3.3.4.2 mass transport in plants

Question 1

xylem

Answer 1

transports water and minerals from the roots, up the plant, to the leaves

Question 2

adaptations of the xylem

Answer 2

elongated, hollow tubes with no end walls
no organelles
walls with lignin for support

Question 3

what is transpiration

Answer 3

evaporation of water from the plant via stomata

Question 4

what is the transpiration pull

Answer 4

when a column of water is pulled up the xylem due to transpiration

Question 5

cohesion-tension hypothesis

Answer 5

water evaporates from the leaves at the top of the plant, reducing water potential in the cells
water moves into the xylem by osmosis.
this creates tension in the column of water
the cohesive forces between water molecules, due to hydrogen bonding, meaning the water is pulled up
adhesive forces between the water and the xylem walls helps the water move up the xylem

Question 6

factors which increase the rate of transpiration

Answer 6

light - more light, more stomata open for photosynthesis
temperature - higher temp, water molecules have more kinetic energy, evaporating from the leaf faster, increasing conc gradient inside and outside of the plant
humidity - lower the humidity, faster the transpiration rate because conc gradient between inside and outside of the plant increase
wind - more wind, greater rate of transpiration as more water molecules are moved away from the stomata, increases gradient

Question 7

why does the diameter of a tree decrease during the day

Answer 7

more light and higher temp.
increases rate of transpiration and transpiration pull
water pulled up via cohesion tension
this pulls the walls of the xylem inwards

Question 8

what is a potometer

Answer 8

apparatus used to estimate transpiration rates

Question 9

what does a potometer measure

Answer 9

water uptake of a plant

Question 10

how does a potometer work

Answer 10

water is pulled up the plant by cohesion tension
causing the bubble to move towards the plant
the more water is lost by transpiration, the more water is uptook, the further the bubble will move

Question 11

how do you set up the potometer

Answer 11

1. cut the shoot underwater - to prevent water entering the xylem. cut it at a slant - to increase surface area
2. assemble the potometer underwater and insert shoot
3. ensure apparatus is water and airtight by sealing it with vaseline
4. dry leaves and allow plant time to acclimatise
5. shut tap to form an air bubble
6. measure the distance of the air bubble and the time taken

Question 12

phloem

Answer 12

transports sugars and amino acids around the plant

Question 13

structure of the phloem

Answer 13

sieve tube - transport solutes
companion cells - produce ATP for each sieve tube element
no organelles except mitochondria

Question 14

what is translocation

Answer 14

movement of solutes to parts of the plant where they are needed

Question 15

explain the terms source and sink

Answer 15

source - usually the leaves. where the solute is made, high conc of the solute
sink - where the solute is used up, low conc of the solute

Question 16

explain the mass flow hypothesis of translocation

Answer 16

1. at the source, solutes are actively transported into the sieve tube of the phloem by companion cells.
2. this decreases the water potential inside the sieve tubes
3. water enters the phloem from the xylem and companion cells by osmosis, increasing the hydrostatic pressure
4. at the sink, solutes diffuse from the phloem into surrounding cells.
5. increasing water potential inside the sieve tubes, water will leave the phloem by osmosis, decreasing the hydrostatic pressure inside the phloem
6. this creates a pressure gradient, pushing solutes from the source to areas with lower pressure at the sink

Question 17

evidence supporting the mass flow hypothesis

Answer 17

1. when sieve tubes are cut at the stem, sap is released quicker at the top, showing there is a greater pressure at the top of the stem.
2. translocation stops when inhibitors that prevent ATP synthesis are added, showing that translocation requires ATP
3. radioactive traces in the form of CO2 can be used to track the movement of organic substances in the plant, showing solutes move from the source to the sink.

Question 18

evidence against the mass flow hypothesis

Answer 18

1. not all solutes move at the same speed, they should if the movement is by mass flow
2. sieve plates create a barrier to mass flow
3. solutes are delivered to all places at the same rate, not areas with a high water potential first.