Biology – 8.1 Transport in plants | e-Consult

The phloem's structure is highly adapted for efficient transport. Key adaptations include:

• Sieve Tubes: These are the main conducting elements. They are long, cylindrical cells with sieve plates at their ends. The sieve plates have pores that allow for the passage of phloem sap. The lack of a nucleus and other organelles in sieve tube elements reduces obstruction to flow.
• Companion Cells: These cells are closely associated with sieve tube elements and provide metabolic support. They load and unload sugars into and out of the sieve tubes, maintaining the pressure gradient. They also provide energy and structural support.
• Phloem Parenchyma: These cells provide storage and structural support within the phloem tissue.
• Sieve Elements: These are specialized sieve tube elements that are more efficient at transport. They have fewer internal structures and more extensive plasmodesmata (connections between cells) to facilitate flow.