1.5 Biological Chemistry
With the exception of water, most of the molecules in living
organisms contain the element carbon. Carbon is the lightest
element that can form four stable covalent bonds with other
elements. This allows it to form the enormous number of
different compounds needed to support life.
Organic Chemistry
Compounds containing carbon atoms bound to other elements such
as hydrogen, oxygen, and nitrogen are called organic compounds,
and the study of organic compounds is called organic chemistry.
Much of the human body is composed of organic compounds. While
there are many different atoms found in the bodies of living
organisms, the vast majority (over 96%) are carbon (C), hydrogen
(H), oxygen (0), and nitrogen (N).
Many organic molecules contain rings or chains of carbon atoms
with additional atoms attached (Figure 1). In many cases, the
additional atoms form clusters called functional groups that
give the organic molecule specific chemical properties. Chemical
reactions between organic molecules usually involve the
molecules' functional groups. Table 1 lists the five most
important functional groups in organic molecules. Can you
identify some in Figure 1?
Biological Compounds
Many organic molecules in living organisms are large molecules
containing dozens of carbon atoms and many functional groups.
Because of their large size, these molecules are called
macromolecules. There are four major groups of biologically
important macromolecules: carbohydrates, lipids, proteins, and
nucleic acids (Figure 2, on the next page).
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Carbohydrates
Carbohydrates are molecules that contain carbon, hydrogen, and
oxygen, and are primarily used by living organisms as a source
of energy. Plants and bacteria called cyanobacteria produce
carbohydrates by the process of photosynthesis (see section
1.17).
Monosaccharides (Simple Sugars)
The simplest carbohydrates are sweet-tasting sugars called
simple sugars, or monosaccharides. (In Greek, monos means
"single" and saccharon means "sweet thing.") The most important
monosaccharides have six carbon atoms and a number of hydroxyl
groups. For example, the primary energy-storage carbohydrate
used by living things is glucose (C SUBSCRIPT 6 H SUBSCRIPT 12 O
SUBSCRIPT 6) (Figure 3). Other six-carbon monosaccharides are
fructose (fruit sugar) and galactose (found in milk). Fructose
is much sweeter than glucose.
Disaccharides (Double Sugars)
Two monosaccharides may link together to form a disaccharide, or
double sugar. Sucrose (table sugar) is a disaccharide formed by
bonding a molecule of glucose to a molecule of fructose. This
occurs when a hydroxyl group of glucose and a hydroxyl group of
fructose react with each other (Figure 4).
Polysaccharides (Complex Carbohydrates)
Monosaccharides such as glucose and disaccharides such as
sucrose are soluble in water because their hydroxyl groups form
hydrogen bonds with the hydrogen and oxygen atoms of water
(Figure 5).
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However, sugars are insoluble in water when attached to each
other in long chains called polysaccharides, or complex
carbohydrates. Plants store chemical energy in the form of a
polysaccharide called starch. Starch is actually a mixture of
two polysaccharides, amylose and amylopectin. Both amylose
(Figure 6) and amylopectin are composed of repeating subunits of
glucose. Animals store some chemical energy in the form of
highly branched chains of glucose called glycogen. Humans store
glycogen in muscle cells, liver cells, and certain types of
white blood cells.
Molecules composed of many linked subunits are called polymers.
Polysaccharides such as amylose, amylopectin, and glycogen are
polymers of glucose. The individual subunits of a polymer are
called monomers. Thus, glucose is the monomer in amylose,
amylopectin, and glycogen.
Plant starch (amylose and amylopectin) is a common component of
food. It is found in large quantities in rice, wheat flour,
cornstarch, and potatoes. Polysaccharides are too large to be
absorbed by the cells in your digestive system. When complex
carbohydrates are eaten, reactions in your digestive system
break the bonds between glucose subunits. Your cells may then
absorb the individual glucose molecules. This process is called
digestion.
Cellulose
Plant cell walls are primarily composed of a polysaccharide
called cellulose. Cellulose is chemically similar to amylose,
but the glucose subunits are bonded in a way that humans cannot
digest. However, animals such as sheep and cows may digest
cellulose. That is why these animals may graze on pastures and
eat hay, but humans cannot. Since cellulose is the major
constituent of plant cell walls, it is the major component of
wood and paper. Cotton is also composed of cellulose. Thus,
cotton clothing is actually composed of long chains of glucose
molecules.
Cellulose fibres pass through the human digestive system
undigested. This means that cellulose has no nutrient value for
humans. However, studies have shown that a diet containing
cellulose (also called fibre or roughage) in fresh fruit,
vegetables, and grains is healthy. Cellulose attracts water and
mucus in the digestive system, and aids in the elimination of
solid wastes, helping to prevent constipation (difficulty in
eliminating wastes).
Chitin
The hard exterior skeletons of insects and crustaceans, such as
lobsters and shrimp, are composed of a modified form of
cellulose called chitin (Figure 7(a)). Chitin is also found in
mushrooms (Figure 7(b)). Chitin is a tough material that humans
find difficult to digest. However, it is used in contact lenses
and stitches used in medical operations that decompose as the
wound heals.
