Mass Transport in Plants

Question 1

What are the 2 specialised plant tissues?

Answer 1

• Xylem : Water & Mineral ions
• Phloem : Organic molecules (Amino acids & Sucrose)

Question 2

XYLEM (Water transport mechanism)

Answer 2

Plants need to conserve water. They obtain water and mineral ions from the soil via their roots. The water passes from the roots to the stem where it is transported up the plant in xylem vessels to the leaves. Water leaves the plant through the stomatal pores by diffusion. This is transpiration.

Transpiration involves the mass transport of water through plants.

Question 3

How does water move in a plant?

Answer 3

• Water enters through the root hair cells in the epidermis.
• It then passes across the cells of the cortex, across the endodermis, and into the xylem vessels.
• The xylem vessels transport water up the plant to the leaves.

Question 4

PHLOEM

Answer 4

transports sugars and organic substances from leaves where they are formed (By PHOTOSYNTHESIS) to where they are needed.

Question 5

The Roots

Answer 5

Root hair cells have (hair like) extensions that increase their surface area to increase the uptake of water and increase the area for channel & carrier proteins.

They also have a thin cell wall so they have a shorter diffusion distance.

Question 6

Mineral Ions movement

Answer 6

1. Mineral ions (Na2+) pumped in by AT, uses carrier proteins, change shape using energy from ATP hydrolysis against conc. gradient (↓to↑)
2. mineral ions ↑ conc. in root hair cell, so lowers water potential of cytoplasm
   valve= water enters by osmosis down water potential gradient through aquaporins
3. Air spaces between soil particles contain O2 needed in aerobic respiration in mitochondria of root hair cell to make ATP

-H2O enters R.H.C by osmosis ↓ WP gradient
-H2O passes from R.H.C to xylem

Question 7

Xylem structure

Answer 7

1. Dead cells form hollow tubes with no cytoplasm= Allows easier water flow
2. End walls break down so xylem forms a continuous tube with no end walls= So water can form a continuous column
3. Cell walls are strengthened with lignin= This makes the xylem waterproof and rigid
4. Xylem pits= These are little holes which allow water to move laterally between xylem vessels

Question 8

Transpiration: Cohesion tension theory

Answer 8

1. Stomata open causes water to diffuse from the air spaces at a higher water potential inside the leaf to a lower water potential of the air outside the leaf. The loss of water from the leaf is known as TRANSPIRATION.
2. The loss of water from the air spaces causes water to move down a water potential gradient from the mesophyll cells to the air spaces.
3. This lowers the water potential of the mesophyll cells, so water moves by osmosis from adjacent mesophyll cells
4. This sets up a water potential gradient across the leaf to the xylem vessels.
5. Water from the xylem enters the leaf, and this causes water to be pulled up UNDER TENSION through the xylem from the roots
6. Water forms a CONTINUOUS COLUMN in the NARROW xylem vessels.
7. Water molecules form weak Hydrogen bonds between them, so they tend to stick together – they have COHESION
8. They are also attracted to the walls of the xylem – there are forces of ADHESION between the water and the xylem.
9. The pulling force is great and the column of water is under TENSION.
10. The movement of water through the plant from the roots to the leaves is known as the TRANSPIRATION STREAM.

5-10 xylem to leaf

Question 9

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

Answer 9

(In the root)
1. Active transport by endodermis;
2. Of ions into xylem
3. Lower water potential in xylem
(Xylem to leaf)
5. transpiration from leaves
6. Creates cohesion
7. Adhesion / water molecules bind to xylem;
8. Creates continuous column of water

Question 10

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

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

Describe how high pressure is produced in leaves

Answer 11

1. Water potential becomes lower
2. Water enters phloem by osmosis
3. Increased volume of water causes increased pressure

Question 12

Light Affecting the Rate of Transpiration

Answer 12

• Light intensity does not affect transpiration directly.
• Stomata open in the light and close in the dark.
• The rate of transpiration is higher in the light.

