Topic 3.4 - Mass Transport in Plants

Question 1

What is the function of the xylem?

Answer 1

• Xylem tissues transports water and minerals in solution. Allows substances to move up the plant from roots to leaves

Question 2

What are xylem vessels apart of?

Answer 2

Part of xylem tissue that transports water and ions

Question 3

Describe the structure of xylem vessels

Answer 3

• They’re long, tube-like structures formed from dead cells joined end to end
  
  • There’s no end walls on these cells

Question 4

Why are there no end walls on the cells that make up xylem vessels?

Answer 4

Makes uninterrupted tube = allows water to pass up through middle easily by cohesion

Question 5

Describe how water gets from the soil to the roots

Answer 5

Water enters root hair cells by osmosis due to the active uptake of mineral ions which has created a water potential gradient (conc. of solutes in soil is lower than in roots)

Question 6

What happens to the water once it has entered the roots?

Answer 6

Water moves through the cortext by osmosis down a water potential gradient gradient

Question 7

Name the mechanisms that help the movement of water up the xylem vessels

Answer 7

1. Mass flow
2. Cohesion tension theory
3. Adhesion

Question 8

Describe mass flow of water through the xylem vessels.

Answer 8

• Whole body of water moving together
• Pressure from water moving into roots = high pressure at base of xylem
• Pressure is higher than top therefore water forced upwards

e.g. similar to water moving up straw

Question 9

Describe cohesion tension theory

Answer 9

• Water molecules are held together by hydrogen bonds (weak bonds)
• Therefore if one molecule moves, it drags another with it
• Attraction between them = cohesion

Question 10

Describe adhesion of water which helps mass flow

Answer 10

• Same bond between water molecules will bind molecules to side of xylem vessels
• Therefore water almost “crawls” up side of vessels

Question 11

Describe how water in the roots moves up the stem

Answer 11

1. Water evaporates from the leaves, creates water potential gradient (lower water potential in leaves)
2. Water drawn out of xylem by osmosis
3. Creates tension on water in xylem
4. Water molecules cohesive, so column of water in xylem move upwards
5. Due to h-bonding, column doesn’t break because of adhesion with xylem walls

Question 12

What is transpiration?

Answer 12

Evaporation of water from plant’s surface through leaves

Question 13

Describe Transpiration

Answer 13

1. Water evaporates from moist cell walls and accumulates air spaces in leaf
2. When stomata open, water vapour diffuses out leaf down concentration gradient
3. (Higher concentration of water vapour inside leaf than outside leaf)

Question 14

Describe the transpiration pull/stream

Answer 14

1. Water evaporates from cell walls of mesophyll
2. Water from xylem vessels replaces this
3. Water moving out of xylem reduces pressure. Therefore water is at a higher pressure so it can move up the xylem vessels

Question 15

Name 4 factors that affect the rate of transpiration

Answer 15

• Light Intensity
• Temperature
• Humidity
• Wind

Question 16

Describe how temperature affects the rate of transpiration

Answer 16

• Higher temp. = faster transpiration rate
• Water molecules have more kinetic energy so therefore evaporate from cells inside leaf faster. This increases water potential gradient so water diffuse out leaf faster

Question 17

Describe how humidity affects the rate of transpiration (3)

Answer 17

• As humidity increases = transpiration rate decreases
• because a higher humidity causes a reduced water potential gradient (Less evaporation)

Question 18

Describe how wind affects the rate of transpiration

Answer 18

• Windier = faster transpiration rate
• Lots of air movement blows away water molecules around stomata
• Increases water potential gradient

Question 19

What does a potometer do?

Answer 19

estimate transpiration rates

Question 20

How does a potometer estimate transpiration rates?

Answer 20

Measures water uptake by plant but assumes water uptake by plant is directly related to water loss by leaves

Question 21

What is the function of the phloem?

Answer 21

Phloem tissue transports solutes (mainly sugar e.g. sucrose) round plants

Question 22

What is the phloem formed from?

Answer 22

It’s formed from cells arranged in tubes

Question 23

Name the 2 cells the phloem is formed from

Answer 23

• Sieve tube elements
• Companion cells

Question 24

Describe sieve tube elements

Answer 24

• Living cells that form tube for transporting solutes
• No nucleus and few organelles

Question 25

Describe companion cells

Answer 25

• Companion cell for each sieve tube element
• Carry out living functions for sieve cells, e.g. providing energy needed for active transport of solutes

Question 26

What is translocation?

Answer 26

Movement of assimilates (solutes) from one area of a plant to another (where they’re needed) by mass flow

Question 27

Translocation requires ____

Answer 27

energy

Question 28

Why does translocation require energy and where does it come from?

Answer 28

Companion cells produce ATP to actively load assimilates into and out of sieve tube elements

Question 29

Translocation moves solutes from ______ to _____

Answer 29

‘sources’ to ‘sinks’

Question 30

What is a source?

