4.3 Translocation
Plants synthesise organic compounds such as sucrose in their leaves through photosynthesis. These essential nutrients must be transported to growing tissues, roots, and storage organs. This movement of manufactured organic substances occurs through specialized vascular tissues called phloem via a process known as translocation.
Learning Objectives
- Define translocation in plants.
- Explain the necessity of translocation in plants.
- Describe the pathway of translocation from source leaves to sink tissues based on hydrostatic pressure gradients.
- Investigate the role of phloem tissue in the transport of organic substances through a stem ringing experiment.
The Translocation Pathway in Phloem and Stem Ringing Demonstration
Illustrate how sucrose loading creates hydrostatic pressure driving sap movement in the sieve tube, and visually explain the physical results of stem ringing.
Source
Leaves that produce sucrose through photosynthesis.
Sink
Growing parts and storage organs that use or store sucrose.
Phloem
Vascular tissue that transports organic substances bidirectionally.
Short Explanation
Definition of Translocation
Translocation is the process of transporting organic substances such as sucrose, amino acids, and hormones in the phloem from the leaves to other parts of the plant such as the roots and stem.
Term analysis: Derived from Greek words (Trans = crossing, Locus = site or place).
Evidence from nature: Aphids feed by inserting their stylets into the phloem of plant stems. Analysis of the sap collected reveals sucrose and amino acids, proving that translocation occurs in the phloem tissue.
Necessity of Translocation
Transports photosynthetic products from the leaves to growing parts that require them for growth and cellular respiration, such as roots, fruits, tips of shoots, or developing flowers.
Transports excess photosynthetic products to storage organs such as rhizomes, tubers, and bulbs.
Pathway of Translocation
The pressure-flow mechanism drives phloem sap from source leaves to sink tissues. Sucrose is actively loaded into sieve tubes, water follows by osmosis, and the resulting hydrostatic pressure pushes sap toward sinks.
Experiment 4.3: The Role of Phloem Tissue in the Transport of Organic Substances
Aim: To investigate the role of phloem tissues in the transport of organic substances.
Hypothesis: The upper part of the removed ring bark from a plant stem becomes swollen and the lower part of the removed ring bark shrinks after a few weeks.
Variables:
- Manipulated variable: The part of the removed ring bark (upper part vs lower part).
- Responding variable: The condition of the upper part and lower part of the removed ring bark from a plant stem.
- Constant variable: Hibiscus plant.
Procedure:
- Cut and remove a complete ring of bark (which contains the phloem tissue while leaving the xylem intact) from a hibiscus branch.
- Apply petroleum jelly to the exposed ringed section to prevent desiccation.
- Water the plant daily and leave it for three weeks.
- Record observations of the stem above and below the ring.
Observations and Explanations:
- Upper part of the ring: Becomes swollen because organic substances (sucrose) manufactured in the leaves continue to travel downwards but are blocked at the cut edge, accumulating there.
- Lower part of the ring: Shrinks or reduces in growth because it is deprived of organic nutrients from the leaves.
Significance & Prediction: Confirms that phloem is located in the bark and transports organic food downwards. If a main trunk is ringed completely for six months, the roots will eventually starve and die from lack of nutrients, causing the entire plant to die.
Try to Answer First
Answer in your mind, then press “Check Answer”.
What substances are transported in the phloem during translocation?
Why does the stem tissue immediately above the ring become swollen in a bark-ringing experiment?
How does the entry of water from the xylem into the sieve tube assist translocation?
Common Mistakes
- Confusing translocation with transpiration (translocation transports organic solutes in phloem; transpiration is water loss as vapour via stomata).
- Assuming translocation only moves substances downwards (translocation moves substances in both directions, from source leaves to both lower sinks like roots and upper sinks like shoot tips and developing fruits).
- Stating that xylem is removed during the ringing experiment (only the outer ring bark containing phloem is removed; xylem remains intact to allow water upward).
Concept Misunderstandings
Phloem transports pure glucose.
Phloem transports carbohydrate mainly in the form of sucrose, which is less reactive than glucose, alongside amino acids and hormones.
Translocation is purely passive.
Sucrose loading into sieve tubes and unloading into sinks occur via active transport requiring energy; only water movement occurs by osmosis.
Summary
- Translocation moves organic nutrients (primarily sucrose, amino acids, hormones) through the phloem from source leaves to sink tissues (growing parts or storage organs).
- Active loading of sucrose into sieve tubes decreases water potential, causing water to enter from the xylem by osmosis.
- This creates high hydrostatic pressure that propels sap toward sink tissues, where sucrose is unloaded.
- Stem ringing experiments confirm that organic substances travel through phloem tissues located in the outer bark.
Short Activity
Objective: test your understanding of translocation, the pressure-flow mechanism and the stem ringing experiment.
A. Objective Quiz
Which of the following causes water to diffuse from the xylem vessel into the sieve tube during translocation?
In a stem-ringing experiment on a hibiscus branch, what is observed at the upper part of the cut ring after three weeks?
B. Fill in the Blanks
Translocation is the process of transporting organic substances such as sucrose, amino acids, and hormones in the from the leaves to other parts of the plant.
The entry of water into the sieve tube increases the pressure, which pushes the phloem sap along the sieve tube to sink organs.
C. Matching / Drag and Drop
Match each step of the translocation pathway to its correct description. On a phone, tap a choice first, then tap the matching box. Tap the Answer Choices area to return a selected item.
C. Match the Step to Its Description
Write the correct step next to each description.
| Description | Step |
|---|---|
| 1. Sucrose is actively transported into the sieve tube via companion cells | |
| 2. Water diffuses from xylem into sieve tube by osmosis due to reduced water potential | |
| 3. High hydrostatic pressure pushes phloem sap along the sieve tube to other organs | |
| 4. Phloem sap is transported into sink tissues like roots and fruits by active transport |
Answer choices: Sucrose loading • Water influx • Mass flow • Sucrose unloading
