All of the following are the features of guard cells except
These are the only epidermal cells that contain chloroplasts
Guard cells can undergo rapid turgor changes
The opening and closing of the stomatal pore is a function of guard cells
Their outer and lateral walls are comparatively thicker than inner wall
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Answer:
'Guard cells are specialized plant cells in the epidermis of leaves, stems and other organs that are used to control gas exchange. They are produced in pairs with a gap between them that forms a stomatal pore. The stomatal pores are largest when water is freely available and the guard cells turgid, and closed when water availability is critically low and the guard cells become flaccid. Photosynthesis depends on the diffusion of carbon dioxide (CO2) from the air through the stomata into the mesophyll tissues. Oxygen (O2), produced as a byproduct of photosynthesis, exits the plant via the stomata. When the stomata are open, water is lost by evaporation and must be replaced via the transpiration stream, with water taken up by the roots. Plants must balance the amount of CO2 absorbed from the air with the water loss through the stomatal pores, and this is achieved by both active and passive control of guard cell turgor pressure and stomatal pore size.[1][2][3][4]
Guard cell function Edit
Guard cells are cells surrounding each stoma. They help to regulate the rate of transpiration by opening and closing the stomata. Light is the main trigger for the opening or closing of stomata.[citation needed]
Guard cells contain phototropin proteins which are serine and threonine kinases with blue-light photoreceptor activity. Phototrophins contain two light, oxygen, and voltage sensor (LOV) domains, and are part of the PAS domain superfamily.[5] The phototropins trigger many responses such as phototropism, chloroplast movement and leaf expansion as well as stomatal opening.[5] Not much was known about how these photoreceptors worked prior to around 1998. The mechanism by which phototropins work was elucidated through experiments with broad bean (Vicia faba). Immunodetection and far-western blotting showed blue light excites phototropin 1 and phototropin 2, causing protein phosphatase 1 to begin a phosphorylation cascade, which activates H+-ATPase, a pump responsible for pumping H+ ions out of the cell.[6] The phosphorylated H+-ATPase allows the binding of a 14-3-3 protein to an autoinhibitory domain of the H+-ATPase at the C terminus.[7] Serine and threonine are then phosphorylated within the protein, which induces H+-ATPase activity.[5] The same experiment also found that upon phosphorylation, a 14-3-3 protein was bound to the phototropins before the H+-ATPase had been phosphorylated.[5] In a similar experiment they concluded that the binding of 14-3-3 protein to the phosphorylation site is essential for the activation of plasma membrane H+-ATPase activity.[7] This was done by adding phosphopeptides such as P-950, which inhibits the binding of 14-3-3 protein, to phosphorylated H+-ATPase and observing the amino acid sequence. As protons are being pumped out, a negative electrical potential was formed across the plasma membrane. This hyperpolarization of the membrane allowed the accumulation of charged potassium (K+) ions and chloride (Cl−) ions, which in turn, increases the solute concentration causing the water potential to decrease. The negative water potential allows for osmosis to occur in the guard cell, so that water entered, allowing the cell to become turgid.
Opening and closure of the stomatal pore is mediated by changes in the turgor pressure of the two guard cells.The turgor pressure of guard cells is controlled by movements of large quantities of ions and sugars into and out of the guard cells. Guard cells have cell walls of varying thickness and differently oriented cellulose microfibers, causing them to bend outward when they are turgid, which in turn, causes stomata to open. Stomata close when there is an osmotic loss of water, occurring from the loss of K+ to neighboring cells, mainly potassium (K+) ions[8][9][10]
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