Lectuer 12: Plant Transport Mechanism

Question 1

Explain how the following are transported in plants:

• H2O
• carbs
• nutrients in seed

Answer 1

• water is transported from root to shoot
• carbs: during photosynthesis from leaf to other tissues (down)
• in young plants: from storage site (roots/seeds) to shoot (upwards)

Question 2

In the xylem and phloem active or passive transport?

Answer 2

• xylem: passive mostly

- phloem: active or passive; depends on with or against gradient

Question 3

Explain what governs water entering and exiting the cell

Answer 3

• direction of h2o depends on pressure potential
• H2O pressure potential is impacted by 2 things: turgor pressure and wall pressure
• turgor pressure: expanding volume of cell pushes the membrane against cell wall
• at equilibrium: osmotic pressure is equal to turgor pressure

Question 4

Answer the following:

• where is water potential highest and lowest
• where does water enter and exit in a plant?
• what drives the transportation of water in a plant (3)?

Answer 4

• highest: roots; lowest: leaves
• enter: roots; exits: leaf through evaporation
• transport is driven by cohesion tension forces, root pressure, and capillary action

Question 5

Explain the 6 steps of cohesion tension theory

• concept: is energy expended?
• how is tension created?

Answer 5

1. Water vapour diffuses out of the leaf
2. Water evaporates inside leaf
3. Water is pulled out of xylem
4. Water is pulled up xylem
5. Water is pulled out of root cortex
6. Water is pulled from soil into root
   - no energy is expended
   - transpiration in the leaves creates tension that pulls water into the roots

Question 6

In roots, water needs to travel through various tissues (3):

Answer 6

• epidermis
• cortex
• into the vascular tissue

Question 7

define: casparian strip

Answer 7

• wax coated (suberin; hydrophobic compound) strip that blocks the route to xylem (esp for the apoplastic route)

Question 8

There are 3 different ways water can enter a plant from the root hair to the xylem. Explain each.

Answer 8

• symplastic route: via plasmodesmata
• transmembrane route: via water channels
• apoplastic route: within porous cell walls; can be blocked by Casparian strip and water must find an alternative route; all ions in soil enter through this route

Question 9

Why have leaves adapted to wilt?

Answer 9

When water is not replaced fast enough the turgor pressure sinks and leaves wilt. To prevent more water loss, stomata close to limit CO2 intake and photosynthesis.

Question 10

Gives some examples of how plants have adapted to dry conditions (2)

Answer 10

• trichomes (aka epidermal hairs) slow down water loss

- cactus/CAM/C4 plants open stomata at night to reduce h2o loss

Question 11

How is sucrose translocated?

Answer 11

• via phloem from source to sink (up and down the plant)

Question 12

Does translocation occur along or against pressure gradient?

Answer 12

• along

Question 13

Where is sugar stored (3)?

Answer 13

• leaves, roots, seeds

Question 14

Explain the composition of the phloem and how phloem sap passes through cell tube elements

Answer 14

• composition: sieve tube elements and companion cells
• plasmodesmata connects the cytoplasm of the sieve tube elements and companion cells
• sap passes through pores in the wall between cell tube elements

Question 15

Explain how sugar is transported from source to sink (make sure you explain how the phloem and xylem is involved)

Answer 15

• see google doc

Question 16

When sucrose moves from the companion cell to the phloem is it active or passive transport

Answer 16

• passive; even though it involves channel proteins that form pores

Question 17

antiporters vs symporters

Answer 17

• both are known as cotransporters; don’t require atp
• antiporters: transport molecules in opposite directions
• symporters: transport molecules in the same direction

Question 18

How are root hairs important?

Answer 18

• expand surface area to facilitate uptake

Question 19

The absorption and concentration of nutrients by plants is _____

Answer 19

• active transport

Question 20

What establishes the electrochemical gradient in a plant

Answer 20

• proton pumps

Question 21

How do cations and anions enter the plant?

Answer 21

• cations: ion channels

- anions: cotransporters

Question 22

How does a proton pump differ from ion channels and cotransporters?

Answer 22

requires atp

Question 23

How are toxic ions kept out of the plant? (ion exclusion); give an example of a toxic ion

Answer 23

• toxic ions are passively excluded
• Na+ is necessary for plants but only in small amounts. plants must keep it out if there is a large amount present in soil

Question 24

How have some plants adapted to tolerate salts?

Answer 24

• ions can be moved into the vacuole

Question 25

Why is nitrogen an important element for plants? How is it acquired by plants?

Answer 25

• necessary for amino acids and nucleotides, etc
• absorbed from soil as NO3-, NH4+ (although much exists in the air, only bacteria can convert nitrogen from the air into soluble salt)
• some plants use symbiotic bacteria to acquire nitrogen (example: legumes use rhizobia)

Question 26

Give the equation of nitrogen fixation

Answer 26

• see google doc

Question 27

Explain the pink nodules on legumes; Explain the symbiotic relationship between legumes and rhizobia

Answer 27

- Roots of infected legumes form
nodules that contain bacteria
infected cells
- Pink due to leghemoglobin
(binds oxygen that would prevent reduction of N2)
Mutual benefits:
- Plants gain nitrogen
- Bacteria gain carbohydrates
and costs

Question 28

How is the process of rhizobia infection on legumes regulated?

Answer 28

• plant secretes flavonoid to attract rhizobia
• Bacteria detect flavonoid
  and produce a
  nodule formation factor
• Nod factor initiates
  infection, cell proliferation,
  and nodule formation
• nod factor: lipopolysaccharide that binds to proteins on root cell membrane
• Bacteria live off nutrients provided by host cells,
  produce NH4+ for use by plant

Question 29

explain the symbiotic relationship between funghi and plants; what are the mutual benefits?

Answer 29

• mycorrhizal fungi digest decaying organic material and provide nutrients to plants (amino acids, phosphate, etc but no energy rich compounds)
• benefits: plants gain nitrogen and phosphate; funghi gain carbohydrates