Medical Physiology – Diffusion

Medical Physiology – Diffusion
The constant movement of molecules in liquids or gases is known as diffusion. The two subtypes of diffusion via the cell membrane are as follows: Simple diffusion is the passage of molecules across a membrane without the need for carrier protein binding. There are two ways that simple diffusion can happen: (1) through the lipid bilayer's interstices, and (2) through watery channels in transport proteins that cross the cell membrane. A carrier protein is necessary for facilitated diffusion. The carrier protein helps molecules move across the membrane, most likely by chemically attaching to them and facilitating their transit in this form.
The diffusion rate of a substance across the cell membrane is directly proportional to its lipid solubility. The lipid solubilities of oxygen, nitrogen, carbon dioxide, anesthetic gases, and most alcohols are sufficiently high to enable direct dissolution in the lipid bilayer and diffusion across the cell membrane.
Lipid-insoluble molecules, including water, diffuse through protein channels in the cell membrane. Water easily permeates the cell membrane and can also traverse transmembrane protein channels. Other lipid-insoluble molecules, primarily ions, can traverse the water-filled protein channels similarly to water molecules, if they are suitably diminutive. Protein channels exhibit selective permeability for the transport of one or more specific molecules. This permeability arises from the attributes of the channel, including its diameter, shape, and the nature of the electrical charges along its internal surfaces. The gating of protein channels facilitates the regulation of their permeability. The gates are considered to be gatelike extensions of the transport protein molecule, capable of closing over the channel opening or being removed from it through a conformational change in the protein molecule. The operation of gates is regulated through two primary method:
Voltage gating - The molecular conformation of the gate reacts to the electrical voltage across the cell membrane. The typical negative charge within the cell membrane prevents the sodium gates from opening. When the inside of the membrane loses its negative charge (becomes less negative), these gates open, permitting sodium ions to enter through the sodium channels. The initiation of sodium channel gate opening is the fundamental cause of action potentials in neurons.
Chemical gating-Certain protein channel gates are activated by the binding of an additional molecule to the protein, resulting in a conformational alteration that opens or closes the gate. This phenomenon is referred to as chemical (or ligand) gating. A significant example of chemical gating is the influence of acetylcholine on the "acetylcholine channel" at the neuromuscular junction.
Facilitated diffusion is often referred to as carrier-mediated diffusion. A chemical conveyed in this manner typically cannot traverse the cell membrane without the aid of a specific carrier protein. • Facilitated dispersion comprises two distinct steps: The molecule designated for transport enters a closed channel and attaches to a specific receptor, resulting in a conformational alteration in the carrier protein, which subsequently opens the channel to the opposite side of the membrane.
Facilitated diffusion is distinct from simple diffusion in a significant manner. The rate of simple diffusion increases in direct proportion to the concentration of the diffusing substance. In assisted diffusion, the diffusion rate approaches a maximum as the substance concentration rises. The maximal rate is determined by the speed at which the carrier protein molecule can undergo conformational change. Glucose and the majority of amino acids are among the most significant molecules that traverse cell membranes via facilitated diffusion.


Determinants Influencing the Net Rate of Diffusion
Substances can permeate the cell membrane in both directions. Consequently, the net diffusion rate of a drug in the intended direction is typically paramount. The net rate is ascertained by the subsequent factors:
• Permeability. The permeability of a membrane for a specific substance is defined as the net diffusion rate of that substance per unit area of the membrane, corresponding to a unit concentration gradient across the membrane, in the absence of electrical or pressure differentials.
• Disparity in concentration. The rate of net diffusion across a cell membrane is directly proportional to the concentration gradient of the diffusing substance between the two sides of the membrane.
• Electric potential. When an electrical potential is supplied across a membrane, ions traverse the membrane according to their electrical charges. When substantial quantities of ions traverse the membrane, a concentration gradient of those ions forms in the direction opposite to the electrical potential difference. When the concentration gradient reaches a sufficiently elevated level, the two effects counterbalance, resulting in a condition of electrochemical equilibrium. The electrical potential that equilibrates a specific concentration gradient can be calculated using the Nernst equation.

Osmosis via Selectively Permeable Membranes
“Net Water Diffusion” Osmosis is the net movement of water resulting from a concentration gradient of water. Water is the most prevalent material to permeate the cell membrane. Nevertheless, the quantity that diffuses in each direction is so meticulously adjusted under standard conditions that not even the slightest net displacement of water transpires. Consequently, the cell's volume stays invariant. A concentration gradient for water may arise across a cell membrane. When this occurs, there is a net movement of water across the cell membrane, resulting in the cell either swelling or shrinking, contingent upon the direction of the net movement. The pressure differential necessary to halt osmosis is termed osmotic pressure. The osmotic pressure exerted by solute particles in a solution is dictated by the particle concentration per unit volume of the solvent, rather than the mass of the particles.

The average kinetic energy of each molecule or ion that impacts a membrane is approximately consistent, irrespective of its molecular size. Thus, the of a solution's osmotic pressure is the concentration of particles per unit volume, rather than the mass of the solute. The osmole quantifies concentration based on the number of particles. One osmole is equivalent to 1 gram of the molecular weight of undissociated solute. Consequently, 180 g of glucose, corresponding to 1 g molecular weight, equates to 1 osmole of glucose, as glucose does not dissociate. A solution containing 1 osmole of solute per kilogram of water is characterized as an osmolality of 1 osmole per kilogram. 1/1000 osmole dissolved per kilogram results in an osmolality of 1 milliosmole per kilogram. The typical osmolality of external and intracellular fluids is approximately 300 milliosmoles per kilogram, while the osmotic pressure of these fluids is around 5500 mm Hg.