Mass Transport In Plants

Question 1

What is transpiration

Answer 1

The loss of water vapour in the stomata via evaporation

Question 2

What are the 4 factors of transpiration

Answer 2

1.light=more light causes more stomata to open
2.temperature=more kinetic energy so faster moving molecules so more evaporation
3.humidity=more water potential in the air so water potential more outside leaf- reduces water potential gradient
4.wind=more wind blows humid air containing water vapour maintains water potential gradient

Question 3

Describe the cohesion theory of water transporting in the xylem

Answer 3

-water molecules lost from leaf because of transpiration and diffusion of water through stomata
-lowers water potential of leaf cells
-water pulled from xylem creating tension
-water molecules cohere together by hydrogen bonds
-forming continuous water column
-adhesion of water molecules to walls of xylem

Question 4

Describe root pressure and how that forces water upwards

Answer 4

-as water moves into the root by osmosis it increases volume of liquid inside the root
-pressure in the root increases forcing water above it upwards

Question 5

What does the xylem transport in plants

Answer 5

-water

Question 6

What does the phloem transport in plants

Answer 6

-organic ions ie sucrose

Question 7

What are the 2 key cells in the phloem that makes it adapted for it’s function

Answer 7

-sieve plates
-companion cells

Question 8

Describe the sieve plate elements of the phloem

Answer 8

● No nucleus / few organelles → maximise space for / easier flow of organic substances

Question 9

Describe the companion cells elements of the phloem

Answer 9

Many mitochondria
High rate of respiration to make ATP for active transport of organic ions

Question 10

Describe the mass hypothesis for the mechanism of translocation in plants

Answer 10

-in the source of the leaf sucrose are actively transported into phloem by companion cells
-this lowers water potential of sieve cells and the tube
-water enters by osmosis
-this creates an increase in hydrostatic pressure causing mass movement towards sink and root
-at the sink sugars(sucrose) used are converted in root for respiration

Question 11

How is transpiration measured in xylem

Answer 11

Using a potometer

Question 12

Using understanding of mass flow hypothesis to explain how pressure is created in phloem tube

Answer 12

-sucrose actively transported in phloem
-lowering water potential
-from the xylem water is transported In the phloem by osmosis

Question 13

What are the 2 methods of investigating translocation

Answer 13

-ringing experiments
-tracer experiments

Question 14

Describe the tracer experiment

Answer 14

1. Leaf supplied with a radioactive tracer eg. CO2 containing radioactive material 1. Leaf supplied with a radioactive tracer eg. CO2 containing radioactive isotope 14C
2. Radioactive carbon incorporated into organic substances during photosynthesis
3. These move around plant by translocation
4. Movement tracked using autoradiography or a Geiger counter

Question 15

Describe the ringing experiment of translocation

Answer 15

1. Remove phloem by removing a ring of bark
2. Bulge forms above ring
3. Fluid from bulge has higher conc. of sugars than below - shows sugar is transported in phloem
4. Tissues below ring die as cannot get organic substances

Question 16

Describe the function of xylem tissue

Answer 16

Transports water and mineral ions through the stem, up the plant to leaves of plants

Question 17

Suggest how xylem tissue is adapted for its function

Answer 17

● Cells joined with no end walls forming a long continuous tube → water flows as a continuous column
● Cells contain no cytoplasm / nucleus → easier water flow / no obstructions
● Thick cell walls with lignin → provides support / withstand tension / prevents water loss
● Pits in side walls → allow lateral water movements

Question 18

Describe how to set up a potometer

Answer 18

1. Cut a shoot underwater at a slant → prevent air entering xylem
2. Assemble potometer with capillary tube end submerged in a beaker of water
3. Insert shoot underwater
4. Ensure apparatus is watertight / airtight
5. Dry leaves and allow time for shoot to
   acclimatise
6. Shut tap
7. Form an air bubble - quickly remove end
   of capillary tube from water

Question 19

Describe how a potometer can be used to measure the rate of transpiration

Answer 19

1. Record position of air bubble
2. Record distance moved in a certain amount of time (eg. 1 minute)
3. Calculate volume of water uptake in a given time:
   ● Use radius of capillary tube to calculate cross-sectional area of water (πr2)
   ● Multiply this by distance moved by bubble
4. Calculate rate of water uptake - divide volume by time taken

Question 20

Suggest limitations in using a potometer to measure rate of transpiration

Answer 20

● Rate of water uptake might not be same as rate of transpiration
○ Water used for support / turgidity
○ Water used in photosynthesis and produced during respiration
● Rate of movement through shoot in potometer may not be same as rate of movement through shoot of whole plant
○ Shoot in potometer has no roots whereas a plant does
○ Xylem cells very narrow

Question 21

Hoe does light intensity effect transpiration rate

Answer 21

Increases rate of transpiration
-● Stomata open in light to let in CO2 for photosynthesis
● Allowing more water to evaporate faster
● Stomata close when it’s dark so there is a low transpiration rate

Question 22

How does temperature effect transpiration rate

Answer 22

Increases rate of transpiration

● Water molecules gain kinetic energy as temperature increases
● So water evaporates faster

Question 23

How does wind intensity effect transpiration rate

Answer 23

Increases rate of transpiration

● Wind blows away water molecules from around stomata
● Decreasing water potential of air around stomata
● Increasing water potential gradient so water evaporates faster

Question 24

How does humidity effect transpiration rate

Answer 24

Decreases rate of transpiration

● More water in air so it has a higher water potential
● Decreasing water potential gradient from leaf to air
● Water evaporates slower

Question 25

Describe the function of phloem tissue

Answer 25

Transports organic substances eg. sucrose in plants

Question 26

What is translocation?

Answer 26

● Movement of solutes such as sucrose
● From source cells (eg. leaves) to sink cells (eg. roots) by mass flow