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Chapter 2 The Chemistry of Life

2–3 Carbon Compounds
Organic chemistry is the study of compounds with bonds between
carbon atoms. Carbon compounds also are known as organic compounds.
Many molecules in living things are very large. Very large
molecules are called macromolecules. Macromolecules form
through polymerization. In this process, smaller units, called
monomers, join to form macromolecules, called polymers.

Four groups of organic compounds found in living things are
carbohydrates, lipids, nucleic acids, and proteins.

Carbohydrates (starches and sugars) are compounds of
carbon, hydrogen, and oxygen. Living things use carbohydrates
as their main energy source. Plants and some animals also use
carbohydrates for structural purposes. Simple sugars are called
monosaccharides. When two or more monosaccharides join, they
are called polysaccharides.

Lipids (fats, oils, and waxes) are made mostly of carbon and
hydrogen. Lipid molecules are made up of compounds of fatty acids
and glycerol.
In the body, lipids are used to:
• store energy
• form parts of membranes
• form waterproof coverings

Nucleic acids contain hydrogen, oxygen, nitrogen, carbon,
and phosphorus. Nucleic acids store and transmit hereditary,
or genetic, information. There are two kinds of nucleic acids:
DNAand RNA.
Proteins are made of nitrogen, carbon, hydrogen, and oxygen.
Proteins are polymers of amino acids.
Proteins are used to:
• control the rate of reactions
• regulate cell processes
• help form bones and muscles
• carry substances into or out of cells
• help fight disease

2–4 Chemical Reactions and Enzymes
Everything that happens in an organism is based on chemical
reactions. A chemical reaction is a process that changes one set
of chemicals into another set of chemicals. The elements or compounds
that enter into the reaction are the reactants. The elements
or compounds produced by the reaction are known as products.

Chemical reactions always involve breaking the bonds in
reactants and forming new bonds in products.
Some chemical reactions release energy; others absorb energy.
Chemical reactions that release energy often occur spontaneously.
Chemical reactions that absorb energy require a source
of energy. Every chemical reaction needs energy to get started.
The energy that starts a chemical reaction is called activation
energy.

Some chemical reactions that make life possible are too slow.
Acatalyst is a substance that speeds up the rate of a chemical
reaction. Catalysts work by lowering a reaction’s activation energy.
Enzymes are proteins that act as biological catalysts. Enzymes
speed up chemical reactions that take place in cells. In an
enzyme-catalyzed reaction, the reactants are known as substrates.

Substrates bind to a site on the enzyme called an active site. The fit
of substrates binding to an active site is so specific that they are
often compared to a lock and key. Substrates remain bound to the
enzyme until the reaction is done. Once the reaction is over, the
products are released.
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