Lesson Video: Transpiration Biology

Video Transcript

In this video, we’ll find out all about the plant process known as transpiration. We’ll explore how it works, which plant structures are involved, and how different environmental conditions affect how quickly it happens. We’ll also find out how a piece of equipment called a potometer can be used in a laboratory to investigate the rate of transpiration. Let’s get started!

You may recall that plants carry out photosynthesis. This chemical reaction, which predominantly takes place in the leaves, uses light energy to convert carbon dioxide and water into glucose and oxygen. The plant then uses the glucose for all its energy requirements. But where do plants get the carbon dioxide and water they need for photosynthesis from in the first place? The water comes from the soil, and it’s absorbed by the plant through its roots. The carbon dioxide diffuses into the plant through tiny pores on the leaves called stomata. A side effect of having these pores is that when they’re open, water can evaporate and then diffuse out of the plant as water vapor. This loss of water from a plant is known as transpiration, and it’s the subject of this video.

Let’s take a closer look at the stomata to fully understand their role in transpiration. As we’ve already said, stomata are tiny pores, and they’re found in the epidermis, the outermost cell layer of the leaves. Each individual pore is known as a stoma, and collectively they’re known as stomata. Usually, leaves have more stomata on their lower surface than their upper surface. Each stoma is made up of two bean-shaped cells called guard cells, which enclose the stomatal pore. The inner walls of the guard cells are thick and elastic, whereas the outer walls are much thinner. The guard cells are responsible for regulating the opening and closing of the stomata.

We can see that the stoma in this diagram is currently closed. When the stomata are required to open, for example, at sunrise, when photosynthesis can begin, solutes such as potassium and chloride ions are pumped into the guard cells by active transport. This lowers the water potential inside the guard cells, causing water to move into them by osmosis. When water enters the guard cells, the pressure within these cells increases and they become swollen or turgid. This pressure causes the inner walls of the guard cells to pull away from each other, opening the stomatal pore through which gases and water vapor can diffuse. The reverse of this process happens when the stomata need to close. The guard cells lose water and therefore lose their turgidity.

Now that we’ve looked at the structure and function of the stomata, let’s trace the path that water takes through the different structures of the plant during transpiration. As we know, plants absorb water from the soil through their roots. The roots have specialized cells called root hair cells, which extend into the soil and maximize the surface area available for the absorption of water. As represented on this diagram, the soil that the plant is growing in usually has a higher water concentration than the root hair cells do. This causes water to move through the cell membranes and into the root hair cells by osmosis.

The osmotic push of water molecules from the soil into the roots causes an upwards pressure, which is known as root pressure. Because of this pressure, the water absorbed from the soil is pushed upwards through the xylem tissue of the stem. The xylem is the vascular tissue responsible for transporting water and dissolved minerals from the roots up to the stem and leaves of the plant. In some plants, this distance can be very long. For example, coastal redwood trees can grow up to a height of 115 meters. However, root pressure only provides enough force to push the column of water up to a height of about a meter. So how is the water transported the rest of the way? The answer is, of course, transpiration, which provides most of the force needed for water transport in plants.

Here, we can see a cross section through a leaf, showing some of the main cells that it’s made of. During transpiration, when the water reaches the leaf and moves out of the xylem vessel, it evaporates in the spongy mesophyll layer of leaf cells. In other words, it changes state and turns from water, which is a liquid, into water vapor, which is a gas. Because the air in the atmosphere surrounding the plant has a lower concentration of water vapor molecules than the space within the stomatal pore, the water vapor molecules diffuse down their concentration gradient through the stomatal pore, out of the leaf, and into the atmosphere.

You might be wondering why the water vapor moves out of the leaf by diffusion when we normally expect water to move by osmosis. Osmosis only happens when water is in a liquid state. But because it’s evaporated into water vapor here, which is a gas, it moves out through the stoma by diffusion instead. While root pressure pushes water molecules up through the xylem tissues, transpiration exerts an upward pull on the water molecules, causing them to move up into the leaves and replace those that have been lost. This is known as the transpiration pull. This force is what allows plants to transport water carrying dissolved minerals to all the places that need it. Transpiration also acts as a cooling mechanism for leaves in hot conditions.

Transpiration doesn’t always happen at the same rate. It’s affected by changes in the environment that the plant is growing in. There are four main factors which affect the rate of transpiration. The first is light intensity. As light intensity increases, the rate of transpiration also increases. This is because when light intensity is high, plants can carry out lots of photosynthesis, converting carbon dioxide and water into glucose and oxygen. The stomata, therefore, open to allow the diffusion of carbon dioxide from the atmosphere into the leaves. Because the stomata are open, transpiration also takes place as water vapor diffuses out of the leaves. This in turn causes more water molecules to be drawn into the leaf from the roots to replace those which have been lost.

Photosynthesis cannot happen in the dark. So, when light intensity is low, the stomata close. This not only prevents carbon dioxide from entering the leaves, but also means very little water vapor can leave via transpiration. We see a similar pattern with temperature. As the temperature increases, the rate of transpiration also increases. This is because as the conditions become hotter, the water molecules inside the leaf gain more energy, meaning they move around more and become more spread out as the weak intermolecular forces between them break. This is evaporation, and it’s what turns the water into water vapor and allows it to diffuse out of the stomata more quickly.

If plants were exposed to high temperatures for very long periods, they could lose so much water that it would start to damage their tissues. In order to limit water loss by transpiration, the stomata on the surface of the leaves close as the temperature increases. The leaves of some plants may also wilt to conserve water. Wilting collapses the leaves, thereby reducing the surface area available for transpiration. Plants such as cacti, which grow in hot, dry environments, have adaptations which help them to survive. For example, they have spines as leaves, which reduces the surface area available for transpiration. They also have thick, fleshy stems which allow them to store water.

