(Terminology review: cations are positively charged ions, anions are negatively charged ions.)
Let's talk about ion concentrations for a moment. Your book shows a figure (27.6) that shows the concentrations of several ions in intracellular fluid (ICF), extracellular fluid (ECF), and blood plasma. Looking at the chart, we notice several interesting things. The interstitial fluid is very high in sodium and chloride, with some bicarbonate. The intracellular fluid is very high in potassium, phosphate, and protien anions. And if we look at calcium, we see a concentration gradient present where calcium is higher outside the cell than inside the cell (although the relative concentration isn't as high as the other ions mentioned).
So outside the cell we have lots of sodium, chloride, and calcium. Inside the cell we have lots of potassium.
These facts are factoring into conversations and lectures in both BIO 137 and BIO 139 right now. Knowing where these ions are in high concentration is helpful for analyzing (there's that critical thinking again) what is going to happen in a physiological situation.
In BIO 137 we have just concluded discussing how neurons work to produce graded and action potentials. In that context, we have discussed depolarizing and hyperpolarizing gradients. The movement of sodium down its concentration gradient (into the cell) creates a depolarizing gradient, while the movement of chloride or potassium down its concentration gradient creates a hyperpolarizing gradient. If these gradients are in graded potentials in the postsynaptic cell, they can be EPSPs or IPSPs.
So knowing where these ions are in high concentration can help you to determine where they will flow if an ion channel opens (because they move down their concentration gradients). That can help you determine the effect on the cell (depolarizing/excitatory or hyperpolarizing/inhibitory).
In BIO 139 we are discussing kidneys and the effect of hormones on urine composition.
