Transmembrane Ion Transport

The cell membrane is a [blank_start]phospholipid bilayer[blank_end] separating the cell interior (the [blank_start]cytoplasm[blank_end]) and from the extracellular environment. The membrane contains numerous [blank_start]proteins[blank_end], and is approximately [blank_start]7.5nm[blank_end] thick. The most important property of the cell membrane is its [blank_start]selective permeability[blank_end]: it allows the passage of some molecules but restricts the passage of others, thereby regulating the passage of materials into and out of the cell. Many substances penetrate the cellular membrane at rates reflected by their [blank_start]diffusive behaviour[blank_end] in a [blank_start]pure phospholipid bilayer[blank_end]. However, certain molecules and ions such as glucose, amino acids and Na+ pass through cell membranes much more [blank_start]rapidly[blank_end], indicating that the membrane proteins [blank_start]selectively facilitate transport[blank_end].

The membrane contains [blank_start]water-filled pores[blank_end] with diameters of about [blank_start]0.8nm[blank_end], and protein-lined pores, called [blank_start]channels[blank_end] or [blank_start]gates[blank_end], which allow the passage of specific molecules. Both the intracellular and extracellular environments comprise (among other things) a dilute aqueous solution of dissolved salts, mainly NaCl and KCl, which dissociate into Na+, K+ and Cl− ions. The cell membrane acts [blank_start]as a barrier[blank_end] to the [blank_start]free flow[blank_end] of these ions and to the flow of water.

Which of the following is an active transport process?

Passive mechanisms by which molecules are transported across the cell membrane include osmosis, diffusion, and carrier-mediated mechanisms. [blank_start]Osmosis[blank_end], i.e. the diffusion of water down its concentration gradient, is the most important mechanism by which water is transported across the cell membrane. Simple [blank_start]diffusion[blank_end] accounts for the passage of small molecules (e.g. Cl−) through pores and or lipid-soluble molecules (such as oxygen and carbon dioxide) through the lipid bilayer. [blank_start]Carrier-mediated diffusion[blank_end] refers to a process by which a molecule “hitches a lift” by binding to a carrier molecule which is lipid soluble and can move readily through the membrane. [blank_start]Carriermediatedtransport”[blank_end] occurs when a protein which sits in the membrane has an active site which may be exposed either on the exterior or interior side of the membrane depending on the conformational state of the protein. A substrate (e.g. glucose and amino acids) may bind to the protein in one conformation: the protein then undergoes a [blank_start]conformational change[blank_end], and the substrate unbinds on the other side of the membrane.

The [blank_start]concentration differences[blank_end] that exist between the intracellular and extracellular environments are set up and maintained by active processes. One of the most important of these is the [blank_start]Na+-K+ pump[blank_end], which uses the energy stored in [blank_start]ATP molecules[blank_end] to pump [blank_start]Na+[blank_end] out of the cell and [blank_start]K+[blank_end] in. Differences in interior and exterior ionic [blank_start]concentrations[blank_end] create a [blank_start]potential difference[blank_end] across the cell which also drives an ionic current down ion-specific membrane channels