1. Cell membrane
Figure 4-1. A: The lipids that make up biological membranes, primarily glycerophospholipids, have a head that is hydrophilic and two tails that are hydrophobic. Two layers of tail-to-tail aligned lipids form a bilayer, so that a wide hydrophobic core is bounded on either side by shallow hydrophilic borders. B: Flipases and scramblases are membrane enzymes that move lipids between the two layers or leaflets of a biological membrane. As a result, the two leaflets of a plasma membrane typically contain somewhat different lipid compositions (not illustrated). Ultimately, membranes prevent the free diffusion of charged molecules, anions (A-) and cations (C+) while allowing gases and ampiphilic molecules, compounds that have both hydrophilic and hydrophobic regions, to freely move between the separated compartments.

2. Ion channels
Figure 4-2. Charged molecules (blue spheres) only traverse biological membranes through specialized membrane-spanning proteins. An ion channel, a transporter, and a gap junction, three different types of membrane proteins, are shown in cross section. A: Ion channels can be either open or closed. In the closed configuration (left), ion channels do not allow ion movement across the membrane. In the open configuration (right), ion channels form a pore that allows ions to cross between the cytosol and the extracellular space.

3. Transporters
Figure 4-2. Charged molecules (blue spheres) only traverse biological membranes through specialized membrane-spanning proteins. An ion channel, a transporter, and a gap junction, three different types of membrane proteins, are shown in cross section. B: Transporters move ions and other small molecules across the membrane without ever forming a membrane-spanning pore. There are several different types of molecular transporters, only one of which is illustrated.

4. Gap junctions
Figure 4-2C. Charged molecules (blue spheres) only traverse biological membranes through specialized membrane-spanning proteins. An ion channel, a transporter, and a gap junction, three different types of membrane proteins, are shown in cross section. Gap junctions form a conduit between the inside of two different cells (in this case cell 1 and cell 2) through which a variety of ions (small dots) and large molecules (larger black stars) can move. At the site of a gap junction, the membranes of the two cells involved are closely juxtaposed, being separated by about 3 nm rather than the normal intercellular separation of 30 nm or so. Complementary membrane proteins, termed connexins, in the two cells join to form an actual pore.

5. Electrochemical balance
Figure 4-3. The steady-state potential, where there is no net flux of potassium ions (spheres), occurs at the potential where the chemical (outward arrow) and electrical (inward arrow) forces exerted on any given ionic species are equal and opposite. The potassium ion concentration inside cells is roughly 30-fold higher than that in the extracellular fluid. Therefore, chemical forces push potassium ions out. Since cells are negative with respect to ground, electrical forces push the positively charged potassium ions in. If we consider potassium ions exclusively, the steady-state potential predicted by the Nernst equation is about -92 mV.  

6. Charge distribution
Figure 4-4. The bulk fluids inside and outside the cell are isopotential, meaning that near to every charged molecule is another molecule or molecules of equal but opposite charge. Yet, a potential difference exists across biological membranes. This potential difference across the membrane of a neuron is carried by extracellular cations, mostly potassium ions, and intracellular anions, mostly chloride ions, that sit in very close proximity to the membrane. A shows an overall view of a cell and the immediate extracellular environment, whereas B shows a magnified view of a short stretch of membrane.

7. Inward and outward currents
Figure 4-5. A: Inward currents make the inside of the cell more positive. Most typically, they arise from a net influx of cations, typically sodium and/or calcium ions or from a reduction (red X) in the efflux of a cation, typically potassium ions. B: Most outward currents, which make the inside of the cell more negative, arise from the influx of anions, typically chloride ions or the efflux of cations, typically potassium ions.

8. Ionotropic receptor Unbound, closed Bound, open
Figure 4-6. Ionotropic receptors, such as the glutamate receptor shown here, possess a pore region through which ions can travel. Ionotropic receptors are a class of ligand-gated channels where a ligand, in this case glutamate, binds directly to the channel and gates a pore. In the absence of glutamate, the pore is shut (left). When two glutamate molecules bind to the two glutamate binding sites, the pore opens, allowing both potassium and sodium ions to pass (right).