Lesson Explainer: Translocation in the Phloem Biology

In this explainer, we will learn how to describe the process of translocation in plants.

Plants are amazing organisms that can not only create their own food but also move it around. They have tube-like transport structures that allow them to move nutrients to different organs. This process can be slow, with substances traveling at speeds ranging from 0.01 metres per hour in the slowest plants to 1 metre per hour in some of the fastest translocating plants. This process tends to be slow in larger plants, such as conifer trees, but they can still transport around 250 kg of sugars down their trunk in just one year. This is far faster than simple diffusion can account for. So, how do these plants transport materials so far and so fast?

Photosynthesis is the process by which plants make their own food. Plants use light energy from the Sun to convert carbon dioxide from atmospheric air and water absorbed via the roots to release sugars, such as glucose, and oxygen. You can see this occurring in Figure 1.

Figure 1: A diagram showing the reactants and products of photosynthesis in plants.

Definition: Photosynthesis

Photosynthesis is the process by which green plants convert carbon dioxide and water into sugars, such as glucose, and oxygen in the presence of sunlight.

Glucose is a sugar that can be used in cellular respiration to release energy. The same process happens in human cells too. Glucose is needed in all plant cells for them to release energy to carry out important life processes, such as growth and division. The question is, how is glucose distributed to all of the cells in the plant?

Translocation is the process by which materials in the plant are moved from where they are made to where they are needed. The prefix trans- means “across,” referring to the directional movement of substances, and the suffix -location refers to the places they are moved from and to. The products of photosynthesis that are moved around are called assimilates.

Plants convert the sugars made during photosynthesis into sucrose by combining glucose with another sugar called fructose. Sucrose is translocated rather than glucose because sucrose is a nonreducing sugar. This means that sucrose does not react with oxygen during aerobic respiration while being transported.

Key Term: Translocation

Translocation is the movement of sucrose around a plant from the source to the sink.

Reaction: Sucrose Formation before Translocation

Glucose+fructosesucrose

The location where the assimilates (the products of photosynthesis) are produced is called the source. During translocation, the assimilates are moved to a location called the sink. The main sources of assimilates are the leaves and stem, which have cells with many chloroplasts for photosynthesis. Major sinks are regions that require a lot of energy, such as the locations where sucrose may be immediately used or organs that will store sugars as starch. For example, a sink may be growing roots that absorb minerals by active transport and, therefore, require sugars to release energy. A sink may also be meristem cells, which actively divide at the tip of the shoot or root. Sinks can also be storage organs, such as developing fruits.

Key Term: Sources

Sources are the areas in a plant where sugars are produced or stored and are, therefore, the starting point of translocation.

Key Term: Sinks

Sinks are the areas in a plant where sugars are transported to be used or stored and are, therefore, the destination points of translocation.

Definition: Active Transport

Active transport is an energy-demanding process by which particles move across a plasma membrane from an area of low concentration to an area of high concentration.

Although materials almost always move from the source to the sink during translocation, the organs that are considered to be the location of the source or sink change throughout the year and throughout the plant’s life. This seasonal change in the sources and sinks in various parts of the plant is shown in Figure 2.

Figure 2: A diagram comparing the main sources and sinks of translocation in the summer and spring.

In the summer, the source of sucrose is the mature leaves and stem of the plant, as this is where the majority of photosynthesis takes place. The sink in the summer is the roots, as the assimilates are stored there while excess sugars can be made during the long daylight hours of the summer. In fact, root vegetables, such as potatoes and carrots, are actually the sugar-storing organs of those plants. This is why these organs caramelize when cooked.

Although plants move sugars in the form of sucrose, they usually store it in the form of starch in these organs. Sugars are stored as starch as starch is a large insoluble molecule. This means that it does not interfere with the water concentration in cells by increasing the solute concentration as sucrose or glucose would. Sinks are also present in the flowers, fruits, stems, and developing leaves.

