A leaf may manufacture sugar during photosynthesis, but that sugar is useful only if it can reach the parts of the plant that need it. Growing roots require materials for respiration and growth. Developing fruits need a supply of organic nutrients. Storage organs may receive food at one time of year and release it again later.
Plants therefore need a transport system for manufactured food. This transport occurs through the phloem and is called translocation. To understand it properly, we need to connect three ideas: what is being transported, where it is moving from and where it is moving to.
Try these five questions before reading the chapter. They test the basic ideas of phloem transport, sources, sinks and evidence for translocation.
- 5 diagnostic questions
- Core plant-transport concepts
- Instant explanation for every answer option
What is translocation?
Plants manufacture glucose during photosynthesis, especially in green leaves exposed to light. Some of the carbohydrate is converted into sucrose, which can be transported to other parts of the plant.
Exam definition: Translocation is the transport of food, mainly sucrose, in the phloem tissue.
Translocation allows food produced or released in one part of the plant to reach tissues that require it elsewhere. The transported sucrose may be used in respiration, converted into other biological molecules for growth, or converted into storage substances.
The phloem is the transport tissue for food
Phloem is a living vascular tissue. Two cell types are particularly important when considering how it transports food: sieve tube elements and companion cells.
| Structure | Important feature | How the feature is useful |
|---|---|---|
| Sieve tube elements | Long cells arranged end-to-end | They form continuous pathways through which phloem sap can move. |
| Sieve plates | Perforated end walls between sieve tube elements | The pores allow phloem sap to pass from one sieve tube element to the next. |
| Mature sieve tube elements | Contain very little cytoplasm and lack a nucleus | There is more internal space available for the movement of phloem sap. |
| Companion cells | Living cells closely associated with sieve tube elements and containing many mitochondria | They support the metabolic activities associated with loading and unloading substances into and from the phloem. |
The important O-Level idea is that sucrose is transported through sieve tubes in the phloem. Companion cells remain metabolically active and support the functioning of the sieve tube elements.
From source to sink
Saying that phloem transports sucrose is only part of the story. To predict where the sucrose goes, we need to identify the source and the sink.
| Term | Meaning | Examples |
|---|---|---|
| Source | A region that supplies sucrose for translocation | A mature photosynthesising leaf; a storage organ releasing stored food |
| Sink | A region that receives sucrose because it is being used or stored there | Growing roots, shoots, flowers, developing fruits and storage organs |
A mature leaf in bright light is often a source because photosynthesis produces more carbohydrate than the leaf itself requires. Sucrose can therefore leave the leaf and be transported through the phloem.
A growing root may be a sink because its cells require organic substances for respiration and growth. A developing fruit can also act as a sink as it receives substances needed for growth and storage.
A useful way to follow any translocation question is: source → phloem → sink.
Sources and sinks can change
A particular organ is not permanently a source or permanently a sink. Its role depends on what is happening in the plant at that time.
For example, a storage organ may act as a sink while food is being stored. Later, when its stored reserves are broken down and transported to developing shoots, the same organ can act as a source.
Can translocation occur upwards and downwards?
Yes. Sucrose may have to reach sinks above or below a particular source. A photosynthesising leaf could supply sucrose to developing tissues near the shoot tip, while another flow of sucrose supplies roots lower down the plant.
This is different from describing the xylem transpiration stream, in which water is transported mainly from the roots towards the leaves.
Important precision: Saying that phloem transport can occur upwards or downwards does not mean that the same sap is simultaneously flowing in opposite directions through the same sieve tube. Different source–sink pathways can transport food in different directions within the plant.
How do we know that translocation occurs in the phloem?
Scientists cannot simply look at an intact stem and watch individual sucrose molecules moving through it. Instead, evidence comes from experiments in which the transport pathway is interrupted, sampled or traced.
Three classic investigations are particularly useful for understanding translocation: ringing experiments, aphid studies and radioactive tracer experiments.
Ringing experiments
In a ringing or girdling experiment, a complete ring of bark is removed from around a woody stem. The phloem lies within the tissues removed with the bark, while the more deeply located xylem is left intact.
| Stage | What happens |
|---|---|
| Setup | A ring containing the phloem is removed while the xylem remains intact. |
| Observation | After some time, a swelling develops above the removed ring. |
| Reason | Sucrose transported from leaves above the ring can no longer continue through the interrupted phloem and accumulates above the ring. |
| Conclusion | The observation provides evidence that manufactured food is transported through the phloem. |
Notice why leaving the xylem intact matters. Water can initially continue to move upwards through the xylem. The experiment therefore distinguishes interruption of food transport from interruption of the water pathway.
