Semipermeable vs Selectively permeable Membranes

Semipermeable vs Selectively Permeable Membranes

The term permeability in biology always refers to membranes. These membranes are made of lipids (phospholipids and cholesterol), integral and peripheral proteins, and carbohydrates (glycolipids and glycoproteins) that all interact with each other to form a barrier between the cell and its environment. The proportion of Carbohydrates, lipids, and fats in membranes vary by cell type and species, but in humans, they are about 50% protein, 40% lipid, and 10% carbohydrates. Cell Membranes are fluid (dynamic in movement) and can regenerate up to a certain degree when damaged. More importantly, cell membranes maintain the electrochemical gradient between the inside of a cell and its environment and can allow smaller charged molecules, water, and metabolic waste to pass in and out of it, making them permeable. This permeability is therefore a vital aspect in maintaining homeostasis.
When referring to membrane permeability there are two types found in living things: semi-permeable and selectively permeable. Both allow molecules and water to move in and out of the cell, as needed to maintain homeostasis. Semipermeable membranes are more simple in function because they are not “picky”, so If molecules are small enough they will pass through the membrane by osmosis, diffusion or following its concentration gradient from an area of higher concentration to an area of lower solute concentration. One example of a semipermeable membrane found in the body would be the tubules of nephrons within the kidney. Blood components like red blood cells, large proteins that are too large to pass through the nephrons will not pass through the tubules, while smaller solutes, Na+, and metabolic waste passes through the kidney to ultimately become filtrate in urine. Patients with renal problems who can’t properly filter blood must undergo dialysis, where an external synthetic semipermeable filter that acts as a membrane is used much like functional kidneys would.
Selectively permeable membranes are more specific (hence the name selective) as to what passes through the membrane, and when. Cell membranes are considered selectively permeable; Some molecules like water can freely pass in and out to regulate solute concentration within the cell, other molecules such as Sodium (Na+), Potassium (K+), carbon dioxide (CO2), hormones and growth factors are regulated. Of course, some molecules are not allowed in at all. Particles that are needed by the cell but cannot diffuse through the membrane on its own can pass through via active transport with the help of integral proteins permanently integrated in the cell wall, and by transport proteins that carry the molecule to wherever it needs to go to be broken down and utilized. The membrane also has pumps that use ATP to expel solutes like Na+ and K+ out of the cell, and receptors (or ligands) that allow for the passage of larger solutes into it.
Though plasma membranes in cells let some molecules like water and sodium pass through freely, they cannot be considered semipermeable because they have a degree of control over what goes in and out to maintain homeostasis. Regulation of this degree can only be done by selectively permeable membranes, and without being selective of what can pass through it the cell would not be able to maintain its inner environment and eventually die.

Semipermeable vs selectively permeable membranes-1e3kmwf

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