You will soon learn that this charge gradient and the sodium-potassium pump are also essential for nerve conduction and muscle contraction. This charge gradient is another source of energy that a cell uses to perform work. The unequal movement of the positively charged sodium and potassium ions makes intracellular fluid more negatively charged than the extracellular fluid. The constant work of the sodium-potassium pump maintains the solute equilibrium and consequently, water distribution between intracellular and extracellular fluids. Every cycle of the pump costs one molecule of ATP (adenosine triphosphate). To maintain charge neutrality on the outside of cells every sodium cation is followed by a chloride anion. 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 restore balance, the sodium-potassium pump transfers sodium back to the extracellular fluid and water follows. Sodium and glucose both move into the cell. The transport protein, called the glucose symporter, uses the sodium gradient to power glucose movement into the cell. The cell (or more specifically the numerous sodium-potassium pumps in its membrane) continuously pumps sodium ions out to establish a chemical gradient. How is the concentration of solutes maintained if they are in a state of flux? This is where electrolytes come into play. 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.Ĭells are constantly transporting nutrients in and wastes out. When water falls through a dam the potential energy is changed to moving energy (kinetic), that in turn is captured by turbines. A concentration gradient is a form of potential energy, like water above a dam. The differences in concentrations of particular substances provide concentration gradients that cells can use to perform work. One equation exemplifying equal concentrations but different volumes is the followingĥ grams of glucose in 1 liter = 10 grams of glucose in 2 liters (5g/L = 5g/L) 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). 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. Solutes at different concentrations on either side of a selectively permeable membrane exert a force, called osmotic pressure. The force driving the water movement through the selectively permeable membrane is the higher solute concentration on the one side. (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 concentration is the amount of particles in a set volume of 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. Cells are about 75 percent water and blood plasma is about 95 percent water.
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