Mass transport in plants

Question 1

which tissue and substances are used by the cohesion theory (transpiration) and in what directions?

Answer 1

XYLEM
transports water and organic ions
upwards from roots to shoots and leaves

Question 2

which tissue and substances are used by the mass flow hypothesis and in what directions?

Answer 2

PHLOEM
transports organic solutes
in any direction : source to sink(s)

Question 3

describe and explain how water is taken up from the soil.

Answer 3

1- water enters through the root hair cells in the epidermis by OSMOSIS

2- root hair cells actively transport ions using energy from the hydrolysis of ATP, from soil via carrier proteins

3- this lowers the water potential of the cytoplasm (its more -ve)

4- water then passes across the cells of the cortex, across the ENDOdermis , into the xylem

Question 4

Describe and explain how the structure of the xylem relates to its function

Answer 4

1- dead cells that form hollow tubes with no cytoplasm (organelles) —> faster water flow / less resistance

2- end walls breakdown so xylem forms a continuous tube with NO END WALLS —> so a continuous column of water can form (due to cohesion)

3-cell walls are strengthened by lignin —> making it waterproof and rigid , providing support , enabling Ahesion / withstanding tension

4- have xylem pits —> little holes which allow to move laterally b/w xylem vessels / get around blocked vesesels

Question 5

describe and explain the cohesion-tension theory.

Answer 5

1- water evaporates from leaves
2- water lost, lowers water potential in leafs cells
3- water replaced from xylem
4- water potential gradient creates tension (pulls up water)
5- hydrogen bonds/ adhesion / cohesion maintains continuous column of water

Question 6

explain how water enters xylem from the endodermis in the root and is then transported to the leaves. (6)

Answer 6

(in the roots)
1. casparian strip blocks apoplast pathway / only allows symplast pathway
2. active transport by endodermis
3. of ions/salts into xylem
4. lower water potential in xylem

(xylem to leaf)
1. evaporation / transpiration from the leaves
2. creates cohesion/ tension / H-bonding between water molecules,/negative pressure
3. adhesion/ water molecules bind to xylem
4. creates continuous columns of water

Question 7

root pressure moves water through the xylem describe what causes root pressure (4)

Answer 7

1. active transport by endodermis
2. ion/salts into xylem
3. lowers water potential in xylem
4. water enters by osmosis

Question 8

describe how a high pressure is produced in the leaves (3)

Answer 8

1. water potential becomes lower ( more -ve)
2. water enters the phloem by osmosis
3. increased volume of water causes increased pressure

Question 9

Name the 5 factors that affect the rate of transpiration

Answer 9

light intensity
temperature
humidity
wind speed
stomatal density

Question 10

how does light intensity affect the rate of transpiration

Answer 10

high light intensity= more stomata open
-water potential of cytoplasm decreases and water enters in by osmosis. the guard cells increase in volume causing the stomata to open

Question 11

how does temperature affect the rate of transpiration

Answer 11

higher temp –> more KE –> higher rate of diffusion and evaporation of water
- dry soil = roots release abscisic acid into the xylem
- causing the closure of stomata to prevent water loss

Question 12

how does humidity affect the rate of transpiration

Answer 12

increased humidity = decreased rate of transpiration
- the greater the difference in humidity between the air spaces and the air outside the leaf, so the greater the rate of diffusion of water water out of the leaf

Question 13

how does wind speed affect the rate of transpiration

Answer 13

increase wind speed = increased R.O.T
- air movement moves the water vapours away from the stomatal pores
- increasing the water potential gradient between inside and outside of leaf

Question 14

how does stomatal density affect the rate of transpiration

Answer 14

higher stomatal density = higher R.O.T

Question 15

why do xerophytes have sunken stomata

Answer 15

to keep a higher humidity outside the stomata, so reduces transpiration

Question 16

what do plants use water for

Answer 16

photosynthesis
storage in vacuoles
support
hydrolysis reactions

Question 17

describe the process of measuring the rate of transpiration

Answer 17

1. a leafy shoot is cut diagonally under water
2. care is taken to prevent water getting on the leaves
3. the potometer is filled completely with water making sure there are no air bubbles
4. using a rubber tube, the leafy shoot is fitted to the potometer under water
5. the potometer is removed from under the water and all joints are sealed with waterproof jelly
6. an air bubble is introduced into the capillary tube
   ( as transpiration occurs, water moves through the tube and air bubbles moves with it )
7. the distance moved over a period of time is recorded and the mean is calculated of a number of repeats
8. the volume of water uptake over a period of time can be calculated

Question 18

suggest a reason why the leaf shoot is cut diagonally

Answer 18

to prevent damage to the xylem and ensures a continuous column of water

Question 19

why is care taken in preventing water getting on the leaves

Answer 19

water could prevent the loss of water vapour through the stomata

Question 20

why is the potometer filled completely with water and why is a rubber tube used.

Answer 20

to prevent air entering the xylem and ensures a continuous column of water

Question 21

why are all the joints sealed with waterproof jelly

Answer 21

to prevent water loss from potometer and prevent air from entering

Question 22

why is an air bubble introduced into the capillary tube

Answer 22

to determine the volume of water entering the shoot

Question 23

give two precautions the students should have taken when setting up the potometer to obtain reliable measurements of water uptake by the plant shoot.

Answer 23

1. seal joints
2. cut shoot under water
3. cut shoot at a slant
4. dry off leaves
5. insert into apparatus underwater
6. ensure no air bubbles are present
7. shut tap
8. note where bubble is at the start

Question 24

what are the adaptations of a xerophyte in reducing water loss by transpiration

Answer 24

1- reduced number of stomata
2- stomata in pits
3- hairs to trap water
4- rolled leaves
5- leaves reduced to spines
6- thick waxy cuticle

Question 25

describe the mass flow hypothesis for the mechanism of translocation in plants

Answer 25

1- in source/leaf sucrose actively transported into phloem
2- by companion cells
3- lowers water potential of sieve cell/ tube and water enters by osmosis
4- increase in pressure causes mass movement (towards sink/root)
5- sucrose (sugars) used/converted in root for respiration for storage