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Lipids
Like carbohydrates, lipids are molecules made up of carbon,
hydrogen, and oxygen, but lipids have a higher proportion of
hydrogen atoms. Lipids store more chemical energy than
carbohydrates, and are used by animals as major energy-storage
molecules. Cells in fat (adipose) tissue are full of lipid
molecules (Figure 8). Lipids are soluble in oils and other
nonpolar solvents, but are insoluble in water and aqueous
solutions. Lipids include oils, fats, waxes, phospholipids, and
steroids.
Oils and Fats
Oils and fats are composed of lipid molecules called
triglycerides. A triglyceride contains four subunits: glycerol
and three fatty acids. Glycerol is a three-carbon molecule with
a hydroxyl group attached to each carbon atom (Figure 9(a)).
Fatty acids are long chains of carbon and hydrogen atoms, with a
carboxyl group at one end (Figure 9(b)).
To form a triglyceride, a fatty acid is attached to each of the
three hydroxyl groups of glycerol (Figure 10).
Saturated and Unsaturated Fats
Fats and oils are mixtures of triglycerides, each differing in
the types of fatty-acid molecules attached to the glycerol
subunits. Fatty-acid molecules differ in length and in the
presence of double bonds between the carbons in their
hydrocarbon chains. Figure 11 shows that a fatty acid with
double bonds between its carbon atoms holds fewer hydrogen atoms
than a fatty acid with only single bonds.
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Triglycerides containing fatty acids with only single bonds are
called saturated triglycerides (saturated fats) (Figure 11(a)).
Triglycerides containing fatty acids with double bonds between
carbon atoms are called unsaturated triglycerides (unsaturated
fats) (Figure 11(b)). Triglycerides containing fatty acids with
more than one double bond are called polyunsaturated
triglycerides. Polyunsaturated triglycerides have low melting
points, and are liquid oils at room temperature. Many plants,
such as sunflower, canola, olive, and corn, produce large
amounts of polyunsaturated triglycerides. These give rise to the
many types of oils used in cooking.
Margarine is a solid fat produced from vegetable oils. Margarine
is made by reacting the polyunsaturated triglycerides in oils
with hydrogen gas. The hydrogen atoms of the gas add to the
double bonds between carbon atoms, causing the fatty acids to
become more saturated. This changes the oil from a liquid to a
solid at room temperature. Margarine is usually white when it is
produced. Manufacturers add yellow food colouring to make it look
like butter.
Animals produce large amounts of saturated triglycerides that
occur as hard fats such as lard and fats in butter. Studies have
shown that diets containing large amounts of saturated fats may
play a part in the development of clogged arteries that increase
the risk of heart attack and stroke.
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Waxes, Phospholipids, and Steroids
Waxes are lipid molecules commonly used by plants and some
animals as waterproof coatings (Figure 12).
Phospholipids are lipid molecules similar to triglycerides.
However, in phospholipids, an additional functional group
containing a phosphate group replaces one of the fatty acids
(Figure 13). Phospholipids play a key role in the structure of
cell membranes.
Steroids are lipid molecules composed of four carbon rings. Many
hormones, such as the male sex hormone testosterone (Figure 14
(a)) and the female sex hormone estradiol, are steroids. Cell
membranes contain the steroid cholesterol (Figure 14 (b)).
Although cholesterol is an essential component of cell
membranes, a high concentration of this steroid in the
bloodstream has been linked to the development of clogged
arteries and an increased risk of heart attack and stroke.
Proteins
What do Jell-O, hair, antibodies, feathers, blood clots, egg
whites, and fingernails all have in common? They are all made of
protein. Proteins are the most diverse and among the most
important molecules in living organisms (Figure 15).
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Although they have various functions, all proteins have the same
basic structure. Proteins are unbranched polymers of amino
acids. Amino acids are small molecules containing a central
carbon atom to which is attached an amino group, a carboxyl
group, a hydrogen atom, and a side chain (Figure 16) (note that
the side chain is labelled "R" in the generalized formula in
Figure 16). The structure of the side chain (R group)
distinguishes one amino acid from another. There are 20
different R groups; thus, there are 20 different amino acids.
Figure 17 illustrates the molecular formulas of some common
amino acids.
Of the 20 amino acids found in the food humans normally eat, 8
are considered to be essential amino acids. This means that the
body cannot produce them from simpler compounds; they must be
obtained from food.
Proteins differ from one another by the number and sequence of
amino acids they contain. Proteins are formed by reactions
between the amino groups and the carboxyl groups of adjacent
amino acids (Figure 18(a)). The linkage that forms between amino
acid subunits is called a peptide bond. A small chain of amino
acids is called a polypeptide. However, most functional proteins
contain hundreds or thousands of amino acids. Proteins may be
broken down into individual amino acids (Figure 18(b)). This
occurs when proteins in your food are digested.
