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    Stomata
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    Stoma Background Information

    Definition

    A stoma is a tiny opening or pore, found mostly on the under-surface (epidermis) of a plant leaf, and used for gas exchange.

    Basics

    In botany, a stoma (also stomate; plural stomata) is a tiny opening or pore that is used for gas exchange.

    Air containing carbon dioxide and oxygen enters the plant through these openings where it is used in photosynthesis and respiration. Waste oxygen produced by photosynthesis exits through these same openings. Also, water vapor gets into the atmosphere through these pores in a process called transpiration.

    The pore is formed by a pair of specialized cells known as guard cells which are responsible for regulating the size of the opening and found mostly on the under-surface (epidermis) of a plant leaf.

    Stoma in Greek means "mouth".

    Topics of Interest

    The stoma pore is formed by a pair of specialized cells known as guard cells which are responsible for regulating the size of the opening. Air containing carbon dioxide and oxygen enters the plant through these openings where it gets used in photosynthesis and respiration. Waste oxygen produced by photosynthesis in the chlorenchyma cells (parenchyma cells with chloroplasts) of the leaf interior exits through these same openings. Also, water vapor is released into the atmosphere through these pores in a process called transpiration.

    Carbon dioxide, a key reactant in photosynthesis, is present in the atmosphere at a concentration of about 384 ppm (as of March 2008). Most plants require the stomata to be open during daytime. The problem is that the air spaces in the leaf are saturated with water vapour, which exits the leaf through the stomata (this is known as transpiration). Therefore, plants cannot gain carbon dioxide without simultaneously losing water vapour.

    CAM plants: A group of mostly desert plants called "CAM" plants (Crassulacean acid metabolism, after the family Crassulaceae, which includes the species in which the CAM process was first discovered) open their stomata at night (when water evaporates more slowly from leaves for a given degree of stomatal opening), use PEPcarboxylase to fix carbon dioxide and store the products in large vacuoles. The following day, they close their stomata and release the carbon dioxide fixed the previous night into the presence of RuBisCO. This saturates RuBisCO with carbon dioxide, allowing minimal photorespiration. This approach, however, is severely limited by the capacity to store fixed carbon in the vacuoles, so it is preferable only when water is severely limiting.

    Opening and closure: Confocal microscopy image of an Arabidopsis thaliana stoma showing two guard cells exhibiting fluorescence from green fluorescent protein and native chlorophyll (red). However, most plants do not have the aforementioned facility and must therefore open and close their stomata during the daytime in response to changing conditions, such as light intensity, humidity, and carbon dioxide concentration. It is not entirely certain how these responses work. However, the basic mechanism involves regulation of osmotic pressure.

    Inferring stomatal behavior from gas exchange: Another way to find out whether stomata are open or closed, or more accurately, how open they are, is by measuring leaf gas exchange. A leaf is enclosed in a sealed chamber and air is driven through the chamber. By measuring the concentrations of carbon dioxide and water vapor in the air before and after it flows through the chamber, one can calculate the rate of carbon gain (photosynthesis) and water loss (transpiration) by the leaf.

    However, because water loss occurs by diffusion, the transpiration rate depends on two things: the gradient in humidity from the leaf's internal air spaces to the outside air, and the diffusion resistance provided by the stomatal pores. Stomatal resistance (or its inverse, stomatal conductance) can therefore be calculated from the transpiration rate and humidity gradient. (The humidity gradient is the humidity inside the leaf, determined from leaf temperature based on the assumption that the leaf's air spaces are saturated with vapor, minus the humidity of the ambient air, which is measured directly.) This allows scientists to learn how stomata respond to changes in environmental conditions, such as light intensity and concentrations of gases such as water vapor, carbon dioxide, and ozone.

    Stomata as pathogenic pathways: Stomata are an obvious hole in the leaf by which, as was presumed for a while, pathogens can enter unchallenged. However, it has been recently shown that stomata do in fact sense the presence of some, if not all, pathogens. However, with the virulent bacteria applied to Arabidopsis plant leaves in the experiment, the bacteria released the chemical coronatine, which forced the stomata open again within a few hours.

    Transpiration is a process similar to evaporation. It is the loss of water from parts of plants, especially leaves but also stems, flowers and roots. Leaf surfaces are dotted with openings called stomata, and in most plants they are more numerous on the undersides of the foliage. The stoma are bordered by guard cells that open and close the pore. Collectively the structures are called stomata. Leaf transpiration occurs through stomata, and can be thought of as a necessary "cost" associated with the opening of the stomata to allow the diffusion of carbon dioxide gas from the air for photosynthesis. Transpiration also cools plants and enables mass flow of mineral nutrients and water from roots to shoots.

    Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License and Creative Commons Attribution-ShareAlike License.)

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