The mechanism of stomata opening and closure
At first glance the mechanism causing this diurnal opening and closing might seem obvious. Unlike other epidermal cells, the guard cells have chloroplasts and at daybreak they start photosynthesizing; this leads to an accumulation of sugar in the guard cells whose osmotic pressure increases. This in turn causes water to be drawn into them from surrounding epidermal cells resulting in the opening of a pore. However this theory is unsatisfactory. It is true that in the light sugar (sucrose mainly) accumulates in the guard cells, but the stomata response is too rapid for it to be explained merely by a resumption of photosynthesis.
So we must look for an alternative explanation. One possible hypothesis depends on the fact that the enzymatic conversion of starch to sugar proceeds more readily when comparatively little acid is present (i.e., at a high pH). The conversion to sugar to starch on the other hand is favored by a comparatively high concentration of acid (low pH). During the night carbon dioxide accumulates in the intercellular spaces of the leaf, and this raises the concentration of carbonic acid. The resulting drop in pH favors the conversion of sugar to starch in the guard cells, thereby decreasing their osmotic pressure and causing the stomata to close. In the morning the resumption of photosynthesis lowers the concentration of carbon dioxide. As a result the level of carbonic acid falls, the pH rises, starch is converted to sugar, the osmotic pressure of the guard cells increases, and the stoma opens.
This theory leaves a number of facts unexplained. For example, starch is absent from the guard cells of certain plants; some guard cells lack chloroplasts but still open and close; and the stomata movements of some plants may not necessarily be related to the time of day; in fact in some plants they open at night and close by day. One possibility is that opening is achieved by ions being actively transported into the guard cells from neighboring epidermal cells, thereby building up the necessary solute concentration for drawing in water by osmosis. There is evidence that in tobacco leaves potassium ions can be actively pumped into guard cells. Alternatively water itself may be pumped into or out of the guard cells.
When the stomata are open carbon dioxide diffuses into the sub-stomata air chambers and thence into the intercellular spaces between mesophyll cells. When it comes into contact with the wet surface of a cell it goes into solution and diffuses into the cytoplasm. The fixation of carbon dioxide in the dark reactions of photosynthesis creates a concentration gradient that carbon dioxide continues to diffuse into the leaf.
Plan:
The aim of this investigation is to try and count the number of stomata, therefore a method has to be devised to try and view the number of stomata. Viewing a leaf under a microscope does not allow the number of stomata to be counted, as the microscope is not powerful enough. Therefore an alternative would be to get an imprint of the leaf. This can be achieved by painting the upper and lower leaf with nail varnish, and when dry to remove the nail varnish and stick it on to some sticky tape and then viewing under a microscope and recording the number of stomata on each side of the leaf.
Fair Test:
To make this investigation a fair test, the test will be carried out on different types of leaves to see if this will affect the number and location of the stomata.
Also three different people will count the number of stomata, so to get an unbiased number and then an average will be taken. The stomata in the field of view will only be counted, to ensure everyone is counting the same surface area. The same magnification will be used when viewing under the microscope.
Safety:
Safety goggles were worn when looking down the microscope, to prevent serious accidents in case someone is pushed.
Apparatus:
Nail Varnish
Sticky tape
Leaves
2 Glass slides
Microscope
Method:
The method was carried out on the following leaves :
Grape Ivy: This plant originates from South Africa and is a climbing plant. It is often found on walls of homes in the country. This plant can cope in moderate to warm conditions.
Crassula: This plant originates from South Africa and is a succulent plant (holds water). This plant tends to grow low on the ground in rocky surroundings.
Spider plant: This plant also originates from South Africa and grows better in dry conditions. It has slim green leaves which then grows clumps of small plants of it. This plant has six fleshy roots which are used to store food. The spider plant can be found in homes in sunny positions.
Begonia: This plant originates from China/Japan and is found in warm to hot conditions. In Britain they are found in greenhouses. The Begonia cannot cope with frosty conditions and therefore cannot grow outside in this country.
Geranium: This is a common plant found in Britain today, however it originates from Iran/Central Asia. It grows better in sunny, warm conditions but can also cope with frosty conditions outdoors.
