What is the primary function of the sieve plates in phloem tissue?

Sieve plates contain pores that allow for the continuous movement of organic substances between adjacent sieve tube elements. They reduce resistance to the mass flow of sap while providing structural support to the tube.

How do sieve tube elements differ from xylem vessels regarding their living state?

Sieve tube elements are living cells at maturity, maintaining a plasma membrane and some cytoplasm, whereas xylem vessels are dead, hollow tubes. This allows phloem to use active transport mechanisms that xylem cannot.

Why do sieve tube elements lack a nucleus and ribosomes at maturity?

The loss of these organelles creates more internal space and reduces friction for the flow of assimilates. This adaptation ensures that the mass flow of sap is as efficient as possible.

What is the role of the companion cell in the phloem complex?

Companion cells provide metabolic support to the sieve tube elements, which lack the machinery for protein synthesis and energy production. They also facilitate the active loading and unloading of sucrose into the sieve tubes.

What is a common misconception regarding the direction of flow in phloem?

A common error is thinking phloem only flows downwards. In reality, translocation is bidirectional, moving assimilates from any source (like leaves) to any sink (like growing buds or roots) depending on the plant's needs.

Why is it incorrect to say sieve tube elements are 'hollow' like xylem?

Sieve tube elements contain a thin layer of living cytoplasm and a plasma membrane. Calling them 'hollow' ignores their living nature and the physiological control they exert over transport.

How does the cell wall of a sieve tube element differ from that of a xylem vessel?

Sieve tube element walls are made of cellulose and are not lignified, making them permeable to water. Xylem walls are heavily lignified, making them waterproof and much more rigid to prevent collapse under tension.

What connects the cytoplasm of a companion cell to a sieve tube element?

They are connected by plasmodesmata. these are channels through the cell walls that allow for the direct exchange of nutrients, ATP, and signaling molecules between the two cells.

What happens to the transport process if the companion cells are damaged?

Transport would cease because the sieve tube elements would lose their source of ATP and metabolic maintenance. Active loading of sucrose would stop, preventing the generation of the pressure gradient needed for flow.

What is the difference between 'source' and 'sink' in the context of phloem transport?

A source is an organ (like a mature leaf) that produces more sugar than it uses, while a sink (like a root or developing fruit) is an organ that consumes or stores sugars. Phloem moves sap from source to sink.

Phloem Sieve Tube Elements | Cambridge International Examinations AS-Level Biology

Revision Notes

AS-Level

Cambridge International Examinations

Biology

7. Transport in Plants

Phloem Sieve Tube Elements

Summary

Phloem sieve tube elements are highly specialized living cells responsible for the long-distance transport of organic assimilates, such as sucrose, through a process called trans Unlike xylem, these cells remain living at maturity but undergo significant structural simplification—losing most organelles—to minimize resistance to the flow of sap, relying on adjacent companion cells for metabolic maintenance.

1. Definition & Core Concepts

Phloem sieve tube elements are the primary conducting cells of the phloem tissue in angiosperms, specialized for the transport of organic nutrients (assimilates).

These cells are arranged end-to-end to form continuous sieve tubes, facilitating the movement of sap from 'sources' (where sugars are produced) to 'sinks' (where sugars are used or stored).

Unlike xylem vessels, sieve tube elements are living cells, although they possess a highly modified and simplified internal structure to optimize transport efficiency.

Diagram showing a longitudinal section of a sieve tube element connected to a companion cell via plasmodesmata, highlighting the sieve plates at the ends.

2. Structural Adaptations for Transport

Reduced Protoplasm: Mature sieve tube elements lack a nucleus, ribosomes, and a large central vacuole, which creates an unobstructed pathway for the mass flow of sap.
Sieve Plates: The end walls of these cells are perforated with sieve pores, allowing the cytoplasm of adjacent cells to be continuous and reducing resistance to longitudinal flow.
Cellulose Walls: The walls are made of cellulose, providing the mechanical strength necessary to withstand the high hydrostatic pressure generated during trans
Peripheral Cytoplasm: A very thin layer of cytoplasm remains at the edge of the cell, containing only essential components like mitochondria and endoplasmic reticulum to maintain cell viability.

3. The Sieve Tube-Companion Cell Complex

Because sieve tube elements lack a nucleus and many organelles, they cannot sustain themselves independently and are functionally linked to companion cells.

Companion cells are metabolically active cells containing a dense cytoplasm, a large nucleus, and numerous mitochondria to produce the ATP required for active loading of sugars.

Communication and transport between these two cell types occur through numerous plasmodesmata, which are microscopic channels crossing the cell walls.

4. Key Distinctions: Phloem vs. Xylem

5. Exam Strategy & Tips