Question

What does it mean to say a membrane channel is selective? What does it mean when we say a membrane channel is gated? Describe two different types of membrane gated channels and explain why we do not always want them open.  

Answer 1

What does it mean to say a membrane channel is selective?

Membrane channels are proteins located on the cell membrane which enable the passive movements of ions or molecules. Channel proteins are the class of facilitative diffusion which allow the flow of ion or molecules down their concentration gradient. Channel proteins form channel like passage which resemble water-filled pores across the membrane to allow diffusion of ions or simply water as in aquaporins. These channel proteins have selective permeability which means it only allows passage of certain substances, not all. Small molecules, few ions and water can cross the membrane through channel proteins. The selectivity might be restricted to f only one ion like potassium to pass across or can be extended for little broad means like cation.

What does it mean when we say a membrane channel is gated? Describe two different types of membrane gated channels and explain why we do not always want them open.

When we say, a membrane is gated which means it allows selective passage of ions/ molecules only when it is open and this opening and closing of gate of any membrane channel is regulated by corresponding stimulus. Most of the biologocal ion channels are gated which means the gate opens or close spontaneously in response to the stimulus such as binding of ligand or voltage difference.

Ligand-gated ion channels are protein transmembrane channels which allows passage of ions such as Na⁺, K⁺, Ca²⁺, and/or Cl⁻ across the membrane in response to binding of chemical messenger called ligand such as neurotransmitter.

Voltage-gated ion channels are similar to ligand gated channels differ by the activation which is regulated by the electrical membrane potential near the channel which brings about the conformational change of the channel proteins renders the opening and closing state. They are usually ion specific such as sodium (Na⁺), potassium (K⁺), calcium (Ca²⁺), and chloride (Cl⁻) and the charge difference between the sides of the membrane triggers the opening and closing of channels.

The gate cannot be open always because the flow of charged ions through channelscauses an electrical current which results in voltage across the membrane by altering the distribution of charge. If the gate remains open, the inward homeostasis is disturbed by the collapse of electrochemical gradient across the membrane. This is very crucial to establish the resting membrane potential, shaping action potentials and other electrical signals by gating the flow of ions across the cell membrane and to maintain cell volume and ultimately homeostasis.