Consider the following statements regarding the transport of phloem sap. Select True or False for each statement.
T/F This is a passive process that is driven by the evaporation of water from leaves.
T/F Sugars tend to move from sources to sinks.
T/F Phloem sap moves through sieve-tube elements under positive pressure.
T/F Sieve-tube elements and vessel elements are commonly involved in the transport of phloem sap.

Verified step by step guidance

1. Understand that phloem transport is primarily an active process, not passive. It involves the movement of sugars and other organic nutrients from sources (like leaves) to sinks (such as roots or fruits). This process is driven by pressure flow, not by the evaporation of water, which is a characteristic of xylem transport.
2. Recognize that sugars indeed move from sources to sinks. Sources are parts of the plant that produce or release sugars, while sinks are parts that consume or store sugars. This movement is facilitated by the pressure flow mechanism in the phloem.
3. Phloem sap moves through sieve-tube elements under positive pressure. This is known as the pressure flow hypothesis or mass flow hypothesis, where the loading of sugars into the phloem creates a high osmotic pressure that drives the flow towards areas of lower pressure.
4. Sieve-tube elements are specialized cells in the phloem that facilitate the transport of phloem sap. Vessel elements, on the other hand, are part of the xylem and are involved in water transport, not phloem sap transport. Therefore, they are not commonly involved in the transport of phloem sap.
5. Review the role of companion cells, which are closely associated with sieve-tube elements. They assist in the loading and unloading of sugars into the phloem, playing a crucial role in the active transport process.

Key Concepts

Phloem Transport Mechanism

Phloem transport, also known as translocation, is the process of moving sugars and other organic nutrients through the plant. Unlike xylem transport, which is driven by negative pressure from water evaporation, phloem transport is an active process driven by positive pressure. This pressure is generated by the loading of sugars into the phloem at source sites, such as leaves, and unloading at sink sites, like roots or fruits.

Source and Sink Dynamics

In plants, 'sources' are the parts that produce or release sugars, typically the leaves, while 'sinks' are the parts that consume or store sugars, such as roots, fruits, and growing tissues. Sugars move from sources to sinks through the phloem, driven by pressure differences. This movement is crucial for distributing energy and nutrients throughout the plant, supporting growth and development.

Sieve-Tube Elements

Sieve-tube elements are specialized cells in the phloem that facilitate the transport of phloem sap. They are elongated cells connected end-to-end, forming a continuous channel. Unlike vessel elements in the xylem, sieve-tube elements are alive at maturity and lack a nucleus, relying on companion cells for metabolic functions. Phloem sap moves through these tubes under positive pressure, a process distinct from the passive transport in xylem.