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Transport across Plasma Membranes

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Transport across Plasma Membranes In cells there are many ways in which molecules can enter and leave cells, these include, Osmosis, diffusion, active transport, exocytosis etc... You may consider that each cell had one plasma membranes which was used to cover the cell and keep everything in, nothing else. This is not true, as structures inside the cells also have plasma membranes, these include endoplasmic reticulums, mitochondria, chloroplasts and there are many more. Also plasma membrane control what goes in and out of the cell. When you may have drawn a cell, you would have drawn the plasma membrane as a straight line, but this is not what it's really like. As it consists of phospholipid bilayer, which means that is has two layers of phospholipid molecules, as you can see from the diagram below, The diagram of a plasma membrane is called a fluid-mosaic model of a membrane structure. The phospholipid molecules have two parts, they the head and the tail. The head is always facing out of the cell and will mix with water but not with fat, this is called a hydrophilic. ...read more.


So they have to go though proteins, which are on both the layer of bilayer. There are two different proteins that work on the bilayer and they are carrier proteins and ion channels. This is also a passive transport, as the molecules are going in the direction of the gradient and does not use any energy Carrier proteins Carrier proteins allow large molecules like glucose, amino acids thorough, which are too large to pass through the phospholipid bilayer. The carrier protein catches the large molecules, and then changes shape, and opens the bottom part of the protein and release the molecules inside the cell. Ion channel This process allows charged particles to enter the cells. The proteins that is used for the ion channels are lined with charged groups. This allows charged molecules like, Ca +, Na+, K+, HCO - and Cl - ions, through which are attracted by the ion channel. Some of the channels are able to control the amount to ions passing into the cells by having gates that are able to open and close, on the top of the ion channels. ...read more.


These protein carriers are found in the phospholipid bilayer. This process it very similar to facilitated diffusion but the proteins work against the gradient. The way that the carrier protein works is, it binds with the molecules, and changes shape by opening the bottom part of the protein and drops the molecules into the cell. The energy that is used it, ATP, which is converted to ADP. Exocytosis This is when the vesicles bud off from the golgi apparatus or from other tubular structures in the cytoplasm. The contents in the vesicles are provided by the ribosomes. The vesicle then travels to the plasma membrane and fuses into the membranes, which secretes their contents out of the cell. Many substances are secreted this way, for example insulin and glucagon. Endocytosis This is when part of the outer membrane bud-off and seals its self and flows back into the cell through the cytoplasm. The plasma membrane, which budded off, contains some substances from outside the cell, which is used inside the cell. During endocytosis, when solid materials are brought into the cell, it is called phagocytosis and when liquids are brought in, they are called pinocytosis. Another example of endocytosis, is when white blood cell engulfs bacteria. ...read more.

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