Aphid stylet studies
Aphids feed by inserting a narrow mouthpart called a stylet into plant tissues. When the stylet reaches a phloem sieve tube, the aphid can obtain phloem sap.
In an investigation, an aphid is allowed to feed and its body is then carefully removed while the stylet remains embedded in the plant. Phloem sap can continue to emerge from the cut stylet and can be collected for analysis.
Analysis shows that the collected phloem sap contains sucrose. This provides direct evidence that sucrose is transported in the phloem.
Radioactive tracer studies
A different approach is to label carbon so that scientists can follow where products of photosynthesis move. A leaf can be supplied with carbon dioxide containing the radioactive isotope carbon-14.
During photosynthesis, the labelled carbon becomes incorporated into carbohydrates. The movement of these labelled substances can then be detected in other parts of the plant.
| Step | Interpretation |
|---|---|
| A leaf receives 14CO₂ | The radioactive carbon can be incorporated into products of photosynthesis. |
| Radioactively labelled sugars are formed | The manufactured food can now be traced. |
| Radioactivity is later detected along phloem pathways and at sinks | This shows that products made in the source leaf have been transported through the plant. |
The strength of this investigation is that the movement of material produced during photosynthesis can be followed from its source towards other plant organs.
What the three translocation studies tell us
| Study | Key observation | What it supports |
|---|---|---|
| Ringing experiment | Food accumulates and swelling develops above a ring where phloem has been removed. | Food is transported through the phloem. |
| Aphid stylet experiment | Sap collected from a stylet inserted into phloem contains sucrose. | Sucrose is present in and transported by phloem sap. |
| Radioactive carbon tracer | Radioactively labelled products formed in a leaf later appear along phloem pathways and at sinks. | Products of photosynthesis are transported from sources to other parts of the plant. |
Translocation and transpiration are not the same process
The names sound similar, so they are easily confused. Their transported substances, tissues and overall functions are different.
| Feature | Translocation | Transpiration stream |
|---|---|---|
| Main substance transported | Food, mainly sucrose | Water and dissolved mineral ions |
| Tissue | Phloem | Xylem |
| Typical pathway | From a source to a sink | From roots towards leaves |
| Possible overall direction in the plant | Can be upwards or downwards depending on the positions of sources and sinks | Mainly upwards from roots to aerial parts |
| Main biological role | Distributes manufactured food to regions where it is used or stored | Supplies water and mineral ions and replaces water lost by transpiration |
Common Mistakes
| Common mistake | Why it is wrong | Better way to think about it |
|---|---|---|
| Saying translocation transports water | Water transport is mainly associated with the xylem and transpiration stream. | Remember: translocation transports food, mainly sucrose, in phloem. |
| Saying translocation occurs in xylem | Xylem and phloem have different transport roles. | Link sucrose with phloem and water with xylem. |
| Saying food always moves from leaves to roots | The destination depends on where sinks are located. | Identify the source and sink before deciding the direction of movement. |
| Assuming every leaf is always a source | Young developing leaves may initially receive food from elsewhere. | A source supplies sucrose; a sink receives it. The role depends on the organ's current activity. |
| Saying sucrose moves both ways through the same sieve tube at the same time | This gives a misleading picture of phloem flow. | Different source–sink pathways can transport sucrose in different directions. |
| Saying a ringing experiment removes the xylem | The experiment is useful because the phloem is interrupted while the xylem is left intact. | Food transport is blocked while water transport can initially continue. |
| Saying swelling above a ring proves that water accumulates there | The important accumulation is manufactured food transported through the phloem. | Relate the swelling to interruption of sucrose transport. |
| Saying radioactive carbon itself is simply pumped through the phloem | The labelled carbon is first incorporated into products of photosynthesis. | The tracer allows scientists to follow labelled manufactured food through the plant. |
| Saying the aphid experiment only shows that aphids eat plants | The important evidence comes from analysing sap obtained from phloem sieve tubes. | Connect the presence of sucrose in phloem sap with translocation. |
What you should be able to do
Define translocation
State that translocation is the transport of food, mainly sucrose, in the phloem tissue.
Identify the transport tissue
Recognise that sucrose is transported through phloem rather than xylem.
Explain source and sink
Identify which part of a plant is supplying sucrose and which part is receiving it.
Predict transport direction
Use the positions of sources and sinks to decide whether sucrose must be transported upwards or downwards.
Describe phloem structure
Relate sieve tube elements, sieve plates and companion cells to the transport of phloem sap.