In the spring and winter, the daylight hours are still short, so there is not much light available for photosynthesis. Therefore, the roots that have been storing sugars become the source of translocation. Sucrose is transported from the source in the roots to the sinks in the leaves so they can restart growth. As the movement of sucrose can be in more than one direction, this movement is described as bidirectional.

Key Term: Bidirectional

The term bidirectional describes how sucrose moves in more than one direction, up and down the plant, during translocation to be delivered from the source to the sink as these change seasonally.

Example 1: Describing the Process of Translocation

Use the terms “sources” and “sinks” to complete the following sentence: Translocation is primarily the movement of sugars from to .

Answer

Translocation is the process by which sugars are moved around the plant, from where they are made to where they are needed.

The place where sugars are produced is called the source, and the place they are moved to is called the sink. The main sources of sugars are the leaves and stem, which have cells with many chloroplasts for photosynthesis. Major sinks are regions that require a lot of energy for life processes, such as the locations where sugars may be immediately used or organs that store sugars as starch.

For example, a sink may be growing roots, which absorb minerals by active transport and, therefore, require sugars to release energy. A sink may also be meristem cells, which actively divide at the tip of the shoot or root. Sinks can also be storage organs, such as developing fruits.

Therefore, translocation is primarily the movement of sugars from sources to sinks.

Translocation occurs in the phloem tissue, which consists of tube-like structures called phloem vessels. These phloem vessels run from the leaves into every other part of the plant and are responsible for transporting dissolved organic solutes, such as sucrose and amino acids, from the sources to the sinks.

Key Term: Phloem

The phloem is a tissue in plants that transports the products of photosynthesis to the cells of the plant.

Example 2: Identifying the Vessel Responsible for Transporting Organic Solutes

Which vessel in the plant is responsible for transporting sugars and amino acids?

Answer

Translocation is the process by which assimilates from photosynthesis are moved around the plant, from where they are made to where they are needed. The place where these assimilates are produced is called the source, and the place they are moved to is called the sink.

Translocation occurs in the phloem tissues, which consist of tube-like vessels called phloem vessels. These phloem vessels run from the leaves into every other part of the plant and are responsible for transporting dissolved organic solutes, such as sucrose and amino acids, from the sources to the sinks.

Therefore, the vessel responsible for transporting sugars and amino acids is the phloem.

Let’s look at the structure of the phloem vessels to understand how they carry out translocation.

Figure 3: A diagram showing the structure of the phloem tissue, including sieve tube members linked to companion cells by plasmodesmata.

The phloem tissue consists of two main types of living cells, which you can see in Figure 3: sieve tube members (or sieve tube elements) and companion cells. They also contain fibers and sclereids that have thick cell walls to provide structural support to the phloem vessels.

Key Term: Sieve Tube Members

Sieve tube members are long, hollow columns of cells fused end to end with porous sieve plates to allow the passage of solutes along the phloem.

Key Term: Companion Cells

Companion cells are specialized cells that provide ATP for sucrose transport and are linked to sieve tube members by plasmodesmata.

Sieve tube members are long, hollow columns of cells fused end to end, and their end walls are partially broken down. There are sieve tube plates between each adjacent sieve tube member, which, much like a sieve, have holes to allow the solutes to pass through. To make the sieve tube members hollow, the majority of their organelles break down, and mature cells have no nucleus. This means that dissolved solutes, such as sucrose and amino acids, can easily flow through the sieve tubes in the phloem.

Companion cells are linked to the sieve tube members by pores in their cell walls called the plasmodesmata, which connect the two cells’ cytoplasm, as you can see in Figure 3. Companion cells contain a nucleus, many ribosomes to synthesize proteins, and mitochondria to release energy through cellular respiration. You can see the mitochondria and nucleus of each companion cell in Figure 3. These cells have many mitochondria, as they demand a high energy output as a result of the active transport that they carry out.

Example 3: Describing the Structure of the Phloem

Which of the following best describes the structure of the phloem?