The next factor we need to consider is humidity. Humidity refers to the concentration of water vapor in the air. If the humidity is high, it means there’s a high concentration of water vapor in the air. As humidity increases, the rate of transpiration decreases. This is because transpiration relies on the diffusion of water vapor out of the leaf through the stomata. And diffusion happens fastest when there’s a steep concentration gradient, in other words when there’s a big difference between the concentration of water vapor inside the leaf and the concentration outside the leaf. As you can see from this diagram, when the conditions are humid, there is already a high concentration of water vapor in the atmosphere. So there isn’t much of a difference between the inside and the outside of the leaf. And therefore the rate of transpiration is low.

The fourth and final factor we’re going to look at is wind speed. As wind speed increases, the rate of transpiration also increases. Similarly to humidity, this is due to maintaining a steep water vapor concentration gradient. At high wind speeds, as soon as the water vapor moves out of the leaf by diffusion, it is immediately carried away by the wind at the same time as more water is drawn into the leaf from the roots. So, there is always a big difference between the concentration of water vapor inside the leaf and the concentration outside the leaf. Therefore, the rate of diffusion and hence transpiration is high.

We can measure the rate of transpiration and investigate how it’s affected by environmental changes using a potometer. As we can see in this diagram, a potometer consists of a long tube with a plant shoot cutting at one end and a beaker of water at the other end. The entire system is full of water, and along its length is a capillary tube, containing an air bubble, which we’ve represented here in pink. The idea behind the potometer is that as water is taken up by the shoot and then leaves the plant by transpiration, the transpiration pull on the water will cause the air bubble to move down the capillary tube towards the plant. By positioning a ruler next to the capillary tube, we can measure this movement to make estimates about the rate of transpiration.

Let’s simulate a simple experiment. First, we make sure the air bubble is positioned at zero on the ruler. We then use a stopwatch to measure how far the air bubble moves in a set period of time. In this example, we timed the experiment for 60 seconds and saw that the air bubble moved 15 millimeters. We can now use the equation shown here, speed equals distance divided by time, to calculate an approximate value for the rate of transpiration in the plant shoot. The reason it’s only an approximate value is that not all the water taken up by the plant will diffuse out of the leaves during transpiration. Some of it is likely to be used for other plant processes such as photosynthesis.

If we do the distance traveled by the air bubble, which was 15 millimeters, divided by the time it took to travel that distance, which was 60 seconds, we can calculate that the air bubble was traveling at a speed of 0.25 millimeters per second, which also gives us a pretty good estimate for the rate of transpiration.

As we’ve already seen, there are several environmental factors which affect the rate of transpiration. We can use the potometer setup we have just demonstrated to measure these effects. If we place a light source, such as a lamp, at different distances from the plant, we can measure the effects of changing light intensity on the rate of transpiration. If we do the same thing but this time use a heater or a fan, we can measure the effects of temperature or wind speed. Finally, if we spray the plant with water and then tie a plastic bag over it to keep the water in, we can simulate a humid environment and measure how this affects the rate of transpiration.

It’s important to note that when we are measuring the effect of each of these different conditions, all the other factors must be kept the same. These are known as control variables. And they allow us to be sure that any changes we see in our results are due to the changes we have made to the specific condition under investigation. Now that we’ve learned all about transpiration, let’s have a go at a practice question.

Which of the following best explains the relationship between temperature and the rate of transpiration? (A) As temperature increases, the rate of transpiration increases because water molecules are actively transported out of the leaf faster. (B) As temperature increases, the rate of transpiration increases because water molecules diffuse from the leaf faster. (C) As temperature decreases, the rate of transpiration increases because water molecules are actively transported out of the leaf faster. Or (D) as temperature increases, the rate of transpiration decreases because water molecules diffuse from the leaf slower.

First, let’s describe the relationship between temperature and the rate of transpiration. As temperature increases, the rate of transpiration also increases. Answer options (C) and (D) both suggest that the opposite relationship is true. So we can rule these out immediately as neither one of them can be correct. Answer options (A) and (B) both state the correct relationship, but only one of them has the correct explanation. So, let’s find out which one it is.

Here is a simple diagram of a leaf side-on. The cells of the leaf contain water because they need it as a reactant for photosynthesis and also because it’s produced as a byproduct of cellular respiration. The leaf also contains tiny pores predominantly on its underside, called stomata. The primary role of the stomata is to allow carbon dioxide to diffuse into the leaf as it’s another reactant for photosynthesis. But they can also allow water molecules to move out of the leaf. And it’s this process that’s known as transpiration.

Before water molecules move out of the leaf, they evaporate. You may recall that this is the process whereby a liquid, in this case water, changes state to become a gas. When water is in its gaseous state, it’s called water vapor. Because there’s a higher concentration of water vapor inside the leaf than outside, the water molecules move out of the leaf by diffusion. As the temperature increases, the water molecules gain more energy. This means they move around more and therefore they evaporate and diffuse out of the leaf faster. This is why the rate of transpiration increases as the temperature increases. We have therefore determined that the answer option which best explains the relationship between temperature and the rate of transpiration is (B). As temperature increases, the rate of transpiration increases because water molecules diffuse from the leaf faster.

Let’s summarize what we’ve learned in this video by reviewing the key points. Transpiration is the loss of water from the leaves of a plant through the stomata. Two guard cells regulate the opening and closing of each stomatal pore. Root pressure and transpiration pull work together to transport water up the xylem. The rate of transpiration is affected by a range of environmental factors. And finally, transpiration rates under different conditions can be measured using a piece of equipment called a potometer.