How exactly do our cell membranes work?
The properties of membranes define how they interact with the molecules around them. Semipermeability describes a characteristic of certain membranes to only allow specific molecules or ions to pass. Osmosis is the process in which molecules of a solvent pass through a semipermeable membrane, from lower concentration to higher concentration, to equalize the concentration. The law of electroneutrality provides a supporting piece of evidence behind how concentration gradients occur to preserve chemical equilibrium in the membranes of neurons. All of this creates the basis of how neurons fire and interact amongst each other.
The Chemical Basis of Neurons
Neurons, which are excitable nerve cells, possess different concentrations of ions around its body. Recall that ions are simply electrically charged atoms or groups of atoms that have either lost or gained an electron. A positively charged ion, such as K+, is called a cation. A negatively charged ion, such as Cl–, is called an anion. Electrical events which regulate signalling in the nervous system, such as action potentials, are dictated by the distribution of ions around the nerve membranes.
Ion Transport in a Neuron
The ion concentration gradients (the difference in concentrations across the membrane) is partially determined by proteins known as active transporters. The function of an active transporter is quite self explanatory—they actively move ions in and out of neurons against the concentration gradient, meaning that ions are moved from high areas of concentration to lower areas of concentration, kind of like pumps. Since they are pumping against the natural concentration gradient, cellular energy (ATP) is required to achieve this process.
Figure 1: Diagram of active transport using an ion pump.
Image source: https://www.expii.com/t/pumps-active-transport-definition-types-10101
Active transporters constitute only half of the equation, as they are not the only mechanism which allows electrical potentials to be generated. Ion channels, which are proteins that only allow specific ions to cross the membrane in the direction of the concentration gradient, contribute to the selectivity of the permeability of membranes. Because of this semipermeability, the ion channels and transporters are essentially working against each other—one works against the concentration gradient, while the other works with the concentration gradient. It is through this competition that things such as the resting membrane potential and action potentials are generated.
Figure 2: Diagram of both ion pumps and ion channels.
The green objects are ion channels, as they transport ions with the concentration gradient. The pink is an ion pump, which facilitates active transport against the concentration gradient.
Image source: https://www.differencebetween.com/difference-between-ion-channel-and-ion-pump/
When analyzing the process of water and solvents crossing membranes, a term frequently used is osmosis. This term describes the diffusion of water across semipermeable membranes from low to high concentration in order to equalize those concentrations. The force behind this movement, named osmotic pressure, is due to the uneven distribution of the molecules on the two sides of the membrane.
Figure 3: Diagram of osmotic pressure and its effect on the semi-permeable membrane.
The Law of Electroneutrality
Closely related to the process of osmosis and chemical equilibriums is the Law of Electroneutrality. This law declares that the total amount of negative charges is equal to the total amount of positive electrical charges. Consider the following example: a potassium chloride (KCl) solution is separated by a semipermeable membrane. To preserve electroneutrality, this would mean that there would be an equal concentration of KCl between the two sides of the membrane. In addition, this equilibrium would have to be preserved on each individual side of the membrane. Of course, if an abundance of KCl is added to one side of the membrane, the concentrations would naturally equalize.
The Link to Neurons
The principles discussed in this article have practical applications in the world of neurons. The plasma membrane of a neuron is semipermeable—it is permeable to K+ ions, while it is only a little permeable to Cl- and Na+ ions. Electroneutrality is attained through a balanced quantity of these negatively and positively charged ions. Osmotic balance is achieved between the extracellular fluid and cytoplasm through the movement of water, to ensure that the number of particles present on either side is even.
All of these concepts contribute to the equilibrium of electrical potential in the neuronal membrane, where the inside is more negative than the outside. In most neurons, this potential is roughly -60 to -75 millivolts. If a change in the membrane potential makes the inside more negative, it is termed hyperpolarization, while an opposite change is called depolarization.
Semipermeability - The trait of only allowing certain ions or molecules to cross a membrane.
Osmosis - The diffusion of water across semipermeable membranes from low to high concentration in order to equalize the concentrations.
Law of Electroneutrality - In any ionic solution, the sum of the negative charges attracts an equal amount of positive charges.
Ions - Electrically charged atoms or groups of atoms that have either lost or gained an electron. Cation: positively charged atom. Anion: negatively charged atom.
Action Potential - Spike of electric activity; occurs information is sent down the axon
Concentration gradient - When a solute is more concentrated in one area than another, creating a gradient of concentration in the area
Active Transport - The movement of molecules from a region of low concentration to high concentration. This movement against the concentration gradient requires energy.
Ion channels - Specialized membrane proteins that only allow specific ions to pass through.
Resting membrane potential - When a neuron is close to the equilibrium potential for K+Osmotic
pressure - Force that causes osmosis to occur
Hyperpolarization - When membrane potential is most negative
Depolarization - When membrane potential is most positive
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