Transport in Plants (xylem and phloem)

Question 1

What is transpiration?

Answer 1

The evaporation of water from the leaves of the plants (closely related to how water is moved throughout the plant)

Question 1

Describe how water moves out of a plant…

Answer 1

• In spongy mesophyll cells, the liquid water evaporates from a higher water potential in the cell to the air spaces in the leaf
• This water vapour moves out of the leaf by diffusion (down a water vapour potential gradient)

Question 2

What are 3 features of the xylem?

Answer 2

• walls lined with lignin
• no end walls
• vessel elements

Question 3

What piece of apparatus could be used to investigate transpiration?

Answer 3

potometer

Question 4

What word describes how the H atoms in water interact with the sides of the xylem vessel?

Answer 4

adhesion

Question 5

Why do xerophytes have smaller leaves?

Answer 5

it reduces the surface area for water to evaporate off of the leaves

Question 6

Outline the cohesion-tension theory

Answer 6

Tension occurs between the water molecules.
Water molecules can form hydrogen bonds, they are cohesive
therefore, they are pulled up as a single column of water

Question 7

What makes up the phloem vessels?

Answer 7

companion cells and sieve tube elements

Question 8

What is translocation?

Answer 8

an energy requiring process which transports sugars from the source cells to the sink cells

Question 9

What are plasmodesmata?

Answer 9

gaps between cell walls which allow flow of substances between cells

Question 10

Describe how sucrose enters companion cells

Answer 10

• H+ ions are actively transported into the source cell, by a H+ ion pump, they then move back into the companion cell in co-transport proteins, carrying with them sucrose

Question 11

How does sucrose enter the sieve tube element?

Answer 11

• simple diffusion of sucrose molecules through the plasmodesmata

Question 12

How does increased concentration of sucrose impact the sieve tube elements?

Answer 12

• decreases water potential
• water moves from the xylem into the sieve tube element by osmosis
• this increases hydrostatic pressure

Question 13

What happens at sink cells?

Answer 13

• sucrose will move into companion cells and sink cells by diffusion or active transport
• this increases water potential, and causes water to move back into the xylem by osmosis
• here it is either stored, or converted back into glucose for respiration

Question 14

How does the release of sap when the phloem is cut provide evidence for mass transport?

Answer 14

it shows that the phloem has high hydrostatic pressure

Question 15

What evidence is there against mass transport?

Answer 15

• sieve tube elements; no clear purpose, could hinder
• not all solutes move at same speed
• sucrose is delivered at similar speeds to all regions

Question 16

What is ringing?

Answer 16

the bark and phloem of a tree are removed leaving just the xylem in the centre.
Overtime the tissues above the missing ring swell with sucrose solution and the tissue below dies

Question 17

What is tracing?

Answer 17

sucrose from plants grown in an environment with radioactive CO2 can be observed moving through the plant

Question 18

Describe the location of xylem and phloem in the tree trunk

Answer 18

Phloem just under the bark, xylem inside this

Question 19

Describe results of a ringing experiment

Answer 19

A bulge containing sugars appears above the ring

Question 20

Describe the conclusions from the ringing experiment

Answer 20

Sugars are transported in the phloem (when phloem is removed sugars accumulate above, tissues below the ring die)

Question 21

Describe how radioactive tracers can be used to locate sugars in plants

Answer 21

Plant grown in radioactive carbon, incorporated into sugars produced in photosynthesis, radiography can locate the position of the sugars in the plant

Question 22

How does water move up the xylem?

Answer 22

• water evaporates from leaves
• water potential gradient causes tension
• cohesion between water molecules maintains a column
• cohesion occurs as water molecules are polar

Question 23

How could water loss be reduced by controlling environmental conditions?

Answer 23

• reduced light intensity
• so stomata close
• reduces transpiration