What is the role of companion cells in the movement of sugars through plants?
a. They are the sites of sugar production by photosynthesis.
d. They are the sites where starch is converted to sucrose.
c. They secrete sucrose, which draws sugars through phloem under negative pressure.
d. They accumulate sucrose, which is then transferred to adjacent sieve-tube elements.

Verified step by step guidance

1. Understand the structure and function of phloem tissue in plants. Phloem is responsible for transporting sugars, primarily sucrose, from the leaves (where photosynthesis occurs) to other parts of the plant. It consists of sieve-tube elements and companion cells.
2. Learn about sieve-tube elements. These are specialized cells in the phloem that form a continuous channel for the transport of sugars. However, they lack nuclei and other organelles, which makes them dependent on companion cells for metabolic support.
3. Explore the role of companion cells. Companion cells are closely associated with sieve-tube elements and are connected to them via plasmodesmata (small channels that allow the exchange of substances). Companion cells actively accumulate sucrose and other solutes, which are then transferred to the sieve-tube elements.
4. Understand the mechanism of sugar movement. Companion cells use active transport to load sucrose into sieve-tube elements. This process creates a high concentration of solutes in the sieve-tube elements, leading to water influx by osmosis. The resulting pressure drives the movement of sugars through the phloem.
5. Review the correct answer based on the explanation. Companion cells accumulate sucrose and transfer it to adjacent sieve-tube elements, facilitating the movement of sugars through the phloem. This matches option 'd'.

Key Concepts

Companion Cells

Companion cells are specialized parenchyma cells found in the phloem of flowering plants. They are closely associated with sieve-tube elements and play a crucial role in the transport of sugars. These cells help maintain the function of sieve-tube elements by providing them with metabolic support, including the loading and unloading of sugars like sucrose.

Phloem Transport

Phloem transport is the process by which nutrients, particularly sugars produced during photosynthesis, are distributed throughout the plant. This movement occurs through a system of sieve tubes and companion cells, where sugars are actively loaded into the phloem and transported to various parts of the plant. The process relies on pressure differences created by the loading and unloading of sugars.

Sugar Loading and Unloading

Sugar loading refers to the process of transferring sucrose from source cells (like those in leaves) into the phloem, primarily facilitated by companion cells. This process often involves active transport mechanisms that require energy. Unloading occurs at sink tissues (like roots or fruits), where sucrose is either utilized for energy or converted into starch for storage, ensuring efficient nutrient distribution.