Which is the major negatively charged electrolyte in intracellular fluid?

Cells are about 75 percent water and blood plasma is about 95 percent water. Why then, does the water not flow from blood plasma to cells? The force of water also known as hydrostatic pressure maintains the volumes of water between fluid compartments against the force of all dissolved substances. The concentration is the amount of particles in a set volume of water. (Recall that individual solutes can differ in concentration between the intracellular and extracellular fluids, but the total concentration of all dissolved substances is equal.)

The force driving the water movement through the selectively permeable membrane is the higher solute concentration on the one side. Solutes at different concentrations on either side of a selectively permeable membrane exert a force, called osmotic pressure. The higher concentration of solutes on one side compared to the other of the U-tube exerts osmotic pressure, pulling the water to a higher volume on the side of the U-tube containing more dissolved particles. When the osmotic pressure is equal to the pressure of the water on the selectively permeable membrane, net water movement stops (though it still diffuses back and forth at an equal rate).

One equation exemplifying equal concentrations but different volumes is the following
5 grams of glucose in 1 liter = 10 grams of glucose in 2 liters (5g/L = 5g/L)

The differences in concentrations of particular substances provide concentration gradients that cells can use to perform work. A concentration gradient is a form of potential energy, like water above a dam. When water falls through a dam the potential energy is changed to moving energy (kinetic), that in turn is captured by turbines. Similarly, when an electrolyte at higher concentration in the extracellular fluid is transported into a cell, the potential energy is harnessed and used to perform work.

Cells are constantly transporting nutrients in and wastes out. How is the concentration of solutes maintained if they are in a state of flux? This is where electrolytes come into play. The cell (or more specifically the numerous sodium-potassium pumps in its membrane) continuously pumps sodium ions out to establish a chemical gradient. The transport protein, called the glucose symporter, uses the sodium gradient to power glucose movement into the cell. Sodium and glucose both move into the cell. Water passively follows the sodium. To restore balance, the sodium-potassium pump transfers sodium back to the extracellular fluid and water follows. Every cycle of the sodium-potassium pump involves the movement of three sodium ions out of a cell, in exchange for two potassium ions into a cell. To maintain charge neutrality on the outside of cells every sodium cation is followed by a chloride anion. Every cycle of the pump costs one molecule of ATP (adenosine triphosphate). The constant work of the sodium-potassium pump maintains the solute equilibrium and consequently, water distribution between intracellular and extracellular fluids.

The unequal movement of the positively charged sodium and potassium ions makes intracellular fluid more negatively charged than the extracellular fluid. This charge gradient is another source of energy that a cell uses to perform work. You will soon learn that this charge gradient and the sodium-potassium pump are also essential for nerve conduction and muscle contraction. The many functions of the sodium-potassium pump in the body account for approximately a quarter of total resting energy expenditure.

Intracellular and extracellular fluid of neurons contain various kinds of charged ions. These include sodium (Na+), potassium (K+), and chloride (Cl-). Additionally, these fluids contain many negatively charged protein molecules called anions (A-). The movement of such ions across neural membranes creates electrical activity. As ions move in and out of the cell, they bring their respective charges (either positive or negative) with them, thus influencing the voltage of cellular membranes.

The resting potential refers to an inactive axon's difference in electrical charge across its membrane, as measured by a voltmeter. The inside of the membrane is -70 millivolts (mV) relative to the extracellular side. This charge across the membrane is a store of potential energy and can be used at a future time. The charged particles mentioned above are unequally distributed across the membrane. As you can see, there are more K+ and anions in the intracellular fluid, and more Na+ and Cl- in the extracellular fluid. While K+ is positively charged and more abundant on the inside, there exists a great amount of negatively charged particles (the anions), accounting for the negative charge inside the membrane.

The cell membrane forms the border of a neuron and acts to control the movement of substances into and out of the cell. This membrane is composed of two layers of lipid (or fat) molecules (phospholipids, in particular).

The intracellular fluid is the clear viscous fluid within a neuron. It composes the bulk of cellular material, and provides a suspension medium for organelles and free-floating molecules.

Na+ is attracted to the inside of neurons at rest by two forces. First, the high concentration of Na+ outside the cell pushes it into the cell down the concentration gradient. Second, the electrical gradient, due to the negative charge within the neuron, tends to pull the positively charged ion inside in the cell. Since there is resistance to the passage of Na+ across the neuronal membrane, an active pump is able to maintain the higher concentration of Na+ outside the neuron.

K+ is found in higher concentrations inside the neuron at rest. Since it moves freely across the neuronal membrane, there is a tendency for K+ to move out of the neuron down the concentration gradient. This concentration gradient is partially offset by the electrostatic pressure induced by the increased negative charge inside the neuron. Therefore, the negative environment inside the neuron tends to attract the oppositely charged K+ ions. Nonetheless, K+ can leak outside the neuron. An active transport channel within the neuronal membrane is necessary to maintain the higher concentration of K+ inside.

Cl- moves freely across the neuronal membrane at rest. The negative charge within the neurons readily pushes Cl- outside the neuron via electrostatic pressure (similar charges repel). As Cl- accumulates outside the neuron, there is an increased tendency for them to move back into the neuron down the concentration gradient.

The protein molecules are large negatively charged proteins (i.e., anions) that are manufactured inside cells. They always remain inside the cell, as there are not membrane channels through which they can leave. Their charge contributes to the negative charge on the intracellular side of the membrane.

The sodium-potassium pump is composed of multiple proteins embedded within the neuronal membrane. The sodium-potassium transporters are individual protein molecules within the pump that transport three (3) Na+ ions out of the neuron in exchange for every two (2) K+ ions transported in. Since both Na+ and K+ are positively charged ions (i.e., cations) with a charge of +1, the net result of the pump's action is the movement of positive charge to the extracellular fluid. Energy produced by the neuron's mitochondria, and stored in the ATP molecule, is needed to fuel the sodium-potassium pump.

See Tutorial 5 for additional information.

Step by Step

1. At resting state, Na+ (positively charged) is attracted by the electrical force into the negatively charged cell. Since the concentration of extracellular Na+ is higher, it tends to be pulled into the cell by the concentration force. However, in the resting state there is almost no Na+ flow.
2. K+ (positively charged) is attracted into the cell by the electrical force, but repelled outwards by the diffusion force.
3. Protein molecules (negatively charged) tend to be driven out by both the electrical force and the diffusion force. However, these molecules are too big to pass through the cellular membrane.
4. Cl- (negatively charged) is repelled outward by the electrical force, but pulled inward by the diffusion force.
5. Due to the overall unequal distribution of the charged particles, the inside of the cell is -70 mV relative to outside of the cell. This is the resting potential of the neuron.

The action potential is a short-lasting but large change in the polarity of the axon's membrane. During the action potential, the voltage across the membrane reverses, resulting in the intracellular side positive relative to the extracellular side. This action is followed by another abrupt voltage change, after which the resting potential is restored. The rapid change in the polarity of the cellular membrane occurs when electrical stimulation causes the electrical potential to drop to approximately -50 mV.

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