🌱 Transport in Plants: Translocation – The Food Delivery System
Hello Biologists! Welcome to the fascinating world of plant transport. We've already looked at how water moves up the plant in the xylem (transpiration). Now we turn our attention to the plant's food delivery network—how it moves the sugars it makes and the amino acids it builds to every living cell that needs them.
This process is called Translocation, and understanding it is key to knowing how plants grow, store energy, and survive!
1. Defining Translocation
In simple terms, translocation is the biological process by which food (specifically sugars) and other organic substances are moved throughout the plant.
What is Transported?
Unlike xylem, which transports water and mineral ions, the phloem transports vital organic molecules:
• Sucrose: This is the main sugar transported. While plants produce glucose during photosynthesis, it is converted into sucrose for transport because sucrose is less reactive and thus a better transport molecule.
• Amino Acids: These are essential building blocks for making proteins, required by every growing cell.
Where does Translocation happen?
Translocation takes place in the phloem tissue, which forms part of the vascular bundles found alongside the xylem in the root, stem, and leaves.
Memory Tip: Think P for Phloem, P for Food (Primary food). Think X for Xylem, X marks the spot for water.
Key Takeaway for Section 1
Translocation is the movement of sucrose and amino acids from their production site to where they are needed, travelling through the phloem.
2. The Journey: From Source to Sink
To fully understand translocation, we need to know where the food starts its journey and where it ends up. We use the terms Source and Sink for the start and end points of the transport pathway.
What is a Source?
A Source is any part of the plant that releases (or loads) sucrose or amino acids into the phloem for transport.
Think of the Source as the "factory" or "storage cupboard" that has a surplus of food.
Examples of Sources:
• Mature Green Leaves: These are the primary sources because they actively carry out photosynthesis, producing large amounts of sucrose.
• Storage Organs (when unloading): For example, a potato tuber or onion bulb that is breaking down its stored starch back into sucrose to feed a new shoot growing in the spring.
What is a Sink?
A Sink is any part of the plant that actively uses or stores sucrose or amino acids.
Think of the Sink as the "customer" or "growth zone" that needs the supplies.
Examples of Sinks (parts requiring food):
• Growing Regions: This includes root tips, developing buds, and young leaves that are not yet green enough (or large enough) to photosynthesise for themselves.
• Storage Organs (when filling): A potato tuber or a carrot root that is actively storing starch during the summer.
• Reproductive Parts: Developing fruits and seeds that need huge amounts of energy and building blocks.
Quick Review Box: Source vs. Sink
Source: Releases food (e.g., photosynthesising leaf).
Sink: Uses or Stores food (e.g., growing root tip, developing fruit).
3. The Dynamic Role: Source and Sink Changes
Don't worry if this seems tricky at first! The most important concept in advanced translocation is that a plant part is not permanently fixed as a source or a sink. Its role can change depending on its age and the time of year.
Why do roles change? (8.4 Supplement 3)
The role changes based on the balance of supply (photosynthesis) and demand (growth/storage).
Example: The Storage Organ Paradox (Tuber/Bulb)
Imagine a potato plant:
1. Summer (Growing Phase): The potato leaves are mature and producing lots of sucrose (Source). This sucrose is moved down to the developing tubers, which are storing the food (Sink).
2. Spring (Dormant Phase/New Growth): The previous year's tuber is buried underground. It now needs to send energy up to the new sprouts before the new leaves are mature enough to photosynthesise. The tuber must break down its stored starch and release the resulting sucrose. The tuber is now acting as a Source, and the new shoot tip is the Sink.
Example: Young vs. Mature Leaves
• Young Leaf: When a leaf is small and still expanding, it requires more sucrose from older, mature leaves than it produces itself. It is a Sink.
• Mature Leaf: Once the leaf is fully expanded and photosynthesising strongly, it produces more sucrose than it needs. It becomes a major Source.
Did You Know?
The direction of translocation in the phloem is bidirectional (up or down the stem), unlike water movement in the xylem, which is strictly unidirectional (only upwards). The phloem always transports food from the source (where sugar is high) to the sink (where sugar is low).
Key Takeaway for Section 3
Parts of the plant, such as storage organs, can switch their role. They are Sinks when filling up with food and Sources when they break down their stored food to support new growth.
4. Comparing Xylem and Phloem Transport
It is very common in IGCSE Biology to be asked to compare the two main transport systems. Use this table for a quick revision:
| Feature | Xylem | Phloem (Translocation) |
| Substances Transported | Water and Mineral Ions | Sucrose and Amino Acids |
| Direction of Flow | Always upwards (Unidirectional) | Upwards and downwards (Bidirectional) |
| Driving Force | Transpiration Pull (powered by evaporation/cohesion) | Source-to-Sink gradient (Pressure Flow is the mechanism, though details aren't required) |
| Vessel Type | Dead, hollow tubes (vessels) | Living sieve tubes and companion cells |
Keep these distinctions clear, and you will master the transport chapter!