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Water and Life

Water is essential for most life processes, including photosynthesis, cellular respiration, and other chemical reactions that occur in organisms.

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Water and Life - Advanced

Condensation. Just in clouds?

Condensation occurs in your cells constantly. It occurs in the form of a chemical reaction. These condensation reactions involve the formation of a water molecule from two other molecules. Water forms when two molecules, such as amino acids or monosaccharides, are joined together. The amino acids join together to form peptides (or polypeptides or proteins) and the monosaccharides join together to form disaccharides or polysaccharides.

Water and Life

Humans are composed of about 60-70 percent water (not counting water in body fat). This water is crucial for normal functioning of the body. Water’s ability to dissolve most biologically significant compounds—from inorganic salts to large organic molecules—makes it a vital solvent inside organisms and cells.

Water is an essential part of most metabolic processes within organisms. Metabolism is the sum total of all body reactions, including those that build up molecules ( anabolic reactions ) and those that break down molecules ( catabolic reactions ). In anabolic reactions, water is generally removed from small molecules in order to make larger molecules. In catabolic reactions, water is used to break bonds in larger molecules in order to make smaller molecules.

Water is central to two related, fundamental metabolic reactions in organisms: photosynthesis and cellular respiration . All organisms depend directly or indirectly on these two reactions. In photosynthesis, cells use the energy in sunlight to change water and carbon dioxide into glucose (C 6 H 12 O 6 ) and oxygen (O 2 ). This is an anabolic reaction, represented by the chemical equation:

6 CO 2 + 6 H 2 O + energy → C 6 H 12 O 6 + 6 O 2 .

In cellular respiration, cells break down glucose in the presence of oxygen and release energy, water, and carbon dioxide. This is a catabolic reaction, represented by the chemical equation:

C 6 H 12 O 6 + 6 O 2 → 6 CO 2 + 6 H 2 O + energy

Two other types of reactions that occur in organisms and involve water are dehydration and hydration reactions. A dehydration reaction occurs when molecules combine to form a single, larger molecule and also a molecule of water. (If some other small molecule is formed instead of water, the reaction is called by the more general term, condensation reaction .) It is a type of anabolic reaction. An example of a dehydration reaction is the formation of peptide bonds between amino acids in a polypeptide chain. When two amino acids bond together, a molecule of water is lost. This is shown in Figure below .

In this dehydration reaction, two amino acids form a peptide bond. A water molecule also forms.

A hydration reaction is the opposite of a dehydration reaction. A hydration reaction adds water to an organic molecule and breaks the large molecule into smaller molecules. Hydration reactions occur in an acidic water solution. An example of hydration reaction is the breaking of peptide bonds in polypeptides. A hydroxide ion (OH - ) and a hydrogen ion (H + ) (both from a water molecule) bond to the carbon atoms that formed the peptide bond. This breaks the peptide bond and results in two amino acids.

Water is essential for all of these important chemical reactions in organisms. As a result, virtually all life processes depend on water. Clearly, without water, life as we know it could not exist.


  • anabolic reaction : Endothermic reaction that occurs in organisms; chemical reaction that builds new molecules and/or stores energy.
  • catabolic reaction : Chemical reaction that breaks down more complex organic molecules into simpler substances; usually releases energy.
  • cellular respiration : Metabolic process which transfers chemical energy from glucose (a deliverable fuel molecule) to ATP (a usable energy-rich molecule); most efficient in the presence of oxygen (aerobic).
  • condensation reaction : A chemical reaction in which two molecules combine to form one single molecule, together with the loss of a small molecule, often water.
  • dehydration reaction : A condensation reaction that involves the loss of water from the reacting molecule (product).
  • hydration reaction : A chemical reaction in which a water molecule is added to a molecule, breaking the reactant into two separate products.
  • metabolism : The sum of all the chemical reactions in a cell and/or organism.
  • photosynthesis : The process by which carbon dioxide and water are converted to glucose and oxygen, using sunlight for energy.



Use this resource to answer the questions that follow.

  1. How is water able to dissolve polar molecules or ions?
  2. Describe the importance of hydrogen bonding.


  1. What percent of humans are composed of water?
  2. Summarize how metabolism in organisms depends on water.
  3. What is a condensation reaction?
  4. Distinguish between anabolic and catabolic reactions.
  5. Distinguish between between hydration and dehydration reactions.

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