Mass Transport In Plants

Question 1

What are the adaptations of xylem vessels?

Answer 1

Spirals of lignin (rigid polymer)
Dead cells
Hollow/no organelles to obstruct flow of water
Continuous tube
Thick walled - he,ps to withstand cohesion tension

Question 2

Explain the cohesion-tension theory of water transport in the xylem?

Answer 2

• Transpiration - water evaporates from leaves via the open stomata (down a water potential gradient)
• Creates a negative pressure in the leaves which pulls water particles forward from the xylem
• Water particles are polar (slightly charged) and cohesive (hydrogen bonding)
• This “pull” or tension acts on the entire continuous column of water in the xylem
• Water is also adhesive and attracted to the xylem walls resulting in upward capillary action

Question 3

What is transpiration?

Answer 3

The evaporation of water from a plant leaves moving down a water potential graident

Question 4

What factors affect the rate of transpiration?

Answer 4

Light intensity
Temperature
Humidity
Wind

Question 5

How does light intensity affect the rate of transpiration?

Answer 5

The stomata open when it light to allow carbon dioxide to enter for photosynthesis causing INCREASED transpiration

Question 6

How does temperature affect the rate of transpiration?

Answer 6

The warmer the water molecules the more kinetic energy they have to evaporate, which increases the water potential gradient causing more water to diffuse out of the leaf

Question 7

How does humidity affect the rate of transpiration?

Answer 7

The higher the humidity the lower the rate of transpiration as there is a lower water potential gradient for water to move down

Question 8

How does wind affect the rate of transpiration?

Answer 8

Air movement blows away water molecules around the stomata which increases the water potential gradient and therefore the rate of transpiration

Question 9

How are root hair cells specialised for their function?

Answer 9

Large surface area for absorption
High density of carrier proteins and mitochondria for ATP to allow active transport

Question 10

What piece of apparatus do we use to estimate the rate of transpiration?

Answer 10

Potometer

Question 11

How does a potometer indicate photosynthesis is occurring?

Answer 11

The air bubble moves towards the plant

Question 12

Why does the air bubble in the potometer sometimes move away from the plant?

Answer 12

The seal is broken so gravity causes the water to to move back to the reservoir

Question 13

What can we tell about the plant if the air bubble in the potometer doesn’t move?

Answer 13

An air bubble has been trapped in the xylem creating an air lock or the plant is too cold/dead

Question 14

Why is it useful the bubble can be moved using the reservoir?

Answer 14

It helps repeat the experiment for a mean rate of transpiration

Question 15

Why is the stem of plant cut underwater at an angle?

Answer 15

Underwater ensures no air bubbles enter the xylem and cause air lock to prevent cohesion in the transpiration stream
At an angle or slant increases the surface area available for uptake

Question 16

Why should other air bubbles in the potometer by removed?

Answer 16

To maintain the full volume of water and allow for correct transpiration calculations

Question 17

What is translocation?

Answer 17

The movement of dissolved sucrose and amino acids in the phloem

Question 18

How do you calculate the rate of transpiration from a potometer?

Answer 18

Volume (Pi R2 L) divided by time
Distance bubble moved/time

Question 19

What are the adaptations of phloem cells?

Answer 19

Companion cells (metabolically active)
Sieve tube elements
Sieve plates with perforation
Plasmodesmata - allows free movement of substances between companion and sieve cells

Question 20

Explain the mass flow theory?

Answer 20

Sucrose is actively transported into the phloem using ATP made by companion cells
This lowers the water potential gradient of the sieve tube element causing water to move into the phloem by osmosis down a water potential gradient
This creates a high hydrostatic pressure at the source (turgid pressure)
At the sink active transport of sucrose out of sieve tube elements increases the water potential of sieve tube elements so water moves by osmosis into xylem creating low hydrostatic pressure
Water moves down the pressure gradient to the sink, where sucrose is stored as starch for growth

Question 21

What two experiments are used to produce evidence for the mass flow theory?

Answer 21

Ringing
Radioactive tracers

Question 22

How does the ringing experiment work to test the mass flow hypothesis?

Answer 22

Ringing experiments that cut a ring of bark (which includes the phloem) in photosynthesising plant. The result in a build up of sucrose above the ring cut and stem swelling yet an absence of sucrose below the ring.

Question 23

How do radioactive tracers work to provide evidence for the mass flow hypothesis?

Answer 23

Plants exposed to radioactively labelled CO2 show the presence of radioactive organic compounds present in the phloem as evidenced by exposure of photographic film over the transverse section of a stem.