GEF

Unit 2  |  Water Science and the Water Cycle 43 The tendency of water molecules to stick together is known as cohesion , which can be observed when molecules join together in a drop of water. The attraction between water mol- ecules creates surface tension , making rain form into drops and making it possible for insects and other organisms to walk on the surface of a pond. Capillary action , the movement of water upward through a tube or small opening, occurs from the ability of water to cling to surfaces. This is another important property for living organ- isms, as it is allows blood to move through blood vessels in animals, and pulls water up from a plant’s roots to its leaves. Polarity not only causes water molecules to be attracted to each other, but also explains why water is a universal solvent , enabling it to dissolve many thousands of substances. Because of its electrical charge, it readily attracts and dissolves other molecules, such as sugar or salt. In organisms, water trans- ports dissolved nutrients and gases throughout the body. Therefore, water is essential to metabolic reactions such as photosynthesis and respiration . Water attracts and dissolves toxic chemicals and other pollutants. COHESION A measure of how well molecules stick to each other, caused by the attractive force between like molecules. SURFACE TENSION A cohesive force on the sur- face of liquids. CAPILLARY ACTION The ability of liquids to flow through narrow spaces, such as blood vessels or plant tis- sues, against gravity. SOLVENT A material that can dis- solve and transport other substances. METABOLIC REACTIONS Chemical reactions that occur within living cells to sustain life. PHOTOSYNTHESIS The production of carbohy- drates from carbon dioxide and hydrogen in the chloro- phyll-containing tissues of plants in the presence of light. RESPIRATION The biological process by which cells use oxygen to break down sugar and pro- duce energy.