diffusion

• Diffusion

  the process by which molecules move from an area of higher concentration to an area of lower concentration
• Diffusion rate factors

  temperature, molecular weight, steepness of concentration gradient, membrane surface area, membrane permeability
• Some of the substances transported in and out of cells by diffusion are oxygen and carbon dioxide in gas exchange, and of the waste product urea from cells into the blood plasma for excretion in the kidney.
• A single-celled organism has a relatively large surface area to volume ratio. This allows sufficient transport of molecules into and out of the cell to meet the needs of the organism.