The four types of cellular transport mechanisms

1. Diffusion - 
   
   Diffusion is the tendency of molecules to spread into an available space. This tendency is a result of the intrinsic thermal energy (heat) found in all molecules at temperatures above absolute zero. Without other outside forces at work, substances will move/diffuse from a more concentrated environment to a less concentrated environment. No work is performed for this to happen, as diffusion is a spontaneous process.
   
2. Osmosis - Osmosis is a special case of passive transport. In osmosis water diffuses from a hypotonic (low solute concentrated) solution to a hypertonic (high solute concentrated) solution. Generally speaking, the direction of water flow is determined by the solute concentration and not by the "nature" of the solute molecules themselves. If the blood cells in the image above are placed in salt water solutions of different concentrations, the following will occur:
   • If the salt water solution is hypertonic it would contain a higher concentration of solute and a lower concentration of water than the blood cells. Fluid would flow from the area of low solute concentration (the blood cells) to an area of high solute concentration (water solution). As a result the blood cells will shrink.
     
   • If the salt water solution is isotonic it would contain the same concentration of solute as the blood cells. Fluid would flow equally between the blood cells and the water solution. As a result the blood cells will remain the same size.
     
   • If the salt water solution is hypotonic it would contain a lower concentration of solute and a higher concentration of water than the blood cells. Fluid would flow from the area of low solute concentration (water solution) to an area of high solute concentration (the blood cells). As a result the blood cells will swell and even burst.
   
3. Facilitated diffusion - Facilitated diffusion is a type of passive transport that allows substances to cross membranes with the assistance of special transport proteins. Some molecules and ions such as glucose, sodium ions and chloride ions are unable to pass through the lipid bilayer of cell membranes.
   
   Through the use of ion channel proteins and carrier proteins that are embedded in the cell membrane these substance can be transported into the cell. Ion channel proteins allow specific ions to pass through the protein channel. The ion channels are regulated by the cell and are either open or closed to control the passage of substances into the cell. Carrier proteins bind to specific molecules, change shape and then deposit the molecules across the membrane. Once the transaction is complete the proteins return to their original position.
   
4. Active transport - the movement of molecules across a cell membrane in the direction against their concentration gradient, i.e. moving from a LOW concentration to a HIGH concentration. 

Usually associated with accumulating high concentrations of molecules that the cell needs, such as ions, glucose and amino acids. If the process uses chemical energy, such as from ATP, it is termed primary active transport. Secondary active transport involves the use of an electrochemical gradient. Active transport uses cellular energy, unlike passive transport, which does not. (So active transport is a good example of a process for which cells require energy.) Examples: the uptake of glucose in the intestines in humans and the uptake of mineral ions into root hair cells of plants.