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PostPosted: 13 Aug 2019 14:21 
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TRANSPORT OF MOLECULES INTO & OUTSIDE CELL
 The composition of the cell and state of homeostasis is maintained because the cell membrane is selectively permeable to small molecules.
 However, most biological molecules cannot pass freely through phospholipid bilayer of the cell membrane which thus forms a protective barrier that prevents free exchange of molecules between the external environment to within the cell.
 In this case, specific transport proteins (carrier proteins and channel proteins) help the selective transport of specific small molecules across the membrane, so that the cell is able to regulate its content
 The simplest way of transport of molecules across the cell membrane is passive diffusion

PASSIVE DIFFUSION
 Passive diffusion is the easiest method by which molecules cross the cell membrane. During passive diffusion, the molecule just dissolves in the phospholipid double layer of the cell membrane, diffuses across it, and then enters the cytoplasm of the cell on the other side.
 Passive diffusion does not involve utilization of energy for transport of the molecule
 In passive diffusion, no carrier or membrane proteins are involved and the direction of transport of the molecule is simply decided by the relative concentrations of the molecule within and outside the cell.
 The net flow of molecule is always from an area of higher concentration to a region of lower concentration of the molecule i.e. down the concentration gradient.
 Gases such as O2 and CO2, and small polar but uncharged molecules such as alcohol and water can also diffuse across the cell membrane

FACILITATED DIFFUSION
 Facilitated diffusion helps transport of charged and polar molecules such as carbohydrates, amino acids, and ions across the plasma membrane.
 Like passive diffusion, facilitated diffusion refers to movement of molecules down the concentration gradient depending on their relative concentrations within and outside the cell. Here also no energy utilization is needed
 However, facilitated diffusion is different from passive diffusion since the transported molecules do not dissolve in the cell membrane’s phospholipid bilayer. Instead, their transport is aided by proteins that help to transport the molecules across the membrane without dissolving in it
 Two classes of proteins help in facilitated diffusion namely carrier proteins and channel proteins.
 Carrier proteins attach to the molecule to be transported on one side of the cell membrane. Certain structural changes then take place in the protein that allows the molecule to be transported to the other side

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 On the other hand, channel proteins or ion channels simply create open pores in the membrane, allowing small molecules of the appropriate charge and size to pass through the membrane
 Some channels open in response to binding of neurotransmitters or other molecules (these are termed ligand gated channels); others open in response to alteration in electric potential across the cell membrane (voltage gated channels).
 Although ion channels occur in all cell membranes, their function has been well characterized in nerve and muscle cells, where they help in conduction of nerve impulses

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