C.9.2

=Describe the importance of the difference in Na+ and K+ concentrations across the cell membrane=

The resting nerve has a high concentration of Na+ ions on the outside of the membrane, and K+ ions and negatively charged non-diffusing proteins on the inside. Because of the movement of K+ ions to the outside, the charge on the membrane is positive on the outside and negative on the inside. The difference in charge between the outside and inside of the cell allows nerve cells to generate electrical impulses which lead to nerve impulses sent down the axons of nerves to the brain.

Mammals have a much higher concentration of potassium ions inside the cell than outside and a much higher concentration of sodium ions outside the cell than inside. The reasons for this are that cell protein synthesis requires high potassium ion concentrations and nerve impulses require a concentration gradient between the inside and outside of a neurone. Because of the uneven concentrations, it would be expected that diffusion of both species of ions across the membrane would occur and the concentration would eventually equalize. The body, however, needs the ions' concentration gradient for cell protein synthesis and for generating nerve impulses. So, the body uses a sodium-potassium pump, which requires energy obtained from the conversion of ATP into ADP, to keep the concentration of Na+ higher on the outside, and K+ higher on the inside.

Protein complexes in the cell membrane act as one-way valves, continuously pumping sodium ions out of the cell and pumping potassium ions into the cell through a type of transport called **active transport**, which requires ATP. The reason is mainly due to charge density. **Both the K+ and Na+ ions have a charge of +1 but the larger potassium ions can pass more easily through a medium of lower polarity, such as that provided by the protein pump because of its larger radius**. The pump itself is a protein molecule, which has three sites where Na+ can bind and two sites where K+ can bind. As it pumps it changes shape, due to ATP phosphorylation, so that Na+ is removed from the cell and K+ is drawn in. This kind of pump or transport controls the water content of the cell and also helps drive the transport of amino acids and sugars. Overall, this process controls a nerve's ability to conduct electricity, contraction of muscles, the absorption of food in the digestive intestines and the production of urine in the kidneys.



Potassium ions (K+) pass more easily through the cell membrane than sodium ions (Na+) because even though they have the same charge they are larger in size