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Passive transport is diffusion of molecules across a concentration gradient from high to low concentration until an equilibrium is achieved on either side of the gradient. An example is diffusion and osmosis which is a special type of diffusion involving water molecules. For example, if more oxygen is in the air sacs, than in the surrounding capillaries, oxygen will move from the air sacs into the capillaries by diffusion. No added energy is required for this to happen.
Active transport involves the use of energy to "push" molecules against a gradient from low to high concentration. An example of this is transport of the waste urea, from the blood to the nephrons in the kidneys. There will always be more urea in the kidney filters(nephrons) in terms of concentration than in the blood where it is more dilute. Energy is needed to cause this substance to be transported against the concentration gradient so that it may be filtered from the blood and excreted.
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