What is the Difference Between Sieve Tubes and Companion Cells

The main difference between sieve tubes and companion cells is that sieve tubes are the long, narrow, pointed tubes present in the phloem of angiosperms whereas companion cells are attached to the sieve tubes, regulating the activity of sieve tubes. Furthermore, sieve tubes do not contain a nucleus or ribosomes while companion cells contain a nucleus and ribosomes.

Sieve tubes and companion cells are two types of closely-associated cells present in the phloem of angiosperms. Their main function is the transport food throughout the plant.

Key Areas Covered

1. What are Sieve Tubes
     – Definition, Structure, Function
2. What are Companion Cells
     – Definition, Structure, Function
3. What are the Similarities Between Sieve Tubes and Companion Cells
     – Outline of Common Features
4. What is the Difference Between Sieve Tubes and Companion Cells
     – Comparison of Key Differences

Key Terms

Companion Cells, Nucleus, Phloem, Sieve Tubes, Transport Carbohydrates

What are Sieve Tubes

Sieve tubes are the most advanced type of sieve cells that only occur in the phloem of angiosperms. They are made up of a series of sieve tube elements arranged end to end in a longitudinal manner, forming a tube. Moreover, the end walls of the sieve tube elements are horizontal and broad. They contain sieve plates with pores whose size can be controlled with time. Sieve plates also allow the transport of higher amounts of materials due to its increased surface area.

Figure 1: Sieve Tube in the Herbaceous Dicot Stem

Additionally, the transverse walls of sieve tube elements contain wide pores, which allow cytoplasmic connections to neighboring cells, moving photosynthetic material and other organic molecules necessary for tissue function.

What are Companion Cells

Companion cells are the cells associated with the sieve tube elements. Unlike sieve tubes, companion cells contain a nucleus and a higher number of ribosomes and mitochondria. Therefore, these cells are responsible for the regulation of the functions of sieve elements. Generally, companion cells are connected to the sieve tubes through plasmodesmata.

Figure 2: Companion Cells Function

Furthermore, companion cells provide ATP and necessary signaling to transport carbohydrates throughout the plant. They also help the bidirectional flow of carbohydrates through the sieve tubes. Additionally, companion cells regulate the transport of carbohydrates into the sieve tubes.

Similarities Between Sieve Tubes and Companion Cells

• Sieve tubes and companion cells are two types of cell found in the phloem of angiosperms.
• Their main function is to transport food throughout the plant.
• Also, both originate from the meristem, and the epigenetic factors drive their differentiation.
• They form the sieve element-companion cell complex.
• And, both are living cells that have an elongated shape.
• Furthermore, they are connected through plasmodesmata.
• Besides, both cells contain mitochondria, ER, and a cell wall made up of cellulose.

Difference Between Sieve Tubes and Companion Cells

Definition

Sieve tubes refer to a series of sieve tube elements placed end to end to form a continuous tube while companion cells refer to a specialized parenchyma cell, located in the phloem of flowering plants and closely associated in development and function with a sieve-tube element. Thus, this is the main difference between sieve tubes and companion cells.

Size 

Another difference between sieve tubes and companion cells is their size. Sieve tubes are large cells while companion cells are small cells.

Sieve Plates

Moreover, sieve tubes contain sieve plates while companion cells do not contain sieve plates.

Pores in the Transverse Wall

The transverse wall of sieve tubes contains pores while the transverse wall of companion cells does not contains pores. Hence, this is one other difference between sieve tubes and companion cells.

Nucleus and Ribosomes

While companion cells contain a nucleus and a large number of ribosomes, sieve tubes do not.

Amount Mitochondria

Moreover, sieve tubes contain fewer mitochondria while companion cells contain a higher number of mitochondria.

Metabolic Activity

Metabolic activity is also a difference between sieve tubes and companion cells. The former shows low metabolic activity while the latter shows high metabolic activity.

Function

Sieve tubes are responsible for the transport of food throughout the plant body while companion cells are responsible for regulating the activity of the sieve element.

Conclusion

Sieve tubes are a component of the phloem of angiosperms, responsible for the transport of food throughout the plant. A series of sieve tube elements forms a sieve tube. Sieve tubes contain pores in its transverse wall and they contain sieve plates at each end of the sieve tube. In contrast, companion cells are associated with each sieve tube element. They contain nuclei and a higher amount of ribosomes and mitochondria. Their main function is to regulate the function of the sieve tubes. Therefore, the main difference between sieve tubes and companion cells is their structure and function.

References:

1. Sengbusch, Peter v. “The Phloem.” Botany Online: Supporting Tissues – Vascular Tissues – Phloem, Available Here.

Image Courtesy:

1. “Herbaceous Dicot Stem: Tangential Section Sieve Tubes in Cucurbita” By Berkshire Community College Bioscience Image Library (Public domain) via Flickr
2. “Figure 30 05 07” By CNX OpenStax (CC BY 4.0) via Commons Wikimedia