What is the natural tendency of molecules to spread from an area of high concentration to an area of low concentration?

Diffusion

Explanation:

Diffusion is a process by which a substance or certain molecule moves from a region of high concentration to a region of low concentration due to the concentration gradient and continues till the concentration become uniform.  

This is a passive mode of transportation.  

For example – Nutrients move across the cell membrane through a lipid bilayer of a cell membrane by the process of diffusion

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the cells uses some of the atoms of the glucose molecules to atp molecules. the atoms of the glucose molecules will then be converted into energy which will be eventually stored in atp. the energy produced which are stored in atp are coming from the glucose’s chemical bonds.