Answer 30

Area where sucrose is moved into phloem (high concentration)

Question 31

Give an example of a source

Answer 31

e.g. source for sucrose is leaves

In winter: roots - convert stored starch back to sugars when needed for growth

Question 32

What is a sink?

Answer 32

Area where sucrose is removed from phloem (low concentration)

Question 33

Give an example of a sink

Answer 33

e.g. food storage organs, meristems (area of growth) in roots, stems and leaves

(In winter: leaf)

Question 34

What do enzymes maintain in translocation & how?

Answer 34

• Enzymes maintain concentration gradient from source to sink by changing solutes at sink
• (e.g. breaking them down or making them into something else)
• Makes sure there’s a lower concentration at sink than at source

Question 35

What does the mass flow hypothesis explain?

Answer 35

Explains how solutes transported from source to sink by translocation

Question 36

Describe how solutes are transported from source to sink by translocation

Answer 36

1. Active transport used to actively load solutes from companion cells into sieve tubes of phloem at source (e.g. leaves)
2. This decreases water potential inside sieve tubes, water enters tubes by osmosis from xylem and companion cells
3. This increases pressure inside sieve tubes = mass movement (towards sink)
4. At sink end, solutes removed from phloem to be used up
5. This increases water potential inside sieve tubes which causes water to leave tubes by osmosis
6. pressure decreases inside sieve tubes
7. Result is pressure gradient from source end to sink end
8. Gradient pushes solutes along sieve tubes towards sink
9. At sink, solutes used for respiration or stored

Question 37

Name 4 pieces of experimental evidence of translocation

Answer 37

• Ringing experiment
• Tracer experiment
• Use of aphids
• Metabolic Inhibitor

Question 38

Describe the ringing experiment

Answer 38

1. Ring of bark/phloem removed from woody stem
   
   • = bulge forms above ring
2. Fluid from bulge has higher concentration of sugars than fluid from below ring
   
   • = evidence that there’s downward flow of sugars

Question 39

Describe the tracer experiment

Answer 39

1. Supply part of plant (e.g. leaf) with carbon-14
2. Carbon-14 incorporated into organic substances produced by leaf (e.g. sucrose) = moved around plant by translocation
3. Movement of substances tracked using autoradiography
4. Results show translocation of substances from source to sink

Question 40

Describe how aphids are used to find evidence of translocation

Answer 40

Pressure in phloem can be investigated using aphids

• They pierce the phloem, leave mouthparts behind which allows sap to flow out
• Sap flows out quicker nearer leaves than further down stem
• Evidence for pressure gradient

Question 41

Describe how a metabolic inhibitor is used to find evidence of translocation

Answer 41

• Stops ATP production, put into phloem and translocation stops
• Evidence that active transport involved

Question 42

Name and describe 2 objections to the mass flow hypothesis

Answer 42

• Sugars travel to many different sinks, not just to one with highest water potential, as model would suggest
• Sieve plates would create barrier to mass flow
  
  • Lots of pressure would be needed for solutes to get through at reasonable rate

Question 43

Explain why the values for the pressure in the xylem are negative

Answer 43

(Inside xylem) lower than atmospheric pressure / (water is under) tension

Question 44

Why is less water lost through upper surface of leaves than through the lower surface?

Answer 44

• More stomata on the lower surface
• (thicker) waxy cuticle on the upper surface

Question 45

Describe and explain 5 adaptions of xerophytic plants to reduce water loss

(xerophytes = plants adapted for life in warm, dry or windy habitats)

Answer 45

• Stomata sunk in pits
  
  • Increases humidity in pits/reduces exposure to the wind
  • Reduced water potential gradient
• Layer of ‘hairs’ on epidermis
  
  • Traps layer of moist air around stomata
  • Reduced water potential gradient
• Curled leaves
  
  • Reduced SA for water loss / stomata covered
• Reduced number of stomata
  
  • Reduced SA for water loss
• Thick waxy cuticle on leaves & stems
  
  • Waterproof = reduces evaporation

Question 46

Explain why the diameter of a tree trunk is smallest at midday (on a sunny, summer day) (6)

Answer 46

1. Midday = warmest & brightest time of day
2. Stomata open in light = more water loss
3. More heat energy for water evaporation
4. Cohesion between water molecules
5. Adhesion between water molecules and walls of xylem vessels
6. Xylem pulled in by tension (faster flow of water)

Summary: ↑ Transpiration = produce higher tension in xylem = reducing diameter

Question 47

Explain why increasing light intensity increases tension in the xylem vessels in the leaves (6)

Answer 47

1. More stomata open
2. Increased evaporation/transpiration
3. Ψw of leaves becomes lower
4. ∴ more water moves from xylem to surrounding cells
5. Down Ψw gradient
6. Cohesion between water molecules

Question 48

Xerophytic Leaf

Explain how the leaf being rounded helps reduce water loss (1)

Answer 48

Smaller SA to volume ratio