CHAPTER 2: BIOCHEMISTRY

Section 2: Water and Solutions

Storage of Energy

Many organisms release excess heat through water evaporation.

Storage of Energy

In organisms, this ability to control temperature enables cells to maintain a constant internal temperature when the external temperature changes.

In this way, water helps cells maintain homeostasis.

Cohesion and Adhesion

Cohesion is attraction between substances of the same kind.

Because of cohesion, water and other liquids form thin films and drops.

Cohesion and Adhesion Molecules at the surface of water

are linked together by hydrogen bonds like a crowd of people linked by holding hands.

This attraction between water molecules causes a condition known as surface tension.

Cohesion and Adhesion

Adhesion is an attraction between different substances.

Adhesion powers a process, called capillary action, in which water molecules move upward through a narrow tube, such as the stem of a plant.

Aqueous Solutions

A solution is a mixture in which one or more substances are evenly distributed in another substances.

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Aqueous Solution

Many important substances in the body have been dissolved in blood or other aqueous fluids.

Because these substances can dissolve in water, they can more easily move within and between.

Polarity

The polarity of water enables many substances to dissolve in water.

When ionic compounds are dissolved in water, the ions become surrounded by polar water molecules.

Polarity

The resulting solution is a mixture of water molecules and ions.

When sodium chloride, NaCl, is dissolved in water, sodium ions, Na+, and chloride ions, Cl-, become surrounded by water molecules, H2O.

Nonpolar

Nonpolar molecules do not dissolve well in water.

Non-polar

The inability of non-polar molecules to dissolve in polar molecules is important to organisms.

For example, the shape and function of cell membranes depend on the interaction of polar water with non-polar membrane molecules.

Acid and Bases

Compounds that form hydrogen ions when dissolved in water are

called acids. When an acid is added

to water, the concentration of

hydrogen ions in the solution is increased

above that of pure water.

Acid and Bases

Compounds that reduce the concentration of hydrogen ions in a solution are called bases.

Many bases form hydroxide ions when dissolved in water.

pH Scale The pH scale measures the concentration

of hydrogen ions in a solution.

Acid BaseNeutral