Chemical Transport Inside Human Body
The movement of chemicals between cells and organs is usually tightly controlled.
Diffusion is the movement of molecules in a fluid phase, in random thermal (Brownian) motion. If two solutions containing the same chemical, one concentrated and the other relatively dilute, are separated by a membrane which is completely permeable and passive, the concentrations of the chemical on either side of the membrane will eventually end up being the same through simple diffusion of solutes. This is because there are many molecules of the chemical on the concentrated side, and therefore a statistically greater probability of movement from the more concentrated side to the more dilute side of the membrane. Eventually, when the concentrations are equal on both sides, the net change on either side becomes zero. Lipophilic molecules such as ethyl alcohol and the steroids, for example estradiol, appear to diffuse freely across all biological membranes.
Facilitated transport is the transport of chemicals across membranes by carrier proteins. The process does not require energy and cannot, therefore, transport chemicals against a concentration gradient. The numbers of transporter proteins may be under hormonal control. Glucose is carried into the cell by transporter proteins whose numbers are increased by insulin.
Active transport uses energy in the form of adenosine triphosphate (ATP) or other metabolic fuels. Therefore chemicals can be transported across the membrane against a concentration gradient, and the transport process can be interrupted by metabolic poisons.
Ion channels mediate active transport, and consist of proteins containing charged amino acids that may form activation and inactivation ‘gates’. Ion channels may be activated by receptors, or by voltage changes through the cell membrane. Channels of the ion Ca2+ can be activated by these two methods.
Osmosis is the passive movement of water through a semipermeable membrane, from a compartment of low solute concentration to one which has a greater concentration of the solute. (‘Solute’ refers to the chemical which is dissolved in the ‘solvent’, usually water in biological tissues.) Cells will shrink or swell depending on the concentrations of the solutes on either side of the membrane.
Phagocytosis and pinocytosis are both examples of endocytosis. Substances can enter the cell without having to pass through the cell membrane. Phagocytosis is the ingestion or ‘swallowing’ of a solid particle by a cell, while pinocytosis is the ingestion of fluid. Receptor - mediated endocytosis is the ingestion of specifically recognized substances by coated pits. These are parts of the membrane which are coated with specific membrane proteins, for example clathrin.
Exocytosis is the movement of substances, such as hormones, out of the cell. Chemicals which are stored in the small vesicles or packets are secreted or released from the cell in which they are stored by exocytosis, when the vesicle fuses with the membrane.