Translocation
What is Translocation?
Translocation is the biological process responsible for the transport of organic substances (specifically sucrose and amino acids) within a plant. Unlike water transport, which moves primarily upwards via the xylem, translocation involves the movement of food materials produced during photosynthesis to parts of the plant that need them.
This process occurs in the phloem tissue. The phloem consists of living cells arranged into tubes called sieve tubes, which allow the flow of sap containing dissolved sugars and proteins.
Sources and Sinks
To understand translocation, you must define two key roles that different parts of a plant play:
| Term | Description |
|---|---|
| Source | A part of the plant that releases sucrose or amino acids into the phloem. Sources are typically sites of active photosynthesis (where food is made) or storage organs releasing stored food. |
| Sink | A part of the plant that uses or stores sucrose or amino acids. Sinks are typically growing tissues (which need energy for respiration and building new cells) or storage organs (which accumulate food). |
Dynamic Roles: It is crucial to understand that a part of a plant is not permanently a source or a sink. Its role depends on its physiological activity at that specific time.
For example, consider a potato plant:
- In summer, the leaves are the source (photosynthesizing) and the tuber (potato) is the sink (storing starch).
- In spring, the tuber acts as the source (releasing stored sugars to fuel new growth) and the growing shoot becomes the sink.
Identifying Sources and Sinks in Different Seasons
Let's apply the concept of dynamic roles to a potato plant throughout the year:
| Time of Year | Plant Part 2 (Role & Reasoning) |
|---|---|
| Summer | |
| Spring |
This explains why some parts of a plant may act as a source and a sink at different times: the role is determined by whether the tissue is currently producing food (source) or consuming/storing it (sink).
⚠︎ Confusing Sources and Sinks
Error: Students often incorrectly assume that leaves are always sources and roots/tubers are always sinks. They may also mistakenly identify growing tips or flowers as sources because they are 'active' parts of the plant.
Correct Understanding: A tissue is a source only if it is releasing sucrose/amino acids (e.g., via photosynthesis or mobilizing stored starch). It is a sink if it is importing these substances for respiration, growth, or storage. Growing tips are sinks because they consume energy to grow. Tubers are sources in spring because they release stored food.
Describing Translocation Changes
When to use this tip: When asked to explain how the roles of sources and sinks change between seasons (e.g., spring vs. summer) or in response to environmental conditions.
Why examiners accept this: Examiners look for specific references to physiological processes (photosynthesis, respiration, storage) rather than just naming the plant parts. You must explicitly state which substance is moving and in which direction relative to the source/sink.
Example of correct usage:
Incorrect: 'The tuber is a source in spring.' (Too vague)
Correct: 'In spring, the tuber acts as a source because stored starch is broken down into sucrose and transported to the growing shoot, which acts as a sink for respiration and growth.'
Incorrect: 'The tuber is a source in spring.' (Too vague)
Correct: 'In spring, the tuber acts as a source because stored starch is broken down into sucrose and transported to the growing shoot, which acts as a sink for respiration and growth.'
Past Paper Style Questions
Q:
Name the two main substances transported by translocation in plants. [2]
A:
- Sucrose (or sugars/carbohydrates/glucose)
- Amino acids
Q:
Explain why a potato tuber acts as a source in spring but a sink in summer. [4]
A:
- In spring, the tuber releases stored food (starch broken down to sucrose) to fuel new growth, so it is a source.
- In summer, the leaves are photosynthesizing and sending sucrose to the tuber for storage as starch, so the tuber is a sink.
Q:
Describe the process of translocation. [1]
A:
The movement of sucrose and amino acids in the phloem from sources to sinks.