Biology – 8.1 Transport in plants | e-Consult

The phloem plays a crucial role in the transport of sucrose, the primary sugar produced during photosynthesis, from the leaves (sources) to other parts of the plant where it is needed for growth, respiration, or storage (sinks).

Sucrose is loaded into the sieve tubes at the source through a process called active transport. This requires energy (ATP) and involves specific carrier proteins that facilitate the movement of sucrose against its concentration gradient. The sucrose concentration in the phloem sap is higher at the source than at the sink, creating a concentration gradient that drives the flow of sucrose.

The rate of translocation can be affected by several factors:

• Temperature: Translocation is most efficient at optimal temperatures. Too low or too high temperatures can reduce the rate of active transport and the viscosity of the phloem sap.
• Water Availability: Water is essential for maintaining the turgor pressure in the phloem. Water stress can reduce turgor pressure and impede sap flow.
• Distance: The longer the distance the sap needs to travel, the slower the rate of translocation.
• Competition: Competition for sucrose between different sinks can reduce the amount of sucrose available for each sink, thus affecting the rate of translocation to each.
• Disease/Damage: Diseases or physical damage to the phloem can block the flow of sap.

The phloem's ability to transport sucrose is vital for the plant's growth and survival, ensuring that all parts of the plant receive the necessary energy for their functions.