Interpret a ringing experiment
Explain why food accumulates above a removed ring of phloem and what this tells us about food transport.
Interpret an aphid study
Explain how collecting sap through an aphid stylet provides evidence that sucrose is present in the phloem.
Interpret radioactive tracer evidence
Explain how labelled carbon incorporated into products of photosynthesis can be followed from a source to other parts of the plant.
Compare xylem and phloem
Distinguish the substances carried, direction of transport and roles of the two vascular tissues.
These ten questions test whether you can apply source–sink relationships and interpret experimental evidence for translocation rather than simply memorising its definition.
- 10 mastery questions
- Source–sink reasoning and experimental evidence
- Instant explanation for every answer option
Frequently asked questions
Translocation: questions students often ask
Most mistakes in translocation questions come from confusing phloem with xylem or treating source and sink as permanent labels. Once those ideas are separated, the experimental evidence becomes much easier to interpret.
What exactly is translocation?
Translocation is the transport of food, mainly sucrose, in the phloem tissue. It distributes manufactured food from regions that supply it to regions where it is used or stored.
Why is sucrose transported instead of simply saying “glucose”?
Glucose is produced during photosynthesis, but plants commonly convert carbohydrate into sucrose for transport in the phloem. For O-Level translocation questions, sucrose is therefore the key substance to name.
What is the difference between a source and a sink?
A source supplies sucrose for transport, whereas a sink receives sucrose because it is being used or stored there. A mature photosynthesising leaf is a common source, while roots, developing fruits and growing shoots can act as sinks.
Is a leaf always a source?
No. A mature, actively photosynthesising leaf commonly acts as a source, but a young developing leaf may initially require imported food and therefore act as a sink. Source and sink describe what an organ is doing at that particular time.
Can a storage organ change from a sink to a source?
Yes. While carbohydrate is being deposited, the storage organ acts as a sink. Later, stored reserves may be mobilised to support new growth, allowing the organ to act as a source.
Does translocation occur only downwards?
No. Food is transported from sources to sinks, and sinks may be located above or below a particular source. Translocation can therefore occur in different overall directions in different parts of the phloem.
Does “two-way transport” mean one sieve tube carries food both ways at the same time?
That is not a good way to picture it. Different source–sink pathways can carry phloem sap in different directions. The important O-Level idea is that the phloem system as a whole is not restricted to the mainly upward direction associated with the xylem transpiration stream.
What does the ringing experiment show?
Removing a ring containing the phloem interrupts food transport while leaving the deeper xylem intact. Food accumulates above the ring and the stem becomes swollen there. This provides evidence that manufactured food is transported through the phloem.
Why does the swelling form above the ring rather than below it?
In the usual experiment, leaves above the ring are supplying sucrose that would normally be transported towards sinks below. Once the phloem pathway is cut, the transported food cannot pass the ring and therefore accumulates above it.
Why does removing the bark not immediately stop water reaching the leaves?
In a correctly performed ringing experiment, the deeper xylem remains intact. Water can therefore continue to move towards the leaves through the xylem initially even though phloem transport has been interrupted.
What does the aphid experiment prove?
When an aphid stylet penetrates a sieve tube, phloem sap can be sampled. Finding sucrose in this sap provides direct evidence that sucrose is transported in the phloem.
How does radioactive carbon provide evidence for translocation?
A leaf is supplied with carbon dioxide containing radioactive carbon. During photosynthesis, the labelled carbon becomes incorporated into organic substances. Detecting the radioactive label later along phloem pathways and at sinks allows scientists to trace the movement of products made in the source leaf.
What is the simplest way to distinguish translocation from transpiration in an exam?
Ask two questions: what is moving? and which tissue carries it? Translocation involves food, mainly sucrose, in the phloem. The transpiration stream involves water and mineral ions in the xylem.
Do I need to describe the detailed pressure-flow mechanism of phloem transport?
For Singapore O-Level Biology 6093, the central requirement is to understand translocation as the transport of food, mainly sucrose, in the phloem and to interpret evidence from translocation studies. Concentrate first on phloem, source–sink relationships and the experimental evidence rather than adding unnecessarily advanced detail.
Translocation is easiest to understand when you follow the food rather than memorising a direction. First identify the source, where sucrose is being supplied. Then identify the sink, where it is being used or stored. The phloem connects the two. Ringing experiments show what happens when that pathway is interrupted, aphid studies reveal what is inside phloem sap, and radioactive tracers allow manufactured food to be followed through the plant. Together, these observations give us strong evidence that food, mainly sucrose, is transported through the phloem.