7.9 Investigating transport in plants

Question 1

What are the two experiments the investigate transport in plants

Answer 1

Ringing experiments

Tracer experiments

Question 2

Where is the phloem found in woody stems

Answer 2

There is a layer of bark around the outside, and then a layer of phloem extends all around the them, with the xylem in the middle

Question 3

Describe what happens in the ringing experiment

Answer 3

. Section of outer layers (phloem and protective layer) are removed around the complete circumference of a woody stem, while it is still attached to to the rest of the plant

. After a while, the region of the stem above the missing tissue will start swelling
.Samples of this accumulating liquid are found to be rich in sugars etc
.Meanwhile, in the region below the ring (roots) wither and die whilst above the ring they keep growing

Question 4

What is the conclusion to the ringing experiment

Answer 4

. The phloem must be the tissue responsible for translocating sugars in plants, rather than the xylem

. This is because the ring of tissue removed didn’t disrupt the xylem so water was still travelling up the stem

. However, the phloem being cut means that the sugars accumulating above the ring couldn’t reach the area with roots so the roots die
Meanwhile above the ring there was still sugar and water so the leaves kept growing

Question 5

Describe the tracer experiment

Answer 5

Using radioactive isotopes to trace movement of substances in plants

Eg isotope 14C can make radioactively labelled 14CO2

If the plant is then grown in an atmosphere containing this 14CO2, the 14C isotope can be incorporated into the sugars produced by photosynthesis

Then trace these sugars as they move within the plant using autoradiography

Question 6

How can you trace the movements of sugars around the plant using autoradiography

Answer 6

. Take thin cross sections of plant stem and place on a piece of x ray film

The film becomes blackened where it has been exposed to the radiation produced by 14C in the sugars

These blackened regions correspond to where the phloem tissue is in the stem, as other tissues don’t blacken the film, meaning they don’t carry sugars, so phloem alone is responsible for translocation

Question 7

Give 2 more pieces of evidence for translocation or organic molecules

Answer 7

. When phloem is cut, a solution of organic molecules flow out

. Aphids are insects that feed on plants from the phloem with needle like mouths
Therefore the contents from the sieve tube they extract can show daily variations in sucrose content of leaves