Mass Transport in Plants

Question 1

Explain how water is transported to the leaves.

Answer 1

(Xylem to leaf)
5. Evaporation / transpiration (from leaves);
6. (Creates) cohesion / tension / H-bonding between water molecules / negative pressure;
7. Adhesion / water molecules bind to xylem;
8. (Creates continuous) column of water

Question 2

Root pressure moves water through the xylem. Describe what causes root pressure. (4) ;

Answer 2

1. Active transport by endodermis;
2. ions/salts into xylem;
3. Lowers water potential (in xylem);
4. (Water enters) by osmosis

Question 3

Name factors that can affect transpiration

Answer 3

Light (intensity) / temperature / air movement / humidity;

Question 4

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 4

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 5

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

Answer 5

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 6

adaptations of the xylem

Answer 6

Dead cells that form hollow tubes with no cytoplasm / organelles so Faster water flow
End walls break down so xylem forms a continuous tube (due to cohesion)
Cell walls are strengthened with lignin, so This makes the xylem waterproof and rigid; provides support / withstand tension / enables adhesion of water

Question 7

how light intensity affect transpiration

Answer 7

Stomata open in the light and close in the dark due to guard cells decreasing the Ψ of their cytoplasm and water enters by osmosis. The guard cells increase in volume which causes the stoma to open

Question 8

where and what is traspiration
(general terms)

Answer 8

During transpiration plants move water from the roots to their leaves from Stromata via xylem vessels.

Question 9

where and what is traslocation (general terms)

Answer 9

Translocatiophloem carries the food produced through photosynthesis in the leaves to the other plant parts.n takes place in the phloem - transport vessels made up of two types of cell, sieve tube elements and companion cells.

Question 10

Online the cohesion tension theory

Answer 10

Water evaporates/is transpired from leaves;
2. Water lost, lowers water potential in leaf cells;
3. Water replaced from xylem;
4. Water potential gradient creates tension (pulls up water) OR Osmosis creates tension/pulls up
water; Accept negative pressure for tension
5. Hydrogen bonds/cohesion/adhesion maintains continuous column of water;

Question 11

Online the cohesion tension theory

Answer 11

Water evaporates/is transpired from leaves;
2. Water lost, lowers water potential in leaf cells;
3. Water replaced from xylem;
4. Water potential gradient creates tension (pulls up water) OR Osmosis creates tension/pulls up
water; Accept negative pressure for tension
5. Hydrogen bonds/cohesion/adhesion maintains continuous column of water;

Question 12

How Structure relates to the function of xylem

Answer 12

S: Dead cells that form hollow tubes with no cytoplasm / organelles
Faster water flow / less resistance / no impediment from organelles
S:End walls break down so xylem forms a continuous tube with no end walls
So a continuous column of water can form (due to cohesion)
Cell walls are strengthened with lignin,
This makes the xylem waterproof and rigid; provides support / withstand tension / enables adhesion of water
Xylem pits
These are little holes which allow water to move laterally between xylem vessels / get around blocked vessels

Question 13

Outline what is transportation

Answer 13

1. Stomataopen(toenablecarbondioxideto enter the leaf for photosynthesis) 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. (Evaporation of water)
2. Thelossofwaterfromtheairspacescauses water to move down a water potential gradient from the mesophyll cells to the air spaces.
3. Thislowersthewaterpotentialofthe mesophyll cells, so water moves by osmosis from adjacent mesophyll cells
4. Thissetsupawaterpotentialgradient across the leaf to the xylem vessels.
5. Waterfromthexylementerstheleaf,andthiscauseswatertobepulledupUNDERTENSION through the xylem from the roots
   Water moving down a Ψ gradient
6. WaterformsaCONTINUOUSCOLUMNinthe NARROW xylem vessels.
7. WatermoleculesformweakHydrogenbondsbetween them, so they tend to stick together – they have COHESION
8. Theyarealsoattractedtothewallsofthexylem–there are forces of ADHESION between the water and the xylem.
9. ThepullingforceisgreatandthecolumnofwaterisunderTENSION.
   10.The movement of water through the plant from the roots to the leaves is known as the
   TRANSPIRATION STREAM.
   This is the COHESION-TENSION theory of the movement of water up a plant.