  1. The phloem is comprised of many dead sieve cells that have pores in their cell walls to allow movement of substances through the plant.
  2. The phloem is comprised of many living sieve cells that have pores in their cell walls to allow movement of substances through the plant.
  3. The phloem is comprised of dead sieve cells that form a long continuous tube.
  4. The phloem is comprised of living sieve cells that form a long continuous tube.

Answer

Generally, the phloem tissue consists of two main types of living cells, which you can see in the diagram below: sieve tube members (or sieve tube elements) and companion cells.

Sieve tube members are long, hollow columns of cells fused end to end, and their end walls are partially broken down. There are sieve tube plates between each adjacent sieve tube member, which, much like a sieve, have holes to allow the solutes to pass through. To make the sieve tube members hollow, the majority of their organelles break down, and mature cells have no nucleus. This means that dissolved solutes, such as sucrose and amino acids, can easily flow through the sieve tubes in the phloem.

Companion cells are linked to the sieve tube members by pores in their cell walls called the plasmodesmata, which connect the two cells’ cytoplasm and allow the movement of substances between them, as you can see in the figure above. Companion cells do contain a nucleus, in addition to many ribosomes to synthesize proteins, as well as mitochondria to release energy through cellular respiration.

Therefore, the best description of the phloem is that it is comprised of many living sieve cells that have pores in their cell walls to allow movement of substances through the plant.

Let’s look in more detail at what is happening between these sieve tube members and companion cells.

Let’s say a palisade mesophyll cell in a plant leaf has synthesized glucose through photosynthesis. This glucose is first converted into sucrose so that it can be easily transported via the phloem without being metabolized during respiration along the way by reacting with oxygen.

Sucrose, the assimilate, is transported by diffusion from the source in the leaf palisade mesophyll cells across their cell walls and spaces between the cells. It eventually reaches the tissues surrounding the phloem in the leaf. You can see this process of diffusion occurring in Figure 4. The sucrose diffuses up until this stage, which does not require energy as there is a higher concentration of sucrose in the palisade cells, where it is made, than in the tissues surrounding the phloem.

Figure 4: A diagram demonstrating how sucrose diffuses from palisade cells, where it is synthesized, to tissues surrounding the phloem.

Sucrose then needs to be actively transported into the companion cells and sieve tube members. This process does require energy, and this is why the companion cells must have many mitochondria to release this energy through cellular respiration. You can see this active process occurring in Figure 5.

Figure 5: A diagram outlining how assimilates move from the cells producing them to the phloem tissues.

Companion cells have membranes with a large surface area due to folding. This large surface area facilitates a high rate of active transport of sucrose into the cell cytoplasm. Once sucrose has enters a companion cell, it can diffuse along the cytoplasmic links between these cells and their adjacent sieve tube members via the plasmodesmata, as you can see in Figure 6.

Figure 6: A diagram showing how water and sucrose move into the sieve tube members for translocation.

As the concentration of sucrose in the phloem tissue increases, the water potential in these cells decreases. This reduced water potential causes water to flow into the companion cells and sieve tube members via osmosis. This builds up turgor pressure inside the cells and allows the sieve tubes to transport sucrose via mass flow through the phloem from areas of high pressure in the source to areas of low pressure in the sinks. The pressure in the phloem is about 125 times higher than in that in a human artery. This is what allows the plants to transport assimilates so far and so fast.

Once the sucrose reaches the sinks, where this turgor pressure is significantly lower, it diffuses into the cells surrounding the phloem, for example, root cells, as you can see in Figure 7.

Figure 7: A diagram outlining how sucrose (pink) diffuses into the cells at the sinks, causing water (blue) to follow via osmosis.

Here, the sucrose is either transported to other adjacent cells or converted into glucose for cellular respiration or into starch for storage. This means that a steep sucrose concentration gradient from the phloem to the surrounding sink tissues is maintained, as the concentration of sucrose in the phloem is kept low. This means that the water potential rises, so water follows the sucrose out of the phloem and into the surrounding cells at the sink, and even into the xylem to be transported back up the plant.

