How would we make a liter (L) of solution consisting of 1 mol of sucrose dissolved in water? We would measure out 342 g of sucrose and then gradually add water, while stirring, until the sugar was completely dissolved. We would then add enough water to bring the total volume of the solution up to 1 L. At that point, we would have a 1-molar (1 M) solution of sucrose. Molaritythe number of moles of solute per liter of solutionis the unit of concentration most often used by biologists for aqueous solutions. Waters capacity as a versatile solvent complements the other properties discussed in this chapter. Since these remarkable properties allow water to support life on Earth so well, scientists who seek life elsewhere in the universe look for water as a sign that a planet might sustain life.

Possible Evolution of Life on Other Planets with Water Humans have probably always gazed skyward, wondering whether other living beings exist beyond Earth. And if life has arisen on other planets, into what form or forms has it evolved? Biologists who look for life elsewhere in the universe (known as astrobiologists) have concentrated their search on planets that might have water. To date, more than 200 planets have been found outside our solar system, and there is evidence for the presence of water vapor on one or two of them. In our own solar system, Mars has been most compelling to astrobiologists as a focus of study. EVOLUTION Like Earth, Mars has an ice cap at both poles. And in the decades since the age of space exploration began, scientists have found intriguing signs that water may exist elsewhere on Mars. Finally, in 2008, the robotic spacecraft Phoenix landed on Mars and began to sample its surface. Years of debate were 52

U N I T O N E The Chemistry of Life (cid:4) Figure 3.9 Subsurface ice and morning frost on Mars. This photograph was taken by the Mars lander Phoenix in 2008. The trench was scraped by a robotic arm, uncovering ice (white in rectangle near bottom) below the surface material. Frost also appears as a white coating in several places in the upper half of the image. This photograph was colorized by NASA to highlight the ice. resolved by the images sent back from Phoenix: Ice is denitely present just under Marss surface, and enough water vapor is in the Martian atmosphere for frost to form (Figure 3.9). This exciting nding has reinvigorated the search for signs of life, past or present, on Mars and other planets. If any life-forms or fossils are found, their study will shed light on the process of evolution from an entirely new perspective. C O N C E P T C H E C K 3.2 1. Describe how properties of water contribute to the

2. Explain the saying Its not the heat; its the humidity. 3. How can the freezing of water crack boulders? 4. The concentration of the appetite-regulating hormone ghrelin is about 1.3 " 10 10 M in a fasting person. How many molecules of ghrelin are in 1 L of blood? 5. WHAT IF? A water strider (which can walk on water) has legs that are coated with a hydrophobic substance. What might be the benet? What would happen if the substance were hydrophilic? For suggested answers, see Appendix A. C O N C E P T 3.3 Acidic and basic conditions affect living organisms