Biological Importance of diffusion Diffusion is an example of passive transport and does not require metabolic energy to occur. Diffusion is defined as the net movement of particles from an area of high concentration to an area of low concentration until evenly distributed. Only small, non-polar molecules like oxygen can diffuse across plasma membranes easily. Larger, charged or polar molecules cannot pass through easily due to the hydrophobic tails in the phospholipid bilayer of a membrane and therefore require channels and carriers. This process is called facilitated diffusion. Osmosis is a type of diffusion involving water, where water molecules move from an area of high water potential to an area of low water potential through a partially permeable membrane. Osmosis is important for cells, for example in plant cells by water moving into the cell by osmosis it creates turgor in the cell and helps plant cells maintain structure. Without water moving into the plant cell by osmosis, plant cells become flaccid or plasmolyzed and do not function properly and the cells die, proving osmosis is crucial for plant cells. There is normally a greater concentration of glucose and amino acids in the ileum in the small intestines than in the blood, which therefore means there is a concentration gradient. Glucose thereby moves down the concentration gradient by facilitated diffusion from inside the ileum to the blood, which is constantly being circulated by the heart, so the glucose being taken up in the blood is continuously being removed by the cells as it used during respiration, helping to further maintain the concentration gradient between inside and outside the ileum. If glucose was not diffused into the blood then there would be no readily available supply for cells to use in respiration and organisms would therefore struggle to carry out simple functions such as respiration or muscle movement. Diffusion is essential in the process of gas exchange. During inhalation oxygen enters the alveoli thereby creating a concentration gradient of high oxygen concentration in the alveoli and low concentration in the blood. Oxygen thereby moves down the gradient out the alveoli into the blood, which is then constantly transported throughout the body to be readily available for respiration. Oppositely, there is high concentration of carbon dioxide in blood capillaries from the waste products of respiration and low concentration of carbon dioxide in alveoli, so carbon dioxide will diffuse out of blood capillaries down the concentration gradient into the alveoli to then be expelled during exhalation. If carbon dioxide was not removed from the body by diffusion the PH of the blood would significantly decrease having affects on the affinity of the haemoglobin which would affect and reduce the rate of respiration in the body. In the action potential of a nerve impulse diffusion is involved as 3 Na+ ions are actively transported out of the axon and 2 K+ actively transported into the axon, via the sodium potassium pump, maintaining the resting potential. During depolarisation the energy of the stimulus causes voltage gated sodium channels to open and therefore sodium diffuses into the axon through these channels along the electrochemical gradient. As more sodium ions diffuse into the axon, more sodium channels open causing an influx of sodium ions. Once the action potential has been reached the sodium channels close stopping any sodium channels to close and the potassium voltage gated channels open which allows potassium to diffuse out of the axon causing more potassium channels to open, the influx of potassium re-polarises the axon. The outwards diffusion of potassium causes a temporary overshoot of the electrical gradient with the inside of the axon being more negative than normal. The gates of the sodium and potassium channels close and the sodium-potassium pump is active again. Action potential relies purely on diffusion for the movement of sodium and potassium into and out of the axon. Diffusion is a crucial part of the transportation of essential substances throughout organisms due to it being a passive process and it not requiring metabolically produced energy. Without diffusion cells would struggle to obtain the molecules it required as rapidly and easily for it to function and organisms would struggle to survive and function properly.