Mass Transport in Plants A1

Question 1

What labels does the cross section of a root have?
(6)

Answer 1

1. phloem
2. xylem
3. epidermis
4. root hair
5. endodermis
6. cortex

Question 2

Describe the features of a root and how they help its function.
(2)

Answer 2

1. extensions to increase surface area to increase uptake of water
2. thin cell wall for a shorter diffusion pathway

Question 3

Explain how roots absorb minerals, ions and water.
(3)

Answer 3

1. actively transport ions from soil across their membrane from soil across their membrane into cytoplasm, via carrier proteins
2. this lowers the water potential of the cytoplasm, to below the water potential in the soil
3. water enters the root hair cells via osmosis down a water potential gradient (high to low) through a partially permeable membrane

Question 4

How does water get into xylem from root hair cell?

Answer 4

Water passes from root hair cells, across the cortex cells to the xylem.

Question 5

Describe the symplastic pathway.
(5)

Answer 5

1. water passes through the cytoplasm of the cell
2. across the cell membranes
3. through plasmodesma (hole in cell wall) from one cell to another
4. via osmosis down a water potential gradient
5. it is relatively slow as there is resistance to the movement of water (cell membrane and solutes)

Question 6

Describe the apoplastic pathway.
(4)

Answer 6

1. water passes through the adjoining cell walls from one cell to another
2. movement is relatively fast (less resistance to water molecules as larger gaps in between cellulose molecules)
3. this is via diffusion
4. the water then reaches the endodermis

Question 7

What do the cells of the endodermis do to water and ions?
(6)

Answer 7

1. the endodermis has a waterproof strip (casparian strip) which prevents water movement through walls of the endodermis
2. water is forced to pass through cytoplasm of cell of the endodermis
3. the cells actively transport ions to xylem
4. this lowers water potential in xylem
5. so water passes into xylem from endodermal cells down a water potential gradient
6. this enables movement of water from root hairs to centre of root, down a water potential gradient.

Question 8

Describe the structure and function of the xylem.
(4)

Answer 8

1. dead cells which form hollow tubes - this allows water potential to not be affected, so water stays within the xylem vessels and doesn’t leave
2. end walls break down so xylem forms a continuous tube - so water can form a continuous column
3. cell walls are strengthened with lignin - this makes the xylem waterproof
4. there are pits in the xylem - these are little holes which allow water to move between xylem vessels.

Question 9

Explain how water enters xylem from the endodermis in the root and is then transported to the leaves.
(8)

Answer 9

(in the roots)
1. casparian strip blocks apoplastic pathway/only allows symplastic pathway
2. active transport by endodermis
3. of ions/salts into xylem
4. lowers water potential in xylem/water enters xylem by osmosis/down a water potential gradient
(xylem to leaf)
5. evaporation / transpiration (from leaves)
6. creates cohesion/tension (H-bonding) between water molecules (negative pressure)
7. adhesion/water molecules mind to xylem
8. creates continuous column of water

Question 10

Root pressure moves water through the xylem.
Describe what causes root pressure.
(4)

Answer 10

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

Question 11

Name a factor that can affect transpiration.

Answer 11

light / temperature / air movement / humidity

Question 12

Describe the process of transpiration.
(2)

Answer 12

1. open stomata (so CO2 can enter leaf) causes water to diffuse from air spaces in leaf to out leaf from high-low water potential (transpiration)
2. this causes a continuous concentration gradient through cells in the plant causing a continuous column in xylem vessels

Question 13

What is the cohesion tension theory of water movement up a plant?
(6)

Answer 13

1. water moves out stomata from a high to low concentration gradient
2. water forms continuous column in narrow xylem vessel
3. water moles have forces of attraction so they stick together - they have cohesion
4. they are also attracted to the walls of the xylem (adhesion)
5. pulling force is great and column is under tension
6. this is called transpiration stream

Question 14

Root pressure moves water through the xylem.
Describe how a high pressure is produced in the leaves.
(3)

Answer 14

1. water potential becomes lower/becomes more negative (as sugar enters phloem)
2. water enters phloem by osmosis
3. increased volume (of water) caused increased pressure

Question 15

What type of pressure aids water movement from root to top of tree?

