Sunday, June 26, 2011


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The aim of this science investigation is to find out where the stomata are located, on the upper or lower epidermis of a leaf.


My prediction is that most of the stomata are found on the lower epidermis of a leaf. I have based this prediction on the function of stomata; to let gases in and out of the leaf i.e. to allow exchange of CO and O between the inside of the leaf and the surrounding atmosphere and to allow the escape of water vapour from the leaf.

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To reduce water loss the leaf has a waxy cuticle on the upper epidermis, which is waterproof, so the leaf uses the lower epidermis for gas exchange.

Science about Stomata

Stomata are pores perforating the epidermis of the leaves and stem. They are usually most numerous in the lower epidermis of the leaf where there may be as many as 400 per mm, there are generally fewer in the upper epidermis and fewer in the stem. Their functions are

(1) To allow exchange of carbon dioxide and oxygen between the inside of a leaf and the surrounding atmosphere.

() To permit the escape of water vapor from the leaf.

Stomata are important in several physiological processes, not just photosynthesis, but they also represent a hazard in that they may permit excessive evaporation from the leaf. In a sense a plant aces a conflict. If it opens its stomata it runs the risk of losing excessive water, particularly if it lives in a dry habitat. On the other hand if it closes them it may run short of carbon dioxide or oxygen. Plants resolve this problem by not opening them for longer than is necessary. This does not mean that a plant never loses more water than it can replace from the soil. The observation that plants frequently wilt in hot weather bears witness to the fact that they often do lose excessive water. However wilting is not disastrous so long as the plant is given an opportunity to recover later. The controlled opening and closing of the stomata resolve the conflicting needs of the plant. essaybank.co.uk

In order to appreciate how the stomata are controlled we must first understand the mechanism by which they open and close, and this necessitates looking at their structure.

A pair of guard cells borders the stomata pore. These are sausage-shaped and, unlike other epidermal cells, contain chloroplasts. There is a sap vacuole and, a point of great importance; the inner cellulose wall (i.e. the wall lining the pore itself) is thicker and less elastic than the thinner outer wall. wwec ecw esececs ayec ecba nec kcec ecuk.

Stomata opening and closure depends on changes in turgor of the guard cells. If water is drawn into the guard cells by osmosis the cells expand and their turgidity is increased. But they do not expand uniformly in all directions. The thick, inelastic inner wall makes them bend. The result is that the inner walls of the two guard cells draw apart from each other and the pore opens. The same effect can be achieved by blowing up a sausage- shaped balloon to which cellotape has been stuck down one side. As it is blown up it will bend over towards the cellotaped side. It is thought that in normal circumstances when a stoma opens the turgidity of the guard cells is increased by their taking up water from the surrounding epidermal cells. Isolated stomata will open when immersed in water, but if placed in a hypertonic solution, they close. wwge gew esgeges ayge geba nge kcge geuk. wwfa faw esfafas ayfa faba nfa kcfa fauk;

When do the stomata open and close? This can be investigated by means of a perometer, an instrument for measuring the resistance to the flow of air through a leaf. If you attach a perometer to a leaf and take measurements of its resistance to airflow at intervals, you will find that there is a generally less resistance during daylight hours than at night. This is because the stomata open during the day and close at night.

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. wwfe few esfefes ayfe feba nfe kcfe feuk.

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. EnNAYQM Visit essaybank bd co bd uk bd for more bd Do not bd redistribute EnNAYQM wwaa aaw esaaaas ayaa aaba naa kcaa aauk

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. wwcc ccw esccccs aycc ccba ncc kccc ccuk!


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.

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