Mass Transport in Plants — Phloem

Phloem

Photosynthesis uses carbon dioxide and water to produce oxygen and glucose. This glucose needs to be transported around the plant so it can be used for respiration, growth or storage.

Phloem is a transport tissue responsible for transporting this glucose around the plant. The process know as translocation, involves the transport of organic substances (mainly sucrose and amino acids) from ‘sources’ to ‘sinks’.

What are ‘sources’ and ‘sinks’?

• Source — where organic substances are produced or released (e.g. leaves where photosynthesis occurs, or storage organs releasing stored sugars)
• Sink — where organic substances are used or stored (e.g. roots, developing fruits, growing tips)

Structure of Phloem

Phloem is made up of two main cell types:

Sieve tube elements:

• Living cells but have very little cytoplasm, no nucleus, few organelles
  • This maximises the space available for the flow of organic substances
• Sieve plates between adjacent sieve tube elements — these are perforated end walls with pores that allow the flow of sap from one element to the next

Companion cells:

• Closely associated with sieve tube elements, connected by many plasmodesmata
• Have a dense cytoplasm, a nucleus, and many mitochondria
• Provide metabolic support to the sieve tube elements (e.g. produce ATP for active loading of sucrose)

Mass Flow Hypothesis

How does the mass flow hypothesis work?

1. Active loading at the source — sucrose is actively loaded into the sieve tube elements at the source (e.g. leaf)

   • This involves active transport of hydrogen ions (H+) out of the companion cell using ATP
   • H+ ions then flow back into the companion cell via co-transport proteins, carrying sucrose with them
   • Sucrose then passes into the sieve tube element through plasmodesmata

2. Water enters from the xylem by osmosis — the high concentration of sucrose in the sieve tube lowers the water potential, so water enters from the surrounding xylem by osmosis.

3. This increases the hydrostatic pressure in the sieve tube elements (in the phloem) at the source end

4. Mass flow (to respiring cells) — the high hydrostatic pressure at the source and lower pressure at the sink creates a pressure gradient. This drives the flow of phloem sap (containing dissolved sucrose) from source to sink

5. Unloading at the sink — sucrose is unloaded from the phloem by active transport (e.g. for respiration or storage). This raises the water potential in the sieve tube, so water leaves by osmosis, lowering the hydrostatic pressure at the sink end

Animation

Evidence for Mass Flow

Radioactive tracers

• Plants are supplied with radioactive carbon dioxide (¹⁴CO₂)
• The plant uses this carbon dioxide in photosynthesis to produce radioactive sugars
• Autoradiography can then be used to track where these radioactive sugars travel in the plant
• Results show that radioactive sugars are found moving up and down the plant in the phloem, providing evidence that phloem transports sucrose from source to sink

Ringing experiments

• A ring of bark (containing the phloem) is removed from the stem of a woody plant, leaving the xylem intact
  • Above the ring: sugars accumulate and the tissue swells
  • Below the ring: sugars are depleted and the tissue eventually dies
• This provides evidence that sucrose is transported in the phloem (not the xylem)
• Water transport is unaffected because the xylem is still intact

Evidence Against Mass Flow

• Not all solutes move at the same rate — mass flow predicts bulk movement of phloem sap, so all dissolved substances should travel together at the same speed. The fact that different solutes move at different rates suggests an additional, selective mechanism is involved.
• Sieve plates may impede flow — sieve plates create resistance that would require a very large pressure gradient to overcome. The pressures measured in phloem are not always large enough to drive flow through sieve plates at the observed rates.
• Bidirectional transport — in some cases, substances can move in opposite directions within the same sieve tube simultaneously, which cannot be explained by a single pressure-driven mass flow.

Exam Questions