12.12 Mass Transport in Plants

Question 1

Xylem

Answer 1

Tissue that transports water and mineral ions in stem and leaves

Question 2

Phloem

Answer 2

Transports sugars and organic substances from leaves to where they are needed in plant

Question 3

What is transpiration?

Answer 3

Water leaves the plant through stomatal pores by diffusion.
Involves mass transport of water through plants

Question 4

Root adaptations

Answer 4

RHC have hair-like extensions- increase SA
Have thin cell wall- short diffuse distance

Question 5

How do root hair cells transport ions?

Answer 5

• Actively transport from soils across their membranes into their cytoplasm via carrier proteins.
• More negative water potential of cytoplasm compared to soil.

Question 6

Osmosis

Answer 6

Movement of water from a region of higher water potential to a region of lower water potential across a partially permeable membrane through aquaporins

Question 7

What direction does water travel in the stem?

Answer 7

Water travels UP stem through xylem vessels

Question 8

Xylem adaptations

Answer 8

• Dead, hollow tubes with no cytoplasm/organelles= Easier water flow
• End walls break down to form a continuous tube with no end walls= Water forms continuous column
• Cell walls strengthened with lignin= Xylem waterproof and rigid to withstand high pressure/tension and provide strength
• Xylem pits= allow water to move laterally between xylem vessels and get around blocked vessels.

Question 9

Outline transpiration/ cohesion-tension theory

Answer 9

1. Stomata will open, to allow co2 to enter for photosynthesis, which causes water to diffuse by osmosis from the air spaces in the leaf to the outside of the leaf
2. Loss of water in the air spaces causes water to move down a WPG from mesophyll cells to air spaces
3. Lowers the WP of the mesophyll cells so water moves via osmosis from the adjacent mesophyll cells
4. Sets up a water potential gradient across the leaf to the xylem
5. Water diffuses from the xylem, enters the leaf, causes water to be pulled up UNDER TENSION through the xylem from the roots
6. Water forms a continuous column
7. Hydrogen bonds between water molecules cause them to stick together SO they have COHESION
8. Water is also attracted to the walls of the xylem so there are forces of ADHESION between the xylem and water

Question 10

Negative pressure

Answer 10

Pressure in leaf is less than pressure outside leaf

Question 11

Describe how high pressure is produced in the leaves (3)

Answer 11

1. Water potential becomes lower/more negative (as sugars enter phloem)
2. Water enters phloem by osmosis
3. Increased volume of water- increased pressure

Question 12

Water potential in a tree

Answer 12

WP decreases/more negative as you go up from soil to leaf to outside air

Question 13

Factors affecting rate of transpiration

Answer 13

Light
Temperature
Humidity
Air movement

Question 14

How does light affect rate of transpiration?

Answer 14

• Increase in light= stomata open
• Water leaves and closes at night
• Rate of transpiration increase and light intensity increase
• Plateaus due to other limiting factors

Question 15

How does temperature affect rate of transpiration?

Answer 15

• Increased temp= increased rate of osmosis
• Water molecules move more rapidly with increased temp= increase in KE
• Rate of transpiration increases as temp increases
• Rate can decrease after temp reaches optimum- proteins (aquaporins) denature- change in tertiary structure

Question 16

How does humidity affect rate of transpiration?

Answer 16

• Air spaces in leaves saturated with water vapour. Air outside contains less water vapour
• Greater rate of osmosis of water vapour out of leaf when there is greater difference in humidity= GREATER RATE OF TRANSPIRATION
• Humidity increases outside leaf= rate of transpiration decreases

Question 17

How does air movement affect rate of transpiration?

Answer 17

• Moves water vapour away from stomatal pores
• Increases water potential gradient between inside and outside of leaf- water vapour moves faster
• Greater rate of air movement, faster movement of water vapour, greater rate of transpiration

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.

Answer 18

1. Seal joints / ensure airtight / ensure 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 the start position;

Question 19

Phloem structure

Answer 19

• Phloem tissue transports organic solutes
• Phloem tissue is formed from cells arranged in tubes

Question 20

Sieve tubes function

Answer 20

• Have no nucleus and few organelles
• There is a companion cell for each sieve tube.
• Sieve tubes are connected through sieve plates

Question 21

Companion cell function

Answer 21

• Carry out the living functions for sieve cells
• Have many mitochondria to synthesise the ATP through aerobic respiration for active transport of solutes

Question 22

What is translocation?

Answer 22

The movement of solutes to where they are needed in a plant, which requires energy from ATP

Question 23

Assimilates

Answer 23

Solutes incorporated into plant tissue

Question 24

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

Answer 24

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. Increase in pressure causes mass movement (towards sink/root);
5. Sugars used/converted in root for respiration for storage;

Question 25

What are the ringing experiments?

Answer 25

• Ring of bark is removed from a woody stem
• A bulge forms above the ring
• Conc in the bulge > conc below the ring

Question 26

Evidence against ringing experiments

Answer 26

• Sugar travels to many different sinks not just to one with the highest water potential
• The sieve plates create a barrier to mass flow
• A lot of pressure would be needed for the solutes to get through a reasonable rate

Question 27

Evidence from radioactive tracers

Answer 27

• Leaves are supplied with radioactive (14)C(CO2) which is converted to glucose then sucrose
• This allows the radioactive (14)C to be tracked through the plant as it is transported throughout the plant
• If inhibitor (which stops ATP production) inserted in phloem, translocation stops= EVIDENCE THAT ACTIVE TRANSPORT INVOLVED
• The movement of these substances tracked using technique called autoradiography
• To reveal where the tracer has spread to in a plant- plant is killed and sections placed onto photographic film