Unit 3 - Mass Transport In Plants

Question 1

Role of the xylem?

Answer 1

They transport water from the roots to the leaves. They’re made out of dead cells.

Question 2

How is xylem tissue adapted for its function?

Answer 2

1) long cells/tubes with no end walls so allows water to move in a continuous column
2) no cytoplasm/no organelles so allows easier water flow
3) thick wall containing lignin so provides support/withstand tension/waterproof
4) pits in walls so allows lateral movement/get around blocked vessels

Question 3

What is transpiration?

Answer 3

Water evaporates from the Mesophyll cells and diffuses out of the open stomata down a water potential gradient.

Question 4

How does water from the soil end up in the xylem?

Answer 4

-water moves up the xylem
-reducing water potential
-water moves through the cortex cells down a water potential gradient
-water drawn into root hair cells down a water potential gradient.

Question 5

What is symplast?

Answer 5

Water molecules move through the cells walls.

Question 6

What is apoplast?

Answer 6

Water moves from cytoplasm to cytoplasm.

Question 7

What is the cohesion tension theory?

Answer 7

-Water evaporates from the Mesophyll cells and diffuses out of the open stomata down a water potential gradient.
-Reduces water potential in the cells in leaf.
-Water is drawn out from the xylem (which has a higher water potential) into the cells via osmosis.
-Water is therefore pulled up the xylem creating a negative pressure called tension.
-Due to hydrogen bonds between water molecules called cohesion.
-Forming a continuous column of water.
-Water molecules are also attracted to the walls of the xylem - called adhesion.

Question 8

On a hot day (rapid transpiration) why does the diameter of a tree trunk reduce slightly?

Answer 8

Due to the adhesion with the walls of the xylem and negative pressure (tension) making the xylem vessels slightly narrower.

Question 9

4 main factors affecting transpiration?

Answer 9

1) light intensity
2) temperature
3) humidity
4) air movement

Question 10

The affect of light intensity on the rate of transpiration?

Answer 10

An increase in light intensity increases the rate of transpiration.
More stomata open in the light to allow more carbon dioxide to enter for photosynthesis.
So more water evaporates out of the stomata causing a higher rate of transpiration.

Question 11

The affect of temperature on the rate of transpiration?

Answer 11

An increase in temperature increases the rate of transpiration
Temperature increases the rate of evaporation and diffusion of water molecules through the stomata.
As the water molecules have a higher kinetic energy.

Question 12

The affect of humidity on the rate of transpiration?

Answer 12

An increase in humidity decreases the rate of transpiration.
A greater humidity reduces the water potential gradient between the inside and outside of the leaf at the stomata, due to the larger number of water vapour molecules in the air.
So the rate of diffusion of water vapour out of the leaf is reduced.

Question 13

The affect of air movement on the rate of transpiration?

Answer 13

An increase in air movement increases the rate of transpiration.
Air movement over a leaf moves the water vapour away from the stomata pores.
This increases the water potential gradient between the inside and the outside of the leaf.
So the greater the rate of transpiration.

Question 14

How does a potometer measure the rate of transpiration?

Answer 14

It measures water uptake by a shoot.

Question 15

Process of using a potometer?

Answer 15

1) A leafy shoot is cut under water to prevent air entering the xylem which would break the continuous water column. The shoot is placed in a rubber tube.
2) The potometer is filled completely with water making sure there are no air bubbles.
3) The potometer is removed from under the water and all joints are sealed with waterproof jelly to prevent water leaking out which would produce an inaccurate result.
4) An air bubble is introduced into the capillary tube.
5) As transpiration occurs, water moves through the capillary tube and into the plant, and the air bubble moves with it.
6) The distance moved over a period of time is recorded and the mean is calculated of a number of repeats.
7) The volume of water lost over a period of time can be calculated by knowing the radius of the tube and the distance the bubble has moved in mm. Volume of a cylinder = pie * radius2 * length

Question 16

How are leaves that live in dry environments adapted to reduce water loss?

Answer 16

1) Reduced number of stomata
2) Stomata in pits to trap water vapour and increase humidity
3) Hairs to trap water vapour and increase humidity
4) Rolled leaves to trap water vapour and prevent exposure of stomata to air movement
5) Leaves reduced to spines to reduce S.A.:Vol
6) Thick waxy cuticles to reduce evaporation

Question 17

How can you start the experiment again?

Answer 17

By emptying the reservoir as it pushes bubble back to the start.

Question 18

Con of the potometer?

Answer 18

Assumes all water taken up is transpired. Some water may be used to make cells turgid for hydrolysis reactions or photosynthesis.

Question 19

Role of the phloem?

Answer 19

Transports organic solutes around plants.

Question 20

What are solutes and assimilates?

Answer 20

Solutes are dissolved substances.
Assimilates are substances that get incorporated into plant tissues.

Question 21

How is phloem tissue adapted for its function?

Answer 21

-Sieve tubes have no nucleus, little cytoplasm and few organelles so it allows unobstructed flow of solutes.
-End walls of sieve tubes perforated with holes so it allows continuous flow of substances through sieve tube.
-Companion cells contain many organelles e.g. mitochondria so it carries out the functions for the sieve tubes.

Question 22

What is translocation?

Answer 22

It is the movement of solutes to where they are needed in a plant. It is an energy requiring process that happens in the phloem.

Question 23

Definition of the source?

Answer 23

It is where the sugars are made/stored and are therefore in high concentration e.g. leaf.

Question 24

Definition of the sink?

Answer 24

It is where the sugars are used in respiration or converted for storage and therefore in lower concentration.

Question 25

What is the mass flow hypothesis?

Answer 25

1) Active transport is used to load solutes from companion cells into the sieve tubes of the phloem at the source.
2) This lowers water potential in sieve tubes.
3) Water enters the sieve tubes by osmosis from xylem.
4) This creates a high hydrostatic pressure inside the sieve tubes at the source.
5) At the sink end, sucrose is removed from the phloem to be used in respiration in repairing cells or stored as starch in storage organs.
6) This increases the water potential inside the sieve tubes so water leaves the tubes by osmosis.
7) This lowers the hydrostatic pressure inside the sieve tubes.

Question 26

Supporting evidence for mass flow?

Answer 26

1) If a ring on bark (which includes the phloem but not the xylem) is removed from a woody stem, a bulge forms above the ring.
2) Pressure in the phloem can be investigated using aphids. They pierce the phloem, then their bodies are removed leaving mouth parts behind, which allows sap to flow out. The sap flows out quicker nearer the leaves than further down the stem - this is evidence that there is a pressure gradient.
3) If a metabolic inhibitor (which stops ATP production) is put into the phloem then translocation stops - this is evidence that active transport is involved.
4) Evidence from radioactive tracers.

Question 27

How does evidence from radioactive tracers work?

Answer 27

-A radioactive tracer such as radioactive carbon dioxide (14C) can be used to track the movement of organic substances in a plant.
-The radioactive carbon is incorporated into sucrose at the source when the cells photosynthesise.
-The radioactive carbon can be traced, proving translocation can occur both up and down the phloem.
-The movement of these substances can be tracked using a technique called auto radiography.
-To reveal where the radioactive tracer has spread to in a plant, the plant is killed and then the whole plant, or sections of it is placed into photographic film - where ever the film turns black, the radioactive substance is present.
-The results demonstrate the translocation of substances form source to sink over time.

Question 28

Evidence against mass flow?

Answer 28

-Sugars move to all areas of the plant, not just from source to sink.
-The sieve plates would create a barrier to mass flow. A lot of pressure would be needed for the solutes to get through at a reasonable rate.