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Transport across Plasma membranes The plasma membrane, known as the Cell surface membrane, ultimately controls the substances that enter and leave the cell. The cell surface membrane is known as the fluid mosaic model; this model controls the movement of substances across the membrane, whether or not they enter via diffusion, osmosis, facilitated diffusion, or active transport. In this essay I will be looking at all of the different types of diffusion and how they enter the plasma membrane, as well as a study on the actual cell membrane itself. The cell membrane consists of phospholipids which make up 75% of the actual lipid; the dual nature of the phospholipids means that a hydrophilic head and a hydrophobic tail (non polar) spontaneously react to give the membrane stability and to make it 'fluid. Lipid soluble molecules can easily pass through the membrane by diffusion whilst hydrophilic substances and ions cannot, and they require facilitated diffusion which requires water-filled pores or channels n the intrinsic proteins. ...read more.


Both however, are the "net movement of molecules or ions from a region of high concentration to a region of low concentration down a concentration gradient". Both do not require an energy source such as ATP, but reply on kinetic energy to diffuse molecules. In facilitated diffusion, the protein channels allow water-soluble molecules or ions to pass through and have a 'gate' for getting the solutes across. Active process is the movement of substances, against a concentration gradient, from a region of higher concentration to a region of lower concentration. Active transport involves carrier proteins, and there are uniport carriers, symport carriers, and antiport carriers, which show the direction in which substances move. Active transport requires ATP (energy from cell metabolism) to move molecules. In active transport, the carrier proteins span the cell surface membrane and accept the molecules or ions to be transported on one side of it. The molecules or ions bind to receptors on the channels of the carrier protein. ...read more.


Macromolecules and larger parcticles, such as bacteria are taken into cells by endocytosis. A bacterium is engulfed in a pocket of plasma membrane which then breaks off and forms a vacuole with the bacterium inside. The small pocket appeared in the plasma membrane is then pinched off to form a vesicle inside the cell. Pinocytosis is similar to phagocytosis, but pinocytosis takes in small droplets of the external solution, forming vesicles. Receptor-mediated endocytosis is a kind of pinocytosis for specific molecules. In receptor-mediated endocytosis, particular molecules to be transported bind to clusters of protein receptors on the outside of the plasma membrane. Large molecules such as proteins are transported out of cells by exocytosis.. The part of the membrane containing the cluster of receptors bulges inwards and pinches off, forming a vesicle containing the particular molecule. Vesicles containing proteins for export break away from the Golgi apparatus. Vesicles migrate to the plasma membrane and the vesicle membrane fuses with the plasma membrane. This process is known as "exocytosis". ?? ?? ?? ?? ...read more.

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