plant transport

Question 1

role of xylem

Answer 1

transports water & mineral ions up the plant from roots to leaves

Question 2

structure of xylem vessel

Answer 2

v long, tube-like structures, formed from dead cells joined end to end
- no end walls, making an uninterrupted tube that allows water to pass through easily

Question 3

cohesion-tension theory

Answer 3

• when water evaporates from leaves at top of xylem vessel, this creates tension, pulling more water into the leaf
• water molecules are cohesive so when some are pulled into the leaf, others follow
• means entire collumn of water from leaves down to the roots moves upwards
• water enters stems through the roots

Question 4

transpiration

Answer 4

the evaporation of water from a plant’s surface

• water evaporates from moist cell walls and accumulate in the spaces between the cells
• when stomata open, water moves down WP gradient, out of the leaf

Question 5

factors that affect the rate of transpiration

Answer 5

• light intensity
- more light means stomata will be open (to let CO2 in for photosynthesis)
- so water can also leave via the stomata

• temperature
- water molecules have more energy so evaporate from cells faster
- creates a larger WP gradient
- so water diffuses out of the leaf faster

• humidity
- if air surrounding the cells is humid (more 💧), this reduces the WP gradient, resulting in a decreased rate of transpiration

• wind intensity
- lots of air movement around the plant blows away water molecules from outside
- increasing the WP gradient
- increasing rate of transpiration

Question 6

phloem

Answer 6

transports organic solutes (e.g. sucrose) around plants

made up of cells arranged in tubes:

• STE - living cells w perferated end walls
  no nucleus + few organelles
  so for each STE, there is a…
• CC - carries out living functions for sieve cells
  e.g. provides ATP required for AT of solutes

Question 7

translocation - mass flow hypothesis

Answer 7

• high conc of sucrose at source cell
• so sucrose diffuses down its conc into the CC via FD
• then moves against its conc grad into the STE via a cotransporter protein, paired w the movement of H+
• influx of sucrose in STE lowers the WP
• so water moves into the phloem from the surrounding xylem vessel via osmosis
• influx of water creates a high HSP
• at sink end, sucrose is removed from phloem to be used up in sink cell for respiration
• loss of sucrose from STE increases the WP
• so water also leaves the tube via osmosis
• loss in water vol lowers the HSP
• diff in HSP between the sounce + sink ends creates a pressure gradient which pushes the liquid containing the sucrose along the sieve tubes, out the phloem + into the sink end where they can either be used in respiration or stored

Question 8

how is a constant supply of solute reaching the sink maintained

Answer 8

enzymes are used to maintain a conc grad from the source to the sink by either breaking down the solutes or converting them (e.g. into starch) to ensure there’s always a lower conc at the sink than at the source

Question 9

MFH evidence - ringing experiment

Answer 9

• ring of bark (incl phloem but not xylem) removed from tree trunk
• results in swelling of trunk above the removed section
• analysis of the fluid shows it has a higher conc of sugar than fluid from below the ring
• shows that when phloem is removed, sugar can’t be transported
• there4 proving that the phloem is responsible for transporting sugars

Question 10

MFH evidence - tracing

Answer 10

• plants only provided w radioactively labelled CO2
• so over time, will create sugars that all contain RLC
• thin slices of stem cut + placed on X-ray film that turns black when exposed to radioactive material
• so when stems are placed on film, the section of stem containing sugars turn black, highlighting where the phloem is
• proving that sugars are transported in the phloem