What are the differences between diffusion, osmosis, facilitated diffusion & active transport? How are these processes related to transport across cell membranes?
1) Diffusion : Is the movement of molecules / ions from a region of higher to lower concentration. It may or may not occur across a semi permeable membrane.
In diffusion there is no challenge involved as it is along the concentration gradient, but in active transport movement of molecules occur against concentration gradient ie; from lower to higher concentration.
2) For active transport, energy is vital for movement, in this case protein molecules function as molecular pumps to enable the cell accumulate glucose/ions, against concentration gradient. Here metabolic energy ATP is required.
3)Facilitated diffusion is a type of passive transport in which ions/molecules cross the semi permeable membrane because permeases present in the membrane facilitate the transport.
Like simple diffusion facilitated diffusion doesn't require metabolic energy and simply occurs across the concentration gradient.
4) Osmosis : is movement of water from hypotonic solution (lower concentration) to hypertonic solution (higher concentration) through a semi permeable membrane.
The cell membrane being permeable to water allows to and fro movement of water molecules along the concentration gradient.