Why is selective permeability important?

Answer 1

Selecive permeability is important because it keeps cells functioning properly by letting only wanted molecules (solutes) in and unwanted solutes out.

In addition to keeping the "bad stuff" out (e.g. bacteria, viruses), selective permeability is essential to the function of our nervous system. Without it, our neurons would not "fire". This is because selective permeability (think sodium potassium protein pump and active transport that requires ATP), creates a negative membrane potential. At rest potassium ions flow out but the membrane is impermeable to sodium ions. Neuron to neuron signaling occurs when there is a depolarization at an axon that causes the permeability to temporarily "switch" so that potassium and sodium ions can enter the cell. This triggers an action potential which jumps along nerve cells. This action potential is converted into a chemical signal as it triggers a calcium ion influx which in turns triggers the production and transportation of neurotransmitter-vesicles, and exocytosis into the synapse between neurons. Receptors on the adjacent neuron receive the neurotransmitter and the "signal" is communicated onwards. Protein pumps return levels of Na, K and CA to "resting" levels awaiting the next signal. Without selective permeability gradients of Na, K, CA and other ions could not be created to "drive" these and other processes.

There is much more that can be said about selective permeability. It allows glycoproteins to sit in the cell membrane and act as antibodies and glycolipids to act as signals on the cell membrane. Proteins embedded in the cell membrane can change shape and respond to feedback loops controlling the influx and efflux of substances and maintaining homeostasis.