Question 13

Temperature Affecting the Rate of Transpiration

Answer 13

An increase in temperature increases the rate of transpiration.

Temperature increases cause the rate of water diffusion to increase.

They move more rapidly with increasing temperature (increase in kinetic energy).

Question 14

Humidity Affecting the Rate of Transpiration

Answer 14

The air outside the leaf contains much less water vapour.

The greater the difference in humidity between the air spaces and the air outside the leaf, the greater the rate of diffusion of water vapour out of the leaf, so the greater the rate of transpiration.

Water leaves the leaf down a water potential gradient.

Question 15

Air Movement Affecting the Rate of Transpiration

Answer 15

Air movement over a leaf moves the water vapour away from the stomatal pores.

This increases the water potential gradient between the inside and the outside of the leaf.

The greater the rate of movement of air, the faster the movement of water vapour, so the greater the rate of transpiration.

Remember xerophytes have sunken stomata. This keeps a higher humidity outside the stomata, so reduces transpiration

Question 16

Name a factor that can affect transpiration

Answer 16

Light (intensity) / temperature / air movement / humidity

Question 17

Measuring the rate of transpiration / uptake of water

Answer 17

using a POTOMETER
1. cut leafy shoot diagonally under water to maintain continuous column and stop xylem walls sticking together

2. prevent water getting on leaves to prevent stomata pores becoming blocked with water
3. make sure no air-bubbles in potometer as may get into xylem and block continuous column H2O, stop transpiration stream
4. leafy shoot fitted to photometer under water so H2O enters xylem and H2O column remains constant
5. All joints sealed with waterproof jelly to stop air getting in and H2O escaping from potometer
6. An air bubble is introduced into the capillary tube. As transpiration occurs, water moves through the capillary tube, and the bubble of air moves with it.
7. The distance moved over a period of time is recorded and the mean is calculated of a number of repeats.
8. The volume of water lost over a period of time can be calculated.
9. return air bubble to start by opening tap (for repeating)

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
2. Cut shoot under water
3. Cut shoot at a slant
4. Dry off leaves

Question 19

Potometer Limitations

Answer 19

Volume of water taken up does NOT always equal volume of water lost through transpiration.
Although over 90% is lost though transpiration.

Also, the potometer is only measuring the uptake of water through the stem (attached) and does not consider the uptake through the roots of a normal plant.

Plants also use water for:
* Photosynthesis
* Storage (in vacuoles)
* Provide turgidity
* Hydrolysis reactions

Question 20

What is a source?

Answer 20

where solutes/assimilates produced (↑ conc.)

Question 21

What is a sink?

Answer 21

where solutes used up (↓ conc.)

Question 22

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

Answer 22

1. In source sugars actively transported into phloem;
2. By companion cells;
3. Lowers water potential of sieve tube and water enters by osmosis;
4. Increase in pressure causes mass movement towards sink
5. Sugars used in root for respiration for storage

Question 23

Mass flow supporting evidence

Answer 23

ringing experiment:
- if ring of bark (including phloem not xylem) removed a bulge forms above ring
- fluid from bulge = ↑ conc. sugar, fluid below = lower conc. sugar
- because sugars cant move past area where bark removed, shows that there is downward flow of sugars

Question 24

Mass flow evidence against

Answer 24

• sugar travels to many different sinks, not just one with highest WP
• sieve plates would create barrier to mass flow
• lot of pressure needed for solutes to get through at reasonable rate

Question 25

Evidence from radioactive tracers

Answer 25

• Leaves are supplied with radioactive 14C which is converted to glucose, then sucrose or other organic substances.
• This allows the radioactive 14C to be tracked through the plant as is is transported throughput the plant.
• If a metabolic inhibitor (which stops ATP production) is put into the phloem then translocation stops – this is evidence that active transport is involved
• autoradiography used
• wherever the film turns black, the radioactive substance is present. The results demonstrate the translocation of organic substances from source to sink over time.