Phloem: The Pressure Flow Hypothesis of Food Movement

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  • 0:04 Review of Phloem
  • 2:17 Sinks and Sources
  • 3:15 Pressure Flow Hypothesis
  • 4:31 Lesson Summary
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Lesson Transcript
Instructor: Danielle Weber

Danielle teaches high school science and has an master's degree in science education.

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. 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
Diagram of phloem

Remember that in the monocot, the phloem is found throughout the stem, while in the dicot, the phloem is found in rings.

Phloem is composed of living cells - unlike xylem, which is made of dead cells. Phloem has several components including sieve plates, sieve tube elements and sieve cells. Let's look at each of these sieve parts individually.

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
image of sieve plates

Sieve cells are simple and found in less advanced vascular plants such as seedless vascular plants and gymnosperms. These cells connect together to create a network allowing for the transport of food.

Sieve tubes are more advanced than sieve cells and are found in flowering vascular plants, which are called angiosperms. Sieve tubes have specialized cells called companion cells located near them. These companion cells carry out energy-consuming metabolic activities for the sieve tube elements, which improve the efficiency of the phloem. Companion cells are derived from parenchyma cells, which are a type of ground tissue in plants.

Sinks and Sources

Now that we know a bit more about the structure of phloem, let's look at how phloem actually transports nutrients throughout plants. Remember that most of the movement is of sugars produced in the leaves down to the roots for storage. We will use specific terms for these areas of production and storage.

Sinks refer to areas in the plant that store or use sugars. Generally, sinks are growing roots, stems and fruits. Sugar is needed for energy in these locations so that the cells can grow and develop. Excess sugar is stored in sinks such as roots.

Sources refer to areas in the plant that produce sugars. The main area of production is in the mesophyll of leaves. This area contains cells with plenty of chloroplasts, which perform photosynthesis. Remember that photosynthesis uses carbon dioxide, water and light energy in order to make sugar and oxygen.

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