# Translocation

> Biology · CIE A-Level
> Source: https://www.owlsprep.com/study/cie-9700-u7-translocation/

This module covers phloem structure, source/sink dynamics, the mass flow hypothesis, and evidence evaluating the theory. Translocation is a common topic for structured and extended response questions in CIE A-Level Biology.

**Prerequisites:** [Transpiration and xylem transport](https://www.owlsprep.com/study/cie-9700-u7-transpiration/); Plant cell structure and membrane transport

## Learning objectives

- Describe how phloem structure is adapted for translocation
- Explain the mass flow hypothesis for solute movement in phloem
- Evaluate evidence for and against the mass flow hypothesis
- Distinguish between sources and sinks in translocation

## Phloem Structure and Key Terminology

**Translocation** — The long-distance transport of organic solutes through phloem tissue of vascular plants, from production/storage sites to sites of use or storage

*Example:* Sucrose from mature leaves is translocated to growing roots and developing fruits

Phloem tissue consists of two key functional cell types: sieve tube elements and companion cells. Unlike xylem, phloem cells are living at functional maturity. Sieve tube elements lose most organelles (nucleus, ribosomes, vacuole) at maturity, forming hollow tubes connected end-to-end by sieve plates with large pores. Companion cells are adjacent to each sieve tube element, connected via plasmodesmata, and retain all organelles to supply ATP and metabolic support.

- Sieve plate pores allow unobstructed bulk flow of solute solution
- Rigid sieve tube cell walls withstand high hydrostatic pressures
- Companion cells have many mitochondria to supply ATP for active loading

**Worked example:** Explain one adaptation of companion cells that suits their role in translocation

1. 1. First, recall the core function of companion cells: they support active loading of sucrose into sieve tubes, which requires energy.
2. 2. The key adaptation is a very high number of mitochondria in companion cells.
3. 3. Mitochondria produce large amounts of ATP via aerobic respiration, which is required for the active transport of sucrose against its concentration gradient into sieve tubes.

## Sources, Sinks and Loading Mechanisms

**Source and Sink** — A source releases organic solutes into phloem for transport. A sink removes solutes from phloem for use or storage.

*Notation:* Source / Sink

*Example:* Mature photosynthetic leaf = source; growing fruit = sink

Loading of sucrose into phloem at the source occurs via two main pathways: apoplastic and symplastic. In apoplastic loading, sucrose moves through cell walls to the companion cell, then is actively transported into the companion cell before entering the sieve tube via plasmodesmata. In symplastic loading, sucrose moves directly between cell cytoplasms via plasmodesmata. Active transport is always required to build up a high sucrose concentration in source sieve tubes.

**Worked example:** Explain how a potato tuber can be both a sink and a source at different times

1. 1. During the growing season, when the tuber is storing starch for later growth:
2. Sucrose is imported from photosynthetic leaves, converted to starch and stored. The tuber removes solutes from phloem, so it acts as a sink.
3. 2. In spring, when the tuber sprouts to produce new shoots:
4. Stored starch is broken down into sucrose, which is released into phloem to supply growing shoots. The tuber supplies solutes to phloem, so it acts as a source.

## The Mass Flow Hypothesis

**Mass Flow Hypothesis** — The leading theory that describes bulk solute movement in phloem as a passive flow driven by hydrostatic pressure differences between source and sink.

The mechanism follows four core steps:

1. Active loading of sucrose into sieve tubes at the source lowers the water potential inside the sieve tube
2. Water moves into the sieve tube from surrounding xylem by osmosis, raising hydrostatic pressure at the source
3. At the sink, sucrose is unloaded, raising the water potential inside the sieve tube
4. Water leaves the sieve tube by osmosis, lowering hydrostatic pressure at the sink

The pressure difference between high-pressure source and low-pressure sink drives continuous bulk flow of solute solution from source to sink.

> **tip**
>
> Only loading and unloading of sucrose require active transport/ATP. The bulk flow of solutes along the sieve tube is a passive process driven by pressure.

