Translocation

1. Overview

Translocation is the process by which plants transport the organic products of photosynthesis and mineral-building blocks to where they are needed for growth or storage. Unlike transpiration, which is a passive process moving water one way, translocation is an active process that moves nutrients in multiple directions through the phloem to ensure every part of the plant survives and thrives.

Key Definitions

• Translocation: The movement of sucrose and amino acids in the phloem from a source to a sink.
• Source: Parts of a plant that release sucrose or amino acids (where they are produced or remobilized).
• Sink: Parts of a plant that use or store sucrose or amino acids (where they are consumed or put into storage).
• Phloem: The vascular tissue responsible for the transport of organic nutrients.
• Sucrose: The transport form of carbohydrates in plants (less reactive and more soluble than glucose).

Core Content

Note: According to the IGCSE syllabus, there are no specific Core-only objectives for this topic; all objectives are categorized under the Supplement (Extended) curriculum. See Section 4 for details.

Extended Content (Extended curriculum only)

Understanding Translocation

Translocation involves the transport of sucrose and amino acids through the phloem tubes. While xylem only moves water and minerals upwards, phloem can transport its contents both upwards and downwards depending on where the sources and sinks are located.

Sources vs. Sinks

The movement of substances in the phloem is always from a source to a sink.

• Sources include:
  • Leaves: The primary source during the growing season as they produce sucrose via photosynthesis.
  • Storage Organs (at the start of a growing season): Such as tubers or bulbs, which release stored starch (converted back to sucrose) to provide energy for new growth.
• Sinks include:
  • Roots: Use nutrients for cellular respiration and active transport of minerals.
  • Growing tips (Shoot and Root apices): High demand for sucrose and amino acids for cell division and growth.
  • Flowers and Fruits: Require high energy for reproduction and seed development.
  • Storage Organs (at the end of a growing season): Such as potatoes or carrots, where sucrose is converted to starch for long-term storage.

Seasonal Changes: Why parts act as both Source and Sink

Some parts of the plant switch roles depending on the time of year or the plant's life cycle.

• Example: The Potato Tuber
  • In Summer: The leaves are photosynthesizing rapidly. They act as the source. The sucrose is sent down to the underground tubers, which act as the sink, storing the energy as starch.
  • In Early Spring: After winter, the plant has no leaves to photosynthesize. The potato tuber now acts as the source. It breaks down its stored starch into sucrose, which is translocated upwards to the growing buds/stems, which act as the sink.

Structure and Function of Phloem

• Sieve tube elements: Living cells joined end-to-end with "sieve plates" (perforated end walls) between them to allow the easy flow of phloem sap.
• Companion cells: Located alongside sieve tubes; they contain many mitochondria to provide the energy (ATP) required to load sucrose into the phloem.

Key Equations

There are no mathematical equations required for the Translocation topic. However, remember the chemical relationship:

• Photosynthesis (in source): $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$
• Glucose is then converted to Sucrose for transport.

Common Mistakes to Avoid

• ❌ Wrong: Thinking phloem only moves nutrients downwards.
• ✓ Right: Phloem moves substances up or down, depending on the location of the source and sink.
• ❌ Wrong: Stating that glucose is transported in the phloem.
• ✓ Right: Glucose is produced in the leaf, but it is converted to sucrose for translocation because sucrose is more stable.
• ❌ Wrong: Confusing Transpiration with Translocation.
• ✓ Right: Transpiration is water/minerals in xylem; Translocation is sucrose/amino acids in phloem.

Exam Tips

• Command Word "Describe": If asked to describe translocation, ensure you mention both sucrose and amino acids, and the direction (source to sink).
• Command Word "Explain": If asked to explain why a part is a source or sink, link it to the process (e.g., "The leaf is a source because it produces sucrose via photosynthesis").
• Real-world Context: Exams often use a "girdling" or "ringing" experiment context (removing a ring of bark). Since phloem is located just under the bark, removing it causes a swell of sucrose above the ring because it can no longer be translocated to the roots (the sink).
• Seasonal Questions: Be prepared to identify the source and sink in a bulb or tuber during different seasons (winter/spring vs. summer).