Water

Most mixtures in our bodies consist of chemicals dissolved or suspended in water. Water constitutes 50% to 75% of your body weight, depending on age, sex, fat content, and other factors. Its structure, simple as it is, has profound biological effects. Two aspects of its structure are particularly important: (1) its atoms are joined by polar covalent bonds, and (2) the molecule is V-shaped, with a 105° bond angle (fig. 2.9a). This makes the molecule as a whole polar, with a slight negative charge (8—) on the oxygen and a slight positive charge (8+) on each hydrogen. Like little magnets, water molecules are attracted to each other by hydrogen bonds (see fig. 2.8). This gives water a set of properties that account for its ability to support life: solvency, cohesion, adhesion, chemical reactivity, and thermal stability.

Solvency is the ability to dissolve other chemicals. Water is sometimes called the universal solvent because it dissolves a broader range of substances than any other liquid. Substances that dissolve in water, such as sugar, are said to be hydrophilic5 (HY-dro-FILL-ic); the relatively few hydro = water + philic = loving, attracted to

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64 Part One Organization of the Body substances that do not, such as fats, are hydrophobic6 (HY-dro-FOE-bic). Virtually all metabolic reactions depend on the solvency of water. Biological molecules must be dissolved in water to move freely, come together, and react. The solvency of water also makes it the body's primary means of transporting substances from place to place.

To be soluble in water, a molecule must be charged so that its charges can interact with those of water. When NaCl is dropped into water, for example, the ionic bonds between Na+ and CP are overpowered by the attraction of each ion to water molecules. Water molecules form a cluster, or hydration sphere, around each sodium ion with the Os~ pole of each water molecule facing the sodium ion. They also form a hydration sphere around each chloride ion, with the Hs+ poles facing it. This isolates the sodium ions from the chloride ions and keeps them dissolved (fig. 2.9b).

Adhesion is the tendency of one substance to cling to another, whereas cohesion is the tendency of molecules of the same substance to cling to each other. Water adheres to the body's tissues and forms a lubricating film on membranes such as the pleura and pericardium. This helps reduce friction as the lungs and heart contract and expand and rub against these membranes. Water also is a very cohesive liquid because of its hydrogen bonds. This is why, when you spill water on the floor, it forms a puddle and evaporates slowly. By contrast, if you spill a nonpolar substance such as liquid nitrogen, it dances about and evaporates in seconds, like a drop of water in a hot dry skillet.

This is because nitrogen molecules have no attraction for each other, so the little bit of heat provided by the floor is enough to disperse them into the air. The cohesion of water is especially evident at its surface, where it forms an elastic layer called the surface film held together by a force called surface tension. This force causes water to hang in drops from a leaky faucet and travel in rivulets down a window.

The chemical reactivity of water is its ability to participate in chemical reactions. Not only does water ionize many other chemicals such as acids and salts, but water itself ionizes into H+ and OH~. These ions can be incorporated into other molecules, or released from them, in the course of chemical reactions such as hydrolysis and dehydration synthesis, described later in this chapter.

The thermal stability of water helps to stabilize the internal temperature of the body. It results from the high heat capacity of water—the amount of heat required to raise the temperature of 1 g of a substance by 1°C. The base unit of heat is the calorie7 (cal)—1 cal is the amount of heat that raises the temperature of 1 g of water 1°C. The same amount of heat would raise the temperature of a nonpolar substance such as nitrogen about four times as much. The difference stems from the presence or absence of hydrogen bonding. To increase in temperature, the molecules of a substance must move around more actively. The hydrogen bonds of water molecules inhibit their movement, so water can absorb a given amount of

Oxygen

  • lt;a> Lydrogen ^^ }
  • a « ÎV»» (b) ^ ^ ^ ^ ^ ^

phobic = fearing, avoiding

Figure 2.9 Water and Hydration Spheres. (a) A water molecule showing its bond angle and polarity. (b) Water molecules aggregate around a sodium ion with their negatively charged oxygen poles facing the Na+ and aggregate around a chloride ion with their positively charged hydrogen poles facing the CF.

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The high heat capacity of water also makes it a very effective coolant. When it changes from a liquid to a vapor, water carries a large amount of heat with it. One milliliter of perspiration evaporating from the skin removes about 500 calories of heat from the body. This effect is very apparent when you are sweaty and stand in front of a fan.

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