Difference Between Source and Sink in Plants

The key difference between source and sink in plants is that source in plants is the site of food production using biological processes while sink in plants is the site of food storage for later use.

Source and sink in plants are two important terminologies used in phloem translocation. Phloem translocation is the process of transportation of the produced food (sucrose) in plants. Thus, sources in plants are the sites that are capable of producing sucrose; plant leaves are the main sources in plants. Whereas, the sites such as stems and roots which store the produced food are the sinks of the plant.

CONTENTS

1. Overview and Key Difference
2. What is Source in Plants
3. What is Sink in Plants
4. Similarities Between Source and Sink in Plants
5. Side by Side Comparison – Source vs Sink in Plants in Tabular Form
6. Summary

What is Source in Plants?

Source of the plant is the site where photosynthesis primarily takes place to produce plant food in the form of sucrose. These sites mainly include the plant leaves. Plant leaves produce sucrose using carbon dioxide and water as their raw materials. However, this process takes place in the presence of sunlight. Chlorophyll pigments trap light energy from the sunlight to produce sucrose. Sucrose is the main form of sugar which can be transported in the phloem tubes. Thus, transportation of sucrose from the production site to its target is called phloem translocation. Phloem loading, which is the process of loading the sucrose to the phloem, takes place at the source.

In addition, the source of a plant can also be a site where nutrient entry takes place; for example, root hairs. The root hairs act as a source for nutrient uptake. Nutrients such as nitrates, nitrites, and phosphates are taken up by plant root hairs. Therefore, they are also source points in plants.

What is Sink in Plants?

Sink in plants is the site where the storage of produced food takes place. Thus, the transportation of produced food in the source will end up in the sink. Therefore, the end point of phloem translocation is the sink. The phloem will unload its contents at the point of the sink. Hence, phloem unloading takes place at the sink. The main sites of the plants which act as a sink are roots, stems, and flowers. The sink stores the produced food as starch. Therefore, the iodine test for starch identification can be used to determine the presence of starch in the sink sites.

In addition, during the amino acid metabolism in plants, the storage of amino acids takes place in root tips. Therefore, root tips also can act as a sink for plants to store amino acids.

What are the Similarities Between Source and Sink in Plants?

• Both source and sink are important in phloem translocation of sucrose.
• Both are present in mature plants which are vascular plants.
• However, non-vascular plants lack both source and sink.
• Moreover, transportation between the source and sink depends on the osmotic pressure.
• And, it takes place by mass flow.

What is the Difference Between Source and Sink in Plants?

In terms of phloem transport, the source and sink play major roles. The source produces the food required for translocation, whereas the sink stores the food brought by translocation. So, this is the key difference between source and sink in plants. Thus, photosynthesis rapidly takes place in the source while photosynthesis does not take place in the sink. Therefore, we can consider this too as a difference between source and sink in plants.

However, the sink is important in storing the produced sucrose, which can be later utilized for the activities of the plant while source does not involve in food storage. Moreover, a major difference between source and sink in plants is that the source loads sucrose for translocation while sink unloads the loaded sucrose.

The below info-graphic presents more information regarding the difference between source and sink in plants.

Summary – Source vs Sink in Plants

Source and sink are important concepts in phloem translocation. Source refers to the site where plants produce their food using photosynthesis. In contrast, sink refers to the site where the plant stores the produced food. Therefore, this is the key difference between source and sink in plants. Furthermore, loading of sucrose to the phloem takes place at the source, whereas unloading of the food takes place at the sink. The typical example of a source site is the plant leaf. Meanwhile, plant roots, stems and flowers are several sinks of a plant.

Reference:

1. “Source and Sink in Phloem Translocation | Plant Physiology.” Biology Discussion, 12 Dec. 2016, Available here.
2. “Transport of Materials from Sources to Sinks at Different Seasons.” Biology Notes for IGCSE 2014, Available here.

Image Courtesy:

1.  “plant, leaves, green” (CC0) via Pixino
2. “Flow and Exchange of Nutrients in the Phloem and Xylem of Plants” By Kl3580 (CC BY-SA 4.0) via Commons Wikimedia