英语论文网

nd glycogen as a storage molecule. The first, insulin, lowers blood glucose levels by promoting its conversion to glycogen. The second, glucagon, increases glucose levels by allowing glycogen to be phosphorylated. Both of these hormones are produced and released by specialised cells in the pancreas known as Islets of Langerhans. Insulin is released from β-cells, and glucagon is released from α-cells.

 

Figure 2: Blood glucose control by insulin and glucagon
If the blood glucose level is too high, more insulin and less glucagon is released. This causes cells to take in glucose from the blood, while the liver converts glucose to glycogen. During low levels of blood glucose however, https://www.51lunwen.org/ glucagon release increases, activating the breakdown of glycogen to glucose in the liver, and glucose is released into the blood. This is a good example of negative feedback control, as the lowering of blood glucose, for example, inhibits further insulin secretion.Importantly, insulin is dependent upon calcium. This is because glucose activates calcium channels. When glucose levels are high, the subsequent release of calcium results in calcium binding to calmodulin. Together, the two molecules promote insulin vesicles to be released from the pancreas. This demonstrates the negative feedback system discussed in the overview.

 

Example 3: Control of water loss by plants
Plants need to balance their need to conserve water with their need to photosynthesise energy. Transpiration causes water to be pulled up through the plant passively as water diffuses out through the leaves. These pores are opened and closed by the action of surrounding guard cells, located as illustrated in Figure 3.

 

Figure 3: Drawing of stoma & guard cells
These guard cells can take on two extremes of conformation; either flaccid, to close the stoma, or turgid, to open the stoma. When guard cells take in water via osmosis, they swell, become turgid, and are forced to bulge outwards into a kidney shape, opening the stoma. They adhere to this shape both because the two cells are attached to each other at either end, and because cellulose microfibrils constrain them. However, if the guard cells lose their water content, they shrink and become flaccid, closing the stoma so that water cannot leave.The opening and closing of the stomata have been shown to be affected by light concentrations. When illuminated, the concentration of solutes in the guard cell vacuoles increases https://www.51lunwen.org/ because starch is converted to malic acid, and a proton pump in the plasma membrane is stimulated. The proton pump removes hydrogen ions (H+) from the guard cells, and in response, potassium ions (K+) flow into the cell. Chloride ions (Cl-) also flow into the cell via another pump in response to the H+ concentration difference. The accumulation of these ions and malate in the vacuole of the guards cells is enough to cause the water potential to drop within the guard cells. Water then flows in by osmosis, leading to the turgidity just described and opening the pore. As this opening process occurs in light, exactly the opposite happens at night. As light is lost, channels open to conduct Cl- and K+ out of the guard cells, water is lost, and the cells become flaccid and close.Another stimulus for the closing of stomata is an emergency response to the plant wilting from lack of water. In this case, CO2 concentration increases inside the leaf cel本论文由英语论文网提供整理，提供论文代写，英语论文代写，代写论文，代写英语论文，代写留学生论文，代写英文论文，留学生论文代写相关核心关键词搜索。