.1 diffusion

• Substances may move into and out of cells across the cell membranes via diffusion.
• Diffusion is the spreading out of the particles of any substance in solution, or particles of a gas, resulting in a net movement from an area of higher concentration to an area of lower concentration
• The rate of diffusion depends on temperature - warmer temperatures increase the kinetic energy of molecules so they diffuse faster
• The rate of diffusion also depends on concentration gradient - steeper gradients result in greater rates of diffusion as there are more molecules moving down their concentration gradient
• The rate of diffusion also depends on surface area - larger surface areas allow more substances to enter/exit the cell at once
• 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.
• Factors which affect the rate of diffusion are:

  the difference in concentrations (concentration gradient)

  the temperature

  the surface area of the membrane
• Water moves from an area of high water potential to an area of low water potential until equilibrium is reached
• 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.