Question
Explain how food (sucrose) is transported through phloem using the pressure flow hypothesis. Why is phloem transport called translocation? How does it differ from xylem transport?
(NEET + CBSE Class 11)
Solution — Step by Step
Translocation is the movement of organic solutes (mainly sucrose) from the site of synthesis (source — usually leaves) to the site of use or storage (sink — roots, fruits, seeds, growing tips) through phloem sieve tubes.
Unlike xylem transport which is always upward, phloem transport can be bidirectional — upward to growing shoots or downward to roots, depending on where the sink is.
The mechanism works through osmotic pressure differences:
-
At the source (leaf): Sucrose is actively loaded into sieve tube elements by companion cells. This increases solute concentration, causing water to enter by osmosis from nearby xylem. Turgor pressure rises.
-
At the sink (root/fruit): Sucrose is actively unloaded from sieve tubes into sink cells. Solute concentration drops, water moves out to xylem. Turgor pressure drops.
-
The pressure gradient from high (source) to low (sink) drives bulk flow of phloem sap through sieve tubes.
| Feature | Xylem transport | Phloem transport |
|---|---|---|
| Substance | Water + minerals | Sucrose + amino acids |
| Direction | Upward only | Bidirectional (source to sink) |
| Driving force | Transpiration pull (passive) | Pressure flow (active at loading/unloading) |
| Cells | Dead tracheids and vessels | Living sieve tubes + companion cells |
| Energy | No ATP needed for flow | ATP needed for sucrose loading |
Pressure Flow Mechanism Flowchart
flowchart TD
A["Source — Leaf"] --> B["Active loading of sucrose into sieve tube"]
B --> C["High solute concentration → water enters from xylem by osmosis"]
C --> D["High turgor pressure at source end"]
D -->|"Bulk flow through sieve tubes"| E["Low turgor pressure at sink end"]
E --> F["Active unloading of sucrose at sink"]
F --> G["Sink — Root/Fruit/Seed"]
G --> H["Water returns to xylem"]
H -.->|"Water recycled"| CWhy This Works
The beauty of the pressure flow hypothesis is that it uses the plant’s own osmotic machinery. No special pump is needed — just a concentration difference between source and sink, maintained by active loading and unloading. The sieve tubes act as pipes, and osmotic pressure difference does the rest.
Evidence supporting this: if you cut phloem (ring bark a tree), sugary sap oozes from the upper cut — confirming positive pressure inside phloem. Aphids, which insert their stylets into sieve tubes, can be used to sample phloem sap — the pressure pushes the sap into the aphid.
Common Mistake
Students assume phloem transport is entirely passive like xylem. It is not — the bulk flow itself is passive (driven by pressure), but sucrose loading and unloading require ATP (active transport by companion cells). This makes phloem transport an active process overall. NEET tests this: “Is phloem transport active or passive?” The answer is active, because energy is required for loading.