The Knowledge

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

Movements across plasma membrane/How molecules move through plasma membrane? Membrane allows specific substances to cross through it (selective permeability).This feature prevent harmful substances to enter inside. There are different types of movements by which molecules enter or exit the cell. We will discuss on each type one by one. 1.Simple diffusion Spontaneous molecular motion triggers the molecules to move randomly from an area of higher concentration to an area of lower concentration. The molecules continue to move until they are equally distributed on both sides and they reach dynamic equilibrium. This type of movement of molecules is called simple diffusion. (L. diffundere , to spread).This type of movement occurs mostly for short-distance transport of substances moving in or out of the cell.  Related topics: Facilitated Diffusion

  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.

What are cells? Cells   Cells are the organizational units of life.In other words we can say that cells are fundamental units of life,in which all of the chemical reactions necessary for the maintenance and reproduction   of life takes place.Because all animals are made of cells,the cell is as fundamental to an understanding of zoology as atom is to an understanding of chemistry.Cells are smallest independent units of life.Structurally speaking,cells are either prokaryotic are eukaryotic. Types of cells on the basis of structure  There are two types of cells on the basis if structure. Prokaryotic cell Eukaryotic cell    Difference between prokaryotes and eukaryotes      All prokaryotes (“before nucleus”) are independent,single-celled organisms(e.g.,bacteria)The word “prokaryote” describe cells in which DNA is localized in a region but is not bound by a membrane.