Three separate experiments which are to be carried out to investigate a plant's unique property of transpiration.

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Joe Harper        Page         04/05/2007

BIOLOGY COURSEWORK – TRANSPIRATION IN PLANTS

AIM:

I have three separate experiments which are to be carried out to investigate a plant’s unique property of transpiration. I will carry out the following three experiments to investigate transpiration:

  • The use of a potometer to investigate the rate of transpiration and the relationship between leaf surface area and water uptake. This experiment will take place over a double lesson.
  • Using a microscope to find the number of stomata on one of the Laurel leaves and then using the results to calculate the number of stomata on each leaf.
  • My third experiment to prove that the substance being lost from the leaves is water by using cobalt chloride paper.

SCIENTIFIC BACKGROUND:

Overall plant structure:

The following diagram shows the overall structure of a plant, and identifies the uses of each different part of the plant:

As you can see by looking at the diagram to the left, there are several key parts to the plant, all of which have a major part in the transport of water in plants (i.e. transpiration).

        The roots can be seen towards the bottom of the diagram. The roots suck up the water from the soil which the plant is in. They do this because of the added effects of transpiration and capillary rise. Capillary rise is where a liquid climbs up a very narrow tube, defying gravity almost. As the water evaporates from the leaf, water must fill up the xylem. This water is sucked up from the ground through the roots.

        It must then travel to the leaves through little vessels in the stalk. It does this by travelling through the xylem tissue in the vascular bundle. The following diagram shows these vessels in the plant and show how the water travels through the xylem.

This diagram shows the microscopic vessels in the plant which transport sugar and water (through the xylem), and food substances (through the phloem).

        You can see how the vascular bundle comes from the roots and to the leaves through the stalk.

        The cells in the xylem which carry water to the leaves from the roots are known as vessels. They are a collection of long cells, one ply thick, positioned end to end so that the water can be passed through them to the leaves. The following diagram is of a vessel (long cells joined together).

As you can see by looking at diagram (a), long thin cells are placed together, end to end, to form a 1-cell-thin vessel which carries water to the leaves. These cells make up the tissue called xylem.

        Notice the thickened bands looping around the tube. These are to support and hold the structure of the vessel. They act like a skeleton, making sure that the vessel is not crushed.

Leaf Structure:

The following diagram shows the cellular structure of a leaf. It is a cross section of a leaf cut along the y-axis. It shows us the different types of cells in the leaf and what different jobs they do.

As you can see on the diagram, the xylem brings water vapour into the leaf. The water vapour then moves throughout the cells (blue arrows), to finally exit the leaf through the stomata.

        I will be measuring the quantity of water that passes through the stomata (blue dotted arrow shows evaporation) and therefore evaporates in experiment number one.

As you can see in the next diagram, there are several different types of cell in the leaf.

Figure 6.6 shows the structure of the cells around the stomata, known as guard cells.

        The guard cell’s function is to prevent CO2 entering the leaf during the night, when photosynthesis is not talking place, and to prevent excess water loss through the stomata. Generally the guard cells are ‘open’ in daylight and ‘closed’ at night time.

        As you can see, the guard cell has a thickened wall, which allows the cell to ‘expand’ and ‘contract’ to close and open the stomata when the time comes for it to do so.

        The following diagram shows the guars cells in their ‘open’ and ‘closed’ positions.

It is not fully understood how the guard cells know when to open or close but we do know that when they do need to open, water passes into the guard cells from the other cells by osmosis, and the cell becomes turgid. When the cell is flaccid, the stomata close. Because the wall facing the stomata is thick, the cell cannot stretch. This causes the cells to stretch in such a way that they ‘bow’ as shown in (a), opening the stomatal pore.

The Factors affecting transpiration:

There are several different factors which affect transpiration. We conducted a preliminary experiment for three reasons, one, to learn the basics of the potometer and how to use it, two, to learn about the factor affecting transpiration, and three, to decide what measurements/times to use in the real experiment. In the preliminary experiment, we didn’t take down results, but made a note of how much water was taken up by the roots (and therefore how much water was lost through the leaves by evaporation).

        We set our potometers up in several different locations (and therefore different circumstances) to identify some of the factors that affected the rate of transpiration.

        We set the potometer up first in the lab, in general conditions (no wind, moderate light and indifferent humidity. We noted the levels of transpiration. Then, we repeated the experiment, but whilst blowing a hot hair dryer on the leaves, to see if air movement (wind) and heat affected transpiration. We came to the conclusion that they do. The heat affects transpiration because it causes the water to evaporate from the leaves much more quickly; therefore the plant sucks up more water, which, in turn, is evaporated from the leaves. Also, the air movement around the leaves affects the rate of transpiration, because it blows the water that has come from the stomata off the leaf; causing water to be lost more quickly, therefore more water is sucked up.

        We then took the potometers outside, to see if the humidity affected the rate of transpiration. We discovered that it did have an affect. This is because, if the air has a high water content, then the water from the leaf will not evaporate into the air as easily as when the air has not much water in it. This is because humidity works along the laws of diffusion, therefore, the water from the leaf is more likely to evaporate because diffusion is the spreading of a substance from an area of high concentration to an area of low concentration.

        Another factor which affects the rate of transpiration is light intensity. Light intensity does not affect the rate of evaporation however, but when it is light, and the plant is photosynthesizing, the stomata are open, which allows more water to pass through them, this, in turn, affects the rate of transpiration because the more water that is lost, the more water has to be taken up to fill the xylem vessels, which helps stop them from collapsing.

PREDICTIONS:

The potometer experiment:

There are several predictions that can be transpired for this experiment.

  • I predict that as transpiration occurs, the bubble will move from left to right on the scale, because when it occurs, water is taken up by the roots to the leaves, so water will be sucked up from the apparatus.
  • Leading on from the previous prediction, I predict that as I remove more leaves from the plant, the bubble will slow down in its movement, however it will still move from left to right. It will slow down as I remove more leaves as I believe that the number of leaves has a direct affect on the water loss, due to the fact that water is lost through the stomata on the leaves, therefore, if there are less leaves (indeed less stomata), then less water will be lost from the leaves (stomata), and so less water will need to be taken up.
  • I predict that the number of leaves removed from the experiment will have a direct, quantitative effect on the rate of transpiration. I predict that the number of leaves will proportionately affect the rate of transpiration. I.e. if the bubble on the potometer travels 10cm in 2 minutes (example figure) when the plant has 100% of its leaves intact, then with 75% of its leaves intact, it will travel 7.5cm in 2 minutes, and with 50% of its leaves intact, it will travel 5cm in 2 minutes etc. These figures are not my guess of what the actual figures will be, but merely an example to explain my theory of the fact that the number of leaves on the plant will have a direct, quantitative effect on the rate of transpiration. Of course, this is dependant on the selection of leaves to be removed having roughly the same number of stomata per removal, i.e. each 25% of leaves removed have the same number of stomata (roughly) as the next 25% to be removed.
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Counting the number of stomata experiment:

  • I predict that many stomata will be found on the bottom of the leaf and none on the top of the leaf. This is because the top of the leaf is covered in a waxy film called a cuticle which prevents water loss; therefore there will be no stomata on the top layer. On the bottom layer, that waxy cuticle still exists but is much thinner, to allow for evaporation from the stomata.
  • I predict that there will be the same number of stomata per square millimetre on each leaf. ...

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