IB 9.2 - Transport in Phloem

Summary

This video explores the movement of sugars through the phloem in plants, a vital process closely linked to water movement in the xylem. Sugars, mainly sucrose, are transported from the leaves (the source) to storage locations (the sink) through active and passive transport mechanisms. The process involves companion cells loading sucrose into phloem tubes, creating a pressure gradient that drives the movement of sap. Water follows the sugar into the phloem via osmosis, and once the sugar is unloaded at the sink, water is recycled back into the xylem. The video also highlights the specialized features of phloem cells and their crucial role in long-distance carbohydrate transport.

Takeaways

• 😀 The phloem is responsible for transporting organic nutrients, particularly sucrose, throughout the plant, complementing the xylem's role in moving water.
• 😀 Phloem transport is an active-passive process involving energy, where sucrose moves through sieve tubes driven by pressure gradients.
• 😀 Sugars produced through photosynthesis in leaves are converted into sucrose for easier transport and are moved to storage locations called 'sink cells'.
• 😀 The movement of water from the xylem to the phloem happens by osmosis due to a high concentration of sucrose in the phloem tubes.
• 😀 Water follows the sugar into the phloem, creating a pressure buildup that moves the sap down to sink cells, where the sugar is stored or used.
• 😀 At sink cells, sucrose is unloaded by companion cells and stored as starch or used by the plant, and the water is then recycled back into the xylem.
• 😀 The process of moving sugars and water from source cells to sink cells is called mass flow, and it relies on both active and passive transport mechanisms.
• 😀 In the apoplast pathway, sucrose moves through the cell wall to companion cells, where active transport and ATP help load the sucrose into the phloem.
• 😀 The symplast pathway involves sucrose moving through the cytoplasm of plant cells, driven by active transport and hydrogen ion pumps that create a concentration gradient.
• 😀 Phloem sieve tubes are living cells with specialized structures, including perforated walls, to withstand the pressure of moving sap and help load and unload sugars effectively.

Q & A

What is the primary role of the phloem in plants?

The phloem is responsible for transporting organic nutrients, particularly sucrose, to various parts of the plant, such as developing fruits, seeds, and growing leaves or stems.

How is the movement of sugars in the phloem connected to the xylem?

The movement of sugars in the phloem is connected to the xylem through pressure gradients. Water moves from the xylem to the phloem by osmosis, helping transport sugars, and this process relies on similar principles of fluid movement as seen in the xylem's movement of water.

What is the role of companion cells in the phloem?

Companion cells are responsible for loading sucrose into the phloem and unloading it at sink cells. They also use energy (ATP) to assist in these processes, making active transport of sugars possible.

Why does water move from the xylem to the phloem?

Water moves from the xylem to the phloem by osmosis due to the high concentration of sugar in the phloem. The movement helps increase the volume of sap and create the pressure needed to transport the sugars through the phloem.

What happens to the sugar once it reaches the sink cell?

Once the sugar reaches the sink cell, it is unloaded by companion cells through active transport. The sugar is then stored as starch in the sink cell, usually in roots or fruits, or used for growth.

How does active transport contribute to sugar movement in the phloem?

Active transport is used to move sucrose into the phloem, requiring energy in the form of ATP. This process creates a concentration gradient, allowing the movement of sucrose against the natural diffusion gradient.

What are the two main pathways for sucrose movement into the phloem?

The two main pathways for sucrose movement into the phloem are the apoplast pathway, where sugars move through the cell wall to companion cells, and the symplast pathway, where sugars move through the cytoplasm of cells.

What is the role of mitochondria in sieve tubes of the phloem?

Mitochondria in sieve tubes provide the energy needed for active transport processes, such as loading and unloading sugars in the phloem. They help generate ATP, which is essential for the movement of sucrose.

What are sieve plates, and how do they function in the phloem?

Sieve plates are perforated walls in sieve tube cells that slow the movement of phloem sap. They have small holes that allow sap to pass through but regulate the flow, ensuring efficient transport of sugars and water.

How does osmosis contribute to the transport of water and sugars in the phloem?

Osmosis helps water move from areas of high water concentration in the xylem to low water concentration in the phloem. This movement of water supports the transport of sugars, as it increases the volume and pressure needed to move the phloem sap toward the sink cells.