Question 14

Describe how a high pressure is produced in the leaves [3]

Answer 14

Water potential becomes lower/becomes more negative (as sugar enters phloem);
2. Water enters phloem by osmosis;
3. Increased volume (of water) causes increased pressure

Question 15

How light intensity affect transpiration

Answer 15

High light intensity =
More stomata open
Light intensity does not affect transpiration directly.
Stomata open in the light and close in the dark due to guard cells decreasing the Ψ of their cytoplasm and water enters by osmosis. The guard cells increase in volume which causes the stoma to open.

Question 16

How temperature affects transportation

Answer 16

Increased temperature =
Increased rate of transpiration
↑ Temperature = ↑ KE = ↑ rate of Diffusion and Evaporation of water
If the soil is dry, then roots will release abscisic acid into the xylem. This triggers the closure of stomata to preserve water los

Question 17

How humidity affects transpiration

Answer 17

Increased humidity =
Decreased rate of transpiration
The air spaces in the leaf are saturated with water vapour. 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 18

How wind speed after a transportation

Answer 18

Increase the wind speed =
Increased rate of transpiration
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.
Remember xerophytes have sunken stomata. This keeps a higher humidity outside the stomata, so reduces transpiration.

Question 19

How stomata density affects transportation

Answer 19

Higher stomatal density =
Higher rate of transpiration
Stomatal density refers to the total number of stomata in a given area

Question 20

How xerophytes adapted

Answer 20

1. Reduced number of stomata 2. Stomata in pits
2. Hairs to trap water
3. Rolled leaves
4. Leaves reduced to spines 6. Thick waxy cuticles
   To reduce water loss in transportation

Question 21

Outline translocation

Answer 21

Translocation is the movement of solutes (e.g. sucrose & amino acids) to where they are needed in a plant. Solutes are sometimes called assimilates (these are substances that get incorporated into the plant tissue).
• Translocation is an energy requiring process (hydrolysis of ATP), that happens in the phloem.
• It moves solutes from SOURCES (where assimilates are produced in higher concentrations) to
SINKS where assimilates are used, (so lower concentration).
• Enzymes maintain a concentration gradient from the source to the sink by converting the
solutes at the sink to other storage substances (e.g., Starch).
• This makes sure there is always a lower concentration at the sink than the source.
• E.g. the source for sucrose is usually the leaves where it is made and the sinks are the storage organs and the meristems (areas of growth) in the roots stems and leaves.
• In potatoes, sucrose is converted to starch, in the sink areas so there is always a low concentration of sucrose at the sink than inside the phloem

Question 22

Describe mass flow

Answer 22

1. In source/leaf sucrose 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. Sucrose (sugars) used/converted in root for respiration for storage;

Question 23

Ringing experiment

Answer 23

If a ring of bark (the phloem) is removed from a woody stem, a bulge forms above the ring.
FOR;
The fluid from the bulge has a higher concentration of sugars than the fluid from below the ring.
• This is because the sugars can’t move past the area where the bark has been removed – this is evidence that there can be a only downward flow of sugars
• Conc. Of sucrose in higher in leaves (source) than roots (sink).m
Tissue below ring due as no sucrose
AGAINST
Sieve plate function unclear.

Question 24

How is xylem and phloem arranged

Answer 24

Xylem towards the centre
Phloem towards outside (hence removed in the ringing exp)

Question 25

Radioactive tracers

Answer 25

Leaves are supplied with radioactive 14C (CO2) which is converted to glucose, then sucrose or other organis substances.
• This allows the radioactive 14C to be tracked through the plant as is is transported throughout the plant.
FOR
Increase in sucrose levels in leaf followed by increase in phloem later
• Metabolic poison or lack of oxygen inhibit translocation in phloem. This is evidence that active transport is involved
• Companion cells
possess many mitochondria to readily produce ATP.