Lesson Video: Translocation in the Phloem Biology

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


Video Transcript

In this video, we’ll discuss an important process that’s happening all the time inside plants, translocation. We’ll find out what it is, how it works, and how different cell types in the plant are adapted to support it. We’ll also analyze some key experiments that have led to our current understanding of translocation. So just sit back, relax, and go with the phloem.

Just like humans have blood vessels to transport substances around the body in the blood, plants have vessels for transport too. You may recall that plants carry out photosynthesis, a chemical reaction in which light energy is used to convert carbon dioxide and water into glucose and oxygen. The water required for photosynthesis is absorbed in the roots and then transported up the plant in the xylem, while the glucose produced by photosynthesis is primarily converted into sucrose and transported around the plant in the phloem. Phloem transport is known as translocation, and it provides all the parts of the plant with the sugars they need for cellular respiration.

When we describe translocation, we often talk about the movement of sucrose from a source to a sink. But what does this actually mean? The source is just the part of the plant from which sucrose originates, and the sink is where it’s being transported to. The source and the sink for translocation are different depending on the time of year. In the summer, plants experience long daylight hours. This means lots of photosynthesis can take place in the leaves, in turn producing lots of glucose. The leaves are therefore the source, and the roots are the sink, as they can store excess sugars in the form of starch.

On the other hand, in the winter, it’s a different story. Because the daylight hours are now short, there is little light available for photosynthesis. Plants therefore rely on the sugars that were stored in the roots during the summer months, meaning the roots become the source instead. And assuming the plant keeps its leaves through the winter, they become the sink. As you can see, translocation is capable of transporting sucrose both up and down the plant. It is therefore described as a bidirectional process.

Now let’s have a look at the different structures which together form the phloem. The main transport vessel of the phloem is the sieve tube, which is made of sieve tube elements stacked end on end. Sieve tube elements are living cells whose nuclei and organelles have broken down to leave a hollow space inside. This space allows dissolved substances, such as sucrose, to easily flow through the sieve tube. The end walls of the sieve tube elements are partially broken down to form what are known as sieve plates. The sieve plates contain pores through which dissolved sucrose can pass, allowing it to travel easily from one sieve tube element to the next.

Closely associated with each sieve tube element is a specialized cell known as a companion cell. Among other adaptations, companion cells contain many mitochondria. This allows them to carry out cellular respiration at a high rate and, hence, release lots of energy. As we’ll see later, translocation requires large quantities of energy because it’s an active process.

You may notice that the companion cells are connected to the sieve tube elements via narrow channels. These channels are called plasmodesmata, with each one being known as a plasmodesma. Plasmodesmata form a cytoplasmic link between the companion cells and the sieve tube elements. And their function is to assist with the movement of sucrose into the sieve tube.

So how does translocation actually work? Let’s imagine it’s the middle of winter and our plant cannot produce sufficient glucose by photosynthesis for all its energy requirements. It’s going to need to transport sugars from the roots, where they’re stored as starch, all the way up to the leaves, providing the plant has kept its leaves through the winter like this one has. So the roots are the source for translocation, and the leaves are the sink. In the source cells of the roots, large insoluble starch is converted into smaller soluble sucrose so it can be transported in the phloem. Sucrose then diffuses from a high concentration in the source cells to a low concentration in the tissues surrounding the phloem. When it reaches these tissues, sucrose is actively transported into the companion cells.

Active transport requires energy, which is why companion cells need so many mitochondria. Because the concentration of sucrose is now higher in the companion cells than it is in the sieve tube elements, it diffuses into the sieve tube aided by the plasmodesmata. This demonstrates why the companion cells play such an important role in translocation. They maintain a steep sucrose concentration gradient, not only between the source cells and the tissues surrounding the phloem, but also between the companion cells and the sieve tube elements, meaning sucrose can move continuously from the source cells into the sieve tube.

The increase in sucrose concentration inside the phloem causes the water potential in these cells to decrease, allowing water from a nearby xylem vessel to move into the phloem by osmosis. When cells contain lots of water, the water molecules exert a high pressure on the cell walls. We call this turgor pressure. And because so much water has moved into the sieve tube elements by osmosis, the turgor pressure inside them is very high. The high turgor pressure allows the sieve tube to transport sucrose by mass flow. Mass flow is just the name given to the movement of fluids down a pressure gradient, in this case from an area of high turgor pressure in the source to an area of low turgor pressure in the sink.

So let’s see what happens when the sucrose reaches the sink. In our example, the leaves are acting as the sink. When the sucrose gets here, it is at a much higher concentration in the sieve tube elements than in the companion cells and sink cells. Therefore, it diffuses from the sieve tube into these cells. It will then either be transported to other adjacent leaf cells or converted into glucose for cellular respiration. As sucrose moves out of the sieve tube elements, the water potential inside them rises. So water follows the sucrose out of the phloem and into the surrounding sink cells by osmosis.

