What is the definition of a partially permeable membrane?
Think of a sieve or colander as an example of the semi-permeable membrane and a mixture of sand and peas as the chemical mixture at one side of the membrane. The membrane will only allow the sand to pass through based on size. In cases such as this, larger molecules could pass through active transport channels and such that may be present on the membrane but these are energy-dependant whereas diffusion across a semi-permeable membrane is not.
