The cell membrane is a phospholipid bilayer that surrounds both eukaryotic and prokaryotic cells. It is made up of a double layer of phospholipids which forms a selectively permeable barrier between two aqueous compartments, allowing only certain molecules to pass. Embedded within this bilayer are proteins which have carry out specific functions. Integral proteins act as pathways for ion and molecules. Peripheral proteins act as cell to cell recognition sites. Transmembrane protein channels and transporters allow nutrients such as sugars and amino acid to enter the cell. Carbohydrates attach to the external surface of integral proteins holds cells together as well as acting as a site where viruses or chemical messengers can attach themselves.
Passive transportation is the transportation of molecules or atoms through a cell membrane without requiring chemical energy. The rate of passive transport is determined by the permeability of the cell membrane.
Diffusion is the movement of molecules from an area of high concentration to an area of lowest concentration. Diffusion takes place at the cellular level of living things. For example, simple animals that do not have internal circulatory systems rely on diffusion to exchange gases and obtain the nutrients need to survive and grow. The root cells of plants obtain their water from the soil by method of diffusion. Diffusion requires no energy from the cell to occur.
Diffusion occurs naturally once certain conditions occur at the molecular level. Molecules move constantly and randomly, mixing together with different types and thus depend on the amount of kinetic energy present in each molecule. As molecules move and mix together, they noticeably form an area of high concentration where they are all gather to an area of lowest concentration, where the fewest are gathered. The overall movement of groups of molecules in this fashion is defined as diffusion.
A good example of diffusion in an everyday situation is the diffusing of perfume through a room. The perfume starts in an area of high concentration, which then begins to spread throughout the room to the lowest concentrated areas.
A cell membrane is composed of molecules that are always in motion. Thus, there are always temporary openings in the cell membranes due to this movement. Living cells are always covered in liquid and if the liquid concentration of a certain type of molecule were higher on the outside of a cell membrane than it is on the inside, the liquid would diffuse through the cell membrane into the cell, as shown in Figure 1. The vice versa can also occur when the liquid concentration inside the cell is higher than what is outside the cell. The rate of diffusion...