Mass Transport in Plants - Topic 3

biology paper 1

Phloem

Transports organic substances (mainly sucrose and amino acids) throughout the plant - from sources to sinks

Sieve tube elements

Living cells joined end to end to form long tubes

No nucleus

Transport of solutes through the plants

Companion cells

One per sieve tube element, have nucleus and mitochondria

Provide metabolic support to sieve tube elements for active transport of solutes

Plasmodesmata 

Connects sieve tube to companion cells

Translocation

Movements of organic solvents (assimilates) such as sucrose from sources to sinks

Source

Area of production or release of sucrose e.g. leaves

Sink

Area of utilisation or storage of glucose e.g. roots

Stage 1 mass flow hypothesis

Photosynthesis in leaves produces glucose → converted to sucrose

Sucrose actively transported into companion cells from mesophyll via co-transport with H+ ions

• H⁺ ions actively pumped out of companion cells → high H⁺ outside.

• H⁺ ions re-enter via a co-transport protein with sucrose.

1. Sucrose diffuses from companion cells → sieve tube elements through plasmodesmata.

➡ This lowers water potential in sieve tubes.

Stage 2 mass flow hypothesis

• Water moves by osmosis from xylem (higher Ψ) into sieve tubes (lower Ψ).

• Creates high hydrostatic (turgor) pressure at the source end.

Stage 3 mass flow hypothesis

1. Sucrose actively transported out of phloem into sink cells (e.g. roots).

2. Sucrose used in respiration or stored as starch.

3. Removal of solute increases water potential in phloem → water moves out back into xylem.

➡ This reduces hydrostatic pressure at the sink end.

Stage 4 mass flow hypothesis

• A pressure gradient is established between source (high pressure) and sink (low pressure).

• Causes mass flow of phloem sap from source → sink.

Evidence for mass flow hypothesis 

1. Radioactive tracer (¹⁴C) studies: show movement of labelled sugars along phloem.
✅ 2. Aphid stylet experiments: show positive pressure in phloem and flow of sap.
✅ 3. Concentration gradient: sucrose concentration higher in source than sink.
✅ 4. Metabolic poisons or lack of ATP inhibit translocation — indicates active process.

Evidence against mass flow hypothesis

❌ Not all substances move at the same rate — some move faster than others.
❌ Sucrose sometimes moves in opposite directions in adjacent sieve tubes.
❌ The sieve plates may actually hinder mass flow

Ringing experiments aim

To show that sugars are transported in the phloem

Ringing experiments method

• Remove a ring of bark (including phloem) from around a woody stem.

• Xylem remains intact (so water transport unaffected).

• Leave for several days.

Ringing experiments observation 

• Swelling forms above the ring.

• Fluid above the ring has a high concentration of sugars.

• Tissues below the ring die (no supply of sugars).

Ringing experiments conclusion

• sugars transported in phloem, not xylem.

• Movement is downwards (from leaves to roots)