Example 4: Describing the Adaptations of Companion Cells

The diagram provided shows an outline of a phloem vessel.

How are companion cells adapted for their role of actively transporting substances in and out of the sieve tube members?

Answer

Phloem tissues consist of two main types of living cells, which you can see in the diagram above: sieve tube members (or sieve tube elements) and companion cells.

Companion cells are linked to the sieve tube members by pores in their cell walls called the plasmodesmata, which link together the two cells’ cytoplasm and allow the movement of substances between them, as you can see in the figure above. Companion cells do contain a nucleus, in addition to many ribosomes to synthesize proteins, as well as mitochondria to release energy through cellular respiration.

Once the sucrose reaches the phloem tissue in a source, it needs to be actively transported into the companion cells and sieve tube members. This process does require energy, and this is why the companion cells must have many mitochondria to release this energy through cellular respiration. Companion cells have membranes with a large surface area due to folding. This large surface area facilitates a high rate of active transport of sucrose into the cell cytoplasm. Once the sucrose enters the companion cells, it can diffuse along the cytoplasmic links between these cells and their adjacent sieve tube members via the plasmodesmata.

Therefore, companion cells are adapted for their role as they contain many mitochondria that provide energy via cellular respiration.

Several experiments have been carried out to improve our understanding of the phloem’s structure and function.

In 1945, Rapeden and Bohr used radioactive 14C to mark the carbon dioxide supplied to a species of bean plant. This radioactive carbon can be traced as it is used in the plant first during photosynthesis to synthesize carbohydrates. It is then either used in respiration or transported into the phloem for translocation to other plant organs. These marked carbohydrates were observed to move both up and down the stem, showing that translocation is bidirectional.

Mittler aimed to investigate the contents of sieve tubes in the phloem. He proposed that aphids feed by piercing into sieve tubes with a sharp mouthpart called a proboscis. These aphids utilize the high turgor pressure of the phloem by the source to feed on the sugary contents, as you can see in the photograph below.

English Grain Aphid (Sitobion avenae) winged adult on barley
Figure 8

Mittler inferred this as the proboscis remained inserted into the sieve tubes by the source when the body of the aphid was removed. Upon inspecting the sample consumed by the aphid’s mouthparts, he discovered that the sieve tubes contain sucrose and amino acids.

Observations by Thain and Canny in 1961 provided evidence that there are long cytoplasmic threads extending from one sieve tube member to its adjacent cell. This introduced the theory of cytoplasmic streaming, as the cytoplasm contained these same organic solutes of sucrose and amino acids. Cytoplasmic streaming suggests that organic solutes are moved around the cytoplasm of sieve tubes and companion cells in a cyclical manner via the plasmodesmata. You can see an outline of this process occurring in Figure 9.

Figure 9: A diagram showing cytoplasmic streaming between sieve tube members and companion cells.

Thain and Canny also suggested that the process of translocation requires ATP, which is supplied by the companion cells. Over the years, developments in microscopy have allowed us to observe the adaptations of the companion cells that support this theory, such as possessing many mitochondria. The presence of a lot of mitochondria in a cell suggests that this cell has high energy demands, such as carrying out active transport, which must be met via cellular respiration. It has also been observed that when the mitochondria of companion cells are poisoned, translocation stops entirely, providing more evidence for the active transport theory. Translocation is slower when the temperature and cellular oxygen content are lower, both of which decrease the rate of respiration. This evidence suggests that Thain and Canny were correct, as less aerobic respiration would result in less ATP, therefore slowing down the rate of translocation.

Let’s recap some of the key points that we have covered in this explainer.

Key Points

  • Translocation is the movement of assimilates around the plant from the source to the sink.
  • Translocation is bidirectional, so dissolved solutes can move either up or down the plant.
  • The phloem consists of sieve tube members and companion cells, which carry out translocation.
  • Companion cells contain nuclei and many mitochondria.
  • Sieve tube members are linked to companion cells via cytoplasmic extensions called the plasmodesmata.

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