Mass Transport in Plants

Question 1

what is transpiration

Answer 1

water is lost from the leaves via evaporation

Question 2

what is translocation

Answer 2

the transport of sugars and organic substances from leaves, where they are formed, to where they are needed

Question 3

give 4 structure features of the xylem and how it helps its function

Answer 3

• no cytoplasm and no organelles = no obstruction to the flow of water
• no end walls = allows continuous columns of water to form
• lignin = strengthens and waterproofs the vessels
• lateral pits in cell wall = allows lateral movements around blockages

Question 4

what is cohesion

Answer 4

water sticks to water via hydorgen bonds

Question 5

what is adhesion

Answer 5

water molecules are attracted to the walls of the xylem

Question 6

explain the cohesion tension theory in the xylem

Answer 6

• water evaporates out the leaves. Known as transpiration
• water lost from the leaf is replaced by the xylem
• the water potentil gradient across the leaf creates tension
• tension pulls water up the xylem, forming a continuous column of water which is held together by hydrogen bonds. This is known as cohesion
• water molcules are also attracted to the walls of the xylem, adhesion, which helps to narrow the walls of the xylem

Question 7

what are 4 factors that affect the rate of transpiration

Answer 7

• light intensity
• temperature
• humidity
• air movement

Question 8

explain how light intensity affects the rate of transpiration

Answer 8

• stomata close in the dark
• the rate of transpiration is higher with increasing light intensity

Question 9

explain how temperature affects rate of transpiration

Answer 9

• increase in kinetic energy which increases movement of water molecules
• this causes the rate of transpiration to increase

Question 10

explain how humidity affects rate of transpiration

Answer 10

• if there is high humidity that means theres lots of water molcules in the air
• therefore there is a lower water potential gradient
• as humidity increases the rate of transpiration will decrease

Question 11

explain how air movement affects transpiration

Answer 11

• air movement over a leaf moves water vapour away from the stomata
• this increases the water potential gradient
• the faster the wind speed, the faster the transpiration

Question 12

what do scientists use to measure the effects of various conditions on the rate of transpiration

Answer 12

potometer

Question 13

give the method to investigate rate of transpiration

Answer 13

• leafy shoot is cut under water
• potometer is filled completely with water
• using a rubber tube, the leafy shoot is fitted to the potometer under water
• the potometer is removed from under the water and all joints are sealed with waterproof jelly
• an air bubble is introduced into the capillary tube
• as transpiration occurs the water moves through the capillary tube
• the distance moved over a period of time is recorded

Question 14

give 3 structure features of the phloem and how this has adapted it for its function

Answer 14

• sieve tube elements have no nucleus and few organelles
• sieve tubes are connected through sieve plates
• companion cells contain many mitochondria to produce ATP

Question 15

explain the mass flow hypothesis

Answer 15

• sucrose is actively transported into the sieve tube by companion cells
• this lowers the water potential in the sieve tubes
• water enters by osmosis
• this produces high hydrostatic pressure inside the sieve tubes
• mass flow to respiring cells
• sucrose is moved into the sinks by active transport or facilitated diffusion

Question 16

what are the 2 experiments that provide evidence for the mass flow hypothesis

Answer 16

• ringing experiment
• experiments with radiotracers

Question 17

explain the ringing experiment that supports the mass flow hypothesis

Answer 17

• if a ring of bark is removed from a woody stem a bulge form above the ring
• the fluid from the buldge has a higher concentration of sugars than the fluid from below the ring
• this is because sugars cant move past the area where the bark has been removed
• this is evidence that there can be a downflow of sugars

Question 18

explain the radiotracer experiment used to provide evidence for the mass flow hypothesis

Answer 18

• carbon dioxidr containing its radioactive isotope is used as a radioactive tracer
• this carbon dioxide can b supplied to a single leaf
• this crbon will then be incorporated into organic sbstances whuch will be moved around the plant by translocation