There are 4 theories to explain stomatal movement: the theory of photosynthesis in guard cells, the theory of glycolate metabolism, and the two that apply to your question, the starch sugar interconversion theory and active K+ transport. Keep in mind that stomata must open to allow the plants to obtain carbon dioxide, a reactant necessary for photosynthesis to proceed. Chlorophyll, water, and light are also required.
The starch sugar interconversion theory contends that it is the conversion of starch to glucose in the presence of light that opens the stomata. Overall, during the day, when the stomata are open, they contain sugar (glucose) and during the night, when stomata are closed, they contain starch (a sugar polymer). When light is available, carbon dioxide is constantly being removed for use in photosynthesis and this increases the pH (makes it more basic) of guard cells. At the same time, starch is being broken down to glucose by enzymes including phosphorylase, phosphoglucomutase, and phosphatase. This conversion increases the osmotic pressure of the guard cells (free glucose results in a higher osmotic pressure than starch) and water moves in to the guard cells due to the osmotic pressure. This increases the turgor pressure of the guard cells and opens the stomata. Again, this provides an opening for carbon dioxide to enter the cells of the leaves.
At night, when light is no longer available, photosynthesis ceases and this results in an increased carbon dioxide concentration within the cells (this decreases the pH). The glucose is also converted back to starch with the enzymes hexokinase and phosphorylase. This conversion decreases the osmotic pressure within the guard cells and water leaves the guard cells and turgor pressure within the guard cells decreases. With decreased turgor pressure, the stomata close.
The theory of active K+ transport shares some components with the previously mentioned theory. In the presence of light, there is a decrease in starch content of the guard cells (see above), but this results in an increased concentration of malic acid in the guard cells. The starch is first converted to hexose sugars (like glucose) and then in the presence of oxygen, the glucose is converted to malic acid. This malic acid dissociates to form malate ions and protons (H+). These protons that are in the guard cells are then exchanged (this requires ATP and the plant hormone cytokinin) for K+ (potassium ions). Thus, H+ decreases within the guard cells and K+ increases within the guard cells. The loss of protons increases the pH of the fluid in the guard cells (this was also a feature of the other theory--see above). The K+ reacts with the malate ions to form potassium malate. The guard cells also take in Cl- (chloride ions) to maintain ionic balance. The potassium malate and Cl- increase the osmotic pressure within the guard cells causing water to move into the cells by osmosis. This increases turgor pressure and causes the stomata to open.
In darkness the process is reversed. The concentration of carbon dioxide increases as photosynthesis ceases, pH decreases, and starch is formed. Both K+ and Cl- are effluxed (moved out of the guard cells) and there is an influx of H+. This results in decreased osmotic pressure within the guard cell and water leaves by osmosis. This decreases the turgor pressure within and the stomata close. Abscisic acid aids in this process because it blocks uptake of K+ and efflux of H+ by guard cells in the dark.
Plants produce carbon dioxide at night as they respire. Just like us, plants need to break down sugars in order to produce ATP (the energy currency of cells). During this process (in plants and animals), oxygen is used to break down glucose resulting in the production of water, carbon dioxide, and ATP (energy). This process takes place in the mitochondria of plants and animals (all eukaryotes). The ATP is then used in processes like building proteins that require energy.
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