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Proteins perform an enormous number of functions within a living
cell and a unique protein is needed for each function. Just as a
huge number of English words are created from the 26 letters of
the alphabet, a very large number of proteins may be produced
from the 20 different amino acids found in living cells.
Cells make proteins by joining amino acids together in a process
called protein synthesis. As the amino acid chain lengthens,
forces of attraction and repulsion between functional groups
cause the growing polypeptide to fold into sheets and wrap into
coils (Figure 19). Proteins with the same sequence of amino
acids will fold into the same shape. If the protein does not
assume its correct shape, it will not work properly. A protein's
shape may be altered by a single misplaced amino acid.
Proteins with lots of sheet structure form fibres such as
keratin in hair and silk in spider webs. Proteins with lots of
coils usually fold into round, or globular, proteins such as the
subunits of hemoglobin in red blood cells.
Enzymes are one of the most important groups of proteins. They
act to speed up chemical reactions. Chemicals, such as enzymes,
that speed up reactions are called catalysts. Enzymes are needed
to catalyze almost every reaction that occurs in living
organisms. Other proteins, such as insulin, act as chemical
messengers (hormones), while still others, such as collagen,
give structural support to bones, cartilage, and tendons.
Denaturation
Proteins may lose their shape if they are subjected to high
temperatures (generally above 40ºC) or are exposed to acidic,
basic, or salty environments. A change in the three-dimensional
shape of a protein caused by changes in environmental factors is
called denaturation. A denatured protein cannot carry out its
biological functions but will usually return to its normal shape
when the harsh environmental factor is removed, so long as the
peptide bonds between amino acids are not broken. If the
environmental factor breaks the peptide bonds that hold the
amino acids together, the protein will be destroyed.
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Protein denaturation can be both dangerous and useful. Fever
above 39ºC could denature critical enzymes in the brain, leading
to seizures and possibly death. Curing meats with high
concentrations of salt or sugar and pickling meats or vegetables
in vinegar preserves the food by denaturing the enzymes in
bacteria that would otherwise cause the food to spoil. Fruits
and vegetables turn brown when exposed to air. This brown colour
is caused by an enzyme reaction. The practice of blanching
fruits and vegetables by quickly dipping them in boiling water
denatures the enzymes that would otherwise cause them to turn
brown. Heat can be used to denature the proteins in hair,
allowing people to temporarily straighten curly hair or curl
straight hair.
Humans may consume animal flesh (meat) as a source of protein.
However, the fibrous proteins contained in muscle cells make raw
meat difficult to chew. Heating denatures the fibrous protein
molecules and partially decomposes them. In many cases, the
primary purpose of cooking is to denature the proteins in food.
Foods are usually a rich mixture of nutrients including
carbohydrates, lipids, proteins, nucleic acids, vitamins, and
minerals.
Nucleic Acids
Organisms store information about the structures of their
proteins in macromolecules called nucleic acids. Nucleic acids
are polymers containing subunits called nucleotides (Figure 20).
Each nucleotide is made up of three smaller components:
1. a five-carbon sugar (ribose in RNA; deoxyribose in DNA)
2. a phosphate group
3. an organic, nitrogen-containing component called a
nitrogenous base
There are four types of nitrogenous bases in DNA: adenine (A),
guanine (G), thymine (T), and cytosine (C). In a nucleic acid
chain, the sugar molecule of one nucleotide is linked to the
phosphate group of the next closest nucleotide, and a
nitrogenous base protrudes from each sugar.
There are two types of nucleic acids in organisms:
deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA is
composed of two long nucleotide strands wound around each other
like the railings of a circular staircase (Figure 20). This
"double helix" structure of DNA causes the nitrogenous bases of
the two nucleotide strands to face each other. Hydrogen bonds
between the nucleotide pairs (dotted lines) hold the double
helix together. In DNA, the bases always pair as follows: A-T
and C-G.
Unlike DNA, RNA is usually composed of a single chain of
nucleotides in the form of a helix. The nitrogenous bases are
not paired, and, instead of thymine (T), RNA contains the base
uracil (U).
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DNA contains genetic (hereditary) information that is passed
from one generation to the next. DNA stores the information for
making proteins. The order of nucleotides in DNA forms a code
that ultimately specifies the order of amino acids in a
particular protein. The set of instructions in DNA that codes a
complete protein is called a gene. Results from the Human Genome
Project, a research study that has determined the number and
sequence of nucleotides in all 46 chromosomes of a human, have
shown that there are more than 3 billion base pairs, and between
30 000 and 35 000 genes in the human genome.
RNA molecules perform many important functions in a cell. RNA
molecules called messenger RNA (mRNA) take the genetic
information in DNA out of the nucleus to ribosomes in the
cytoplasm where proteins are produced. DNA molecules remain in
the nucleus, where they are protected from enzymes in the
cytoplasm that might break them down. RNA molecules also form
part of the structure of ribosomes.
Table 2 summarizes the characteristics of biological
macromolecules.