Cells and Movement of Substances

All living things are formed by cells, the structure of these cells varying depending on their function. Animal and plant cells have common features such as a nucleus, cytoplasm and cell membrane. Dissolved substances pass into and out of cells by diffusion Diffusion Diffusion definition: Diffusion is the movement of molecules from a high concentration to a low concentration. Dissolved substances have to pass through the cell membrane to get into or out of a cell.
Diffusion is one of the processes that allows this to happen There are many examples of diffusion occurring in our bodies; two of these are: 1. Digested food particles in the gut moving from the gut cavity to the blood in capillary of villus 2. Oxygen particles in the lungs moving from the Alveolar air space to the blood circulating around the lungs Particles continue to move from a high to a low concentration while there is a concentration gradient*.
In the lungs, the blood will continue to take in oxygen from the alveolar air spaces, so long as the concentration of oxygen there is greater than that in the blood. Oxygen diffuses across the alveolar walls and goes into the blood, where the circulation takes the oxygen-rich blood away. Osmosis Osmosis Definition: A process by which molecules of a solvent tend to pass through a semi permeable membrane* from a less concentrated solution into a more concentrated one, thus equalizing the concentrations on each side of the membrane.

Eventually the level on the more concentrated side of the membrane rises as the molecules move through the membrane, causing the level on the less concentrated side to fall. When the concentration is the same on both sides of the membrane the movement of water will be the same in both directions, and a balance has, in effect, been found. At this point, the net exchange rate of water is zero and consequently there is no more change in the level of the liquids.