Chapter 9: Transport in Plants

Question 1

Why is lignin essential to wall of xylem?

Answer 1

• Provides strength + support.
• Prevents vessel from collapsing by keeping it open.
• Transpiration produces a tension/negative pressure.
• Adhesion between water molecules and xylem wall.
• Limit lateral flow of water.
• Waterproofing.
• So cell dies/contents decay.
• To create a hollow tube/continuous column.

Question 2

What is the symplast?

Answer 2

• Continuous cytoplasm of living plant cells connected by plasmodesmata.

Question 3

What is the apoplast?

Answer 3

• Cell walls + intracellular spaces.

Question 4

Explain route of water in apoplast pathway.

Answer 4

1. Water moves through cellulose cell walls by cohesive forces between water molecules +as a result of the transpiration pull up the xylem.
2. Moves into symplast pathway in endodermis due to the Casparian strip.
3. Needs active pumping of ions into xylem followed by osmosis of water before water moves back into the apoplast.

Question 5

Explain route of water in symplast pathway.

Answer 5

1. Relies on osmosis as water moves through cell membranes and cytoplasm.
2. Water moves into RHC from soil by osmosis, raising WP or this RHC compared to next cell along.
3. Once again water moves to next RHC by osmosis.
4. Active transport of ions needed to move water from endodermis into xylem by osmosis.

Question 6

Define transpiration.

Answer 6

• Loss of water vapour from leaves by evaporation from mesophyll and diffusion through stomata.

Question 7

Define transpiration stream.

Answer 7

• Movement of water molecules up xylem vessels from roots to leaves/air surrounding leaves.

Question 8

Define transpiration pull.

Answer 8

• Pull of column of water molecules up xylem held by cohesive forces between water molecules as a result of evaporation from mesophyll + diffusion through stomata.
• Replace water lost in evaporation or diffusion out of stomata.

Question 9

Explain the cohesion-tension theory.

Answer 9

1. Water moves into xylem by osmosis down water vapour potential gradient.
2. Root pressure at bottom of xylem.
3. Water vapour loss/evaporation at top of plant/leaves –> lowers WP.
4. Creates low HP at top of xylem.
5. Water under tension –> column of water pulled up in a continuous stream –> replace water lost by evaporation.
6. Cohesion between water molecules.
7. Adhesion of water molecules to carbohydrates in vessel walls.
8. Water exhibits capillary action.
9. Water moves up xylem by mass flow from high HP to low HP.

Question 10

Define translocation.

Answer 10

• The movements of the products of photosynthesis (assimilates) for where they are produced/stored (source) to where they are needed (sink).

Question 11

Outline translocation via the apoplast route (active route).

Answer 11

1. Phloem utilises companion cells + sieve tube elements.
2. Companion cells actively pump H+ ions out of cell using ATP.
3. H+ ions move back into companion cells down conc. gradient using a co-transport protein.
4. Co-transport protein –> shuttles sucrose back into cell down conc. gradient via facilitated diffusion.
5. From the companion cell, the assimilates move into the phloem by diffusion.

Question 12

Outline translocation via the symplast route (mass flow).

Answer 12

1. Sucrose in sieve tubes.
2. Assimilates/Sucrose enters phloem/sieve tubes at source by diffusion through plasmodesmata –> lowering WP inside sieve tube.
3. Water moves into phloem/sieve tube by osmosis –> increasing HP.
4. Assimilates/sucrose leave phloem/sieve tubes at sink by diffusion through plasmodesmata –> increasing WP inside sieve tube.
5. Water leaves phloem/sieve tube by osmosis down WP gradient –> decreasing HP.
6. Assimilates move from high HP to low HP down pressure gradient.

Question 13

Adaptations of companion cells for exchange?

Answer 13

• Infolding in cell membrane –> increase s.a. to vol ratio –> more rapid active transport + osmosis.
• Mitochondria –> provide ATP for active transport of H+ ions.
• Co-transport proteins.
• Ribosomes.
• Plasmodesmata.

Question 14

What are the various adaptations of xerophytes to reduce water loss?

Answer 14

• Thick waxy cuticle –> reduce transpiration + relatively impermeable + waterproof.
• Sunken stomata –> microclimate of still, humid air that decreases water vapour potential gradient + traps warm, moist air –> reduce water loss by transpiration..
• Reduced no. of stomata –> less diffusion out of stomata + reduce transpiration.
• Curled leaves.
• Reduced no. of leaves –> reduce s.a. to vol. –> reduce water loss by transpiration.
• Hairy leaves.
• Avoiding the problem.
• Long widespread roots.
• More stomata on the lower surface of the plant.
• Leaf loss.
• Succulents –> use water in dry conditions + store water when in plentiful supply.

Question 15

Various adaptations of hydrophytes to their environment?

Answer 15

• Very thin or no waxy cuticle.
• Wide flat leaves on water surface.
• Arenchyma –> many air spaces –> makes leaves + stems more buoyant.
• Small roots.
• Large s.a. of roots + stems underwater.
• Many always open stomata.
• Reduced structure to plant.
• Air sacs –> enable flotation + prevent water logging.

Question 16

Define vascular bundle.

Answer 16

• Transport system in plants.

- Composed of xylem + phloem.

Question 17

Describe + explain what happens when a ring of bark is removed from a tree.

Answer 17

• Bulge above the ring.
• Translocation past ring prevented.
• Sucrose accumulates.
• Lowers WP.
• Water moves into area by osmosis.