What Is an Example in a Living System of How Molecular Shape Is Critical?

Molecular shape is an essential feature of living things. Neurons are nerve cells that communicate with other cells by sending electrical signals. They do this by allowing salt ions to flow in and out of them, which changes the electrical charge inside and outside the cell. Every aspect of a neuron’s function in communicating with other cells requires that certain proteins only fit certain molecules, so that only the right molecule is moved here and not there. Also, the exact fit of a molecule to a protein is how certain proteins are only turned on or off at the right time.

A neuron that is at rest and ready to send an electrical signal needs to maintain a negative electrical charge on its inside and a positive electrical charge on its outside. How does it do this? The inside of the cell has many organic acids, which have negative electrical charges. Additionally, the neuron actively pumps sodium ions (Na+) out, while pumping potassium ions (K+) in. The combination of having organic acids on the inside, less sodium inside than outside, and more potassium inside than outside makes the inside of a resting neuron negative while its outside is positive. The neuron has a protein pump on its surface called the sodium-potassium pump. This pump moves sodium ions out and then potassium ions in. It only fits three sodium ions at once or two potassium ions at once. No other ions in the body fit into the pockets in this pump.

A neuron generates an electrical signal by opening a protein channel on its surface membrane. This channel is a sodium channel, meaning that when the lid opens, only sodium ions, but no other ions, can flow through it. Since there are many sodium ions outside the cell, sodium will naturally want to rush into the cell through the sodium channel -- much like water soaking into a dry sponge. The rush of sodium ions into the cell switches the electrical charge on either side of the cell membrane. The cell is now positive on the inside and negative on the outside. This switch happens all along the neuron's membrane, which is how the electrical signal moves across a neuron. Generating an electrical signal through the movement of sodium ions works because the sodium channel only fits sodium ions.

Once the electrical signal moves down the arm of a neuron and reaches its fingertips, it causes the fingertips to release chemicals called neurotransmitters. The fingertips are right next to and nearly touching a neighboring cell. The released chemicals flow from the fingertips and bind to ion channels on the neighboring cell’s membrane. Binding causes the channels to open, which starts an electrical signal that will move from the surface to the command center of the cell. Acetylcholine is the main “go” neurotransmitter that controls muscle contraction. Gamma aminobutyric acid (GABA) is the main “stop” neurotransmitter. Each neurotransmitter has a certain shape that opens only certain ion channels. This ensures that a neurotransmitter sends a very specific message.

Molecular shape is the reason why certain chemical weapons work. Sarin gas is a chemical weapon that kills people by blocking the activity of an enzyme called acetylcholinesterase. Acetylcholine is a neurotransmitter that is involved in telling your skeletal muscles to contract. After release from the fingertips of a neuron, it must be quickly destroyed so that it cannot keep stimulating a neighboring neuron. Acetylcholinesterase is the enzyme that stops the activity of acetylcholine. Sarin gas binds to the mouth of acetylcholinesterase, the place that normally binds and breaks acetylcholine, and prevents the enzyme from attaching to its target.