As we’ve seen, translocation is quite complicated, so let’s have a brief recap of the process. First, sugars produced by source cells are converted into sucrose, which diffuses into cells surrounding the phloem. Next, the sucrose is actively transported into the companion cells, where it can then diffuse into the sieve tube elements via the plasmodesmata. The low water potential generated in the sieve tube elements causes water to move into the phloem by osmosis. This generates a high turgor pressure, which transports sucrose up or down the sieve tube by mass flow.

When it arrives at the sink, sucrose diffuses out of the phloem tissues, where it can ultimately be used for respiration or storage. The loss of sucrose increases the water potential in the sieve tube, causing water to leave by osmosis. Water also sometimes moves back into the xylem to be transported up the plant.

Now, let’s have a look at some of the experiments that have led to our current understanding of translocation.

The first experiment was carried out in 1945 by two scientists called Rapeden and Bohr. In this investigation, they supplied one leaf of a green bean plant with carbon dioxide containing carbon-14. Carbon-14 is a radioactive version of carbon, which can be easily traced using a detector. When the leaf carried out photosynthesis, the sugars it produced contained the radioactive carbon. As the scientists traced the subsequent movement of the sugars in the plant, they saw that it was transported both up and down the stem. Rapeden and Bohr could therefore conclude from this experiment that translocation is bidirectional. It can transport sucrose in both directions.

The next experiment we’re going to look at was carried out by a different scientist called Mittler. Mittler wanted to find out exactly what substances were being transported by the sieve tube of the phloem. To do this, he used small insects called aphids, which feed directly on the sugars in the phloem. They can feed this way by inserting their sharp mouth part known as a proboscis into the stem of the plant and utilizing the high turgor pressure of the phloem to extract their food.

In Mittler’s investigation, he separated the proboscis of an aphid from its body while it was feeding. He then analyzed the contents of the proboscis as well as examining the region of the plant where it had been inserted by the aphid. Mittler found that the sample contained sucrose as well as amino acids, which are the building blocks of all proteins. He therefore concluded that translocation involves the transport of sucrose and amino acids in the sieve tube.

Now that we found out all about translocation in the phloem, let’s have a go at a couple of practice questions.

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

You may recall that plants produce sugars in the form of glucose during photosynthesis. Photosynthesis predominantly takes place in the leaves of plants. But the sugars that are produced are needed by all parts of the plant for processes such as cellular respiration, the building of strong cell walls, and for storage. These sugars therefore need to be transported around the plant. And they do this through a process called translocation.

Any part of a plant which produces or releases sugars for translocation is known as a source. And a sink is any part of the plant that these sugars are transported to. Although we often think of photosynthesis in the leaves as the source and storage in the roots as the sink, this is mainly the case in the summer months when the plant is receiving lots of light energy. In the winter, when there’s far less light available to the plant, stored sugars in the roots will become the source, and respiring organs, such as the leaves, assuming they’ve been retained through the winter, will become the sink.

Now we can answer the question. The completed sentence would be “Translocation is primarily the movement of sugars from sources to sinks.”

Let’s try another question.

Which of the following best describes the structure of the phloem? (A) The phloem is comprised of living sieve cells that form a long, continuous tube. (B) The phloem is comprised of many dead sieve cells that have pores in their cell walls to allow movement of substances through the plant. (C) The phloem is comprised of dead sieve cells that form a long, continuous tube. Or (D) the phloem is comprised of many living sieve cells that have pores in their cell walls to allow movement of substances through the plant.

Let’s remove the multiple-choice options for now and remind ourselves about the structure of the phloem. You may recall that translocation is the process whereby substances such as sucrose are transported around a plant in the phloem. The structure of the phloem can be represented by this simple diagram. The main transport vessel of the phloem is called the sieve tube. The sieve tube is made of living sieve cells, which are also known as sieve tube elements. These are stacked end on end to form one continuous tube.

The end walls of the sieve tube elements are called sieve plates. These contain pores which allow substances to move from one sieve tube element to the next. Sieve tube elements also contain plasmodesmata. Plasmodesmata are narrow pores located between each sieve tube element and its associated companion cell. And they allow substances to more easily diffuse into the sieve tube for transport in the phloem.

We now have enough information to answer the question. The statement which best describes the structure of the phloem is (D). The phloem is comprised of many living sieve cells that have pores in their cell walls to allow movement of substances through the plant.

Let’s summarize what we’ve learnt in this video by reviewing the key points. Translocation is the movement of sucrose around a plant from a source to a sink. It’s bidirectional, which means it can transport substances both up and down the plant. Translocation occurs in a specialized tissue called the phloem, which consists of sieve tube elements and companion cells. Companion cells contain many mitochondria to provide energy for active transport. Sieve tube elements are linked to companion cells via cytoplasmic extensions called plasmodesmata. And finally, sucrose is transported in the phloem by a process called mass flow.

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