Translocation of Manufactured Food

• The leaves are the main organs responsible for photosynthesis, producing the food that needs to be transported to other parts of the plant.
• Other parts of the plant (non-photosynthetic areas) such as roots, developing buds, fruits, and storage organs (like roots and stems) require food for growth, which is delivered through the phloem.
• Phloem is a vascular tissue that carries food from the leaves (source) to other parts of the plant (sink).

1. Large quantity of materials — Phloem carries large amounts of food, as the plant needs to transport substantial quantities of stored food to growing or storage parts (e.g., roots, fruits, etc.).
2. High rate of flow — The rate of flow of materials through the phloem sieve tubes is high, meaning that food is transported efficiently and rapidly.
3. Long transport distance — In tall plants, food may need to travel long distances, such as from the leaves at the top to the roots or storage organs at the bottom of the plant.
4. Small size of phloem — The phloem tissue is relatively small, consisting of a thin layer of inner bark in trees. Only newly formed phloem tissue is capable of translocating food, as older phloem becomes stretched and dies over time.
5. Structure of sieve tubes — Sieve tubes are specialized structures in the phloem, adapted for efficient transport. They lack a nucleus at maturity and have sieve plates (pores) that allow easy flow of food materials between cells.

The mass flow hypothesis (also known as the pressure flow hypothesis) as a model for describing how food materials, primarily sugars, are transported in plants through the phloem. This hypothesis was proposed by Münch in 1930 and remains a widely accepted explanation for the mechanism of phloem transport.

Key points of the mass flow hypothesis

1. Active transport of sugars:
   • Sugars (mainly sucrose) are actively transported into the phloem sieve tubes from the source (e.g., leaves where photosynthesis occurs).
   • This active transport creates a hypertonic solution in the phloem, leading to the movement of water into the phloem from the adjacent xylem by osmosis.
2. Pressure gradient:
   • The influx of water increases osmotic pressure (Ψ) in the phloem at the source.
   • This high pressure drives the flow of the sugary sap through the phloem to areas of lower pressure, typically the sink (e.g., roots, fruits, or developing shoots).
3. At the sink:
   • Sugars are actively removed from the phloem at the sink for use or storage.
   • This reduces the osmotic pressure in the sink, and water exits the phloem, often returning to the xylem.
4. Continuous flow:
   • The constant production of sugars at the source and their usage at the sink maintains the pressure gradient, allowing continuous mass flow of solutes.

Münch demonstrated the mechanism of the mass flow hypothesis using a model with two containers (A and B) connected by a tube.

Container A (Source):

1. Contained a concentrated sugar solution.
2. Represented the source (leaves), where sugars are actively loaded into the phloem.
3. Water moved into container A due to osmosis, creating high hydrostatic pressure.

Container B (Sink):

1. Contained a less concentrated sugar solution.
2. Represented the sink (roots, fruits, or young shoots), where sugars are removed for storage or metabolism.
3. Water moved out, creating lower hydrostatic pressure.

Tube C:

1. Connected the two containers, representing the phloem.
2. Water and solutes flowed through the tube due to the pressure difference.

As water entered container A and pressure built up, it forced the solution to move through the tube to container B, demonstrating mass flow due to a pressure gradient.

TOPIC 1: TRANSPORTATION | BIOLOGY FORM 6

Container A (Source):

• Represents leaves, where sugars are actively loaded into the phloem.
• Water is drawn into the phloem from the xylem, increasing pressure.

Container B (Sink):

• Represents areas such as roots or fruits, where sugars are unloaded.
• Sugars are stored or used, reducing the osmotic pressure, and water exits the phloem.

Continuous movement:

• As sugars are constantly loaded at the source and unloaded at the sink, equilibrium is never reached, maintaining the flow.

1. Mass flow observed in phloem sap — When the phloem is cut, sap flows out under pressure, indicating the existence of a pressure gradient in the sieve tubes.
2. pH evidence — Phloem sap has a higher pH than expected because hydrogen ions are actively transported out of the phloem. This demonstrates active processes within the phloem tissue.
3. Aphid experiment — When an aphid is anesthetized and its stylet (mouthpart) is cut, phloem sap continues to flow out of the stylet. This prolonged flow suggests the presence of hydrostatic pressure within the sieve tubes.
4. Faster rate than diffusion — The rate of transport in the phloem is faster than would be expected if substances were moving by simple diffusion, indicating a mass flow mechanism driven by pressure differences.
5. Correlation between pressure and transport rate — The rate of solute movement in the phloem matches closely with the pressure differences measured between sources (e.g., leaves) and sinks (e.g., roots or fruits).

1. Neglects the living nature of phloem — The hypothesis does not explain why sieve tube cells must be living and metabolically active, even though the process seems to rely primarily on passive pressure gradients.

2. Transport rates vary for different substances — The hypothesis assumes that all solutes (e.g., sugars, amino acids) are transported at the same rate, but experiments show that different substances move at varying speeds.

3. Bidirectional movement — The hypothesis struggles to explain how bidirectional movement of solutes occurs in the phloem simultaneously (e.g., movement from source to sink and from sink to source).

4. Effect of environmental conditions — Translocation is significantly affected by temperature, metabolic inhibitors, and other environmental factors, which the hypothesis does not fully address.

5. Role of metabolism — The hypothesis overlooks the role of active metabolic processes in:

   • Loading sucrose into sieve elements at the source.
   • Unloading sucrose at the sink.

   These processes require energy and are critical for maintaining the concentration gradients necessary for translocation.