7: Mass Transport in Plants

Question 1

What is Mass Transport?

Answer 1

• movement of materials over large distances, due to pressure differences

Question 2

What + Where does the Phloem transport?
What + Where does the Xylem transport?

Answer 2

1. concerned with the transport of organic substances such as sucrose and other soluble products of photosynthesis from the source (leaves) to the sinks (rest of plant)
2. concerned with the transport of water + dissolved minerals from the soil, through the roots, stem and to the leaves

Question 3

What is Transpiration?
What is Transpiration Stream?

Answer 3

• process of water movement through a plant and its evaporation from leaves
• continuous columns of water that move from the soil through the roots, stems and leaves to air

Question 4

How is Transpiration Rate linked to Gas Exchange?

Answer 4

• when stomata are open for exchange of gases for metabolic reactions, water is lost (diffuses out from air spaces out of stomata)

Question 5

Explain the Movement of water THROUGH THE LEAVES:

Answer 5

1. when stomata are open, water vapour diffuses from air spaces in the leaf out through the stomata down a water potential gradient (transpiration
2. to replace this water lost, water evaporates from mesophyll cells into air spaces, forming water vapour build up in air spaces
3. this reduces water potential in mesophyll cells surrounding the air spaces
4. this results in a water potential gradient formed across the leaf (between xylem vessels + mesophyll cells)
5. therefore, water is drawn out from xylem vessels in the leaf, and diffuses into mesophyll cells by osmosis (to replace lost water)

Question 6

What are 4 Structural features of Xylem Vessels?
- explain the function:

Answer 6

1. cell walls contain lignin:
   - this strengthens xylem walls against tension within them, as well as making them waterproof
2. lignified cell walls causes cell content to die:
   - this leaves a hollow lumen with no cytoplasm, therefore offering little resistance to mass flow of water + minerals
3. xylem vessel walls contain tiny holes called pits:
   - if a vessel becomes blocked or damaged, the water can be diverted laterally, so the upwards movement of water will continue in adjacent vessels
4. vessels lose end walls:
   - forms continuous columns for water movement from roots to leaves

Question 7

What is the advantage of the Cell Wall in Xylem Vessels being Spiral?

Answer 7

• uses less material: so is less wasteful
• uses less material: therefore allows the xylem to be flexible, and have a lower mass

Question 8

Explain how Cohesion is formed in Water Molecules?

Answer 8

• water molecules are dipolar, with many weak hydrogen bonds between molecules
• this allows them to stick together (cohesion)

Question 9

What is Adhesion?

Answer 9

• water molecules forming hydrogen bonds with the walls of the xylem vessels

Question 10

Explain the Movement of Water from Roots to Leaves:

Answer 10

1. transpiration occurs in leaves. So water is evaporated from mesophyll cells into air spaces, reducing water potential of mesophyll cells
2. water vapour builds up in air spaces, then diffuses out through the stomata
3. a water potential gradient is formed across whole leaf. Water then leaves the xylem vessel in the leaf, and diffuses into mesophyll cells by osmosis, replacing lost water
4. this water diffusing out of mesophyll cells by osmosis, causes negative pressure/tension to build up at the top of xylem vessels
5. this negative pressure cause water (under tension) in the xylem to be pulled up towards the leaves
6. this water is uptaken towards the leaves in continuous columns, due to cohesion between water molecules
7. there is also adhesion between water molecules and the walls of the xylem vessels. this creates an inward pull on the xylem vessels as the water is pulled up, which means the diameter of xylem vessels to decrease

Question 11

Is ATP required for Transpiration?
Explain your Answer:

Answer 11

• no, because the only energy needed is the heat to evaporate the water from the leaves

Question 12

What is the Evidence for the Theory for Transpiration?
(hint- 4 points)

Answer 12

• tension has been measured in xylem as plants transpire
• when columns of water in xylem have been broken, air bubbles form preventing any further movement of water, as air bubbles prevent cohesion
• respiratory inhibitors will not inhibit this process (no ATP required)
• diameters of trees decrease when transpiring, (tension + adhesion pull xylem walls in), and more so when light intensity + temperature increases. this is measured with dendrometers

Question 13

What can Diameter of Trees be measured with?

Answer 13

• dendrometer

Question 14

Why would a tree trunk diameter be lowest at noon?

Answer 14

• stomata would open due to maximum sunlight for photosynthesis
• transpiration rates are highest, increased tension, as water is pulled up the xylem faster and more frequently to replace lost water in leaf
• therefore, walls of the xylem are pulled in more and xylem vessels decrease in diameter

Question 15

What is the benefit of a “waxy cuticle” in xerophytes?

Answer 15

• waxy cuticle causes reduced water loss from the epidermis
• this is due to the greater thickness, which would increase the length of diffusion pathway for water to reach the atmosphere
• waxy cuticle is waterproof

Question 16

What are 3 structural advantages that Marram Grass contain for lower water conditions?
Explain:

Answer 16

• stomata sunken in pits: water vapour is held above the stomatal pore, so therefore the water potential gradient between air spaces in the leaf and the atmosphere decreases, so lower rate of diffusion
• hairs on lower epidermis of leaf: water vapour would be trapped between the hairs, which decreases water potential gradient between air spaces inside the leaf and outside atmosphere, so lower rate of diffusion
• leaf rolled up to trap humid air: water vapour trapped in rolled leaf, reducing water potential gradient between air spaces in leaf and outside atmosphere, so lower rate of diffusion

Question 17

What are the adaptations of other xerophytes?

