3.4.10

Investigating Plant Transport

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Investigating Mass Transport in Plants

Mass transport can be investigated in two ways.

Ringing

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

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

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

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

  • 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

Living phloem cells

  • Phloem cells are living and xylem cells are non-living.
  • The purpose of phloem cells being living is unclear.
Jump to other topics
1

Biological Molecules

2

Cells

3

Substance Exchange

4

Genetic Information & Variation

5

Energy Transfers (A2 only)

6

Responding to Change (A2 only)

7

Genetics & Ecosystems (A2 only)

8

The Control of Gene Expression (A2 only)

9

Mathematical Skills

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