Facilitated Diffusion

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What is facilitated diffusion? In membrane, there are protein channels (pores) that allow polar molecules (soluble in water but not soluble in lipids) to move through them. The molecules that move through these channels never come into contact with the hydrophobic layer or the membrane's polar surface because these channels provide a continuous pathway for molecules that are moving through them. In this type of diffusion, like simple diffusion, no energy is consumed. Large and polar molecules require help to pass through the membrane. This help is done by a type of protein called carrier (transport) protein . To pass through the membrane, a molecules binds (temporarily) with carries protein in lipid bilayer and is transported from an area of higher concentration to an area of lower concentration.  Related topics: Simple Diffusion
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Why are most cells small?The Knowledge

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Why are most cells small?

Most cells(See cells) are small and can be seen only with the aid of a microscope. (Exceptions include egg of most vertebrate [fishes, amphibians, reptiles, and birds] and long some nerve cells.) One reason for the smallness of cells is that the ratio of the volume of the cell’s nucleus to the volume of its cytoplasm must not be so small that the nucleus, the cell’s major control center, can not control the cytoplasm.

Another aspect of cell volume works to limit cell size, as the radius of a cell lengthens, cell volume increases more rapidly than cell surface area.The need  for nutrients and the rate of waste production are proportional to cell volume.The cell takes up nutrients and eliminates wastes through its surface plasma membrane.If cell volume becomes to large, the surface area to-volume ratio is too small for an adequate exchange of nutrients and wastes.

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