Importance Of Cholesterol In Cell Membranes

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Cholesterol is one of type of steroids which is one the major group of lipids (Citovsky, Lecture 3). Cholesterol plays a role as a major part of cell membranes (Campbell et al., pg. 90). Cholesterol will be placed in between phospholipid bilayer of the cell membrane. It plays a major role in maintaining the membrane fluidity at low temperature and high temperature. The presence of cholesterol in the membrane will stabilize and restrict the movement of the membrane by lowering the membrane’s fluidity at high temperature. On the other hand, cholesterol interrupts the structure of phospholipid bilayer and increases the spacing in between. This prevents phospholipid bilayer’s solidification at low temperature (Campbell et al., pg. 126).

Choice
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Pinocytosis is the mechanism for liquid droplet. It has similar mechanism with phagocytosis. The liquid droplet will be engulfed into cell and is followed by the membrane around it. (Campbell et al., pg. 138). This is why cellular drinking is another name for pinocytosis. (Campbell et al., pg 137). Since pinocytosis does not involve the transport of solid molecules, choice C is not a correct answer.

Choice D, osmosis, is an incorrect answer. Osmosis is a type of passive transport that does not require the ATP hydrolysis (Campbell et al., pg. 131, pg. 139). It involves the movement of water along its concentration gradient. Water will move to a more concentrated solution in order to dilute them (Citovsky, Lecture 14). Such mechanism enabled cells to gain or lose water based on the surrounding environment (Campbell et al., pg. 132). Since osmosis does not involve transport of large molecules, osmosis (Choice D) is an incorrect answer.

Choice E, exocytosis, is an incorrect answer. Exocytosis is a type of mechanism that allows components exit from the cells to the surrounding environment. The vesicles will fuse with plasma membrane and secrete the components out from the cells (Campbell et al., pg. 139). Exocytosis is involved in the movement of macromolecules out from the cells (Campbell et al., pg. 207). Therefore, choice E is an incorrect
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Renal proximal tubular epithelial cells (PTECs) are one of which. PTECs reabsorb and metabolize substances such as albumin and low molecular weight protein through receptor-mediated endocytosis. The endocytosis of substances into PTECs are regulated by a receptor megalin. Megalin is a member of the glycoproteins family which could be found on the membrane of PTECs (Saito et. al, pg. 1). Mutation of megalin will cause malfunction in reabsorption of albumin and low molecular weight. This could be proved with studies that discover a high amount of albumin and low molecular weight protein in urine of patients who suffered from megalin’s mutation. In addition to that, the preservation of vitamin and trace elements such as vitamin A, vitamin D and iron are regulated by megalin as well (Saito et.al, pg 2). In conclusion, receptor-mediated endocytosis not only helps in the movement of cholesterol into cells, it is also an important mechanism in preserving nutrients from being excreting into

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