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In And Out Of Cells - Membrane Structure & Passive Trasport.

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IN AND OUT OF CELLS - MEMBRANE STRUCTURE & PASSIVE TRASPORT For cells to carry out their functions, molecules must be able to pass in and out of them. Every cell has a partially permeable membrane that regulates those substances that enter the cell and those that do not. The complex cell membrane is shown in the fluid - mosaic model, proposed by Singer and Nicolson in 1927.1 The name is derived from the nature of the membrane. Al the particles of the membrane are able to move around in their respective layer past each other, giving the name 'fluid'. These particles are also randomly distributed to produce a patchwork, mosaic-like effect. ...read more.


The concentration of substances varies inside and outside the cell. Diffusion is the movement of such substances from an area of high concentration to a low concentration for even distribution. Consequently, the molecules are moving down its concentration gradient. The rate of diffusion is affected by other factors. A rise in temperature increase the kinetic energy that the molecules have, which means they have increased speed of movement resulting in faster diffusion. However temperature is not always changed and then other factor determine the rate. A greater surface area, eg, with microvilli, across which diffusion takes place increases rate, as does a greater difference in concentration between regions and a smaller length of diffusion path. ...read more.


They travel across the membrane through carrier protein molecules present in the membrane. The lipid - insoluble molecules attach themselves to the carrier protein, which then changes shape and deposits the molecules inside the cell. This process is reversible and depends of the concentration gradient of the substance. As this is a form of diffusion, the cell does not have to provide energy for this reaction to take place. Ions are unable to pass through the phospholipid layer as well. So that necessary ions, eg, Na and Cl, can be brought into the cell there are channel protein in the membrane as well. The hydrophilic water filled pores let ions and water - soluble molecules pass through. This always happen down as concentration gradient as this process is passive, ie, it require no energy to take place. ...read more.

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