?

Answer 21

The windier it is, the faster the transpiration rate. Lots of air movement blows away water molecules from around the stomata.
This increases the concentration gradient, which increases the rate of transpiration.

Question 22

What can a potometer be used to estimate?

Answer 22

It can be used to estimate transpiration rates. It usually measures water uptake by a plant.

Question 23

What does a potometer assume?

Answer 23

It assumes that water uptake by the plant is directly related to water loss by the leaves.

Question 24

A potometer can estimate how different factors affect the …

Answer 24

… transpiration rate.

Question 25

State the steps you would carry out to use a potometer to estimate the transpiration rate:
[8 steps]

Answer 25

1] Cut a shoot underwater to prevent air from entering the xylem. Cut it at a slant to increase the surface area available for water uptake.
2] Assemble the potometer in water and insert the shoot underwater, so no air can enter.
3] Remove the apparatus from the water but keep the end of the capillary tube submerged in a beaker of water.
4] Check the apparatus is watertight and airtight.
5] Dry the leaves, allow time for the shoot to acclimatise, and then shut the tap.
6] Remove the end of the capillary tube from the beaker of water until one air bubble has formed, then put the end of the tube back into the water.
7] Record the starting position of the air bubble.
8] Start a stopwatch and record the distance moved by the bubble per unit time, e.g. per hour. The rate of air bubble movement is an estimate of the transpiration rate.

Question 26

What do you need to remember when using a potometer to estimate the transpiration rate?

Answer 26

To only change one variable at a time and keep all other conditions the same.
[E.g. change the temperature but keep the light and humidity the same].

Question 27

What part of a plant would you look at under a microscope to see the phloem and xylem?

Answer 27

A part of a plant stem.

Question 28

How would you disinfect the plant and prepare a section of the tissue to look as the xylem/phloem under a microscope?
[4 steps]

Answer 28

1] Use a scalpel to cut a thin [better for viewing under a microscope] cross-section of the stem.
2] Use tweezers to gently place the cut sections in water until you come to use them [stops them from drying out].
3] Transfer each section to a dish containing a stain, and leave for one minute. E.g. you could use TBO [toluidine blue O], this stains the lignin in the walls of the xylem vessels blue/green. This will let you see the position of the xylem vessels and examine their structure.
4] Rinse off the sections in water and mount each on onto a slide.

Question 29

What can you use to see different parts of the cell?

Answer 29

You can use different stains.

Question 30

What are solutes?

Answer 30

Dissolved substances.

Question 31

What are important cell types in phloem tissue?

Answer 31

Sieve tube elements and companion cells.

Question 32

What are sieve tube elements?

Answer 32

Living cells that form the tube for transporting solutes. They have no nucleus and few organelles and this is where companion cells come in.

Question 33

What are companion cells?

Answer 33

There is a companion cell for each sieve tube element. They carry out living functions for sieve cells. E.g. providing the energy needed for the active transport of solutes.

Question 34

Translocation in simple terms:

Answer 34

The movement of solutes [e.g. sugars like sucrose, and amino acids] to where they’re needed in a plant.

Question 35

What are solutes sometimes called?

Answer 35

Assimilates.

Question 36

Does translocation require energy?

Answer 36

Yes

Question 37

Where does translocation happen?

Answer 37

In the phloem.

Question 38

Where does translocation move solutes from and to?

Answer 38

From ‘sources’ to ‘sinks’.

Question 39

What is a ‘source’ of a solute?

Answer 39

It is where it’s made [so it’s at a high concentration there].

Question 40

What is a ‘sink’ of a solute?

Answer 40

It is where it is used up [so it’s at a lower concentration there].

Question 41

What is an example of the ‘source’ and ‘sink’?

[hint = sucrose]

Answer 41

The source for sucrose is usually the leaves [where it’s made], and the sinks are the other parts of the plant, especially the food storage organs and the meristems [areas of growth] in the roots, stems and leaves.

