Читать книгу Neurobiology For Dummies - Frank Amthor - Страница 72

What are phospholipids and how do they form membranes? First, consider the common phospholipid molecule, as shown in Figure 2-1. To understand the membrane function, remember that phospholipid molecules have a hydrophilic (meaning, “water liking”) “head” and two hydrophobic (meaning “water fearing”) “tails.” Molecules with this property are called amphipathic.

Figure 2-1: Phospho-lipid molecular structure.

Why is a molecule hydrophilic? Water molecules (H₂O) are polarized because most of their electrons are concentrated around the oxygen atom, leaving the two hydrogen atoms with a deficiency of electric charge. So, the two hydrogen ends of the water molecule are slightly positive, with a slightly negative charge around the oxygen in the middle. Although water molecules move freely in the liquid state, most of the time adjacent water molecules have the hydrogen atoms in one water molecule near the oxygen atoms in adjacent ones.

Common table salt (NaCl) dissolves in water because the positively charged sodium atom (Na⁺) is attracted to the negative oxygen region of the water molecules and “lets go” of the chloride (Cl^–) to which it is bonded. The chloride, in turn, tends to stick to the hydrogen ends of the water molecule.

So, how does the hydrophilic water molecule relate to phospholipid membranes? When phospholipid molecules are floating around in water, the polar heads are attracted to and bind water molecules. But the hydrophobic tails don’t bind water; instead, they’re more likely to interact with each other. The most stable way for these molecules to arrange themselves, then, is for the hydrophobic tails to bind together, with the heads protruding outward from a bilayer and contacting water. The schematic structure of the molecule viewed this way is shown in Figure 2-2, and the stable bilayer (molecular layer two molecules thick) is shown in Figure 2-3.

Figure 2-2: A simplified ball-and-stick model of a single phospholipid molecule.

The last piece of this puzzle is that, unlike what might be implied in Figure 2-3, phospholipid bilayers won’t form infinitely large sheets in aqueous environments. Instead, the only stable configuration is for the bilayer to close on itself and form a sphere. The size of this sphere is a function of the water forces and the chemistry of the phospholipid.

It’s interesting that the extracellular fluid around cells in animals resembles the seawater in which cells originally evolved. If you think this wasn’t an accident, you’re right!

Figure 2-3: Phospho-lipid molecules assemble to form the plasma membrane.