Leaves this lesson, we will look at how

Leaves produce sugars and stems; roots and fruits use these sugars for energy. In this lesson, we will look at how these sugars move throughout vascular plants, including the importance of phloem and the pressure flow hypothesis in the process.

Review of Phloem

You may remember that vascular tissue is the tissue used to transport water and nutrients throughout a plant.

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There are two types of vascular tissue: xylem and phloem. Phloem transports food, while xylem transports water and dissolved minerals. We will focus on the structure of phloem as well as how this vascular tissue actually transports food throughout a plant.Let’s first review a few basic aspects of phloem. You may remember that phloem is found in vascular plants but not in non-vascular plants. Since phloem is a type of vascular tissue, this presence or absence of phloem makes sense. Within vascular plants, the arrangement of phloem is different depending on the specific type of flowering plant.

In monocots, such as grasses, phloem is found in paired bundles with xylem throughout the stem. In dicots, such as trees, the phloem is found in concentric rings. The xylem is on the inside of the ring, and the phloem is on the outside of the ring. We can see the layout of phloem in a monocot and in a dicot here.

Phloem in a monocot and in a dicot
Phloem transports food and is made of living cells with several components
Phloem living cells

Sieve plates are areas of the phloem with large pores.

These openings allow for food products to get into and out of the phloem for transport to different areas of the plant. We can see the sieve plates below. Note the large pores in this structure.

Sieve plates have large pores
The main area of production is in the mesophyll of leaves

Pressure Flow Hypothesis

Now that we’ve covered sinks and sources, let’s look at the pressure flow hypothesis. This model of how phloem works is based on the relationship between sinks and sources.

The pressure flow hypothesis of food movement states that dissolved sugars flow from sources and are released at sinks where they are used. This means that dissolved sugars enter the phloem at sources (such as the mesophyll layer of leaves) and move through the phloem until they reach a sink (such as a growing root) where they are needed.This movement of sugars creates a pressure difference between the source and the sink, allowing for continued movement of dissolved sugars in the phloem. The pressure difference is sort of like supply and demand. When some of the sugar is used in the sink, it will demand more from the source. This pressure for more sugar will help new sugar enter the phloem at the source and move to the sink in order to meet the demand.

If the sink does not need sugar, there will be a pressure resistance, which will cause the sugar to move to a different sink. For example, if the stem is growing slowly, it will not need much sugar for energy. This means that, rather than going to the growing stem, the sugar will instead go to a storage sink such as a root.

Lesson Summary

Phloem is vascular tissue that moves food throughout the plant. It does this through a series of tubes that connect sugar sources (such as leaves) to sugar sinks (such as growing fruits, stems and roots).

Phloem can be made of sieve cells, sieve tubes and sieve plates. Some advanced vascular plants also have companion cells that help improve the effectiveness of phloem. The pressure flow hypothesis helps explain how dissolved sugars move from sugar sources to sugar sinks. When sinks need sugar, the pressure difference between the source and sink causes dissolved sugars to move to the area of need.

Excess sugars can be stored in areas such as roots to be used later.

Learning Outcomes

At the conclusion of this lesson, you will be able to:

  • Describe the structure and function of phloem
  • Define sinks and sources
  • Explain the pressure flow hypothesis

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