The photosynthetic carbon reduction cycle (PCR)

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**Explain how CO2 enters leaves. What environmental factors control stomatal movement? How are these factors related to physical and chemical properties that control the opening and closing of stomata?

The main route by which carbon dioxide (CO2) enters any plants is through the special part of the plant called leaves, which are primarily its most important structure. Leaves have broad, flat, and thin shapes, which allow them to have an excellent surface areas for photosynthesis (2). Photosynthesis is defined as the production of oxygen and glucose from carbon dioxide (CO2), water, and light (Bailey, About.com Guide, 1). Carbon dioxide (CO2) enters leaves through pores called stomata located on the underside of all the leaves (2). At night, when there is no photosynthesis and no sunlight, the stomata tend to close. In addition, when there is extreme heat in the environment, the plants are in danger of losing too much water from its leaves through evaporation; a process called transpiration and the stomata will close (2). However, in the daytime when there is sunlight and photosynthesis, the stomata will open allowing CO2 to enter plants for photosynthesis ( http://en.wikipedia.org/wiki/Stoma, 2).

The opening and closing of stomata is control by the specialized cells called the guard cells, which are located in the epidermic on the leaf of plants. These pair of guard cells surround tiny stomata airway pores (2). The action of shrinking and swelling by the guard cells of the stomata dictates the action of closing and opening of the stomata, respectively. Guard cells control how the plant takes in CO2 from the environment for photosynthesis by regulating the opening and closing of the stomata. Guard cells also regulate the amount of water evaporating or transpired from the plants by controlling opening and closing of stomata. In addition, the tugor pressure of guard cells control the opening and closing of stomata (2).

What is turgor pressure? It is pressure that changes inside and outside the plant guard cells as a result of the movement of ions and sugars into and out of the guard cells (2). When the guard cells are open, they take up ion solutes (Potassium, chloride, sugars, etc), which lead to a decrease in water potential inside the cells causing water flow into the guard cells (2). As a result, the osmotic flow of water into guard cells increases the turgor pressure causing the swelling of the guard cells (2). Finally, swelling of guard cells cause stomata to open (2). When guard cells are closed, they do not take up solute ions which leads to an increase in water potential, and cause water to flow out of cells; this lead to a decrease in the turgor pressure causing shrinkage of guard cells (2).
This property of turgor pressure is exactly what drives the opening and closing of the stomata; that is ions and sugars that moves into the guard cells causing a change in turgor pressure and drive the opening of the stomata pores allowing take up of solutes and ions for the plant.

The physical factors that control the opening and closing of stomata are the turgor pressure and ion movement that cause an imbalance between the inside and outside of the guard cells. This change in turgor pressure is what drives stomata opening and closing.

The environmental factors controlling stomatal movement are water stress, such as drought and salt stress. In response to drought, a plant hormone, abscisic acid (ABA) is produced, which causes the stomatal pores to close in response to dry environment preventing the transpiration of water or water loss (2). This closing of stomata by ABA reduces plant water loss via transpiration, and allows plants to slow down their water loss during the period of droughts (2).

Stomatal openings and closing is cause by other physical factors such as the potassium pump on the guard cells (2). When potassium ...