7.9- INVESTIGATING TRANSPORT IN PLANTS

Question 1

What do woody stems have outside?

Answer 1

outer protective layer of bark on inside of which is layer of phloem that extends all around stem

Question 2

What is inside the phloem layer?

Answer 2

xylem

Question 3

What is done at the start of the ringing experiment?

Answer 3

section of outer layer (protective layer + phloem) removed around complete circumference of woody stem while it’s still attached to rest of plant

Question 4

What happens after section of outer layer (protective layer + phloem) removed around complete circumference of woody stem while it’s still attached to rest of plant ? (ringing experiment)

Answer 4

after a period of time, region of stem immediately above missing ring of tissue seen to swell

Question 5

What are samples of the liquid that has accumulated in the swollen region found to be rich in? (ringing experiment)

Answer 5

found to be rich in sugars + other dissolved organic substances

Question 6

What happens to some non-photosynthetic tissue in the region below the ring (towards roots) and what about the ones above? (ringing experiment)

Answer 6

found to wither + die, while those above ring continue to grow

Question 7

What has the ringing experiment suggested a about removing the phloem around the stem leading to? (2)

Answer 7

sugars of phloem accumulating above ring, leading to swelling in this region

interruption of flow of sugars to region below ring + death of tissues in this region

Question 8

What is the conclusion from the ringing experiment?

Answer 8

phloem, rather than xylem, is tissue responsible for translocating sugars in plants

as ring of tissue removed had not extended into xylem, its continuity has not been broken

if it were tissue responsible for translocating sugars, would not have expected sugars to accumulate above ring nor tissues below it to die

Question 9

What are useful for tracing the movement of substances in plants? (tracer experiments)

Answer 9

radioactive isotopes

Question 10

What can isotope 14C be used to make? (tracer experiments)

Answer 10

used to make radioactively labelled carbon dioxide (14CO2)

Question 11

What will happen is a plant is grown in an atmosphere containing 14CO2? (tracer experiments)

Answer 11

14C isotope will be incorporated into sugars produced during photosynthesis

Question 12

What can be done to the radioactive sugars of plants? (tracer experiments)

Answer 12

can then be traced as they move within plant using autoradiography

Question 13

What does tracing the radioactive sugars in plants using autoradiography involve? (tracer experiments)

Answer 13

taking thin cross-sections of plant stem + placing them on piece of X-ray film

Question 14

What happens to the film (autoradiography) where it has been exposed to radiation produced by 14C in sugars? (tracer experiments)

Answer 14

becomes blackened

Question 15

What are the blackened regions of the film (autoradiography) found to correspond to? (tracer experiments)

Answer 15

correspond to where phloem tissue is in stem

Question 16

As other tissues other than phloem do not blacken the film (autoradiography) what follows? (tracer experiments)

Answer 16

follows they don’t carry sugars + phloem alone responsible for their translocation

Question 17

Evidence that translocation of organic molecules occur in phloem: what happens when phloem cut?

Answer 17

when phloem cut, solution of organic molecules flow out

Question 18

Evidence that translocation of organic molecules occur in phloem: what can plants provided with radioactive carbon dioxide be shown to have?

Answer 18

plants provided with radioactive carbon dioxide can be shown to have radioactively labelled carbon in phloem after a short time

Question 19

Evidence that translocation of organic molecules occur in phloem: what are aphids? what do aphids have + what they do?

Answer 19

aphids are type of insect that feed on plants
have needle-like mouthparts which penetrate phloem
so can be used to extract contents of sieve tubes
contents show daily variation in sucrose contents of leaves that’re mirrored a little later by identical changes in sucrose content of phloem

Question 20

Evidence that translocation of organic molecules occur in phloem: what happens when the ring of phloem removed around the whole circumference of stem?

Answer 20

removal of ring of phloem from around whole circumference of stem leads to accumulation of sugars above ring + their disappearance from below it