Answer 17

• lower SA:vol ratio of leaves
• stomata on underside of leaf as it is cooler and exposed to less sunlight
• stomata closes during the day, through guard cells
• succulent stems/leaves for storage of water

Question 18

Give 2 reasons why Sand Dunes need Xerophyte-like features even if rain is surplus?

Answer 18

• rain would rapidly drain through sand, which would be out of reach of roots
• more likely in windy conditions, so water vapour “wafted” away frequently so there would be an increased water potential gradient

Question 19

Why is it necessary for sucrose + amino acids to be transported from one part of the plant to another?

Answer 19

• sucrose is produced by photosynthesis in the leaf of the plant and needs to be transported to other cells in the plant to be hydrolysed and used for respiration
• sucrose also need to be transported and be stored as starch as energy storage
• amino acids needed for photosynthesis

Question 20

Why aren’t as many reducing sugars transported in the phloem?

Answer 20

• they are too reactive (as they donate electrons), so would be altered before arriving at the destination

Question 21

Describe the Structure of the Phloem:

Answer 21

• each sieve tube element links to the next via a sieve plate with perforated pores to allow phloem sap to pass through
• sieve tube elements has little cytoplasm, no nucleus and few organelles apart from some mitochondria
• sieve tubes still alive, as the have cytoplasmic connections with companion cells (plasmodesmata)
• these companion cells contain nucleus, vacuole and other organelles

Question 22

Where is the “usual” source for Sucrose?
How is Sucrose formed?

Answer 22

1. in the mesophyll cells of the leaf
2. • through photosynthesis
   • formed via the condensation of glucose + fructose

Question 23

Why does the Sinks of the plant have a lower Concentration of Sucrose?

Answer 23

• growing points such as meristems of roots, shoots and leaves, would require much more respiration for ATP production for growth.
• therefore, sucrose must be transported to these areas where it is hydrolysed (hence why the concentration is low) into fructose + glucose for respiration useage

Question 24

How can fruits, seeds and organic flowers act as sinks?

Answer 24

• they can store starch converted from sucrose for energy storage

Question 25

What direction is Translocation?

Answer 25

-bidirectional

Question 26

What is translocation?

Answer 26

• the mass transport of organic solutes from source to sink via the phloem

Question 27

Explain the Transfer of Sucrose from Photosynthesising Tissue to Sieve Tubes:

Answer 27

• sucrose produced in the mesophyll cells of the plant (leaf/source) will be actively transported from the mesophyll cells to the companion cells and then into sieve tube elements (active loading)
• this process requires ATP + carrier proteins in companion cell membranes
• this increases concentration of sucrose in phloem sap of the sieve tubes in this region

Question 28

Explain the Mass Flow of Sucrose through Sieve Tube Elements into Sink Cells:

Answer 28

1. (Source):
   - this increased concentration of sucrose (and other solutes) in the sieve tubes, lowers the water potential of the phloem sap in the sieve tubes.
   - therefore, water enters the sieve tubes via osmosis from xylem and companion cells
   - this increase in volume within sieve tube elements around the source causes an increase in hydrostatic pressure
2. (Sinks):
   - sucrose is used up in respiration or converted into starch for storage, so sucrose concentration is very low
   - therefore, sucrose is actively transported from sieve tubes into companion cells and into sink cells
   - this increases the water potential of the sieve tube cells, so water leaves the sieve tube elements into the xylem and sink cells via osmosis
   - due to the decrease in volume of sieve tube elements near the sinks, there is a decrease in hydrostatic pressure
3. therefore, there is now a pressure gradient from source to sink, so there is a mass flow of sucrose down the pressure gradient in sieve tubes of the phloem
4. sucrose now near the sinks, would be actively transported from the phloem (sieve tubes) into companion cells and then into sink cells

Question 29

Explain the process of Autoradiography:

Answer 29

• expose a plant with only ‘radioactive carbon’, from carbon dioxide
• this will enter and be absorbed by the plant when stomata are open, and ultimately used as a substrate in photosynthesis to produce sugars (still containing the radioactive carbon)
• the movement of these carbohydrates can be traced by cutting tissue from different areas of the plant into thin sections and placed with photographic film
• the film blackens when exposed to the radioactive carbon
• this reveals the location of which the carbohydrates (sucrose etc.) are travelling and hence proves the phloem is where sucrose is transported

Question 30

Explain the Ringing Experiment:

Answer 30

• a ring of bark + phloem are peeled and removed from the tree trunk
• this means any sugars present in phloem cannot travel down the trunk
• therefore, the trunk directly above the ring will swell with sugar
• to prove this is sugar, the solutions present can be tested to prove that a sugar has been present
• this shows that when the phloem is removed the sugar cannot be transported in the plant

Question 31

What is a potometer used for?

Answer 31

• measures the rate of uptake of water

Question 32

Why is the assumption “rate of uptake of water is the same as the rate of which water leaves the plant” incorrect?

Answer 32

• some water used in photosynthesis/made in respiration
• some water used to keep cell turgid

Question 33

Why cut the stem of leafy shoot underwater?

Answer 33

• prevent air bubbles entering the xylem.

Question 34

Why submerge the potometer and fill with water?

Answer 34

• prevent air bubbles entering the xylem or the potometer itself

Question 35

How do you stop water vapour escaping the potometer?

Answer 35

• seal joints around rubber tube with vaseline to maintain airtight seal

Question 36

How to repeat potometer experiment?

Answer 36

• turn reservoir tap on, which pushes the air bubble back to the start for repeats

Question 37

What is the granulated capillary tube used for in potometers?

Answer 37

• to measure diameter moved by air bubble in a certain time
• the further it travels the higher the rate of transpiration

Question 38

State 2 factors affecting rate of transpiration:

Answer 38

• wind speed
• air temperature