Question 42

What maintains a concentration gradient from the ‘source’ to the ‘sink’? + how? + what does this ensure?

Answer 42

Enzymes.
They do this by changing the solutes at the sink [e.g. by breaking them down or making them into something else].
It ensures that there is always a lower concentration at the sink than at the source.

Question 43

What is an example of enzymes maintaining a concentration gradient from the source to the sink?

[hint = potatoes]

Answer 43

In potatoes, sucrose is converted to starch in the sink areas, so there’s always a lower concentration of sucrose at the sink than inside the phloem. This makes sure a constant supply of new sucrose reaches the sink from the phloem.

Question 44

What is the theory which best supports how solutes are transported from source to sink by translocation?

Answer 44

The mass flow hypothesis.

Question 45

State the process of the mass flow hypothesis:
[3 main steps with 3 steps in each]

Answer 45

1
1] AT is used to actively load the solutes [e.g. sucrose from photosynthesis] from companion cells into the sieve tubes of the phloem at the source [e.g. the leaves].
2] This lowers the WP inside the sieve tubes, so water enters the tubes by osmosis from the xylem and companion cells.
3] This creates a high pressure inside the sieve tubes at the source end of the phloem.

2
1] At the sink end, solutes are removed from the phloem to be used up.
2] This increases the WP inside the sieve tubes, so water also leaves the tubes by osmosis.
3] This lowers the pressure inside the the sieve tubes.

3
1] The result is a pressure gradient from the source end to the sink end.
2] This gradient pushes solutes along the sieve tubes towards the sink.
3] When they reach the sink the solutes will be used [e.g. in respiration] or stored [e.g. as starch].

Question 46

What are the pieces of evidence for [supporting] mass flow?

Answer 46

• If a ring of bark [which includes the phloem, but not the xylem] is removed from a woody stem, a bulge forms above the ring. The fluid from the bulge has a higher concentration of sugars than the fluid below the ring - this is evidence that there’s a downward flow of sugars.
• A radioactive tracer such as radioactive carbon [14C] can be used to track the movement of organic substances in a plant.
• Pressure in the phloem can be investigated using aphids [they pierce the phloem, then their bodies are removed leaving the mouthparts behind which allows the sap to flow out]. The sap flows out quicker nearer the leaves than further down the stem - this is evidence that there’s a pressure gradient.
• If a metabolic inhibitor [which stops ATP production] is put into the phloem, then translocation stops - this is evidence that active transport is involved.

Question 47

What are the pieces of evidence against [objecting] mass flow?

Answer 47

• Sugar travels to many different sinks, not just to the one with the highest WP, as the model would suggest.
• The sieve plates would create a barrier to mass flow. A lot of pressure would be needed for the solutes to get through at a reasonable rate.

Question 48

In what experiment can the translocation of solutes in a plant be modelled?

Answer 48

In an experiment using radioactive tracers.

Question 49

Photosynthesis produces glucose. This is converted into _ _ _ _ _ _ _ for transport around the plant

Answer 49

Sucrose.

Question 50

How can radioactive tracers be used to model the translation of solutes in plants?
[4 steps]

Answer 50

1] This can be done by supplying part of a plat [leaf] with an organic substance that has a radioactive label.
[E.g. CO2 containing the radioactive isotope 14C. This radioactively-labelled CO2 can be supplied to a single leaf by being pumped into a container which completely surrounds the leaf].
2] The radioactive carbon will then be incorporated into organic substances produced by the leaf [e.g. sugars produced by photosynthesis], which will be moved around the leaf by translocation.
3]The movement of these substances can be tracked using a technique called autoradiography. To reveal where the radioactive tracer has spread thin the plant, the plant is killed and then the whole plant or sections of it is placed on photographic film - the radioactive substance is present wherever the film turns black.
4] The results demonstrate the translocation of substances from source to sink over time. E.g. autoradiographs of plants killed at different times show an overall movement of solutes from the leaves towards the roots.