WHAT IS OSMOSIS? How else do molecules move in and out of cells? Besides diffusion, molecules like nutrients and waste products move in and out of cells through osmosis and active transport. Diffusion is the net movement of ions or molecules, called the solute, from a region where they are higher in concentration to a region where they are lower, that is, down a concentration gradient. Diffusion has been discussed in our video titled ‘What is Diffusion?’ Now, we will talk about osmosis. Osmosis is a special type of diffusion that allows the movement of a solvent, mainly water, from a region where it is higher in concentration to a region where it is lower through a partially permeable membrane. The membrane here is the cell surface membrane. The cellulose component of the cell wall makes it permeable and allows most substances to pass through itself. In our daily life, people make use of osmosis principles to preserve foods like pickles or other salted food. Salt or sugar is used to lower the concentration of water in the jar or bowl so the higher water contents in cucumber flow outside. We have covered active transport in our other video, but let’s refresh! Sometimes living cells can absorb certain substances even though the concentration of the substances is higher inside the cell than their environment outside. That means it’s against the concentration gradient. This process is called active transport and it is possible by spending energy in the form of ATP. Whenever a molecule is passing across the membrane into a higher concentration region, some ATP or adenosine triphosphate is utilized. In living organisms, the flow of some substances depends on active transport. This includes the absorption of dissolved mineral salts by root hair. It is also used in the uptake of glucose and amino acids by the cells in the human small intestine.