Experimental evidence including tracer and ringing experiments

Joe Wolfensohn

Teacher

Joe Wolfensohn

Recall Questions

This topic requires prior knowledge of transport in plants. You can test your knowledge on this below.

What is the cohesion-tension theory in the xylem?

It explains how water is pulled up the xylem due to transpiration creating tension in the xylem. Cohesion between water molecules keep the column of water unbroken.

How does sucrose enter the sieve tube elements of the phloem at the source?

It is actively transported into the sieve tube elements via the companion cells.

Why do companion cells have many mitochondria?

To provide the ATP required for the active transport of sucrose into the sieve tube elements.

Topic Explainer Video

Check out this @JoeDoesBiology video that explains experimental evidence including tracer and ringing experiments or read the full notes below. Once you've gone through the whole note, try out the practice questions!

Evidence for the Cohesion-Tension Theory (Xylem Transport)

1. Changes in Tree Trunk Diameter

  • During the day, transpiration pulls water up the xylem, creating tension in the xylem, making the tree trunk diameter smaller.

  • At night, transpiration slows down, reducing tension, and the trunk diameter expands slightly.

  • This supports the cohesion-tension theory, as water movement is linked to transpiration rates.

2. Broken Xylem and Air Intake

  • If a xylem vessel is broken, air enters, and the plant can no longer draw up water.

  • This shows that water transport relies on tension and cohesion, as an air break disrupts the continuous water column.

3. No Leakage When Xylem is Cut

  • If water movement was due to high pressure, cutting the stem should cause water to leak out.

  • Instead, air is drawn in, proving that water is being pulled up under negative pressure or tension.

Evidence for the Mass Flow Hypothesis (Phloem Transport)

1. Ringing Experiment

  • A ring of bark (containing phloem vessels but not xylem) is removed from a tree stem.

  • (Phloem vessels are found on the outer side of the stem, whereas xylem vessels are towards the middle).

  • Sucrose accumulates above the ring, causing swelling, while the area below the ring dies due to lack of sugars.

2. Radioactive Tracer Experiment (Carbon-14 Labelling)

  • A plant is exposed to radioactive carbon dioxide (¹⁴CO₂).

  • The plant uses the carbon-14 in photosynthesis to produce radioactively labelled sucrose.

  • Autoradiography (X-ray imaging) shows that radioactive sucrose moves through the phloem, confirming translocation occurs in the phloem and follows a source-to-sink pattern.

3. Aphid Stylet Experiment

  • Aphids (sap-sucking insects) pierce the phloem with their mouthparts.

  • The phloem sap continues to flow out, even when the aphid is removed, showing that translocation occurs under pressure.

  • This supports the mass flow hypothesis, as movement is due to hydrostatic pressure gradients in the phloem.

Evidence That Contradicts the Mass Flow Hypothesis

While the Mass Flow Hypothesis is the most widely accepted explanation for phloem transport, some evidence suggests that translocation is more complex than simple pressure-driven mass flow.

 

1. Different Solutes Move at Different Rates

  • According to mass flow, all solutes in the phloem should move at the same speed if transported by bulk flow.

  • However, different organic molecules (e.g., sucrose, amino acids) move at different rates, suggesting that other processes might be involved in regulating transport.

2. Bidirectional Flow in the Same Phloem Vessel

  • The mass flow hypothesis suggests all solutes move in one direction per sieve tube (from high to low pressure).

  • However, some experiments show that solutes can move in opposite directions in the same sieve tube element, which mass flow cannot explain.

  • This suggests that active processes or additional control mechanisms might influence translocation.

3. Phloem Transport is Faster Than Diffusion

  • The speed of solute movement in the phloem is faster than can be explained by passive mass flow alone.

  • This suggests that more active processes  may assist transport.

4. The Role of Sieve Plates

  • The perforated sieve plates between sieve tube elements should slow down flow, acting as a resistance barrier.

  • However, translocation still occurs efficiently, suggesting sieve plates may have a more active role in regulating flow, rather than just being passive channels.

Key Terms

  • Cohesion-Tension Theory: The explanation for water movement in the xylem due to transpiration, cohesion, and tension.

  • Mass Flow Hypothesis: The theory that sucrose moves in the phloem due to pressure differences between source and sink.

  • Ringing Experiment: A method that removes phloem but not xylem, proving sucrose transport occurs in the phloem.

  • Radioactive Tracer (Carbon-14): A technique using radioactively labelled CO₂ to track sucrose movement in the phloem.

  • Aphid Stylet Experiment: A method showing phloem sap flows under pressure, supporting the mass flow hypothesis.

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Exam Tip

You may have to evaluate experimental data linked to cohesion tension or mass flow. When you see the command word “evaluate” make sure that you have given evidence both for and against. You will likely be expected to get this information from tables and graph that have been provided but will also need to use your own knowledge.

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Phloem-sap-feeders are passive feeders, they do not need to use their jaw muscles to take up sap from phloem.

Explain why they can take up sap without using their jaw muscles. (3 marks)

1. Contents of the phloem vessel pushed into insect’s mouth by high pressure. (1)

2. High pressure in phloem vessel is caused by loading of sucrose into phloem. (1)

3. And the resulting entry of water by osmosis. (1)

Phloem pressure is reduced during the hottest part of the day. Use your understanding of transpiration and mass flow to explain why. (3 marks)

1. High rate of transpiration/evaporation during the hottest part of the day. (1)

2. More water is lost through the stomata. (1)

3. Less water movement from xylem to phloem. (resulting in lower phloem pressure) (1)

Practice Question

Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!