AQA Mass Transport in Plants | Maths and Physics Tutor

Root pressure is a force that is partly responsible for the movement of water through xylem in stems.

Explain how the active transport of mineral ions into xylem vessels in the roots results in water entering these vessels and then being moved up the xylem tissue.

1. Water potential in xylem reduced (by entry of ions)

2. Water potential gradient established between xylem and surrounding cells

3. Plasma membranes of surrounding cells are partially permeable

4. Water enters xylem by osmosis

5. Volume of water in xylem increases

6. Cannot move back due to gradient

7. Pressure in xylem increases (and forces water upwards)

The presence of an air bubble in a xylem vessel in the stem blocks the movement of water through that vessel.

Use the cohesion-tension theory to explain why.

1. Evaporation from leaves / transpiration

2. Water in xylem under tension*/negative pressure/pulled up

3. Water molecules cohere*/stick together/form hydrogen bonds

4. So water a single column

5. Air bubble breaks column / prevents cohesion

Water vapour diffuses through open stomata into the atmosphere. Describe two structural adaptations of the leaves of xerophytes that reduce this loss.

Using Fick's law, explain how these two adaptations reduce the rate of diffusion of water vapour into the atmosphere.

1. reduced number of stomata; reduced surface area

2. (epidermal) hairs; reduce diffusion gradient

3. thick waxy cuticle; increases diffusion distance

4. sunken stomata; reduce concentration difference

5. leaves reduced to spines; reduced surface area

6. curled leaves; reduced concentration gradient

Statement of Fick's law:

Rate of diffusion = Surface area of exchange surface x concentration / difference across surface thickness of exchange surface

Explain how water enters a plant root from the soil and travels through to the endodermis.

1. water enters root hair cells by osmosis

2. because active uptake of mineral ions has created a WP gradient

3. water moves through the cortex

4. (by osmosis) down a WP gradient

5. through cell vacuoles and cytoplasms / symplastic pathway

6. through cell walls / apoplastic pathway

From the root, water is transported upwards through the stem.

Explain how evaporation from the leaves can cause the water to move upwards.

1. WP in leaf cells decreases / becomes more negative

2. therefore water moves out of xylem (into surrounding tissues) by osmosis

3. this creates a pull/tension on the water in xylem

In daylight, most of the water evaporates from the leaves but some is used by the plant.

Describe the ways in which this water could be used by the plant.

1. which is in a continuous column / water molecules cohere

2. cohesion due to H bonding

3. column doesn't break because of adhesion with xylem walls

4. (water is used in) the light-dependent reactions of photosynthesis

5. electrons from water enable ATP production / H+ are used to reduce NADP / produces O2

6. (water can be used in) hydrolysis reactions within the plant

7. to create turgor;

8. as a solvent for transport

9. as a medium for chemical reactions

10. component of cells / cytoplasm

The diameter of a tree is less during the day, when the tree is transpiring, than it is at night.

Explain how this

(i) supports the cohesion-tension theory.

(ii) does not support the root pressure theory.

(i)

- Evaporation from leaves during daytime only/mainly

- tension/negative pressure (on water) in xylem creates inward pull (on walls of xylem vessel)

- xylem vessels become narrower due to adhesion of water molecules (to walls of xylem vessels)

(ii)

- root pressure gives outward force/push on walls of xylem vessels

- tree would become wider/stay same diameter

- xylem vessels become wider/stay same diameter

Change in the circumference of tree trunks can be related to change in the diameter of xylem vessels. The graph shows changes in circumference of three different species of trees measured on a hot dry day in June.

Use your knowledge of the cohesion-tension theory to explain the changes in the circumference of tree A.

1. Increase in light (from 06.00) causes stomata to open

2. water evaporates/lost by transpiration, from substomatal space/ mesophyll/air space/stomata

3. (causing) water to move across leaf in apoplast / symplast / by osmosis

4. (transpiration/evaporation) exerts force causing tension/ pulling force in water columns

5. (hydrogen) bonding between molecules/

cohesion holds water (columns) together

6. bonding/ adhesion between walls of xylem vessels and water molecules

7. tension/pulling causes water columns to decrease diameter, narrowing/stretching

xylem vessels (and hence tree trunk)

8. Decreases in light (from 20.00), water evaporation slows, tension falls allowing water columns to collapse

Describe two features you would expect in the leaves of a tree adapted to a dry environment. Explain how each feature helps the tree's survival.

1. Sunken stomata;

water evaporation into pit creates local humidity; increased humidity reduces gradient for water evaporation;

2. close arrangement of stomata;

diffusion shells of individual stomata overlap; interferes with water diffusion and slows evaporation;

3. restriction of stomata to lower side of leaf;

rate of air movement below leaf less/ heating effect of sun less; gradient for water evaporation reduced/ water molecules have less kinetic energy;

4. thick cuticle/wax/suberin (on upper surface); (wax/suberin )waterproof;

water unable to diffuse onto surface to evaporate,

5. presence of trichomes/ hairs;

surface traps water close to leaf surface;

increased humidity reduces gradient for water evaporation;

6. reduced leaves/spines/small surface area to volume; less surface area for evaporation;

more distance across leaf for water to diffuse;

7. rolled leaves; stomata enclosed in localised humidity; increased humidity reduces gradient for water evaporation;

Explain how the rate of water movement through the apoplast pathway is affected by increased humidity of air around the leaves.

1. decreases as less lost by leaves/by transpiration

2. high humidity due to decreased diffusion/water potential gradient

Weeds can be killed by watering the soil around the weed with a concentrated salt solution. Explain in terms of water potential how the weed is killed.

