Cell diffusion
This report is aimed to show the process of diffusion in a cell. Diffusion is where the substances required for cellular processes e.g. ions and molecules enter/ leave the cell, the process is driven by the concentration gradient either side of the cell membrane (www.rpi.edu 2013).
Diffusion according to Crank can be worked out using the equation below, (Crank, 1975)
“The amount of a property passing through a unit area per unit time is called a flux. e.g. moles m–2s–1. The flux of matter is directly proportional to the gradient of matter.” (Crank, 1975)
There are two ways in which a substance can enter/ leave a cell i.e. passive and active transport. Passive includes simple diffusion and osmosis, this happens when water is present. Active transport deals with substances binding together for diffusion to take place. Passive diffusion of particles e.g. atoms, ions and molecules does not require energy. Diffusion is where the substances are transported from a region of higher concentration to a region of lower concentration randomly continuing until the concentration is balanced and the particles are evenly distributed. Diffusion is one of several transport phenomena that occurs in nature. A distinguishing feature of diffusion is that it results in mixing or mass transport, without requiring much movement. Diffusion is relatively slow, the higher the concentration the faster it goes. Heat and some movement can speed it up (e.g. stirring a liquid or a breeze). Bigger objects take longer to diffuse through because the volume to surface area ratio is too small. This is why mammals have a respiratory system to carry oxygen to the furthest tissues and why amoebas can get the oxygen they need from diffusion. (www.encyclopedia.com 2013)
Fig 1 (yellowtang 2010)
Diffusion of one compound is independent to diffusion of others.
Simple passive diffusion is where small molecules, ions and atoms cross the lipid bilayer of a cell membrane which does not require energy.
Some examples of diffusion in biology as discussed in biologymad.com (2005) are
Gas exchange at the alveoli of the lungs —
Where oxygen moves from the air to the blood in the alveoli and carbon dioxide moves from the blood to the alveoli.
Gas exchange at cellular level —
Oxygen diffuses from the oxygenated blood to tissue cells and carbon dioxide moves in the opposite direction in the deoxygenated blood.
Fick’s law has proven the high diffusion rate is based upon, the properties of a cell, the diffusing molecule, a short distance, a larger surface area a big concentration difference.
High temperatures increase diffusion, but the larger the molecules that need to be diluted the slower the rate of diffusion (bio-medicine 2003-2012)

Oxygen diffuses across the thin alveolar wall into the blood to be taken and distributed throughout the body (BBC 2013). The picture below shows the movement of both gases during inhalation and exhalation. During inhalation oxygen is diffused from the alveoli being an area of high concentration. It is absorbed into the blood stream being an area of low concentration. On exhalation carbon dioxide follows the same process in reverse.

Fig 2
(learningon 2013)

Adaptations of lungs for gaseous exchange:
- The inner walls of the lungs are folded to increase the surface area