3.3.4.2 - Mass transport in plants

Question 1

Define transpiration

Answer 1

evaporation of water vapour from a plants surface - especially the leaves

Question 2

How does water move out of the stomata ?

Answer 2

• water evaporates from cell walls and accumulates in air spaces between leaf cells
  * water potential gradient between air spaces and atmosphere - water molecules diffuse out of air spaces

Question 3

How does water move through a leaf?

Answer 3

1. mesophyll cells lose water via evaporation - heat from sun
2. cells have a lower WP so water enters by osmosis from neighbouring cells
3. this repeats as long as there is a water potential gradient

Question 4

How does water move up the xylem?

Answer 4

COHESION - TENSION THEORY
1. transpiration - water evaporates from the leaves
2. cohesion- water molecules form hydrogen bonds with each other resulting in a continuous unbroken column
3. **transpiration pull **- as water evaporates more molecules are drawn up behind it
4. xylem is under tension due to the transpiration pull

Question 5

What are the three main pieces of evidence which support the cohesion-tension theory ?

Answer 5

1. diameter of tree trunks changes - Day = smallest diameter due to high tension caused by transpiration pull - Night - largest diameter due to reduced tension
2. if xylem vessel breaks tree will not draw up water - cohesion between water molecules has been broken
3. if xylem vessel breaks water does not leak and air is drawn in which shows the column is under tension

Question 6

What is a potometer used for?

Answer 6

estimate transpiration rates

Question 7

How do you reset the air bubble in a potometer ?

Answer 7

open tap on resevoir and push down syringe till bubble is returned to the start of the scale

Question 8

How do you carry out an experiment with a potometer?

Answer 8

1. cut shoot at a slant under water
2. assemble potometer under water - no air bubbles
3. fit shoot to potometer underwater
4. remove potometer from underwater and seal with waterproof jelly
5. record distance moved by the bubble per unit time
6. remember to only change one variable

Question 9

Define translocation

Answer 9

transport of organic molecules and mineral ions from one part of the plant to another

Question 10

What are sieve tube elements?

Answer 10

• living cells which form a tube with sieve plates
• no nucleus or organelles

Question 11

What are companion cells ?

Answer 11

• carry out living functions for sieve tube elements
• large nucleus
• dense cytoplasm and lots of mitochondria (ATP for sugar loading)

Question 12

How does sucrose move into the sieve elements from photosynthesising tissue?

Answer 12

CO-TRANSPORT
1.** sucrose diffuses** (facilitated diffusion) down the concentration gradient** into the companion cells**
2. H+ ions are actively transported from CC into spaces in cell walls
3. **H+ ions diffuse into sieve tube elements **via carrier proteins
4. sucrose molecules are transported in along with the H+ ions

Question 13

How does sucrose move through the sieve tube elements?

Answer 13

MASS FLOW
1. sucrose molecules lower the water potential in sieve tubes so **water enters via osmosis **from xylem and companion cells
2. creates a **high hydrostatic pressure at the source end **of the phloem
3. sink end - solutes are removed via respiration or conversion into starch
4. sucrose is actively transported into respiring cells (from sieve tubes) causing an **increased water potential at sink end **so water leaves via osmosis
5. hydrostatic pressure in sieve tubes is reduced
6. so there is a mass flow of sucrose solution down the hydrostatic pressure gradient

Question 14

Give four pieces of supporting evidence for the mass flow hypothesis

Answer 14

1. if a **ring of bark is removed **a bulge forms above the ring - **higher concentration of sugars than area below ring **which is evidence for the downward flow of sugars in the phloem from source to sink over time
2. a radioactive tracer can be used to** track movement of organic substances in phloem**
3. pressure is shown via aphid mouthpieces - sap flows quicker near leaves than further down which is evidence for the pressure gradient
4. if a **metabolic inhibitor **is put in phloem , translocation stops which is evidence for active transport

Question 15

Give two objections to the mass flow hypothesis

Answer 15

1. sugar travels to many different sinks not just the one with the highest water potential as evidence would suggest
2. sieve plates create a barrier - a lot of pressure would be needed for solutes to pass through at a reasonable rate