Answer 15

Hydrostatic pressure is higher in roots than top of tree so aids water movement upwards

Question 16

Name and explain the factors affecting the rate of transpiration.
(4)

Answer 16

1. light - doesn’t directly affect it, stomata open in light and closed in dark (rate is higher in light)
2. temperature - increases kinetic energy, water diffusion increases
3. humidity - greater difference in humidity, the greater rate of diffusion
4. air movement - moves water away from stomatal pores increases potential gradient and diffusion rate

Question 17

Explain how to measure the rate of transpiration in plants using a potometer.
(8)

Answer 17

1. leafy shoot cut under water
2. potometer completely filled with water so no air bubbles
3. leafy shoot fitted to potometer under water using rubber tube
4. potometer removed from under water and all joints sealed with waterproof jelly
5. air bubble introduced to capillary
6. as transpiration occurs water moves through tube and bubble moves with it
7. distance recorded over a period of time, mean is calculated from n. of repeats
8. volume of water lost over a period of time calculated

Question 18

Give two precautions the students should have taken when setting up the potometer to obtain reliable measurements of water uptake by the plant shoot.
(8)

Answer 18

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

Question 19

Name some xerophyte adaptations plants have to reduce water loss.
(6)

Answer 19

1. reduced number of stomata
2. stomata trapped in pits (maintain water vapour in pits) - less concentration gradient
3. hair to trap water
4. rolled leaves
5. leaves reduced to spines
6. thick waxy cuticles

Question 20

What is translocation?
(5)

Answer 20

1. the movement of solutes (assimilates) to where they are needed in a plant
2. its an energy requiring process that happens in the phloem
3. moves solutes from sources (where assimilates are produced, so high concentrations) to sinks (where assimilates are used up, so low concentrations)
4. enzymes maintain a concentration solutes at sink to other storage substances
5. this makes sure there is always a lower concentration at the sink than source

Question 21

Describe the mass flow hypothesis for the mechanism of translocation in plants.
(6)

Answer 21

(transporting substances from source to sink in plant)
1. in source/leaf sugars actively transported into phloem
2. by companion cells
3. lowers water potential of sieve cell/tube and water enters by osmosis
4. increases in pressure causes mass movement (toward sink/root) - active transport down concentration gradient
5. sugars used/converted in root for respiration for storage
6. this causes a lower pressure in sieve tube, creating a pressure gradient from source to sink, continuing the pattern

Question 22

Give supporting evidence of mass flow.
(5)

Answer 22

1. if a ring of bark (which includes a phloem not xylem) is removed from a woody stem, a bulge forms above the ring
2. the fluid in bulge has a high concentration of sugar that fluid below ring because sugars can’t move past area where bark removed (provided there is a downflow of sugars)
3. when an aphid pierces a leaf further down the stem, sap flows out quicker, evidence that there is a pressure gradient
4. radioactive tracers eg. carbon (14C) can be put in organic substances in plant
5. if metabolic inhibitor (which stops ATP production) put in phloem, translocation stops - providing active transport is involved

Question 23

Give evidence again the mass flow theory.
(2)

Answer 23

1. sugar travels to many sinks, not just the one with the highest water potential
2. sieve plates would create a barrier to mass flow. A lot of pressure needed for a solutes to get through at reasonable rate

Question 24

Give the evidence from radioactive tracers for mass flow theory.
(5)

Answer 24

1. done by supplying part of a plant (leaf) with an organic substance that has a radioactive label eg CO2 (gets in sugars during photosynthesis etc)
2. then tracking its movement by autoradiography to reveal where tracker has spread
3. plant is killed (frozen using liquid nitrogen) then placed onto photographic film
4. where the film turns black, radioactive substances are present
5. this demonstrates translocation of substances from source to sink over time (during photosynthesis)

Question 25

Describe correlation and causation relationships.

Answer 25

1. data from an experiment may show a continuous pattern with results eg. width of bark compared to mg of carbohydrate transported to bottom of tree
2. however this does not mean it is caused by the width of bark
3. other factors may affect the results eg. more bark taken away means less phloem, reducing the amount of carbohydrates that can be transported etc.

Question 26

Sodium chloride and potassium chloride had different effects for the opening and closing of stomata, explain why.

Answer 26

1. K+ (potassium) ions lower water potential inside / Na+ (sodium) ion lowers water potential outside
2. different guard cell wall thickness causing opening stomata.