7.9 Investigating transport in plants

Question 1

Describe the structure of a woody-stemmed plant.

Answer 1

• a protective layer of bark
• within this is the phloem, spanning the whole circumference of the stem
• within that is the xylem

Question 2

Describe the process of a ringing experiment.

Answer 2

1) a section of bark and phloem are removed while the rest is still attached
2) the region directly above swells and becomes rich in sugars and dissolved organic substances
3) non-photosynthetic layers below wither and die

Question 3

What does the process suggest happens when the phloem around the stem was removed?

Answer 3

• phloem sugars above the stem accumulated

- interruption of sugar flow below the ring, killing those tissues

Question 4

What is the conclusion of this?

Answer 4

the phloem is the tissue that transports sugars, because the xylem wasn’t broken and if it were you would not expect sugar to build up above the ring or tissue to die below it.

Question 5

Describe a tracer experiment.

Answer 5

• radioactive isotope 14C can be used to make radioactively labelled CO2 (14CO2)
• plants grown in an atmosphere containing this will have 14C incorporated into their sugars from photoynthesis
• the sugars can then be traced as they move through the plant

Question 6

What evidence is there that transport of organic molecules occurs in the phloem?

Answer 6

• when phloem is cut, a solution of organic molecules flows out
• plants provided with radioactive CO2 have radioactively labelled carbon in the phloem shortly after
• Aphid’s needle mouths penetrate the phloem at certain times which shows daily variation in leaf sucrose levels, shortly followed by phloem sucrose levels
• removal of a ring of phloem leads to an accumulation of sugars above the ring.