3.3.4.1 Mass Transport in Plants

Question 1

What is transpiration?

Answer 1

Loss of water vapour from the stomata.

Question 2

What are the 4 factors affecting transpiration

Answer 2

Light intensity

Temperature

Humidity

Wind

Question 3

How does light intensity affect rate of transpiration?

Answer 3

More light causes stomata to open. Larger surface area for evaporation.

Question 4

How does temperature affect rate of transpiration?

Answer 4

More heat means more kinetic energy, faster moving molecules so more evaporation.

Question 5

How does humidity affect rate of transpiration?

Answer 5

More water vapour in the air makes the water potential higher outside the leaf which reduces the water potential gradient.

Question 6

How does wind affect rate of transpiration?

Answer 6

More wind blows away humid air containing water vapour so maintains a water potential gradient.

Question 7

What three effects are required for the cohesion-tension theory?

Answer 7

Cohesion

Adhesion/capillarity

Root pressure

Question 8

How does cohesion work?

Answer 8

Water is a dipole with O being partially negative and H partially positive. These can form hydrogen bonds between the molecules.

Question 9

Why is cohesion in water helpful?

Answer 9

As it allows for a continuous water column.

Question 10

How does capillarity/adhesion work?

Answer 10

Water sticks to other molecules and can adhere xylem walls.

Question 11

How does the size of the xylem lumen affect capillarity?

Answer 11

Narrower the xylem lumen, bigger the impact of capillarity

Question 12

What is root pressure?

Answer 12

Water moves into the roots by osmosis which increases the volume of liquid inside.

Question 13

Why does a high root pressure mean?

Answer 13

The roots forced water above it upwards by positive pressure.

Question 14

Movement of water up the xylem process?

Answer 14

Water vapour evaporates out of the stomata on the leaves by transpiration. The loss of water creates a higher pressure.

More water is pulled up the xylem to replace the lost water by negative pressure.

Cohesive water molecules create a column of water in the xylem.

Water molecules adhere to xylem walls which helps pull the water column upwards.

As water is pulled up, tension is created so xylem walls become narrower.

Question 15

Sieve tube element structure

Answer 15

Living cells
No nucleus
Contain few organelles

Question 16

Sieve tube function? How does the structure help?

Answer 16

Allows continuous flow of sugar solution.

More hollow time allows maximised flow.

Question 17

Companion cell function

Answer 17

Provides ATP required for active transport of organic substances

Question 18

What is the mass flow hypothesis?

Answer 18

Mass flow from source of production to the sink where it is used up.

Question 19

Example of source cell

Answer 19

Photosynthesising leaf cell

Question 20

Example of sink cell

Answer 20

Respiring cell

Question 21

What is the process at the source cell during mass flow?

Answer 21

Produces sucrose which lowers water potential and so water moves in by osmosis.

Increases in hydrostatic pressure.

Question 22

What is the process at the sink cell during mass flow?

Answer 22

Uses up sucrose which rises the water potential and so water moves out by osmosis.

Decreases in hydrostatic pressure.

Question 23

How does hydrostatic pressure differences in the source and sink cell cause movement?

Answer 23

Source cell has a higher hydrostatic pressure than the sink cell so the solution is forced towards the sink via the phloem.

Question 24

What happens in Translocation [1] summarised?

Answer 24

Sucrose transports from source to the sieve tube element.

Question 25

What is the process of Translocation [1]?

Answer 25

Photosynthesis creates organic substances. High concentration of sucrose at production site/ Sucrose diffuses don gradient into companion cells by facilitated diffusion. Active transport of H+ ions from companion cells to cell wall spaces using energy from ATP. Creates a conc gradient of H+ ions and they move down it via carrier proteins into sieve tube elements. Co-transport of sucrose its H+ ions moving by co-transporter proteins so sucrose also moves into sieve tube elements.

Question 26

What happens in Translocation [2] summarised?

Answer 26

Movement of sucrose within sieve tube elements

Question 27

What is the process of Translocation [2]?

Answer 27

Increase of sucrose in the sieve tube elements which lowers the water potential. Water enters the sieve tube elects from surrounding xylem vessels by osmosis. Increase in water volume in sieve tube element increases hydrostatic pressure - forcing the liquid towards the sink.

Question 28

What happens in Translocation [3] summarised?

Answer 28

Transport of sucrose to the sink

Question 29

What is the process of Translocation [3]?

Answer 29

At the sink, sucrose is used, stored or converted into insoluble starch. Sucrose actively transported into sink cell, decreasing conc gradient of water. Osmosis of water from sieve tube element to sink cell. Removal of water decreases the volume of the sieve tube element decreases hydrostatic pressure. Movement of soluble organic substances is due to differences in hydrostatic pressure between source and sink.

Question 30

Two methods of investigating translocation

Answer 30

Tracers Ringing Experiment

Question 31

How do tracers work?

Answer 31

Plants and provided and synthesis with radioactively labelled carbon dioxide. This creates sugars with radioactively labelled carbon. Thin slices of stem are cut are placed on x-ray film that turns back when exposed to radioactive material so the sugars show up which have been transported by the phloem.

Question 32

How does the ringing experiment work?

Answer 32

Ring of bark and phloem are peeled and removed. Trunk swells above and the liquid is analysed. Shows that the sugars cannot be transported.