7.8 - Transport Of Organic Substances In The Phloem

Question 1

What is translocation?

Answer 1

The process by which organic molecules and some mineral ions are transported from one part of the plant to another

Question 2

What are sources?

Answer 2

Having produced sugars during photosynthesis, the plants that transports them from the site of production

Question 3

What are sinks?

Answer 3

The place where sugar is transported to where they will be used directly or stored for future use

Question 4

Does the phloem flow in one direction?

Answer 4

No it can go in both directions

Question 5

What is the theory of translocation?

Answer 5

Mass flow theory

Question 6

What are the 3 phases of the mass flow theory?

Answer 6

1. Transfer of sucrose into sieve elements from photosynthesis tissue
2. Mass flow of sucrose through sieve tube elements
3. Transfer of sucrose from the sieve tube elements into storage or other sink cells

Question 7

Why cant substances be transported by diffusion in the phloem?

Answer 7

The rate of movement is too fast

Question 8

What are sieve tube elements?

Answer 8

Living cells that form the tube for transporting solutes

Question 9

What is the role of companion cells?

Answer 9

Releases energy for active loading/transport of solutes

Question 10

Translocation moves solutes from ____to____

Answer 10

Sources, sinks

Question 11

How do enzymes maintain a concentration gradient between source and sink in the phloem?

Answer 11

They change the solutes at the sink (break them down/store them) this lowers the concentration at the sink

Question 12

Mass flow hypothesis: describe how sucrose is transferred into sieve tube elements from photosynthesising and companion cells

Answer 12

• facilitated diffusion from photosynthesising into companion cells
• H+ ions actively transported from companion cells, into spaces within cell walls

Question 13

Mass flow hypothesis: Describe the mass flow of sucrose through sieve tube elements

Answer 13

• sucrose from the source end in the sieve tubes decrease there water potential
• this causes water to move into sieve tubes from the xylem (at the source end of the tube) increasing hydrostatic pressure
• at the sink cells sucrose is broken down by enzymes for storage therefore these cells have a low sucrose content, to sucrose is actively transported from sieve tubes to sink cells
• this lowers the seive tubes water potential, and water moves out of the sieve tubes
• hydrostatic pressure at the sink end is low, and high at the source

Question 14

State how sucrose is transfered from sieve tubes to elements into storage or other sink cells

Answer 14

Sucrose is actively transported by companion cells from sieve tubes into sink cells

Question 15

What is the evidence supporting mass flow hypothesis?

Answer 15

• pressure in sieve tubes shown by sap they are pierced by aphids
• concentration of sucrose higher in leaves (source) than at sink
• downward flow in phloem occurs in daylight, but not at night
• increase in sucrose levels in leaf are often followed by an increase in levels in the phloem

Question 16

What is evidence against mass flow hypothesis (be able to evaluate this)

Answer 16

• function of sieve plates unclear, as they would hinder mass flow
• not all solutes move at same sped - they should by mass flow
• sucrose is delivers at the same rate to all regions, rather than going to areas with lower sucrose concentration faster - as mass flow would suggest

Question 17

How are sieve cells adapted for mass transport?

Answer 17

• large vacuole/ few organelles/ little cytoplasm/ cytoplasm at edge/ more room/ hollow
• easier flow

Question 18

Why do companion cells have many mitochondria?

Answer 18

To release energy for active loading

Question 19

Name two types of experiments for investigation transport in plants?

Answer 19

• ringing experiments
• tracer experiments