After looking at different plants you would expect most of then to be well adapted to there habitat in which the live, most of the plants are from hot and dry places like Asia and Africa where for a plant it is essential to prevent water loss as water is scarce in these kinds of places and the loss of excessive water could lead to problems for the plant. From the list of plants above which we have used in our experiment Geranium is a very common plant in our houses and can live in cold and frosty conditions like Britain where water is plentiful.
Method:
A leaf was taken and clear nail varnish was painted on the upper and lower surface of the leaf. This was allowed to dry (ca. 1 minute) and then sticky tape was placed over the nail varnish, the sticky tape was peeled off, bringing with it the varnish which had an imprint of the surface of the leaf. This was repeated both for the upper and lower surface of the leaf.
The sticky tape was then placed on a slide and observed under a microscope and was viewed under 100x magnification. The stomata were counted in the field of view. Three different people counted the number of stomata in the same field of view, an agreement was reached and an average was taken.
View under the Microscope: Stomata
Results:
* Groups which carried out the experiment out once
PLANT GROUPS UPPER
EPI-DERMIS AVERAGE LOWER
EPI-DERMIS AVERAGE
GRAPE IVY
1 0 1 0 2.50
73 46 44 62.0
2 9 * * 85 * *
CRUSSULA
1 26 17 12 18.33 37 31 24 30.67
ONLY ONE GROUP
SPIDER PLANT 1 0 0 0 0.00 66 77 75 71.33
2 0 0 0 66 70 74
BEGONIA
1 0 0 0 0.00 53 48 45 53.33
2 0 0 0 66 55 53
GERRANIUM 1 29 * * 29.00 32 * * 32.00
ONLY ONE GROUP
Graph:
A Graph to Show the number of Stomata on the Upper and Lower Epidermis of
Different types of Leaves.
Results and Discussion
From the graph it can be seen very clearly that most of the stomata will be found on the lower epidermis. Although there is slight variation depending on the type of leaf for e.g. Crassula has an average of 18 stomata on the upper epidermis, but it has nearly double the amount on the lower epidermis. The geranium was the only leaf that did not show a great variation but a clear conclusion can not be drawn from this as the number of stomata were only counted once on both the upper and lower epidermis. The spider plant and begonia had no stomata on the upper epidermis and although the grape ivy showed an average of 2.5 stomata, group 1 only counted 1 stomata but group 2 counted 9, a clear conclusion cannot really be drawn from this because group 2 only carried out the test once.
Evaluation:
The sources of error in my investigation could have been; different people counted the number of stomata, error could have occurred if someone did not know what a stomata looked like or they did not look in the same field of view. To try and overcome this error everyone was given a picture of the stomata before the investigation.
Although magnification was kept constant someone may have adjusted it. When peeling the nail varnish, it was difficult to peel it off completely and there was a chance of mixing up slides. Another similar experiment, which could be carried out, is using cobalt thiocyanate. In the anhydrous state cobalt thiocyanate is blue, but when hydrated it turns pink. A piece of cobalt thiocyanate paper is placed on each side of a leaf and sandwiched between two glass slides clamped together, and then a stop clock started you would measure the time it takes for the cobalt thiocyanate to go pink as this indicates that water has escaped out of the leaf. The time varies in which the colour change takes place depending on the temperature and humidity. Generally the pink colour develops more rapidly on the lower epidermis of the leaf than upper surface, the reason already being discussed in the investigation.
Conclusion:
My prediction that the greatest number of stomata will be found on the lower epidermis was proved correct, as was seen by all the leaves inspected. Other than the Crassula all the other leaves proved without doubt that the greatest number of stomata are found on the lower epidermis. The grape ivy, spider plant, begonia and the geranium were all thin leaves without a visible waxy cuticle so the stomata are located on the lower epidermis to prevent excessive water loss as they have no waxy cuticle to protect them. Also they are relatively thin leaves so the exchange of CO2 and O2 can occur relatively quickly and easily through the stomata of the lower epidermis.
Further Work:
If this investigation was to be carried out again I would use a greater variation of leaves, different shapes, sizes, thickness and leaves from different habitats to see what affect this would have. Also when peeling off the nail varnish the area would be calculated so that everyone was counting in the same area also make sure that everyone repeated the test. Attempts should be made to carry out similar investigations