**Worked example:** Use the mass flow hypothesis to explain why translocation stops if phloem is punctured

1. 1. Translocation relies entirely on the hydrostatic pressure gradient between source and sink to drive flow.
2. 2. Puncturing a sieve tube allows the pressurised solution inside to leak out into surrounding tissue.
3. 3. This equalises the pressure inside the sieve tube with external pressure, eliminating the pressure gradient between source and sink.
4. 4. Without a pressure gradient, there is no force to drive bulk flow of solutes, so translocation stops.

## Evidence For and Against Mass Flow

CIE regularly asks to evaluate the mass flow hypothesis, so you need to remember key pieces of supporting and conflicting evidence:

- For: Sieve tubes have higher hydrostatic pressure at sources than sinks, matching predictions
- For: Aphid feeding experiments confirm sap is under positive pressure in phloem
- Against: Different solutes move at different speeds in the same phloem, which mass flow cannot explain
- Against: Sieve plates create resistance to flow that would require higher pressure than is actually observed

**Worked example:** Describe how aphid experiments provide evidence for the mass flow hypothesis

1. 1. Aphids feed on phloem sap by inserting their sharp stylet mouthparts into sieve tubes.
2. 2. If the aphid is anaesthetised and removed, leaving the stylet in place, sap continues to exude from the cut end.
3. 3. This confirms that phloem sap is under positive hydrostatic pressure, which is a core prediction of the mass flow hypothesis.

## Common pitfalls

- **Wrong:** Confusing translocation with transpiration, or stating translocation occurs in xylem
  - Why it fails: Transpiration is water/mineral transport in xylem; translocation is organic solute transport in phloem
  - Correct: Remember: Xylem = water and minerals up, Phloem = organic solutes from source to sink
- **Wrong:** Claiming mass flow itself is an active process that requires ATP
  - Why it fails: Only loading and unloading of sucrose require active transport. The bulk flow of solutes down the pressure gradient is passive
  - Correct: Active transport is only needed for loading/unloading; mass flow is driven by passive pressure differences
- **Wrong:** Claiming all sources are leaves and all sinks are roots
  - Why it fails: Any plant structure can be a source or sink depending on growth stage
  - Correct: A source releases solutes to phloem, a sink removes solutes, regardless of their location in the plant
- **Wrong:** Stating sieve tube elements are dead at maturity like xylem vessels
  - Why it fails: Xylem vessels are dead at maturity, but sieve tubes are alive, supported by companion cells
  - Correct: Sieve tube elements are living cells that retain a plasma membrane, even without most organelles
- **Wrong:** Stating hydrostatic pressure is higher at the sink than the source
  - Why it fails: Sucrose loading at the source increases pressure, while unloading at the sink decreases it
  - Correct: Pressure is higher at the source, lower at the sink, driving flow from source to sink

## Cheatsheet

| Term | Key Definition | Core Exam Fact |
| --- | --- | --- |
| Translocation | Organic solute transport | Occurs in living phloem tissue |
| Source | Releases solutes to phloem | Can be leaf or sprouting storage organ |
| Sink | Removes solutes from phloem | Can be root, fruit, bud or growing tuber |
| Mass Flow | Bulk flow driven by pressure gradient | Passive flow; loading/unloading need ATP |
| Aphid Experiment | Tests phloem pressure | Confirms positive hydrostatic pressure in phloem |

## What's next

Translocation is a core topic in CIE A-Level plant transport, and is often tested in extended response questions that compare transport in xylem and phloem, or evaluate the mass flow hypothesis. It links closely to topics including membrane transport, photosynthesis, and plant growth and development. Mastering the key terminology, mechanism, and evidence will help you score full marks on common exam questions. Explore the related topics below to build a connected understanding of the whole unit.

- [Transpiration and Xylem Transport](https://www.owlsprep.com/study/cie-9700-u7-transpiration/)
- [Xerophyte adaptations](https://www.owlsprep.com/study/cie-9700-u7-xerophyte-adaptations/)
- [Transport in Mammals](https://www.owlsprep.com/study/cie-9700-u8-overview/)

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