Difference Between Sieve Tubes and Companion Cells

The key difference between sieve tubes and companion cells is that the sieve tubes are the phloem sieve elements that conduct food in angiosperms whereas, companion cells are the associated cells of sieve tubes. Moreover, sieve tubes have pores in the transverse walls while companion cells do not have pores.

Phloem is one of the two types of vascular tissues. It is the vascular tissue that conducts food from photosynthetic parts to other parts of the plant. Phloem consists of different cells. Among them, sieve cells and sieve tubes are the two types of sieve elements that carry out food transportation throughout the plant. Sieve cells are present mostly in seedless vascular plants and gymnosperms. However, sieve tubes are present only in angiosperms. Sieve tubes are associated with living companion cells. Moreover, these sieve tubes and companion cells are unique to angiosperms.

CONTENTS

1. Overview and Key Difference
2. What are Sieve Tubes
3. What are Companion Cells
4. Similarities Between Sieve Tubes and Companion Cells
5. Side by Side Comparison – Sieve Tubes vs Companion Cells in Tabular Form
6. Summary

What are Sieve Tubes?

Sieve tubes are the sieve elements present in angiosperms. They are elongated large cells that have pores in their transverse walls. Moreover, at the young stage, sieve tubes have a nucleus. But with maturity, the nucleus disappears from the sieve tube cells. They have a few number of mitochondria. However, they lack ribosomes, unlike companion cells.

Sieve tube cells are always associated with companion cells for efficient transportation of foods. Sieve cells occur end on end fashion and make a long tube to facilitate the conduction. Furthermore, sieve tube cells connect with each other by plasmodesmata. Compared to companion cells, sieve tubes have a low metabolic rate.

What are Companion Cells?

Companion cells are elongated cells associated with sieve tube elements of angiosperms. These cells have a lot of mitochondria and ribosomes. Furthermore, they have a nucleus throughout their life time. Hence, these companion cells are metabolically highly active. However, these cells are small cells compared to sieve tubes.

Similar to sieve tubes, companion cells are present only in angiosperms. Unlike sieve tubes, companion cells do not have pores in their transverse walls.

What are the Similarities Between Sieve Tubes and Companion Cells?

• Sieve tubes and companion cells are unique to angiosperms.
• Both are phloem elements.
• Also, they are living cells.
• Both are involved in the conduction of food (sucrose) from photosynthetic parts to other parts of the plant.
• Furthermore, they are elongated cells.
• Besides, both types of cells contain mitochondria, ER and cellulose cell walls.
• Moreover, there are plasmodesmata between them.

What is the Difference Between Sieve Tubes and Companion Cells?

Sieve tubes are the phloem sieve elements that conduct food in angiosperms. Whereas, companion cells are the associated cells of sieve tubes. So, this is the key difference between sieve tubes and companion cells. Furthermore, a significant difference between sieve tubes and companion cells is that the sieve tubes have pores in their transverse cell walls while companion cells do not have pores.

Moreover, companion cells have a lot of mitochondria, ribosomes and a nucleus. Hence, a further difference between sieve tubes and companion cells is that the companion cells are metabolically highly active than the sieve tubes. Also, generally, sieve tubes are larger cells than the companion cells. Therefore, we can consider this also as a difference between sieve tubes and companion cells.

 Below infographic summarizes the difference between sieve tubes and companion cells.

Summary – Sieve Tubes vs Companion Cells

Sieve tubes are the cells that conduct food in angiosperms. These cells are wider cells arranged in end on end fashion to make a tube. They are associated with small nucleated cells called companion cells. Companion cells are metabolically highly active since they possess a lot of mitochondria and ribosomes. Sieve tube cells have pores or sieve plates in their transverse walls. But companion cells do not have pores. Both sieve tubes and companion cells are living cells. They are unique to angiosperms. Thus, this is the summary of the difference between sieve tubes and companion cells.

Reference:

1. “Sieve Tube.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 3 Feb. 2011, Available here.

Image Courtesy:

1. “Figure 30 05 07” By CNX OpenStax –(CC BY 4.0) via Commons Wikimedia
2. “Figure 30 05 06” By CNX OpenStax – (CC BY 4.0) via Commons Wikimedia