(USMLE topics) Cell transport: permeability of the cell membrane to various molecules, types of ion channels and transporters.

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All animal cells are enclosed in a plasma membrane, which consists of 2 layers of phospholipids. The hydrophobic nature of the cell membrane makes it intrinsically permeable to small NON-polar and uncharged polar molecules, but NON-permeable to large polar molecules and CHARGED particles. Charged particles, such as ions, must use special channels to move through the membrane.
Transport of a molecule can be passive or active. PASSIVE transport does NOT require energy input because it moves the molecules “DOWNHILL”, for example, from HIGHER to LOWER concentration. ACTIVE transport, on the other hand, moves the molecules AGAINST their gradients and therefore requires ENERGY expenditure.
Ion channels permit PASSIVE transport of ions. These are transmembrane proteins that form PORES for ions to pass through. Most ion channels are SPECIFIC for a certain type of ion.
Ion channels can be classified by how they change their OPEN-CLOSED state in RESPONSE to different factors of the environment. Common types of ion channels include:
- LEAK channels: these channels are almost always OPEN allowing more or less steady flow of ions; examples are potassium and sodium leak channels in neurons.
- LIGAND-gated ion channels: these channels OPEN upon BINDING of a LIGAND. They are most commonly found at synapses, where neurons communicate via chemical messages, or neurotransmitters. An example is the GABA receptor, a chloride channel located on POST-synaptic neurons. It OPENS upon binding to GABA, a neurotransmitter released by the PRE-synaptic neuron, and allows chloride ions to flow into the cell.
- VOLTAGE-gated ion channels: these channels are REGULATED by membrane voltage. They OPEN at some values of the membrane potential and CLOSE at others. These are the channels that underlie ACTION POTENTIALS in neurons and cardiac muscles.
ACTIVE transport of ions is carried out by ion transporters, or ion PUMPS. These are transmembrane proteins that PUMP ions AGAINST their concentration gradient using cellular ENERGY, such as ATP. Most notable example is the sodium-potassium pump which maintains the resting potential in neurons by pumping two potassium IN and three sodium OUT of the cell.
Another type of ion transporters, known as SECONDARY transporters, do NOT use ATP directly. Instead, they move ONE ion DOWN its concentration gradient and use THAT ENERGY to POWER the transport of a SECOND ion. Symporters transport the two ions in the same direction, while antiporters pump the coupled molecule in the OPPOSITE direction.