Investigating Plant Transport
Biology: AQA A Level, topic 3.4.10
Investigating Mass Transport in Plants
Mass transport can be investigated in two ways.
Ringing
- Ringing can be used to investigate mass transport in the phloem.
- Phloem vessels are located outside the xylem vessels in the stem of a plant.
- If a ring is cut around the outside of a stem, this halts transport in the phloem but allows transport in the xylem to continue.
- Eventually, the tissue above the ring begins to swell and growth stops below the ring because sucrose cannot be transported.
Tracing
- Sucrose produced during photosynthesis can be tracked by exposing the leaves of a plant to carbon dioxide containing radioactive carbon.
- The plant is frozen very quickly in liquid nitrogen and placed onto photographic film.
- The pathway travelled by the radioactive sucrose down the phloem can be traced.
Evidence for mass transport
- Tracing and ringing experiments provide evidence for the mass flow theory.
- Both types of investigation show a causal link between the phloem and sucrose transport.
Evidence Against Mass Flow
Although ringing and tracing provide evidence for the mass flow hypothesis, there are some anomalies.
Direction of flow
- Sap can move up or down the phloem vessels.
- The hydrostatic pressure gradient that is set up by water entering the source and leaving at the sink does not explain how sap can move in both directions.
Sieve plates
- Sieve plates are the pores between sieve tube elements.
- Increased hydrostatic pressure is required for water to flow through the pores.
- This means that sieve plates should present a problem for the mass flow.
Living phloem cells
- Phloem cells are living and xylem cells are non-living.
- The purpose of phloem cells being living is unclear.
Test yourself on Investigating Plant Transport
Which of the following statements about ringing are true?
What substance are plants exposed to when using the tracing technique?
Which of these is NOT evidence against mass flow?