translocation and mass flow experiments

what is the function of the phloem tissues? (3)

• transport organic substances from the leaves (source)

• to the rest of the plant (sink)

• by the process of translocation

describe the structure of the phloem tissues (3)

• consists of sieve elements and companion cells

• sieve elements are joined end to end to form sieve tubes

• next to each sieve element is a companion cell with dense cytoplasm and many mitochondria

what is the function of companion cells? (1)

provides/produces ATP required for active transport of organic substances

what is the adaptation of sieve tubes? (1)

don’t contain any organelles to increase the area for mass flow

what is the source in translocation? (1)

the leaves

what is the sink in translocation? (1)

the roots / any actively growing or dividing tissue

describe the mass flow hypothesis for translocation (7)

1. in the source, sugars are actively transported into phloem

2. by companion cells

3. this lowers the water potential of the sieve tube

4. water enters by osmosis

5. this creates a high turgor pressure which causes mass movement towards roots

6. in the sink, sugars are being used up in respiration

7. which lowers the turgor pressure and the cycle continues

describe the process of translocation (6)

1. photosynthesis occurring in chloroplasts of leaves creates organic substances (e.g. sucrose)

2. sucrose is actively transported into the sieve tube element by companion cells

3. the increase of sucrose in the sieve tube decreases the water potential

4. water enters the sieve tube elements from the surrounding xylem by osmosis

5. the increase in water volume in the sieve tube element increases the hydrostatic pressure

6. this causes the liquid to be forced towards the sink

what is the ringing experiment used for? (1)

to show the transport in plants via the phloem

describe the use of ringing experiment to show transport in plants via the phloem (2)

1. involves removing a complete ring of phloem around the stem

2. preventing transport through the phloem at this point

what does swelling in the ringing experiment indicate? (3)

• a build up of photosynthetic products from the leaf

• which were prevented from being transported past the ring

• due to the removal of the phloem

describe how you would use tracers to show the transport in plants via both the phloem and xylem (2)

• radioactive substances are supplied to the leaf of a plant

• after a while, you can use autoradiography to detect where the radioactive substances have moved

what radioactive substance is used to show transport in plants via xylem vessels? (1)

mineral ions

• e.g. Na⁺

what radioactive substance is used to show transport in plants via phloem? (1)

carbon dioxide

how can you determine where the radioactive substances have moved? (2)

• take thin horizontal sections of plant tissues/stem

• carry out autoradiography by placing against photographic film in a dark room for several hours

what does autoradiography reveal about the action of phloem in plants? (2)

• photosynthetic products are present in growing regions

• which indicates translocation through the phloem

what are some evidences that support the mass flow hypothesis? (4)

• cutting the stem of a plant results in phloem sap being released

• indicating hydrostatic pressure in the sieve tubes

• lowering the temperature or the use of respiratory inhibitors reduces the rate of translocation

• indicating an active transport mechanism is involved

what are some evidences that are against the mass flow hypothesis? (4)

• not all solutes move at the same speed

• they should do if it is mass flow

• in young phloem tissue, substances have been observed moving in opposite directions

• in the same sieve tube