1. water potential decreases /becomes more negative in soil solution /outside root

2. water unable to enter root (cells)/root(cells) lose water to soil

The four species had similar numbers of stomata per mm2 per leaf surface.

Explain two adaptations of leaf structure which could account for the differences in the amount of water lost by these plants.

1. stomata sunken in pits creates local humidity/ decreases exposure to air currents

2. presence of hairs creates local humidity next to leaf/ decreases exposure to air currents

3. stomata mainly located on underside of leaf so less exposed to air currents/ heat from sun

4. stomata close midrib so more sheltered from air currents

5. stomata close together so diffusion shells overlap

6. thick waxy cuticle makes more waterproof impermeable to water

7. double palisade layer increases diffusion distance

8. stomata on inside of rolled leaf creates local humidity/ decreases exposure to air currents

Another method of investigating the rate of transpiration involves the regular weighing of detached leaves over a period of time. The graph shows the results of such an investigation.

Describe and explain the change in mass of the leaves over the period of this investigation

1. rapid loss then decreases

2. loss in mass as water loss is not replaced

3. initially stomata are open / later the stomata close

4. higher water potential of leaf cells compared with atmosphere / diffusion of water from leaf to atmosphere

5. water potential of leaf reduced / diffusion gradient decreased

Describe how the structure of xylem is related to its function.

Vessels

1. Have no end walls / hollow / no cytoplasm;

Allows unrestricted flow of water

2. Lignification; Provides support / strength / impermeability

3. Pits allow lateral transport

4. Tracheids with porous end walls.

Explain the change in mass of the leaves over the period of this investigation.

1. (Loss in mass as) water is not replaced

2. Initially stomata are open / later stomata close

3. Higher water potential of leaf / diffusion of water from leaf to atmosphere

4. Water potential of leaf reduced / diffusion gradient decreased

All water passes through the endodermis by the same pathway.

Explain what causes this.

1. Casparian strip / waterproof walls

2. so water must go through cytoplasm / vacuole / symplast

Describe and explain how water moves through the trunk of a tree to the leaves

1. In xylem

2. evaporation / transpiration from leaves through stomata

3. cohesion of water molecules

4. leaf cells have more negative water potential, so water enters from xylem

5. water drawn up as column/continuous stream

6. adhesion of water to walls

7. capillarity due to narrow lumen of xylem (vessels)

8. lignified walls keep xylem (vessels) open

9. root pressure forces (some) water up

Describe and explain the relationship between humidity and transpiration rate.

1. increased humidity leads to decreased transpiration

2. high humidity means more water in the air / increased saturation / increased water potential

3. reduced diffusion gradient / water potential gradient

4. slower rate of water loss / less evaporation

Explain how the structure of the endodermis affects the passage of water by this pathway.

1. Casparian bands

2. which are impermeable/waterproof

3. lower water potential in the cytoplasm of endodermis cell

4. enters symplastic pathway / cytoplasm of cell

5. by osmosis;

Describe and explain the changes in rate of flow of water in the branch over the 24 hour period

1. rate of flow increases to max at 1200 and then decreases

2. increasing transpiration/evaporation from leaves

3. transpiration creates tension / increases transpirational pull

4. water molecules are cohesive/stick together

5. produces a water column

Explain why the diameter of the branch decreased during the first 12 hours.

1. (increase transpiration) produce a higher tension / reduces the pressure in the xylem reducing the diameter

2. adhesive forces between xylem and water;

A stem was cut from a transpiring plant. The cut end of the stem was put into a solution of picric acid, which kills plant cells. The transpiration stream continued.

Suggest an explanation for this observation.

1. water moves in dead cells / xylem is non-living tissue

2. the process is passive / no energy is needed

Describe how water is moved through a plant according to the cohesion-tension hypothesis.

1. water evaporates/transpires from leaves

2. reduces water potential in cell /water potential/osmotic gradient across

cells

3. water is drawn out of xylem

4. creates tension

5. cohesive forces between water molecules

6. water pulled up as a column

What is the advantage of using the same plant in every treatment?

same surface area of leaf / number of leaves / age/thickness of cuticle

Explain how each of the following is related to the function of xylem tissue.

(i) Xylem tissue contains hollow tubes.

(ii) Lignin is present in xylem cell walls

(i)

unrestricted/free/quick/easy water flow/continuous column / maintains transpiration stream

(ii)

resists tension in water (column) / provides support/strength / maintains column of water/adhesion / prevents water loss

Some xerophytic plants have sunken stomata.

Explain the advantage of this adaptation.

1. conserves water / reduces water loss / reduces transpiration / reduces evaporation

2. high humidity (in pit) / reduced water potential gradient / less water blown

away / increased diffusion pathway;

Explain how xylem tissue is adapted for its function.

1. Long cells / tubes with no end walls

2. continuous water columns

3. no cytoplasm / no organelles/named organelle

4. to impede/obstruct flow / allows easier water flow

5. thickening/lignin

6. support / withstand tension / waterproof / keeps water in cells

7. pits in walls

8. allow lateral movement / get round blocked vessels

Explain the increase in the flow rate between 1000 and 1400 hours.

1. increase in light (intensity) increases transpiration rate/evaporation

2. greater stomatal aperture / more stomata open

3. increase in flow rate due to cohesion/attraction of water molecule

The diameter of the trunk decreased during the same period, reaching its minimum when the flow rate was highest.

Use your knowledge of the cohesion-tension theory to suggest an explanation for this decrease.

1. adhesion/attraction of water molecules to walls of xylem

2. results in tension as water pulled up stem

3. pulling in walls