Translocation
BiologyΒ· Unit 7: Transport in PlantsΒ· 25 min read
1. Phloem Structure and Key Terminologyβ β ββββ± 5 min
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
Explain one adaptation of companion cells that suits their role in translocation
- 1
- First, recall the core function of companion cells: they support active loading of sucrose into sieve tubes, which requires energy.
- 2
- The key adaptation is a very high number of mitochondria in companion cells.
- 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.
2. Sources, Sinks and Loading Mechanismsβ β β βββ± 7 min
Source and Sink
A source releases organic solutes into phloem for transport. A sink removes solutes from phloem for use or storage.
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.
Explain how a potato tuber can be both a sink and a source at different times
- 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
- 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.
3. The Mass Flow Hypothesisβ β β βββ± 8 min
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:
Active loading of sucrose into sieve tubes at the source lowers the water potential inside the sieve tube
Water moves into the sieve tube from surrounding xylem by osmosis, raising hydrostatic pressure at the source
At the sink, sucrose is unloaded, raising the water potential inside the sieve tube
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.
Use the mass flow hypothesis to explain why translocation stops if phloem is punctured
- 1
- Translocation relies entirely on the hydrostatic pressure gradient between source and sink to drive flow.
- 2
- Puncturing a sieve tube allows the pressurised solution inside to leak out into surrounding tissue.
- 3
- This equalises the pressure inside the sieve tube with external pressure, eliminating the pressure gradient between source and sink.
- 4
- Without a pressure gradient, there is no force to drive bulk flow of solutes, so translocation stops.
4. Evidence For and Against Mass Flowβ β β β ββ± 5 min
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
Describe how aphid experiments provide evidence for the mass flow hypothesis
- 1
- Aphids feed on phloem sap by inserting their sharp stylet mouthparts into sieve tubes.
- 2
- If the aphid is anaesthetised and removed, leaving the stylet in place, sap continues to exude from the cut end.
- 3
- This confirms that phloem sap is under positive hydrostatic pressure, which is a core prediction of the mass flow hypothesis.
5. Common Pitfalls
Wrong move:
Confusing translocation with transpiration, or stating translocation occurs in xylem
Why:
Transpiration is water/mineral transport in xylem; translocation is organic solute transport in phloem
Correct move:
Remember: Xylem = water and minerals up, Phloem = organic solutes from source to sink
Wrong move:
Claiming mass flow itself is an active process that requires ATP
Why:
Only loading and unloading of sucrose require active transport. The bulk flow of solutes down the pressure gradient is passive
Correct move:
Active transport is only needed for loading/unloading; mass flow is driven by passive pressure differences
Wrong move:
Claiming all sources are leaves and all sinks are roots
Why:
Any plant structure can be a source or sink depending on growth stage
Correct move:
A source releases solutes to phloem, a sink removes solutes, regardless of their location in the plant
Wrong move:
Stating sieve tube elements are dead at maturity like xylem vessels
Why:
Xylem vessels are dead at maturity, but sieve tubes are alive, supported by companion cells
Correct move:
Sieve tube elements are living cells that retain a plasma membrane, even without most organelles
Wrong move:
Stating hydrostatic pressure is higher at the sink than the source
Why:
Sucrose loading at the source increases pressure, while unloading at the sink decreases it
Correct move:
Pressure is higher at the source, lower at the sink, driving flow from source to sink
6. Quick Reference 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 |
7. Frequently Asked
Is translocation only the movement of sugars?
No. While sucrose is the main translocated solute, phloem also transports amino acids, hormones, and organic minerals from source to sink.
Can a structure be both a source and a sink?
Yes. For example, a potato tuber is a sink when storing starch during growth, and becomes a source when sprouting releases sugars for new shoots.
When this came up on past exams
AI-estimated based on syllabus patterns β cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2022 Β· 22
Evidence for mass flow hypothesis
- 2021 Β· 12
Phloem sieve tube structure
- 2023 Β· 11
Compare xylem and phloem transport
Going deeper
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.