Investigating Transport In Plants

Question 1

How is a ringing experiment used to investigate transport in plants?

Answer 1

• a ring of bark is removed from the tree deep enough to remove the phloem but not the xylem
• later the region of the stem above the missing ring starts swell as excess sugars are collected
• non photosynthetic tissues below the ring are found to wither and die while those above the ring continue to grow

Question 2

What has removing the phloem around the stem lead to?

Answer 2

• the sugars in the phloem accumulating above the ring leading to swelling
• the interruption of flow of sugars to the region below the ring and the death of tissues in this region

Question 3

What conclusion can be drawn from ringing experiments?

Answer 3

-it’s the phloem not the xylem tissue responsible for translocating sugars

Question 4

How are tracer experiments used to investigate transport in plants?

Answer 4

• radioactive isotopes are useful for tracing the movement of substances in plants
• the plant is grown in an atmosphere containing 14CO2 so 14C isotope will be incorporated into the sugars produced during photosynthesis
• the radioactive sugars an be traced as they move within the plant using autoradiography
• a thin cross section of the plant is taken
• the blackened regions are found to correspond to where the phloem tissue is in the stem showing this is where translocation takes place
• plants use carbon to photosynthesise and make sugars which means the carbon isotope with the radioactive tags allow you to use X-ray film to look at the area of the plant where sugars and therefore carbon isotopes are found
• the area where carbon would be shows up as black marks on X-ray paper

Question 5

Which evidence supports that translocation of organic molecules occurs in phloem?

Answer 5

• when the phloem is cut a solution of organic molecules flow out
• plants provided with radioactive carbon dioxide can be shown to have radioactively labelled carbon in phloem after a short time
• aphids extract the contents of sieve tubes which show daily variations in the sucrose content of leaves that are mirrored a little later by identical changes in the sucrose content of the phloem
• the removal of a ring of phloem from around the while circumference of a stem leads to the accumulation of sugars above the ring and their disappearance from below it

Question 6

What are the layers of the stem in trees?

Answer 6

• protective outer layer
• phloem
• xylem
• pith

Question 7

What are aphids?

Answer 7

• a type of insect that feeds on plants

- they have needle like mouthparts which penetrate the phloem and can be used to extract the contents of the sieve tubes