Diffusion is the movement of any molecules from a region of high concentration to a region of low concentration, until equilibrium is reached.
Osmosis is a type of diffusion which is the movement of solvent molecules through a semipermeable membrane.
Essentially, it is the process by which water moves through the cell membrane.
Active transport is the movement of molecules from a region of low concentration to a region of high concentration gradient, this movement involves the use of energy. This movement goes against the concentration gradient.
When a large particle wants to enter the cell, the cell membrane can change its shape to surround the particle and engulf it by the process of endocytosis.
Exocytosis is the process by which special substances are transported to the external environment of the cell.
Many uncharged molecules such as ethanol, can easily penetrate the cell membrane because they dissolve in the phospholipid bilayer. Hydrophilic charged ions such as sodium and potassium cannot cross the hydrophobic centre of the membrane.
Size and shape affect the movement of substances across the cell membrane.
Glucose and amino acids are examples of large molecules that use carrier proteins to move through the cell membrane. However, very large molecules move out of the cell through the process of endocytosis and exocytosis.
If the concentration gradient is high then the substance will diffuse rapidly. However, if it is low, then it would take ages for it to diffuse.
When the concentration reaches equilibrium there will be no movement between the cells.
The surface area divided by the volume is called the surface area to volume ratio.